Efavirenz is an NNRTI. Efavirenz activity is mediated predominantly by noncompetitive inhibition of HIV-1 reverse transcriptase (RT). Emtricitabine, a synthetic nucleoside analog of cytidine, is phosphorylated by cellular enzymes to form emtricitabine 5'-triphosphate, which inhibits the activity of the HIV-1 RT by competing with the natural substrate deoxycytidine 5'-triphosphate and by being incorporated into nascent viral DNA, resulting in chain termination. Tenofovir DF is an acyclic nucleoside phosphonate diester analog of adenosine monophosphate. Tenofovir DF requires initial diester hydrolysis for conversion to tenofovir and subsequent phosphorylations by cellular enzymes to form tenofovir diphosphate. Tenofovir diphosphate inhibits the activity of HIV-1 RT by competing with the natural substrate deoxyadenosine 5'-triphosphate and terminates the chain after incorporation into DNA. [#]

In HIV-infected patients, time-to-peak plasma concentrations (Tmax) of efavirenz were approximately 3 to 5 hours and steady-state plasma concentrations were reached in 6 to 10 days. In 35 patients receiving efavirenz 600 mg once daily, steady-state peak plasma concentration (Cmax) was 12.9 ± 3.7 μM (mean ± SD), plasma trough concentration (Cmin) was 5.6 ± 3.2 μM, and the area under the plasma concentration-time curve (AUC) was 184 ± 73 μM•hr. Efavirenz is highly bound (approximately 99.5–99.75%) to human plasma proteins, predominantly albumin. In vitro studies suggest cytochrome P450 (CYP) 3A4 and CYP2B6 are the major isozymes responsible for efavirenz metabolism. Efavirenz has been shown to induce CYP enzymes, resulting in induction of its own metabolism. Efavirenz has a terminal half-life of 52 to 76 hours after single doses and of 40 to 55 hours after multiple doses. Following administration of 14C-labeled efavirenz, 14% to 34% of the dose was recovered in the urine (mostly as metabolites) and 16% to 61% was recovered in feces (mostly as parent drug). [#]

Following oral administration, emtricitabine is rapidly absorbed, with the Cmax occurring at 1 to 2 hours post-dose. Following multiple-dose, oral administration of emtricitabine to 20 HIV-infected patients, the steady-state mean Cmax was 1.8 ± 0.7  μg/mL, and the mean AUC over a 24-hour dosing interval was 10.0 ± 3.1 μg (hr)/mL. The mean steady state Cmin at 24 hours post-dose was 0.09 μg/mL.The mean absolute bioavailability of emtricitabine was 93%. In vitro binding of emtricitabine to human plasma proteins is less than 4% and is independent of concentration over the range of 0.02 to 200 μg/mL. Emtricitabine is eliminated by a combination of glomerular filtration and active tubular secretion. Following a single oral dose, the half-life is approximately 10 hours. Following administration of radiolabelled emtricitabine, approximately 86% is recovered in the urine, and 13% is recovered as metabolites. The metabolites of emtricitabine include 3′-sulfoxide diastereomers and their glucuronic acid conjugate. [#] 

Following oral administration of a single, 300-mg dose of tenofovir DF to fasting patients, the mean Cmax (achieved in approximately 1 hour) was 296 ± 90  ng/mL, and the mean AUC was 2,287 ± 685  ng(h)/mL. The oral bioavailability of tenofovir from tenofovir DF in fasting patients is approximately 25%. In vitro binding of tenofovir to human plasma proteins is less than 0.7% and is independent of concentration over the range of 0.01 to 25 μg/mL. Tenofovir is eliminated by a combination of glomerular filtration and active tubular secretion with a renal clearance in adults with normal renal function of 243 ± 33 mL/min (mean ± SD). Following a single oral dose, the terminal elimination half-life is approximately 17 hours. Approximately 70% to 80% of an IV dose of tenofovir is recovered unchanged in the urine. [#]

Atripla has not been evaluated in the presence of food. Administration of efavirenz tablets with a high fat meal increased the mean AUC and Cmax of efavirenz by 28% and 79%, respectively, compared to administration in the fasted state. Compared to fasted administration, dosing of tenofovir DF and emtricitabine in combination with either a high fat meal or a light meal increased the mean AUC and Cmax of tenofovir by 35% and 15%, respectively, without affecting emtricitabine exposures. [#]

The pharmacokinetics of efavirenz has not been studied in subjects with renal insufficiency; however, less than 1% of efavirenz is excreted unchanged in the urine, so the impact of renal impairment on efavirenz elimination should be minimal. The pharmacokinetics of emtricitabine and tenofovir DF are altered in subjects with renal impairment. In subjects with creatinine clearance of less than 50 mL/min, Cmax and AUC0-∞ of emtricitabine and tenofovir were increased. [#]

Atripla is in FDA Pregnancy Category D. There are no adequate and well-controlled studies of Atripla in pregnant women. Pregnancy should be avoided in women receiving Atripla. Barrier contraception should always be used in combination with other methods of contraception. Because of the long half-life of efavirenz, use of adequate contraceptive measures for 12 weeks after discontinuation of Atripla is recommended. Women of childbearing potential should undergo pregnancy testing before initiation of Atripla. If this drug is used during the first trimester of pregnancy, or if the patient becomes pregnant while taking this drug, the patient should be apprised of the potential harm to the fetus. Atripla should be used during pregnancy only if the potential benefit justifies the potential risk to the fetus, such as in pregnant women without other therapeutic options. To monitor fetal outcomes of pregnant women, an Antiretroviral Pregnancy Registry has been established. Physicians are encouraged to register patients who become pregnant online at http://www.APRegistry.com or by calling 1-800-258-4263. [#]

As of July 2009, the Antiretroviral Pregnancy Registry has received prospective reports of 661 pregnancies exposed to efavirenz-containing regimens, nearly all of which were first-trimester exposures (606 pregnancies). Birth defects occurred in 14 of 501 live births (first-trimester exposure) and 2 of 55 live births (second/third-trimester exposure). One of these prospectively reported defects with first-trimester exposure was a neural tube defect. A single case of anophthalmia with first-trimester exposure to efavirenz has also been prospectively reported; however, this case included severe oblique facial clefts and amniotic banding, a known association with anophthalmia. There have been six retrospective reports of findings consistent with neural tube defects, including meningomyelocele. All mothers were exposed to efavirenz-containing regimens in the first trimester. Although a causal relationship of these events to the use of efavirenz has not been established, similar defects have been observed in preclinical studies of efavirenz. [#]

HIV-1 isolates with reduced susceptibility to the combination of emtricitabine and tenofovir have been selected in cell culture and in clinical studies. Genotypic analysis of these isolates identified the M184V/I and K65R amino acid substitutions in the viral reverse transcriptase. [#]

In a clinical study of treatment- naïve subjects (Study 934) resistance analysis was performed on HIV-1 isolates from all confirmed virologic failure subjects with greater than 400 copies/mL of HIV-1 RNA at Week 144 or early discontinuations. Genotypic resistance to efavirenz, predominantly the K103N substitution, was the most common form of resistance that developed. Resistance to efavirenz occurred in 13/19 analyzed subjects in the emtricitabine + tenofovir DF group and in 21/29 analyzed subjects in the zidovudine/lamivudine fixed-dose combination group. The M184V amino acid substitution, associated with resistance to emtricitabine and lamivudine, was observed in 2/19 analyzed subject isolates in the emtricitabine + tenofovir DF group and in 10/29 analyzed subject isolates in the zidovudine/lamivudine group. Through 144 weeks of Study 934, no subjects developed a detectable K65R substitution in their HIV-1 as analyzed through standard genotypic analysis. [#]

In a clinical study of treatment- naïve subjects, isolates from 8/47 (17%) analyzed subjects receiving tenofovir DF developed the K65R substitution through 144 weeks of therapy; 7 of these occurred in the first 48 weeks of treatment and one at Week 96. In treatment experienced subjects, 14/304 (5%) of tenofovir DF treated subjects with virologic failure through Week 96 showed greater than 1.4 fold (median 2.7) reduced susceptibility to tenofovir. Genotypic analysis of the resistant isolates showed a substitution in the HIV-1 RT gene resulting in the K65R amino acid substitution. [#]

Cross-resistance has been recognized among NNRTIs. Cross resistance has also been recognized among certain NRTIs. The M184V/I and/or K65R substitutions selected in cell culture by the combination of emtricitabine and tenofovir are also observed in some HIV-1 isolates from subjects failing treatment with tenofovir in combination with either lamivudine or emtricitabine, and either abacavir or didanosine. Therefore, cross-resistance among these drugs may occur in patients whose virus harbors either or both of these amino acid substitutions. [#]

Emtricitabine: Emtricitabine, a synthetic nucleoside analog of cytidine, is phosphorylated by cellular enzymes to form emtricitabine 5'-triphosphate. Emtricitabine 5'-triphosphate inhibits the activity of the HIV-1 RT by competing with the natural substrate deoxycytidine 5'-triphosphate and by being incorporated into nascent viral DNA which results in chain termination. Emtricitabine 5′-triphosphate is a weak inhibitor of mammalian DNA polymerase α, β, ε, and mitochondrial DNA polymerase γ.

Rilpivirine: Rilpivirine is a diarylpyrimidine non-nucleoside reverse transcriptase inhibitor of HIV-1 and inhibits HIV-1 replication by non-competitive inhibition of HIV-1 RT. Rilpivirine does not inhibit the human cellular DNA polymerases α, β, and mitochondrial DNA polymerase γ.

Tenofovir Disoproxil Fumarate: Tenofovir DF is an acyclic nucleoside phosphonate diester analog of adenosine monophosphate. Tenofovir DF requires initial diester hydrolysis for conversion to tenofovir and subsequent phosphorylations by cellular enzymes to form tenofovir diphosphate. Tenofovir diphosphate inhibits the activity of HIV-1 RT by competing with the natural substrate deoxyadenosine 5′-triphosphate and, after incorporation into DNA, by DNA chain termination. Tenofovir diphosphate is a weak inhibitor of mammalian DNA polymerases α, β, and mitochondrial DNA polymerase γ.

Pharmacodynamics

Effects on Electrocardiogram
The effect of rilpivirine at the recommended dose of 25 mg once daily on the QTcF interval was evaluated in a randomized, placebo and active (moxifloxacin 400 mg once daily) controlled crossover study in 60 healthy adults, with 13 measurements over 24 hours at steady state. The maximum mean time-matched (95% upper confidence bound) differences in QTcF interval from placebo after baseline-correction was 4.8 (8.2) milliseconds (i.e., below the threshold of clinical concern). When supratherapeutic doses of 75 mg once daily and 300 mg once daily of rilpivirine were studied in healthy adults, the maximum mean time-matched (95% upper confidence bound) differences in QTcF interval from placebo after baseline-correction were 10.7 (15.3) and 23.3 (28.4) milliseconds, respectively. Steady-state administration of rilpivirine 75 mg once daily and 300 mg once daily resulted in a mean steady-state Cmax approximately 2.6-fold and 6.7-fold, respectively, higher than the mean Cmax observed with the recommended 25 mg once daily dose of rilpivirine.

Pharmacokinetics

Complera: Under fed conditions (total calorie content of the meal was approximately 400 kcal with approximately 13 grams of fat), rilpivirine, emtricitabine and tenofovir exposures were bioequivalent when comparing Complera to Emtriva capsules (200 mg) plus Edurant tablets (25 mg) plus Viread tablets (300 mg) following single-dose administration to healthy subjects (N=34). Single-dose administration of Complera tablet to healthy subjects under fasted conditions provided approximately 25% higher exposure of rilpivirine compared to administration of Emtriva capsules (200 mg) plus Edurant tablets (25 mg) plus Viread tablets (300 mg), while exposures of emtricitabine and tenofovir were comparable (N=15).

Emtricitabine: Following oral administration, emtricitabine is absorbed with peak plasma concentrations occurring at 1–2 hours post-dose. Following multiple dose oral administration of Emtriva to 20 HIV-1 infected subjects, the mean steady-state plasma emtricitabine Cmax was 1.8 ± 0.7 μg per mL and the AUC over a 24-hour dosing interval was 10.0 ± 3.1 μg•hr per mL. The mean steady state plasma trough concentration at 24 hours post-dose was 0.09 μg per mL. The mean absolute bioavailability of Emtriva capsules was 93%. Less than 4% of emtricitabine binds to human plasma proteins in vitro over the range of 0.02 to 200 μg per mL. Following administration of radiolabelled emtricitabine, approximately 86% is recovered in the urine, approximately 14% in the feces and 13% is recovered as metabolites in the urine. The metabolites of emtricitabine include 3′-sulfoxide diastereomers (approximately 9% of the dose) and the glucuronic acid conjugate (approximately 4% of the dose). Emtricitabine is eliminated by a combination of glomerular filtration and active tubular secretion with a renal clearance in adults with creatinine clearance >80 mL per minute of 213 ± 89 mL per minute (mean ± SD). The plasma emtricitabine half-life is approximately 10 hours.

Rilpivirine: The pharmacokinetic properties of rilpivirine have been evaluated in adult healthy subjects and in adult antiretroviral treatment-naive HIV-1 infected subjects. Exposure to rilpivirine was generally lower in HIV-1 infected subjects than in healthy subjects. After oral administration, the Cmax of rilpivirine is achieved within 4–5 hours. The absolute bioavailability of rilpivirine is unknown. Rilpivirine is approximately 99.7% bound to plasma proteins in vitro, primarily to albumin. In vitro experiments indicate that rilpivirine primarily undergoes oxidative metabolism by the cytochrome CYP3A system. The terminal elimination half-life of rilpivirine is approximately 50 hours. After single dose oral administration of 14Crilpivirine, on average 85% and 6.1% of the radioactivity could be retrieved in feces and urine, respectively. In feces, unchanged rilpivirine accounted for on average 25% of the administered dose. Only trace amounts of unchanged rilpivirine (less than 1% of dose) were detected in urine.

Tenofovir Disoproxil Fumarate: Following oral administration of a single 300 mg dose of Viread to HIV-1 infected subjects in the fasted state, Cmax was achieved in one hour. Cmax and AUC values were 0.30 ± 0.09 μg per mL and 2.29 ± 0.69 μg•hr per mL, respectively. The oral bioavailability of tenofovir from Viread in fasted subjects is approximately 25%. Less than 0.7% of tenofovir binds to human plasma proteins in vitro over the range of 0.01 to 25 μg per mL. Approximately 70-80% of the intravenous dose of tenofovir is recovered as unchanged drug in the urine within 72 hours of dosing. Tenofovir is eliminated by a combination of glomerular filtration and active tubular secretion with a renal clearance in adults with creatinine clearance >80 mL per minute of 243.5 ± 33.3 mL per minute (mean ± SD). Following a single oral dose, the terminal elimination half-life of tenofovir is approximately 17 hours.

Effects of Food on Oral Absorption
Take Complera with a meal. A food effect trial was not conducted for Complera. Therefore, the specific effect of food with Complera tablets on rilpivirine, emtricitabine and tenofovir exposure has not been established. The recommendation to administer Complera with a meal is based on the increased exposure that was observed when rilpivirine tablets were administered under fed conditions.

Special Populations
Pediatric Patients
Emtricitabine has been studied in pediatric subjects from 3 months to 17 years of age. Tenofovir DF has been studied in adolescent subjects (12 to less than 18 years of age). The pharmacokinetics of rilpivirine in pediatric subjects have not been established.

Patients with Renal Impairment
Emtricitabine and Tenofovir Disoproxil Fumarate: The pharmacokinetics of emtricitabine and tenofovir DF are altered in subjects with renal impairment. In subjects with creatinine clearance below 50 mL per minute or with end stage renal disease requiring dialysis, Cmax, and AUC of emtricitabine and tenofovir were increased. Rilpivirine: Population pharmacokinetic analysis indicated that rilpivirine exposure was similar in HIV-1 infected subjects with mild renal impairment relative to HIV-1 infected subjects with normal renal function. There is limited or no information regarding the pharmacokinetics of rilpivirine in patients with moderate or severe renal impairment or in patients with end-stage renal disease, and rilpivirine concentrations may be increased due to alteration of drug absorption, distribution, and metabolism secondary to renal dysfunction.

Patients with Hepatic Impairment
Emtricitabine: The pharmacokinetics of emtricitabine have not been studied in subjects with hepatic impairment; however, emtricitabine is not significantly metabolized by liver enzymes, so the impact of liver impairment should be limited. Rilpivirine: Rilpivirine is primarily metabolized and eliminated by the liver. In a study comparing 8 subjects with mild hepatic impairment (Child-Pugh score A) to 8 matched controls and 8 subjects with moderate hepatic impairment (Child-Pugh score B) to 8 matched controls, the multiple dose exposure of rilpivirine was 47% higher in subjects with mild hepatic impairment and 5% higher in subjects with moderate hepatic impairment. Tenofovir Disoproxil Fumarate: The pharmacokinetics of tenofovir following a 300 mg dose of Viread have been studied in non-HIV infected subjects with moderate to severe hepatic impairment. There were no substantial alterations in tenofovir pharmacokinetics in subjects with hepatic impairment compared with unimpaired subjects.

Hepatitis B and/or Hepatitis C Virus Coinfection
Pharmacokinetics of emtricitabine and tenofovir DF have not been fully evaluated in hepatitis B and/or C virus-coinfected patients. Population pharmacokinetic analysis indicated that hepatitis B and/or C virus coinfection had no clinically relevant effect on the exposure to rilpivirine.

Antiviral Activity

Emtricitabine, Rilpivirine, and Tenofovir Disoproxil Fumarate: The triple combination of emtricitabine, rilpivirine, and tenofovir was not antagonistic in cell culture.

Emtricitabine: The antiviral activity of emtricitabine against laboratory and clinical isolates of HIV-1 was assessed in lymphoblastoid cell lines, the MAGI-CCR5 cell line, and peripheral blood mononuclear cells. The 50% effective concentration (EC50) values for emtricitabine were in the range of 0.0013–0.64 μM. Emtricitabine displayed antiviral activity in cell culture against HIV-1 clades A, B, C, D, E, F, and G (EC50 values ranged from 0.007–0.075 μM) and showed strain specific activity against HIV-2 (EC50 values ranged from 0.007–1.5 μM). In drug combination studies of emtricitabine with nucleoside reverse transcriptase inhibitors (abacavir, lamivudine, stavudine, tenofovir, zidovudine), non-nucleoside reverse transcriptase inhibitors (delavirdine, efavirenz, nevirapine, and rilpivirine), and protease inhibitors (amprenavir, nelfinavir, ritonavir, saquinavir), no antagonistic effects were observed.

Rilpivirine: Rilpivirine exhibited activity against laboratory strains of wild-type HIV-1 in an acutely infected T-cell line with a median EC50 value for HIV-1IIIB of 0.73 nM. Rilpivirine demonstrated limited activity in cell culture against HIV-2 with a median EC50 value of 5220 nM (range 2510 to 10830 nM). Rilpivirine demonstrated antiviral activity against a broad panel of HIV-1 group M (subtype A, B, C, D, F, G, H) primary isolates with EC50 values ranging from 0.07 to 1.01 nM and was less active against group O primary isolates with EC50 values ranging from 2.88 to 8.45 nM. The antiviral activity of rilpivirine was not antagonistic when combined with the NNRTIs efavirenz, etravirine or nevirapine; N(t)RTIs abacavir, didanosine, emtricitabine, lamivudine, stavudine, tenofovir or zidovudine; the PIs amprenavir, atazanavir, darunavir, indinavir, lopinavir, nelfinavir, ritonavir, saquinavir or tipranavir; the fusion inhibitor enfuvirtide; the CCR5 coreceptor antagonist maraviroc or the integrase strand transfer inhibitor raltegravir.

Tenofovir Disoproxil Fumarate: The antiviral activity of tenofovir against laboratory and clinical isolates of HIV-1 was assessed in lymphoblastoid cell lines, primary monocyte/macrophage cells and peripheral blood lymphocytes. The EC50 values for tenofovir were in the range of 0.04–8.5 μM. Tenofovir displayed antiviral activity in cell culture against HIV-1 clades A, B, C, D, E, F, G, and O (EC50 values ranged from 0.5–2.2 μM) and showed strain specific activity against HIV-2 (EC50 values ranged from1.6 μM–5.5 μM). In drug combination studies of tenofovir with NRTIs (abacavir, didanosine, emtricitabine, lamivudine, stavudine, and zidovudine), NNRTIs (delavirdine, efavirenz, nevirapine, and rilpivirine), and PIs (amprenavir, indinavir, nelfinavir, ritonavir, saquinavir), no antagonistic effects were observed.

Resistance

In Cell Culture
Emtricitabine and Tenofovir Disoproxil Fumarate: HIV-1 isolates with reduced susceptibility to emtricitabine or tenofovir have been selected in cell culture. Reduced susceptibility to emtricitabine was associated with M184V/I substitutions in HIV-1 RT. HIV-1 isolates selected by tenofovir expressed a K65R substitution in HIV-1 RT and showed a 2–4 fold reduction in susceptibility to tenofovir. Rilpivirine: Rilpivirine-resistant strains were selected in cell culture starting from wild-type HIV-1 of different origins and subtypes as well as NNRTI resistant HIV-1. The frequently observed amino acid substitutions that emerged and conferred decreased phenotypic susceptibility to rilpivirine included: L100I, K101E, V106I and A, V108I, E138K and G, Q, R, V179F and I, Y181C and I, V189I, G190E, H221Y, F227C and M230I and L.

In Treatment-Naïve HIV-1-infected Subjects
In a pooled resistance analysis for subjects receiving rilpivirine in combination with emtricitabine/tenofovir DF in clinical trials C209 and C215, the emergence of resistance among subjects was greater in the rilpivirine arm compared to the efavirenz arm. In the combined studies, 44% (34/77) of the virologic failures in the rilpivirine arms had genotypic and phenotypic resistance to rilpivirine compared to 23% (10/43) of the virologic failures in the efavirenz arms who had genotypic and phenotypic resistance to efavirenz. Moreover, phenotypic and/or genotypic resistance to emtricitabine and tenofovir emerged in 51% (39/77) and 9% (7/77) of the virologic failures, respectively, in the rilpivirine arms compared to 16% (7/43) and 9% (4/43) in the efavirenz arms. Emerging NNRTI substitutions in the rilpivirine virologic failures included V90I, K101E/P/T, E138K/G, V179I/L, Y181I/C, V189I, H221Y, F227C/L and M230L, which were associated with a rilpivirine phenotypic fold change range of 2.6–621. The E138K substitution emerged most frequently on rilpivirine treatment commonly in combination with the M184I substitution. The emtricitabine and lamivudine resistance-associated substitutions M184I or V emerged more frequently in rilpivirine virologic failures compared to efavirenz virologic failures.

Cross Resistance

Emtricitabine, Rilpivirine, and Tenofovir Disoproxil Fumarate:
In Cell Culture
No significant cross-resistance has been demonstrated between rilpivirine-resistant HIV-1 variants and emtricitabine or tenofovir, or between emtricitabine- or tenofovirresistant variants and rilpivirine.

Rilpivirine:
Site-Directed NNRTI Mutant Virus
Cross-resistance has been observed among NNRTIs. The single NNRTI substitutions K101P, Y181I and Y181V conferred 52-fold, 15-fold and 12-fold decreased susceptibility to rilpivirine, respectively. The combination of E138K and M184I showed 6.7-fold reduced susceptibility to rilpivirine compared to 2.8-fold for E138K alone. The K103N substitution did not show reduced susceptibility to rilpivirine. Combinations of 2 or 3 NNRTI resistance-associated substitutions gave decreased susceptibility to rilpivirine (fold change range of 3.7–554) in 38% and 66% of mutants, respectively.

In Treatment-Naïve HIV-1-infected Subjects
Considering all of the available cell culture and clinical data, any of the following amino acid substitutions, when present at baseline, are likely to decrease the antiviral activity of rilpivirine: K101E, K101P, E138A, E138G, E138K, E138R, E138Q, V179L, Y181C, Y181I, Y181V, H221Y, F227C, M230I or M230L. Cross-resistance to efavirenz, etravirine and/or nevirapine is likely after virologic failure and development of rilpivirine resistance. In the pooled analysis for subjects receiving rilpivirine in combination with emtricitabine/tenofovir DF in clinical trials C209 and C215, 34 virologic failure subjects had evidence of rilpivirine resistance. Of these subjects, 91% (N=31) were resistant to etravirine and efavirenz, and 65% (N=22) were resistant to nevirapine. In the efavirenz arm, none of the 10 efavirenz-resistant virologic failures were resistant to etravirine at failure. Subjects experiencing virologic failure on rilpivirine developed more NNRTI resistance-associated substitutions conferring more cross-resistance to the NNRTI class and had a higher likelihood of cross-resistance to all NNRTIs in the class than subjects who failed on efavirenz.

Emtricitabine: Emtricitabine-resistant isolates (M184V/I) were cross-resistant to lamivudine but retained susceptibility in cell culture to didanosine, stavudine, tenofovir, zidovudine, and NNRTIs (delavirdine, efavirenz, nevirapine, and rilpivirine). HIV-1 isolates containing the K65R substitution, selected in vivo by abacavir, didanosine, and tenofovir, demonstrated reduced susceptibility to inhibition by emtricitabine. Viruses harboring substitutions conferring reduced susceptibility to stavudine and zidovudine (M41L, D67N, K70R, L210W, T215Y/F, K219Q/E), or didanosine (L74V) remained sensitive to emtricitabine. HIV-1 containing the substitutions associated with NNRTI resistance K103N or rilpivirine-associated substitutions were susceptible to emtricitabine.

Tenofovir Disoproxil Fumarate: The K65R substitution selected by tenofovir is also selected in some HIV-1 infected patients treated with abacavir or didanosine. HIV-1 isolates with the K65R substitution also showed reduced susceptibility to emtricitabine and lamivudine. Therefore, cross-resistance among these NRTIs may occur in patients whose virus harbors the K65R substitution. HIV-1 isolates from patients (N=20) whose HIV-1 expressed a mean of 3 zidovudine-associated RT amino acid substitutions (M41L, D67N, K70R, L210W, T215Y/F, or K219Q/E/N) showed a 3.1-fold decrease in the susceptibility to tenofovir. Subjects whose virus expressed an L74V substitution without zidovudine resistance associated substitutions (N=8) had reduced response to Viread. Limited data are available for patients whose virus expressed a Y115F substitution (N=3), Q151M substitution (N=2), or T69 insertion (N=4), all of whom had a reduced response. HIV-1 containing the substitutions associated with NNRTI resistance K103N and Y181C, or rilpivirine-associated substitutions were susceptible to tenofovir.

Pregnancy
Complera (emtricitabine/rilpivirine/tenofovir disoproxil fumarate) is in FDA pregnancy category B. Emtricitabine: The incidence of fetal variations and malformations was not increased in embryofetal toxicity studies performed with emtricitabine in mice at exposures (AUC) approximately 60 times higher and in rabbits at approximately 120-times higher than human exposures at the recommended daily dose. Rilpivirine: Studies in animals have shown no evidence of embryonic or fetal toxicity or an effect on reproductive function. In offspring from rat and rabbit dams treated with rilpivirine during pregnancy and lactation, there were no toxicologically significant effects on developmental endpoints. The exposures at the embryo-fetal No Observed Adverse Effects Levels in rats and rabbits were respectively 15 and 70 times higher than the exposure in humans at the recommended dose of 25 mg once daily. Tenofovir Disoproxil Fumarate: Reproduction studies have been performed in rats and rabbits at doses up to 14 and 19 times the human dose based on body surface area comparisons and revealed no evidence of impaired fertility or harm to the fetus due to tenofovir.

There are, however, no adequate and well-controlled studies in pregnant women. Because animal reproduction studies are not always predictive of human response, Complera should be used during pregnancy only if the potential benefit justifies the potential risk to the fetus. Antiretroviral Pregnancy Registry: To monitor fetal outcomes of pregnant women exposed to Complera, an Antiretroviral Pregnancy Registry has been established. Healthcare providers are encouraged to register patients by calling 1-800-258-4263 or online at http://www.APRegistry.com.

Nursing Mothers
The Centers for Disease Control and Prevention recommend that HIV infected mothers not breastfeed their infants to avoid risking postnatal transmission of HIV. Studies in rats have demonstrated that tenofovir is secreted in milk. Studies in lactating rats and their offspring indicate that rilpivirine was present in rat milk. It is not known whether emtricitabine, rilpivirine, or tenofovir is excreted in human milk. Because of both the potential for HIV transmission and the potential for serious adverse reactions in nursing infants, mothers should be instructed not to breastfeed if they are receiving Complera.

For complete information, see full prescribing information for Complera.

Pharmacodynamics
In a monotherapy study raltegravir (400 mg twice daily) demonstrated rapid antiviral activity with mean viral load reduction of 1.66 log₁₀ copies/mL by Day 10.

In the randomized, double-blind, placebo-controlled, dose-ranging trial, Protocol 005, and Protocols 018 and 019, antiviral responses were similar among subjects regardless of dose.

Effects on Electrocardiogram
In a randomized, placebo-controlled, crossover study, 31 healthy subjects were administered a single oral supratherapeutic dose of raltegravir 1600 mg and placebo. Peak raltegravir plasma concentrations were approximately 4-fold higher than the peak concentrations following a 400 mg dose. ISENTRESS did not appear to prolong the QTc interval for 12 hours postdose. After baseline and placebo adjustment, the maximum mean QTc change was -0.4 msec (1-sided 95% upper Cl: 3.1 msec).

Pharmacokinetics
Absorption
Raltegravir is absorbed with a T_max of approximately 3 hours postdose in the fasted state. Raltegravir AUC and C_max increase dose proportionally over the dose range 100 mg to 1600 mg. Raltegravir C_12hr increases dose proportionally over the dose range of 100 to 800 mg and increases slightly less than dose proportionally over the dose range 100 mg to 1600 mg. With twice-daily dosing, pharmacokinetic steady state is achieved within approximately the first 2 days of dosing. There is little to no accumulation in AUC and C_max. The average accumulation ratio for C_12hr ranged from approximately 1.2 to 1.6.

The absolute bioavailability of raltegravir has not been established.

In subjects who received 400 mg twice daily alone, raltegravir drug exposures were characterized by a geometric mean AUC_0-12hr of 14.3 μM(hr) and C_12hr of 142 nM.

Considerable variability was observed in the pharmacokinetics of raltegravir. For observed C_12hr in Protocols 018 and 019, the coefficient of variation (CV) for inter-subject variability = 212% and the CV for intra-subject variability = 122%.

Effect of Food on Oral Absorption
ISENTRESS may be administered with or without food. Raltegravir was administered without regard to food in the pivotal safety and efficacy studies in HIV-1-infected patients. The effect of consumption of low-, moderate- and high-fat meals on steady-state raltegravir pharmacokinetics was assessed in healthy volunteers. Administration of multiple doses of raltegravir following a moderate-fat meal (600 Kcal, 21 g fat) did not affect raltegravir AUC to a clinically meaningful degree with an increase of 13% relative to fasting. Raltegravir C_12hr was 66% higher and C_max was 5% higher following a moderate-fat meal compared to fasting. Administration of raltegravir following a high-fat meal (825 Kcal, 52 g fat) increased AUC and C_max by approximately 2-fold and increased C_12hr by 4.1-fold. Administration of raltegravir following a low-fat meal (300 Kcal, 2.5 g fat) decreased AUC and C_max by 46% and 52%, respectively; C_12hr was essentially unchanged. Food appears to increase pharmacokinetic variability relative to fasting.

Distribution
Raltegravir is approximately 83% bound to human plasma protein over the concentration range of 2 to 10 μM.

Metabolism and Excretion
The apparent terminal half-life of raltegravir is approximately 9 hours, with a shorter α-phase half-life (~1 hour) accounting for much of the AUC. Following administration of an oral dose of radiolabeled raltegravir, approximately 51 and 32% of the dose was excreted in feces and urine, respectively. In feces, only raltegravir was present, most of which is likely derived from hydrolysis of raltegravir-glucuronide secreted in bile as observed in preclinical species. Two components, namely raltegravir and raltegravirglucuronide, were detected in urine and accounted for approximately 9 and 23% of the dose, respectively. The major circulating entity was raltegravir and represented approximately 70% of the total radioactivity; the remaining radioactivity in plasma was accounted for by raltegravir-glucuronide. Studies using isoform-selective chemical inhibitors and cDNA-expressed UDP-glucuronosyltransferases (UGT) show that UGT1A1 is the main enzyme responsible for the formation of raltegravir-glucuronide. Thus, the data indicate that the major mechanism of clearance of raltegravir in humans is UGT1A1-mediated glucuronidation.

Special Populations
Pediatric
The pharmacokinetics of raltegravir in pediatric patients has not been established.

Age
The effect of age on the pharmacokinetics of raltegravir was evaluated in the composite analysis. No dosage adjustment is necessary.

Race
The effect of race on the pharmacokinetics of raltegravir was evaluated in the composite analysis. No dosage adjustment is necessary.

Gender
A study of the pharmacokinetics of raltegravir was performed in healthy adult males and females. Additionally, the effect of gender was evaluated in a composite analysis of pharmacokinetic data from 103 healthy subjects and 28 HIV-1 infected subjects receiving raltegravir monotherapy with fasted administration. No dosage adjustment is necessary.

Hepatic Impairment
Raltegravir is eliminated primarily by glucuronidation in the liver. A study of the pharmacokinetics of raltegravir was performed in subjects with moderate hepatic impairment. Additionally, hepatic impairment was evaluated in the composite pharmacokinetic analysis. There were no clinically important pharmacokinetic differences between subjects with moderate hepatic impairment and healthy subjects. No dosage adjustment is necessary for patients with mild to moderate hepatic impairment. The effect of severe hepatic impairment on the pharmacokinetics of raltegravir has not been studied.

Renal Impairment
Renal clearance of unchanged drug is a minor pathway of elimination. A study of the pharmacokinetics of raltegravir was performed in subjects with severe renal impairment. Additionally, renal impairment was evaluated in the composite pharmacokinetic analysis. There were no clinically important pharmacokinetic differences between subjects with severe renal impairment and healthy subjects. No dosage adjustment is necessary. Because the extent to which ISENTRESS may be dialyzable is unknown, dosing before a dialysis session should be avoided.

UGT1A1 Polymorphism
There is no evidence that common UGT1A1 polymorphisms alter raltegravir pharmacokinetics to a clinically meaningful extent. In a comparison of 30 subjects with *28/*28 genotype (associated with reduced activity of UGT1A1) to 27 subjects with wild-type genotype, the geometric mean ratio (90% CI) of AUC was 1.41 (0.96, 2.09).

Drug Interactions
(See Drug and Food Interactions below. For additional information, consult the Isentress complete prescribing information).

Microbiology

Mechanism of Action
Raltegravir inhibits the catalytic activity of HIV-1 integrase, an HIV-1 encoded enzyme that is required for viral replication. Inhibition of integrase prevents the covalent insertion, or integration, of unintegrated linear HIV-1 DNA into the host cell genome preventing the formation of the HIV-1 provirus. The provirus is required to direct the production of progeny virus, so inhibiting integration prevents propagation of the viral infection. Raltegravir did not significantly inhibit human phosphoryltransferases including DNA polymerases α, β, and γ.

Antiviral Activity in Cell Culture
Raltegravir at concentrations of 31 ± 20 nM resulted in 95% inhibition (EC₉₅) of viral spread (relative to an untreated virus-infected culture) in human T-lymphoid cell cultures infected with the cell-line adapted HIV-1 variant H9IIIB. In addition, 5 clinical isolates of HIV-1 subtype B had EC₉₅ values ranging from 9 to 19 nM in cultures of mitogen-activated human peripheral blood mononuclear cells. In a single-cycle infection assay, raltegravir inhibited infection of 23 HIV-1 isolates representing 5 non-B subtypes (A, C, D, F, and G) and 5 circulating recombinant forms (AE, AG, BF, BG, and cpx) with EC₅₀ values ranging from 5 to 12 nM. Raltegravir also inhibited replication of an HIV-2 isolate when tested in CEMx174 cells (EC₉₅ value = 6 nM). Additive to synergistic antiretroviral activity was observed when human T-lymphoid cells infected with the H9IIIB variant of HIV-1 were incubated with raltegravir in combination with non-nucleoside reverse transcriptase inhibitors (delavirdine, efavirenz, or nevirapine); nucleoside analog reverse transcriptase inhibitors (abacavir, didanosine, lamivudine, stavudine, tenofovir, zalcitabine, or zidovudine); protease inhibitors (amprenavir, atazanavir, indinavir, lopinavir, nelfinavir, ritonavir, or saquinavir); or the entry inhibitor enfuvirtide.

Resistance
The mutations observed in the HIV-1 integrase coding sequence that contributed to raltegravir resistance (evolved either in cell culture or in subjects treated with raltegravir) generally included an amino acid substitution at either Y143 (changed to C, H, or R) or Q148 (changed to H, K, or R) or N155 (changed to H) plus one or more additional substitutions (i.e., L74M, E92Q, T97A, E138A/K, G140A/S, V151I, G163R, H183P, Y226C/D/F/H, S230R, and D232N).

Treatment-Naïve Subjects: By Week 96 in the STARTMRK trial, the primary raltegravir resistance-associated substitutions were observed in 4 (2 with Y143H/R and 2 with Q148H/R) of the 10 virologic failure subjects with evaluable genotypic data from paired baseline and raltegravir treatment-failure isolates.

Treatment-Experienced Subjects: By Week 96 in the BENCHMRK trials, at least one of the primary raltegravir resistance-associated substitutions, Y143C/H/R, Q148H/K/R, and N155H, was observed in 76 of the 112 virologic failure subjects with evaluable genotypic data from paired baseline and raltegravir treatment-failure isolates. The emergence of the primary raltegravir resistance-associated substitutions was observed cumulatively in 70 subjects by Week 48 and 78 subjects by Week 96, 15.2% and 17% of the raltegravir recipients, respectively. Some (n=58) of those HIV-1 isolates harboring one or more of the primary raltegravir resistance-associated substitutions were evaluated for raltegravir susceptibility yielding a median decrease of 26.3-fold (mean 48.9 ± 44.8-fold decrease, ranging from 0.8- to 159-fold) compared to the wild-type reference.

USE IN SPECIFIC POPULATIONS

Pregnancy
Pregnancy Category C
ISENTRESS should be used during pregnancy only if the potential benefit justifies the potential risk to the fetus. There are no adequate and well-controlled studies in pregnant women. In addition, there have been no pharmacokinetic studies conducted in pregnant patients.

Developmental toxicity studies were performed in rabbits (at oral doses up to 1000 mg/kg/day) and rats (at oral doses up to 600 mg/kg/day). The reproductive toxicity study in rats was performed with pre-, peri-, and postnatal evaluation. The highest doses in these studies produced systemic exposures in these species approximately 3- to 4-fold the exposure at the recommended human dose. In both rabbits and rats, no treatment-related effects on embryonic/fetal survival or fetal weights were observed. In addition, no treatment-related external, visceral, or skeletal changes were observed in rabbits. However, treatment-related increases over controls in the incidence of supernumerary ribs were seen in rats at 600 mg/kg/day (exposures 3-fold the exposure at the recommended human dose).

Placenta transfer of drug was demonstrated in both rats and rabbits. At a maternal dose of 600 mg/kg/day in rats, mean drug concentrations in fetal plasma were approximately 1.5-to 2.5-fold greater than in maternal plasma at 1 hour and 24 hours postdose, respectively. Mean drug concentrations in fetal plasma were approximately 2% of the mean maternal concentration at both 1 and 24 hours postdose at a maternal dose of 1000 mg/kg/day in rabbits.

Antiretroviral Pregnancy Registry
To monitor maternal-fetal outcomes of pregnant patients exposed to ISENTRESS, an Antiretroviral Pregnancy Registry has been established. Physicians are encouraged to register patients by calling 1800-258-4263.

Nursing Mothers
Breast-feeding is not recommended while taking ISENTRESS. In addition, it is recommended that HIV-1-infected mothers not breast-feed their infants to avoid risking postnatal transmission of HIV-1.

It is not known whether raltegravir is secreted in human milk. However, raltegravir is secreted in the milk of lactating rats. Mean drug concentrations in milk were approximately 3-fold greater than those in maternal plasma at a maternal dose of 600 mg/kg/day in rats. There were no effects in rat offspring attributable to exposure of ISENTRESS through the milk.

Peak efavirenz plasma concentrations of 1.6-9.1 μM were attained by 5 hours following single oral doses of 100 mg to 1600 mg administered to uninfected volunteers. Dose-related increases in peak plasma drug concentration (Cmax) and area under the plasma concentration-time curve (AUC) were seen for doses up to 1600 mg; the increases were less than proportional suggesting diminished absorption at higher doses. In HIV-1-infected patients at steady state, mean Cmax, mean trough plasma drug concentration (Cmin), and mean AUC were dose proportional following 200-mg, 400-mg, and 600-mg daily doses. Time-to-peak plasma concentrations were approximately 3to 5 hours and steady-state plasma concentrations were reached in 6 to10 days. In 35 patients receiving efavirenz 600 mg once daily, steady-state Cmax was 12.9 ± 3.7 μM (mean ± SD), steady-state Cmin was 5.6 ± 3.2 μM, and AUC was 184 ± 73 μM(h). [#]

It is recommended that efavirenz be taken on an empty stomach. The increased efavirenz concentrations observed following administration of efavirenz with food may lead to an increase in frequency of adverse reactions. Administration of a single 600-mg dose of efavirenz capsules with a high-fat/highcaloric meal (894 kcal, 54 g fat, 54% calories from fat) or a reduced-fat/normal-caloric meal (440 kcal, 2 g fat, 4% calories from fat) was associated with a mean increase of 22% and 17% in efavirenz AUC∞ and a mean increase of 39% and 51% in efavirenz Cmax, respectively, relative to the exposures achieved when given under fasted conditions. Administration of a single 600-mg efavirenz tablet with a high-fat/high-caloric meal (approximately 1000 kcal, 500-600 kcal from fat) was associated with a 28% increase in mean AUC∞ of efavirenz and a 79% increase in mean Cmax of efavirenz relative to the exposures achieved under fasted conditions. [#]

Efavirenz is highly bound (approximately 99.5% to 99.75%) to human plasma proteins, principally albumin. In HIV infected patients who received 200 mg to 600 mg of efavirenz once a day for at least 1 month, cerebrospinal fluid concentrations ranged from 0.26% to 1.19% (mean 0.69%) of the corresponding plasma concentration. This proportion is approximately threefold higher than the nonprotein-bound (free) fraction of efavirenz in plasma. [#]

Studies in humans and in vitro studies using human liver microsomes have demonstrated that efavirenz is principally metabolized by the cytochrome P450 system to hydroxylated metabolites with subsequent glucuronidation of these hydroxylated metabolites. These metabolites are essentially inactive against HIV-1. The in vitro studies suggest that CYP3A and CYP2B6 are the major isozymes responsible for efavirenz metabolism. Efavirenz has been shown to induce CYP enzymes, resulting in the induction of its own metabolism. Multiple doses of 200 mg to 400 mg per day for 10 days resulted in a lower than predicted extent of accumulation (22% to 42% lower) and a shorter terminal half-life of 40 to 55 hours (single dose half-life 52 to 76 hours). [#]

Efavirenz has a terminal half-life of 52 to76 hours after single doses and 40 to 55 hours after multiple doses. A one-month mass balance/excretion study was conducted using 400 mg per day with a 14C-labeled dose administered on Day 8. Approximately 14% to 34% of the radiolabel was recovered in the urine and 16% to 61% was recovered in the feces. Nearly all of the urinary excretion of the radiolabeled drug was in the form of metabolites. Efavirenz accounted for the majority of the total radioactivity measured in feces. [#]

The pharmacokinetics of efavirenz have not been studied in patients with renal insufficiency; however, less than 1% of efavirenz is excreted unchanged in the urine, so the impact of renal impairment on efavirenz elimination should be minimal. A multiple-dose study showed no significant effect on efavirenz pharmacokinetics in patients with mild hepatic impairment (Child-Pugh Class A) compared with controls. There were insufficient data to determine whether moderate or severe hepatic impairment (Child-Pugh Class B or C) affects efavirenz pharmacokinetics. [#]

Efavirenz has been shown in vivo to cause hepatic enzyme induction, thus increasing the biotransformation of some drugs metabolized by CYP3A. In vitro studies have shown that efavirenz inhibited CYP isozymes 2C9, 2C19, and 3A4 with Ki values (8.5 to 17 μM) in the range of observed efavirenz plasma concentrations. In in vitro studies, efavirenz did not inhibit CYP2E1 and inhibited CYP2D6 and CYP1A2 (Ki values 82 to 160 μM) only at concentrations well above those achieved clinically. The inhibitory effect on CYP3A is expected to be similar between 200-mg, 400-mg, and 600-mg doses of efavirenz. Coadministration of efavirenz with drugs primarily metabolized by 2C9, 2C19, and 3A isozymes may result in altered plasma concentrations of the coadministered drug. Drugs which induce CYP3A activity would be expected to increase the clearance of efavirenz resulting in lowered plasma concentrations. [#]

Efavirenz is in FDA Pregnancy Category D. Efavirenz may cause fetal harm when administered during pregnancy, especially in the first trimester of pregnancy. If efavirenz is used during the first trimester of pregnancy or if pregnancy occurs while the patient is taking efavirenz, the patient should be appraised of the potential harm to the fetus. No adequate and well-controlled studies have been performed in pregnant women. In prospective reports, birth defects have occurred in 14 of 501 live births (first-trimester exposure) and 2 of 55 live births (second/third-trimester exposure). One of these reported defects with first-trimester exposure was a neural tube defect. A single case of anophthalmia with first-trimester exposure to efavirenz has also been reported: however, this case included severe oblique facial clefts and amniotic banding, a known association with anopthalmia. Six retrospective reports identified findings consistent with neural tube defects, including meningomyelocele, in mothers exposed to efavirenz during the mother's first trimester. Although a causal relationship has not been established, similar defects have been observed in preclinical studies of efavirenz. [#]

Pregnancy should be avoided in women receiving efavirenz. Two methods of birth control, with a barrier method in combination with a nonbarrier method such as an oral or other hormonal contraceptive, should be used to avoid pregnancy in women taking efavirenz. Because of the long half-life of efavirenz, use of adequate contraceptive measures for 12 weeks after discontinuation of the drug is recommended. Before initiating therapy with efavirenz, women of childbearing potential should undergo pregnancy testing. It is recommended that efavirenz not be given to pregnant women except in situations in which there are no therapeutic alternatives. An Antiretroviral Pregnancy Registry has been established to monitor the outcomes of pregnant women exposed to efavirenz. Physicians may register patients online at http://www.APRegistry.com or by calling 1-800-258-4263. The Centers for Disease Control and Prevention recommend that HIV-infected mothers not breastfeed their infants to avoid risking postnatal transmission of HIV. Although it is not known if efavirenz is secreted in human milk, efavirenz is secreted into the milk of lactating rats. Because of the potential for HIV transmission and the potential for serious adverse effects in nursing infants, mothers should be instructed not to breastfeed if they are receiving efavirenz. [#]

In cell culture, HIV-1 isolates with reduced susceptibility to EFV (>380-fold increase in EC90 value) emerged rapidly in the presence of drug. Genotypic characterization of these viruses identified single amino acid substitutions L100I or V179D, double substitutions L100I/V108I, and triple substitutions L100I/V179D/Y181C in RT. [#]

Clinical isolates with reduced susceptibility in cell culture to EFV have been obtained. One or more RT substitutions at amino acid positions 98, 100, 101, 103, 106, 108, 188, 190, 225, and 227 were observed in patients failing treatment with EFV in combination with IDV, or with ZDV plus LAM. The mutation K103N was the most frequently observed. Long-term resistance surveillance (average 52 weeks, range 4 to106 weeks) analyzed 28 matching baseline and virologic failure isolates. Sixty-one percent (17/28) of these failure isolates had decreased EFV susceptibility in cell culture with a median 88-fold change in EFV susceptibility (EC50 value) from reference. The most frequent NNRTI substitution to develop in these patient isolates was K103N (54%). Other NNRTI substitutions that developed included L100I (7%), K101E/Q/R (14%), V108I (11%), G190S/T/A (7%), P225H (18%), and M230I/L (11%). [#]

Cross-resistance among NNRTIs has been observed. Clinical isolates previously characterized as EFV-resistant were also phenotypically resistant in cell culture to DLV and NVP compared to baseline. DLV- and/or NVP-resistant clinical viral isolates with NNRTI resistance-associated substitutions (A98G, L100I, K101E/P, K103N/S, V106A, Y181X, Y188X, G190X, P225H, F227L, or M230L) showed reduced susceptibility to EFV in cell culture. Greater than 90% of NRTI-resistant clinical isolates tested in cell culture retained susceptibility to EFV. [#]

Pooled data from Phase III trials through Week 48 of rilpivirine 25 mg administered once daily in antiretroviral treatment-naïve HIV-1-infected subjects (n = 679) demonstrate a mean area under the plasma concentration-time curve from time of administration up to 24 hours (AUC24h) of 2397 ± 1032 ng(h)/mL and a mean pre-dose plasma concentration (C0h) of 80 ± 37 ng/mL. [#]

After oral administration, the maximum plasma concentration of rilpivirine is generally achieved within 4 to 5 hours. The absolute bioavailability of rilpivirine is unknown. The exposure to rilpivirine was approximately 40% lower when rilpivirine was taken in a fasted condition compared to a normal caloric meal (533 kcal) or high-fat high-caloric meal (928 kcal). When rilpivirine was taken with only a protein-rich nutritional drink, exposures were 50% lower than when taken with a meal. [#]

Rilpivirine is approximately 99.7% bound to plasma proteins in vitro, primarily to albumin. The distribution of rilpivirine into compartments other than plasma (e.g., cerebrospinal fluid, genital tract secretions) has not been evaluated in humans. In vitro experiments indicate that rilpivirine primarily undergoes oxidative metabolism mediated by the cytochrome P450 (CYP) 3A system. The terminal elimination half-life of rilpivirine is approximately 50 hours. After single dose oral administration of 14C-rilpivirine, on average 85% and 6.1% of the radioactivity could be retrieved in feces and urine, respectively. In feces, unchanged rilpivirine accounted for on average 25% of the administered dose. Only trace amounts of unchanged rilpivirine (< 1% of dose) were detected in urine. [#]

Rilpivirine is primarily metabolized and eliminated by the liver. In a study comparing 8 subjects with mild hepatic impairment (Child-Pugh score A) to 8 matched controls, and 8 subjects with moderate hepatic impairment (Child-Pugh score B) to 8 matched controls, the multiple-dose exposure of rilpivirine was 47% higher in subjects with mild hepatic impairment and 5% higher in subjects with moderate hepatic impairment. No dose adjustment is required in patients with mild or moderate hepatic impairment. Rilpivirine has not been studied in subjects with
severe hepatic impairment (Child-Pugh score C). [#]

Population pharmacokinetic analysis indicated that rilpivirine exposure was similar in HIV-1-infected subjects with mild renal impairment compared to HIV-1-infected subjects with normal renal function. No dose adjustment is required in patients with mild renal impairment. There is limited or no information regarding the pharmacokinetics of rilpivirine in patients with moderate or severe renal impairment or in patients with end-stage renal disease, and rilpivirine concentrations may be increased due to alteration of drug absorption, distribution, and metabolism secondary to renal dysfunction. The potential impact is not expected to be of clinical relevance for HIV-1-infected subjects with moderate renal impairment, and no dose adjustment is required in these patients. Rilpivirine should be used with caution and with increased monitoring for adverse effects in patients with severe renal impairment or end-stage renal disease. Because rilpivirine is highly bound to plasma proteins, it is unlikely that it will be significantly removed by hemodialysis or peritoneal dialysis. [#]

Rilpivirine is in FDA Pregnancy Category B. No adequate and well-controlled or pharmacokinetic studies of rilpivirine use in pregnant women have been conducted. Studies in animals have shown no evidence of relevant embryonic or fetal toxicity or an effect on reproductive function. In offspring from rat and rabbit dams treated with rilpivirine during pregnancy and lactation, there were no toxicologically significant effects on developmental endpoints. The exposures at the embryo-fetal no observed adverse effects levels (NOAELs) in rats and rabbits were respectively 15 and 70 times higher than the exposure in humans at the recommended dose of 25 mg once daily. Rilpivirine should be used during pregnancy only if the potential benefit justifies the potential risk to the fetus. To monitor maternal-fetal outcomes of pregnant women exposed to rilpivirine, an Antiretroviral Pregnancy Registry has been established. Physicians may register patients either online at http://www.APRegistry.com or by calling 1-800-258-4263. The Centers for Disease Control and Prevention recommend that HIV-infected mothers not breastfeed their infants to avoid risking postnatal transmission of HIV. Studies in lactating rats and their offspring indicate that rilpivirine was present in rat milk. It is not known whether rilpivirine is secreted in human milk. Because of both the potential for HIV transmission and the potential for adverse reactions in nursing infants, mothers should be instructed not to breastfeed if they are receiving rilpivirine. [#]

Two Phase III trials (TMC278-C209: ECHO and TMC278-C215: THRIVE) compared rilpivirine to efavirenz in antiretroviral-naïve HIV-1 infected subjects with HIV-1 RNA ≥ 5000 copies/mL and no NNRTI resistance. Both trials were identical in design, with the exception of the background regimen. In TMC278-C209, the background regimen was fixed to the nucleoside (tide) reverse transcriptase inhibitors (N(t)RTIs), tenofovir disoproxil fumarate plus emtricitabine. In TMC278-C215, the background regimen consisted of 2 investigator-selected N(t)RTIs: tenofovir disoproxil fumarate plus emtricitabine or zidovudine plus lamivudine or abacavir plus lamivudine. In both trials, randomization was stratified by screening viral load. In TMC278 C215, randomization was also stratified by N(t)RTI background regimen. In the pooled resistance analysis from the Phase III Studies C209 and C215, the emergence of resistance among subjects was greater in the rilpivirine arm compared to the efavirenz arm. In the combined studies, 41% (38/92) of the virologic failures in the rilpivirine arms had genotypic and phenotypic resistance to rilpivirine compared to 25% (15/60) of the virologic failures in the efavirenz arms who had genotypic and phenotypic resistance to efavirenz. Moreover, resistance to a background drug (emtricitabine, lamivudine, tenofovir, abacavir or zidovudine) emerged in 48% (44/92) of the virologic failures in the rilpivirine arms compared to 15% (9/60) in the efavirenz arms. [#]

Emerging NNRTI substitutions in the rilpivirine virologic failures included V90I, K101E/P/T, E138K/G, V179I/L, Y181I/C, V189I, H221Y, F227C/L, and M230L, which were associated with a rilpivirine phenotypic fold change range of 2.6 to 621. The E138K substitution emerged most frequently on rilpivirine treatment commonly in combination with the M184I substitution. The emtricitabine and lamivudine resistance-associated substitutions M184I or V and the tenofovir resistance-associated substitutions K65R or N emerged more frequently in rilpivirine virologic failures compared to efavirenz virologic failures. [#]

Pharmacokinetics
Pharmacokinetics in Adults:
EPZICOM: In a single-dose, 3-way crossover bioavailability study of 1 EPZICOM Tablet versus 2 ZIAGEN Tablets (2 x 300 mg) and 2 EPIVIR Tablets (2 x 150 mg) administered simultaneously in healthy subjects (n = 25), there was no difference in the extent of absorption, as measured by the area under the plasma concentration-time curve (AUC) and maximal peak concentration (C_max), of each component.

Abacavir: Following oral administration, abacavir is rapidly absorbed and extensively distributed. After oral administration of a single dose of 600 mg of abacavir in 20 subjects, C_max was 4.26 ± 1.19 mcg/mL (mean ± SD) and AUC_∞ was 11.95 ± 2.51 mcg•hr/mL. Binding of abacavir to human plasma proteins is approximately 50% and was independent of concentration. Total blood and plasma drug-related radioactivity concentrations are identical, demonstrating that abacavir readily distributes into erythrocytes. The primary routes of elimination of abacavir are metabolism by alcohol dehydrogenase to form the 5′-carboxylic acid and glucuronyl transferase to form the 5′-glucuronide.

Lamivudine: Following oral administration, lamivudine is rapidly absorbed and extensively distributed. After multiple-dose oral administration of lamivudine 300 mg once daily for 7 days to 60 healthy volunteers, steady-state C_max (C_max,ss) was 2.04 ± 0.54 mcg/mL (mean ± SD) and the 24-hour steady-state AUC (AUC_24,ss) was 8.87 ± 1.83 mcg•hr/mL. Binding to plasma protein is low. Approximately 70% of an intravenous dose of lamivudine is recovered as unchanged drug in the urine. Metabolism of lamivudine is a minor route of elimination. In humans, the only known metabolite is the trans-sulfoxide metabolite (approximately 5% of an oral dose after 12 hours). The steady-state pharmacokinetic properties of the EPIVIR 300-mg tablet once daily for 7 days compared with the EPIVIR 150-mg tablet twice daily for 7 days were assessed in a crossover study in 60 healthy volunteers. EPIVIR 300 mg once daily resulted in lamivudine exposures that were similar to EPIVIR 150 mg twice daily with respect to plasma AUC_24,ss; however, C_max,ss was 66% higher and the trough value was 53% lower compared with the150-mg twice-daily regimen. Intracellular lamivudine triphosphate exposures in peripheral blood mononuclear cells were also similar with respect to AUC_24,ss and C_max24,ss; however, trough values were lower compared with the 150-mg twice-daily regimen. Inter-subject variability was greater for intracellular lamivudine triphosphate concentrations versus lamivudine plasma trough concentrations. The clinical significance of observed differences for both plasma lamivudine concentrations and intracellular lamivudine triphosphate concentrations is not known.

In humans, abacavir and lamivudine are not significantly metabolized by cytochromeP450 enzymes.

The pharmacokinetic properties of abacavir and lamivudine in fasting subjects are summarized below.

Pharmacokinetic Parameters^a for Abacavir and Lamivudine in Adults

• Oral bioavailability (%) – Abacavir: 86 ± 25, n=6; Lamivudine: 86 ± 16, n=12
• Apparent volume of distribution (L/kg) – Abacavir: 0.86 ± 0.15, n=6; Lamivudine: 1.3 ± 0.4, n=20
• Systemic clearance (L/hr/kg) – Abacavir: 0.80 ± 0.24, n=6; Lamivudine: 0.33 ± 0.06, n=20
• Renal clearance (L/hr/kg) – Abacavir: .007 ± .008, n=6; Lamivudine: 0.22 ± 0.06, n=20
• Elimination half-life (hr) – Abacavir: 1.45 ± 0.32, n=20; Lamivudine: 5 to 7^b

a Data presented as mean ± standard deviation except where noted.
b Approximate range.

Effect of Food on Absorption of EPZICOM:
EPZICOM may be administered with or without food. Administration with a high-fat meal in a single-dose bioavailability study resulted in no change in AUC_last, AUC_∞, and C_max for lamivudine. Food did not alter the extent of systemic exposure to abacavir (AUC_∞), but the rate of absorption (C_max) was decreased approximately 24% compared with fasted conditions (n = 25). These results are similar to those from previous studies of the effect of food on abacavir and lamivudine tablets administered separately.

Special Populations:
Renal Impairment: EPZICOM: Because lamivudine requires dose adjustment in the presence of renal insufficiency, EPZICOM is not recommended for use in patients with creatinine clearance <50 mL/min.

Hepatic Impairment: EPZICOM: EPZICOM is contraindicated for patients with hepatic impairment because EPZICOM is a fixed-dose combination and the dosage of the individual components cannot be adjusted. Abacavir is contraindicated in patients with moderate to severe hepatic impairment, and dose reduction is required in patients with mild hepatic impairment.

Pregnancy: Abacavir and Lamivudine: No data are available on the pharmacokinetics of abacavir or lamivudine during pregnancy.

Nursing Mothers: Abacavir: No data are available on the pharmacokinetics of abacavir in nursing mothers.
Lamivudine: Samples of breast milk obtained from 20 mothers receiving lamivudine monotherapy (300 mg twice daily) or combination therapy (150 mg lamivudine twice daily and 300 mg zidovudine twice daily) had measurable concentrations of lamivudine.

Pediatric Patients: EPZICOM: The pharmacokinetics of EPZICOM in pediatric subjects are under investigation. There are insufficient data at this time to recommend a dose.

Geriatric Patients: The pharmacokinetics of abacavir and lamivudine have not been studied in subjects over 65 years of age.

Gender: Abacavir: A population pharmacokinetic analysis in HIV-1-infected male (n = 304) and female (n = 67) subjects showed no gender differences in abacavir AUC normalized for lean body weight.

Lamivudine: A pharmacokinetic study in healthy male (n = 12) and female (n = 12) subjects showed no gender differences in lamivudine AUC_∞ normalized for body weight.

Race: Abacavir: There are no significant differences between blacks and Caucasians in abacavir pharmacokinetics.
Lamivudine: There are no significant racial differences in lamivudine pharmacokinetics.

Drug Interactions:
The drug interactions described are based on studies conducted with the individual nucleoside analogues. In humans, abacavir and lamivudine are not significantly metabolized by cytochrome P450 enzymes nor do they inhibit or induce this enzyme system; therefore, it is unlikely that clinically significant drug interactions will occur with drugs metabolized through these pathways.

Abacavir: Lamivudine and Zidovudine: Fifteen HIV-1-infected subjects were enrolled in a crossover-designed drug interaction study evaluating single doses of abacavir (600 mg), lamivudine (150 mg), and zidovudine (300 mg) alone or in combination. Analysis showed no clinically relevant changes in the pharmacokinetics of abacavir with the addition of lamivudine or zidovudine or the combination of lamivudine and zidovudine. Lamivudine exposure (AUC decreased 15%) and zidovudine exposure (AUC increased 10%) did not show clinically relevant changes with concurrent abacavir.

Methadone: In a study of 11 HIV-1-infected subjects receiving methadone-maintenance therapy (40 mg and 90 mg daily), with 600 mg of ZIAGEN twice daily (twice the currently recommended dose), oral methadone clearance increased 22% (90% CI: 6% 557 to 42%).

Lamivudine: Zidovudine: No clinically significant alterations in lamivudine or zidovudine pharmacokinetics were observed in 12 asymptomatic HIV-1-infected adult subjects given a single dose of zidovudine (200 mg) in combination with multiple doses of lamivudine (300 mg q 12 hr).

Ribavirin: In vitro data indicate ribavirin reduces phosphorylation of lamivudine, stavudine, and zidovudine. However, no pharmacokinetic (e.g., plasma concentrations or intracellular triphosphorylated active metabolite concentrations) or pharmacodynamic (e.g., loss of HIV-1/HCV virologic suppression) interaction was observed when ribavirin and lamivudine (n = 18), stavudine (n = 10), or zidovudine (n = 6) were coadministered as part of a multi-drug regimen to HIV-1/HCV co-infected subjects.
(For additional information, consult the Epzicom complete prescribing information).

Microbiology
Mechanism of Action:
Abacavir: Abacavir is a carbocyclic synthetic nucleoside analogue. Abacavir is converted by cellular enzymes to the active metabolite, carbovir triphosphate (CBV-TP), an analogue of deoxyguanosine-5′-triphosphate (dGTP). CBV-TP inhibits the activity of HIV-1 reverse transcriptase (RT) both by competing with the natural substrate dGTP and by its incorporation into viral DNA. The lack of a 3′-OH group in the incorporated nucleotide analogue prevents the formation of the 5′ to 3′ phosphodiester linkage essential for DNA chain elongation, and therefore, the viral DNA growth is terminated. CBV-TP is a weak inhibitor of cellular DNA polymerases α, β, and γ.
 
Lamivudine: Lamivudine is a synthetic nucleoside analogue. Intracellularly lamivudine is phosphorylated to its active 5′-triphosphate metabolite, lamivudine triphosphate (3TC-TP). The principal mode of action of 3TC-TP is inhibition of RT via DNA chain termination after incorporation of the nucleotide analogue. CBV-TP and 3TC-TP are weak inhibitors of cellular DNA polymerases α, β, and γ.

Antiviral Activity:
Abacavir: The antiviral activity of abacavir against HIV-1 was evaluated against a T-cell tropic laboratory strain HIV-1_IIIB in lymphoblastic cell lines, a monocyte/macrophage tropic laboratory strain HIV-1_BaL in primary monocytes/macrophages, and clinical isolates in peripheral blood mononuclear cells. The concentration of drug necessary to effect viral replication by 50 percent (EC₅₀) ranged from 3.7 to 5.8 μM (1 μM = 0.28 mcg/mL) and 0.07 to 1.0 μM against HIV-1_IIIB and HIV-1_BaL, respectively, and was 0.26 ± 0.18 μM 595 against 8 clinical isolates. The EC₅₀ values of abacavir against different HIV-1 clades (A-G) ranged from 0.0015 to 1.05 μM, and against HIV-2 isolates, from 0.024 to 0.49 μM. Ribavirin (50 μM) had no effect on the anti–HIV-1 activity of abacavir in cell culture.

Lamivudine: The antiviral activity of lamivudine against HIV-1 was assessed in a number of cell lines (including monocytes and fresh human peripheral blood lymphocytes) using standard susceptibility assays. EC₅₀ values were in the range of 0.003 to 15 μM (1 μM = 0.23 mcg/mL). HIV-1 from therapy-naive subjects with no amino acid substitutions associated with resistance gave median EC₅₀ values of 0.429 μM (range: 0.200 to 2.007 μM) from Virco (n = 92 baseline samples from COLA40263) and 2.35 μM (1.37 to 3.68 μM) from Monogram Biosciences (n = 135 baseline samples from ESS30009). The EC₅₀ values of lamivudine against different HIV-1 clades (A-G) ranged from 0.001 to 0.120 μM, and against HIV-2 isolates from 0.003 to 0.120 μM in peripheral blood mononuclear cells. Ribavirin (50 μM) decreased the anti–HIV-1 activity of lamivudine by 3.5 fold in MT-4 cells.

The combination of abacavir and lamivudine has demonstrated antiviral activity in cell culture against non-subtype B isolates and HIV-2 isolates with equivalent antiviral activity as for subtype B isolates. Abacavir/lamivudine had additive to synergistic activity in cell culture in combination with the nucleoside reverse transcriptase inhibitors (NRTIs) emtricitabine, stavudine, tenofovir, zalcitabine, zidovudine; the non-nucleoside reverse transcriptase inhibitors (NNRTIs) delavirdine, efavirenz, nevirapine; the protease inhibitors (PIs) amprenavir, indinavir, lopinavir, nelfinavir, ritonavir, saquinavir; or the fusion inhibitor, enfuvirtide. Ribavirin, used in combination with interferon for the treatment of HCV infection, decreased the anti-HIV-1 potency of abacavir/lamivudine reproducibly by 2- to 6-fold in cell culture.

Resistance:
HIV-1 isolates with reduced susceptibility to the combination of abacavir and lamivudine have been selected in cell culture and have also been obtained from subjects failing abacavir/lamivudine-containing regimens. Genotypic characterization of abacavir/lamivudine-resistant viruses selected in cell culture identified amino acid substitutions M184V/I, K65R, L74V, and Y115F in HIV-1 RT.

Genotypic analysis of isolates selected in cell culture and recovered from abacavir-treated subjects demonstrated that amino acid substitutions K65R, L74V, Y115F, and M184V/I in HIV-1 RT contributed to abacavir resistance. Genotypic analysis of isolates selected in cell culture and recovered from lamivudine-treated subjects showed that the resistance was due to a specific amino acid substitution in HIV-1 RT at codon 184 changing the methionine to either isoleucine or valine (M184V/I). In a study of therapy-naive adults receiving ZIAGEN 600 mg once daily (n = 384) or 300 mg twice daily (n = 386) in a background regimen of lamivudine 300 mg and efavirenz 600 mg once daily (Study CNA30021), the incidence of virologic failure at 48 weeks was similar between the 2 groups (11% in both arms). Genotypic (n = 38) and phenotypic analyses (n = 35) of virologic failure isolates from this study showed that the RT substitutions that emerged during abacavir/lamivudine once-daily and twice-daily therapy were K65R, L74V, Y115F, and M184V/I. The abacavir- and lamivudine-associated resistance substitution M184V/I was the most commonly observed substitution in virologic failure isolates from subjects receiving abacavir/lamivudine once daily (56%, 10/18) and twice daily (40%, 636 8/20).

Thirty-nine percent (7/18) of the isolates from subjects who experienced virologic failure in the abacavir once-daily arm had a >2.5-fold decrease in abacavir susceptibility with a median-fold decrease of 1.3 (range: 0.5 to 11) compared with 29% (5/17) of the failure isolates in the twice-daily arm with a median-fold decrease of 0.92 (range: 0.7 to 13). Fifty-six percent (10/18) of the virologic failure isolates in the once-daily abacavir group compared with 41% (7/17) of the failure isolates in the twice-daily abacavir group had a >2.5-fold decrease in lamivudine susceptibility with median-fold changes of 81 (range: 0.79 to >116) and 1.1 (range: 0.68 to >116) in the once-daily and twice-daily abacavir arms, respectively.

Cross-Resistance:
Cross-resistance has been observed among NRTIs. Viruses containing abacavir and lamivudine resistance-associated amino acid substitutions, namely, K65R, L74V, M184V, and Y115F, exhibit cross-resistance to didanosine, emtricitabine, lamivudine, tenofovir, and zalcitabine in cell culture and in subjects. The K65R substitution can confer resistance to abacavir, didanosine, emtricitabine, lamivudine, stavudine, tenofovir, and zalcitabine; the L74V substitution can confer resistance to abacavir, didanosine, and zalcitabine; and the M184V substitution can confer resistance to abacavir, didanosine, emtricitabine, lamivudine, and zalcitabine.

The combination of abacavir/lamivudine has demonstrated decreased susceptibility to viruses with the substitutions K65R with or without the M184V/I substitution, viruses with L74V plus the M184V/I substitution, and viruses with thymidine analog mutations (TAMs: M41L, D67N, K70R, L210W, T215Y/F, K219 E/R/H/Q/N) plus M184V. An increasing number of TAMs is associated with a progressive reduction in abacavir susceptibility.

USE IN SPECIFIC POPULATIONS
Pregnancy
EPZICOM: Pregnancy Category C. There are no adequate and well-controlled studies of EPZICOM in pregnant women. Reproduction studies with abacavir and lamivudine have been performed in animals (see Abacavir and Lamivudine sections below). EPZICOM should be used during pregnancy only if the potential benefits outweigh the risks.

Abacavir: Studies in pregnant rats showed that abacavir is transferred to the fetus through the placenta. Fetal malformations (increased incidences of fetal anasarca and skeletal malformations) and developmental toxicity (depressed fetal body weight and reduced crown-rump length) were observed in rats at a dose which produced 35 times the human exposure, based on AUC. Embryonic and fetal toxicities (increased resorptions, decreased fetal body weights) and toxicities to the offspring (increased incidence of stillbirth and lower body weights) occurred at half of the above-mentioned dose in separate fertility studies conducted in rats. In the rabbit, no developmental toxicity and no increases in fetal malformations occurred at doses that produced 8.5 times the human exposure at the recommended dose based on AUC.

Lamivudine: Studies in pregnant rats showed that lamivudine is transferred to the fetus through the placenta. Reproduction studies with orally administered lamivudine have been performed in rats and rabbits at doses producing plasma levels up to approximately 35 times that for the recommended adult HIV dose. No evidence of teratogenicity due to lamivudine was observed. Evidence of early embryolethality was seen in the rabbit at exposure levels similar to those observed in humans, but there was no indication of this effect in the rat at exposure levels up to 35 times those in humans.

Antiretroviral Pregnancy Registry: To monitor maternal-fetal outcomes of pregnant women exposed to EPZICOM or other antiretroviral agents, an Antiretroviral Pregnancy Registry has been established. Physicians are encouraged to register patients by calling 1-800-258-4263.

Nursing Mothers
The Centers for Disease Control and Prevention recommend that HIV-1-infected mothers not breastfeed their infants to avoid risking postnatal transmission of HIV-1 infection.
Abacavir: Abacavir is secreted into the milk of lactating rats.
Lamivudine: Lamivudine is excreted in human breast milk and into the milk of lactating rats.

Because of both the potential for HIV-1 transmission and the potential for serious adverse reactions in nursing infants, mothers should be instructed not to breastfeed if they are receiving EPZICOM.

Pediatric Use
Safety and effectiveness of EPZICOM in pediatric patients have not been established. EPZICOM is not recommended for use in patients aged <18 years because it cannot be dose adjusted.

Geriatric Use
Clinical studies of abacavir and lamivudine did not include sufficient numbers of subjects aged 65 and over to determine whether they respond differently from younger subjects. In general, dose selection for an elderly patient should be cautious, reflecting the greater frequency of decreased hepatic, renal, or cardiac function, and of concomitant disease or other drug therapy.

Patients With Impaired Renal Function
EPZICOM is not recommended for patients with impaired renal function (creatinine clearance <50 mL/min) because EPZICOM is a fixed-dose combination and the dosage of the individual components cannot be adjusted.

Patients With Impaired Hepatic Function
EPZICOM is contraindicated for patients with hepatic impairment because EPZICOM is a fixed-dose combination and the dosage of the individual components cannot be adjusted.

Antiviral Activity:
The antiviral activity in cell culture of emtricitabine against laboratory and clinical isolates of HIV was assessed in lymphoblastoid cell lines, the MAGI-CCR5 cell line, and peripheral blood mononuclear cells. The 50% effective concentration (EC₅₀) value for emtricitabine was in the range of 0.0013–0.64 μM (0.0003–0.158 μg/mL). In drug combination studies of emtricitabine with nucleoside reverse transcriptase inhibitors (abacavir, lamivudine, stavudine, tenofovir, zalcitabine, zidovudine), non-nucleoside reverse transcriptase inhibitors (delavirdine, efavirenz, nevirapine), and protease inhibitors (amprenavir, nelfinavir, ritonavir, saquinavir), additive to synergistic effects were observed. Emtricitabine displayed antiviral activity in cell culture against HIV-1 clades A, B, C, D, E, F, and G (EC₅₀ values ranged from 0.007–0.075 μM) and showed strain specific activity against HIV-2 (EC₅₀ values ranged from 0.007–1.5 μM).

Resistance:
Emtricitabine−resistant isolates of HIV have been selected in cell culture and in vivo. Genotypic analysis of these isolates showed that the reduced susceptibility to emtricitabine was associated with a mutation in the HIV reverse transcriptase gene at codon 184 which resulted in an amino acid substitution of methionine by valine or isoleucine (M184V/I).

Emtricitabine-resistant isolates of HIV have been recovered from some patients treated with emtricitabine alone or in combination with other antiretroviral agents. In a clinical study of treatment-naive patients treated with EMTRIVA, didanosine, and efavirenz, viral isolates from 37.5% of patients with virologic failure showed reduced susceptibility to emtricitabine. Genotypic analysis of these isolates showed that the resistance was due to M184V/I mutations in the HIV reverse transcriptase gene.

In a clinical study of treatment-naive patients treated with either EMTRIVA, VIREAD^®, and efavirenz or zidovudine/lamivudine and efavirenz, resistance analysis was performed on HIV isolates from all virologic failure patients with >400 copies/mL of HIV-1 RNA at Week 48 or early discontinuations. Development of efavirenz resistance-associated mutations occurred most frequently and was similar between the treatment arms. The M184V amino acid substitution, associated with resistance to EMTRIVA and lamivudine, was observed in 2/12 (17%) analyzed patient isolates in the EMTRIVA + VIREAD group and in 7/22 (32%) analyzed patient isolates in the lamivudine/zidovudine group. Through 48 weeks of Study 934, no patients have developed a detectable K65R mutation in their HIV as analyzed through standard genotypic analysis. Insufficient data are available to assess the development of the K65R mutation upon prolonged exposure to this regimen.

Cross Resistance:
Cross-resistance among certain nucleoside analog reverse transcriptase inhibitors has been recognized. Emtricitabine-resistant isolates (M184V/I) were cross-resistant to lamivudine and zalcitabine but retained sensitivity in cell culture to didanosine, stavudine, tenofovir, zidovudine, and NNRTIs (delavirdine, efavirenz, and nevirapine). HIV-1 isolates containing the K65R mutation, selected in vivo by abacavir, didanosine, tenofovir, and zalcitabine, demonstrated reduced susceptibility to inhibition by emtricitabine. Viruses harboring mutations conferring reduced susceptibility to stavudine and zidovudine (M41L, D67N, K70R, L210W, T215Y/F, K219Q/E) or didanosine (L74V) remained sensitive to emtricitabine. HIV-1 containing the K103N mutation associated with resistance to NNRTIs was susceptible to emtricitabine.

Pharmacodynamics:
The in vivo activity of emtricitabine was evaluated in two clinical trials in which 101 patients were administered 25–400 mg a day of EMTRIVA as monotherapy for 10–14 days. A dose-related antiviral effect was observed, with a median decrease from baseline in plasma HIV-1 RNA of 1.3 log₁₀ at a dose of 25 mg QD and 1.7 log₁₀ to 1.9 log₁₀ at a dose of 200 mg QD or BID.

Pharmacokinetics in Adults:
The pharmacokinetics of emtricitabine were evaluated in healthy volunteers and HIV-infected individuals. Emtricitabine pharmacokinetics are similar between these populations. (For additional information, consult the Emtriva complete prescribing information).

Absorption:
Emtricitabine is rapidly and extensively absorbed following oral administration with peak plasma concentrations occurring at 1–2 hours post-dose. Following multiple dose oral administration of EMTRIVA Capsules to 20 HIV-infected subjects, the (mean ± SD) steady-state plasma emtricitabine peak concentration (C_max) was
1.8 ± 0.7 μg/mL and the area-under the plasma concentration-time curve over a 24-hour dosing interval (AUC) was 10.0 ± 3.1 hr•μg/mL. The mean steady state plasma trough concentration at 24 hours post-dose was 0.09 μg/mL. The mean absolute bioavailability of EMTRIVA Capsules was 93% while the mean absolute bioavailability of EMTRIVA Oral Solution was 75%. The relative bioavailability of EMTRIVA Oral Solution was approximately 80% of EMTRIVA Capsules.

The multiple dose pharmacokinetics of emtricitabine are dose proportional over a dose range of 25–200 mg.

Effects of Food on Oral Absorption:
EMTRIVA Capsules and Oral Solution may be taken with or without food. Emtricitabine systemic exposure (AUC) was unaffected while C_max decreased by 29% when EMTRIVA Capsules were administered with food (an approximately 1000 kcal high-fat meal). Emtricitabine systemic exposure (AUC) and C_max were unaffected when 200 mg EMTRIVA Oral Solution was administered with either a high-fat or low-fat meal.

Distribution:
In vitro binding of emtricitabine to human plasma proteins was <4% and independent of concentration over the range of 0.02–200 μg/mL. At peak plasma concentration, the mean plasma to blood drug concentration ratio was ~1.0 and the mean semen to plasma drug concentration ratio was ~4.0.

Metabolism:
In vitro studies indicate that emtricitabine is not an inhibitor of human CYP450 enzymes. Following administration of ¹⁴C-emtricitabine, complete recovery of the dose was achieved in urine (~86%) and feces (~14%). Thirteen percent (13%) of the dose was recovered in urine as three putative metabolites. The biotransformation of emtricitabine includes oxidation of the thiol moiety to form the 3’-sulfoxide diastereomers (~9% of dose) and conjugation with glucuronic acid to form 2’-O-glucuronide (~4% of dose). No other metabolites were identifiable.

Elimination:
The plasma emtricitabine half-life is approximately 10 hours. The renal clearance of emtricitabine is greater than the estimated creatinine clearance, suggesting elimination by both glomerular filtration and active tubular secretion. There may be competition for elimination with other compounds that are also renally eliminated.

Special Populations:
Race, Gender and Elderly
The pharmacokinetics of emtricitabine were similar in adult male and female patients and no pharmacokinetic differences due to race have been identified. The pharmacokinetics of emtricitabine have not been fully evaluated in the elderly.

Hepatic Impairment
The pharmacokinetics of emtricitabine have not been studied in patients with hepatic impairment, however, emtricitabine is not metabolized by liver enzymes, so the impact of liver impairment should be limited.

Pediatrics
The pharmacokinetics of emtricitabine at steady state were determined in 77 HIV-infected children, and the pharmacokinetic profile was characterized in four age groups (see below). The emtricitabine exposure achieved in children receiving a daily dose of 6 mg/kg up to a maximum of 240 mg oral solution or a 200 mg capsule is similar to exposures achieved in adults receiving a once-daily dose of 200 mg.

The pharmacokinetics of emtricitabine were studied in 20 neonates born to HIV-positive mothers. Each mother received prenatal and intrapartum combination antiretroviral therapy. Neonates received up to 6 weeks of zidovudine prophylactically after birth. The neonates were administered two short courses of emtricitabine oral solution (each 3 mg/kg QD x 4 days) during the first 3 months of life. The AUC observed in neonates who received a daily dose of 3 mg/kg of emtricitabine was similar to the AUC observed in pediatric patients ≥3 months to 17 years who received a daily dose of emtricitabine as a 6 mg/kg oral solution up to 240 mg or as a 200 mg capsule (see below).

Mean ± SD Pharmacokinetic Parameters by Age Groups for Pediatric Patients and Neonates Receiving EMTRIVA Capsules or Oral Solution

HIV-exposed Neonates
Age 0–3 mo (N=20¹; formulation: capsule [n=0], oral solution [n=20]) – Dose (mg/kg)²: 3.1 (2.9-3.4); C_max (μg/mL): 1.6 ± 0.6; AUC (hr•μg/mL): 11.0 ± 4.2; T_1/2 (hr): 12.1 ± 3.1.

HIV-infected Pediatric Patients
Age 3–24 mo (N=14; formulation: capsule [n=0], oral solution [n=14]) – Dose (mg/kg)²: 6.1 (5.5−6.8); C_max (μg/mL): 1.9 ± 0.6; AUC (hr•μg/mL): 8.7 ± 3.2; T_1/2 (hr): 8.9 ± 3.2.

Age 25 mo–6 yr (N=19; formulation: capsule [n=0], oral solution [n=19]) – Dose (mg/kg)²: 6.1 (5.6−6.7); C_max (μg/mL): 1.9 ± 0.7; AUC (hr•μg/mL): 9.0 ± 3.0; T_1/2 (hr): 11.3 ± 6.4.

Age 7–12yr (N=17; formulation: capsule [n=10], oral solution [n=7]) – Dose (mg/kg)²: 5.6 (3.1−6.6); C_max (μg/mL): 2.7 ± 0.8; AUC (hr•μg/mL): 12.6 ± 3.5; T_1/2 (hr): 8.2 ± 3.2.

Age 13–17 yr (N=27; formulation: capsule [n=26], oral solution [n=1]) – Dose (mg/kg)²: 4.4 (1.8−7.0); C_max (μg/mL): 2.7 ± 0.9; AUC (hr•μg/mL): 12.6 ± 5.4; T_1/2 (hr): 8.9 ± 3.3.

1. Two pharmacokinetic evaluations were conducted in 20 neonates over the first 3 months of life. Median (range) age of infant on day of pharmacokinetic evaluation was 26 (5–81) days.
2. Mean (range)

Renal Impairment
The pharmacokinetics of emtricitabine are altered in patients with renal impairment. In adult patients with creatinine clearance <50 mL/min or with end-stage renal disease (ESRD) requiring dialysis, C_max and AUC of emtricitabine were increased due to a reduction in renal clearance (see below). It is recommended that the dosing interval for EMTRIVA be modified in adult patients with creatinine clearance <50 mL/min or in adult patients with ESRD who require dialysis. The effects of renal impairment on emtricitabine pharmacokinetics in pediatric patients are not known.

Mean ± SD Pharmacokinetic Parameters in Adult Patients with Varying Degrees of Renal Function

Creatinine Clearance (mL/min) >80 (N=6) – Baseline creatinine clearance (mL/min): 107 ± 21; C_max (μg/mL): 2.2 ± 0.6; AUC (hr•μg/mL): 11.8 ± 2.9; CL/F (mL/min): 302 ± 94; CLr (mL/min): 213 ± 89.

Creatinine Clearance (mL/min) 50–80 (N=6) – Baseline creatinine clearance (mL/min): 59.8 ± 6.5; C_max (μg/mL): 3.8 ± 0.9; AUC (hr•μg/mL): 19.9 ± 1.2; CL/F (mL/min): 168 ± 10; CLr (mL/min): 121 ± 39.

Creatinine Clearance (mL/min) 30–49 (N=6) – Baseline creatinine clearance (mL/min): 40.9 ± 5.1; C_max (μg/mL): 3.2 ± 0.6; AUC (hr•μg/mL): 25.1 ± 5.7; CL/F (mL/min): 138 ± 28; CLr (mL/min): 69 ± 32.

Creatinine Clearance (mL/min) <30 (N=5) – Baseline creatinine clearance (mL/min): 22.9 ± 5.3; C_max (μg/mL): 2.8 ± 0.7; AUC (hr•μg/mL): 33.7± 2.1; CL/F (mL/min): 99 ± 6; CLr (mL/min): 30 ± 11.

Creatinine Clearance (mL/min) ESRD¹ <30 (N=5) – Baseline creatinine clearance (mL/min): 8.8 ± 1.4; C_max (μg/mL): 2.8 ± 0.5; AUC (hr•μg/mL): 53.2 ± 9.9; CL/F (mL/min 64 ± 12; CLr (mL/min): NA².

1. ESRD patients requiring dialysis
2. NA = Not Applicable

Hemodialysis: Hemodialysis treatment removes approximately 30% of the emtricitabine dose over a 3-hour dialysis period starting within 1.5 hours of emtricitabine dosing (blood flow rate of 400 mL/min and a dialysate flow rate of 600 mL/min). It is not known whether emtricitabine can be removed by peritoneal dialysis.

Drug Interactions:
At concentrations up to 14-fold higher than those observed in vivo, emtricitabine did not inhibit in vitro drug metabolism mediated by any of the following human CYP 450 isoforms: CYP1A2, CYP2A6, CYP2B6, CYP2C9, CYP2C19, CYP2D6, and CYP3A4. Emtricitabine did not inhibit the enzyme responsible for glucuronidation (uridine-5’-disphosphoglucuronyl transferase). Based on the results of these in vitro experiments and the known elimination pathways of emtricitabine, the potential for CYP450 mediated interactions involving emtricitabine with other medicinal products is low.

EMTRIVA has been evaluated in healthy volunteers in combination with tenofovir disoproxil fumarate (DF), zidovudine, indinavir, famciclovir, and stavudine. (For additional information, consult the Emtriva complete prescribing information).

Pregnancy:
Emtricitabine is in FDA pregnancy category B. The incidence of fetal variations and malformations was not increased in embryofetal toxicity studies performed with emtricitabine in mice at exposures (AUC) approximately 60-fold higher and in rabbits at approximately 120-fold higher than human exposures at the recommended daily dose. There are, however, no adequate and well-controlled studies in pregnant women. Because animal reproduction studies are not always predictive of human response, EMTRIVA should be used during pregnancy only if clearly needed. To monitor fetal outcomes of pregnant women exposed to emtricitabine, an antiretroviral Pregnancy Registry has been established. Healthcare providers are encouraged to register patients by calling 1–800–258–4263.

Nursing Mothers:
The Centers for Disease Control and Prevention recommend that HIV-infected mothers not breast-feed their infants to avoid risking postnatal transmission of HIV: It is not known whether emtricitabine is secreted into human milk. Because of both the potential for HIV transmission and the potential for serious adverse reactions in nursing infants, mothers should be instructed not to breast-feed if they are receiving EMTRIVA.

Mechanism of Action
TRUVADA is a fixed-dose combination of antiviral drugs emtricitabine and tenofovir disoproxil fumarate.

Pharmacokinetics
TRUVADA: One TRUVADA tablet was bioequivalent to one EMTRIVA capsule (200 mg) plus one VIREAD tablet (300 mg) following single-dose administration to fasting healthy subjects (N=39).

Emtricitabine: The pharmacokinetic properties of emtricitabine are summarized below. Following oral administration of EMTRIVA, emtricitabine is rapidly absorbed with peak plasma concentrations occurring at 1–2 hours post-dose. Less than 4% of emtricitabine binds to human plasma proteins in vitro and the binding is independent of concentration over the range of 0.02–200 μg/mL. Following administration of radiolabelled emtricitabine, approximately 86% is recovered in the urine and 13% is recovered as metabolites. The metabolites of emtricitabine include 3′-sulfoxide diastereomers and their glucuronic acid conjugate. Emtricitabine is eliminated by a combination of glomerular filtration and active tubular secretion. Following a single oral dose of EMTRIVA, the plasma emtricitabine half-life is approximately 10 hours.

Tenofovir Disoproxil Fumarate: The pharmacokinetic properties of tenofovir disoproxil fumarate are summarized below. Following oral administration of VIREAD, maximum tenofovir serum concentrations are achieved in 1.0 ± 0.4 hour. Less than 0.7% of tenofovir binds to human plasma proteins in vitro and the binding is independent of concentration over the range of 0.01–25 μg/mL. Approximately 70–80% of the intravenous dose of tenofovir is recovered as unchanged drug in the urine. Tenofovir is eliminated by a combination of glomerular filtration and active tubular secretion. Following a single oral dose of VIREAD, the terminal elimination half-life of tenofovir is approximately 17 hours.

Single Dose Pharmacokinetic Parameters for Emtricitabine and Tenofovir in Adults^a

• Fasted Oral Bioavailability^b (%) – Emtricitabine: 92 (83.1–106.4); Tenofovir: 25 (NC–45.0)
• Plasma Terminal Elimination Half-Life^b (hr) – Emtricitabine: 10 (7.4–18.0); Tenofovir: 17 (12.0–25.7)
• C_max ^c (μg/mL) – Emtricitabine: 1.8 ± 0.72d; Tenofovir: 0.30 ± 0.09
• AUC^c (μg•hr/mL) – Emtricitabine: 10.0 ± 3.12^d; Tenofovir: 2.29 ± 0.69
• CL/F^c (mL/min) – Emtricitabine: 302 ± 94; Tenofovir: 1043 ± 115
• CL_renal ^c (mL/min) – Emtricitabine: 213 ± 89; Tenofovir: 243 ± 33

a. NC = Not calculated
b. Median (range)
c. Mean (± SD)
d. Data presented as steady state values.

Effects of Food on Oral Absorption
TRUVADA may be administered with or without food. Administration of TRUVADA following a high fat meal (784 kcal; 49 grams of fat) or a light meal (373 kcal; 8 grams of fat) delayed the time of tenofovir C_max by approximately 0.75 hour. The mean increases in tenofovir AUC and C_max were approximately 35% and 15%, respectively, when administered with a high fat or light meal, compared to administration in the fasted state. In previous safety and efficacy trials, VIREAD (tenofovir) was taken under fed conditions. Emtricitabine systemic exposures (AUC and C_max) were unaffected when TRUVADA was administered with either a high fat or a light meal.

Special Populations

Race
Emtricitabine: No pharmacokinetic differences due to race have been identified following the administration of EMTRIVA.

Tenofovir Disoproxil Fumarate: There were insufficient numbers from racial and ethnic groups other than Caucasian to adequately determine potential pharmacokinetic differences among these populations following the administration of VIREAD.

Gender
Emtricitabine and Tenofovir Disoproxil Fumarate: Emtricitabine and tenofovir pharmacokinetics are similar in male and female subjects.

Pediatric Patients
TRUVADA should not be administered to pediatric patients less than 12 years of age or weighing less than 35 kg (less than 77 lb).

Emtricitabine: The pharmacokinetics of emtricitabine at steady state were determined in 27 HIV-1-infected pediatric subjects 13 to 17 years of age receiving a daily dose of 6 mg/kg up to a maximum dose of 240 mg oral solution or a 200 mg capsule; 26 of 27 subjects in this age group received the 200 mg EMTRIVA capsule. Mean (± SD) C_max and AUC were 2.7 ± 0.9 μg/mL and 12.6 ± 5.4 μg•hr/mL, respectively. Exposures achieved in pediatric subjects 12 to less than 18 years of age were similar to those achieved in adults receiving a once daily dose of 200 mg.

Tenofovir Disoproxil Fumarate: Steady-state pharmacokinetics of tenofovir were evaluated in 8 HIV-1 infected pediatric subjects (12 to less than 18 years). Mean (± SD) C_max and AUC_tau are 0.38 ± 0.13 μg/mL and 3.39 ± 1.22 μg•hr/mL, respectively. Tenofovir exposure achieved in these pediatric subjects receiving oral daily doses of VIREAD 300 mg was similar to exposures achieved in adults receiving once-daily doses of VIREAD 300 mg.

Geriatric Patients
Pharmacokinetics of emtricitabine and tenofovir have not been fully evaluated in the elderly (65 years of age and older).

Patients with Impaired Renal Function
The pharmacokinetics of emtricitabine and tenofovir are altered in subjects with renal impairment. In adult subjects with creatinine clearance below 50 mL/min, C_max, and AUC_0-∞ of emtricitabine and tenofovir were increased. It is recommended that the dosing interval for TRUVADA be modified in patients with creatinine clearance 30–49 mL/min. TRUVADA should not be used in patients with creatinine clearance below 30 mL/min and in patients with end-stage renal disease requiring dialysis.

Patients with Hepatic Impairment
The pharmacokinetics of tenofovir following a 300 mg dose of VIREAD have been studied in non-HIV infected subjects with moderate to severe hepatic impairment. There were no substantial alterations in tenofovir pharmacokinetics in subjects with hepatic impairment compared with unimpaired subjects. The pharmacokinetics of TRUVADA or emtricitabine have not been studied in subjects with hepatic impairment; however, emtricitabine is not significantly metabolized by liver enzymes, so the impact of liver impairment should be limited.

Assessment of Drug Interactions
The steady state pharmacokinetics of emtricitabine and tenofovir were unaffected when emtricitabine and tenofovir disoproxil fumarate were administered together versus each agent dosed alone.

In vitro studies and clinical pharmacokinetic drug-drug interaction trials have shown that the potential for CYP mediated interactions involving emtricitabine and tenofovir with other medicinal products is low.

No clinically significant drug interactions have been observed between emtricitabine and famciclovir, indinavir, stavudine, tenofovir disoproxil fumarate, and zidovudine. Similarly, no clinically significant drug interactions have been observed between tenofovir disoproxil fumarate and abacavir, efavirenz, emtricitabine, entecavir, indinavir, lamivudine, lopinavir/ritonavir, methadone, nelfinavir, oral contraceptives, ribavirin, saquinavir/ritonavir, and tacrolimus in trials conducted in healthy volunteers.

Following multiple dosing to HIV-negative subjects receiving either chronic methadone maintenance therapy or oral contraceptives, or single doses of ribavirin, steady state tenofovir pharmacokinetics were similar to those observed in previous trials, indicating lack of clinically significant drug interactions between these agents and VIREAD.

Coadministration of tenofovir disoproxil fumarate with didanosine results in changes in the pharmacokinetics of didanosine that may be of clinical significance. Concomitant dosing of tenofovir disoproxil fumarate with didanosine buffered tablets or enteric-coated capsules significantly increases the C_max and AUC of didanosine. When didanosine 250 mg enteric-coated capsules were administered with tenofovir disoproxil fumarate, systemic exposures of didanosine were similar to those seen with the 400 mg enteric-coated capsules alone under fasted conditions. The mechanism of this interaction is unknown.
(For additional information, consult the Truvada complete prescribing information).

Microbiology

Mechanism of Action
Emtricitabine: Emtricitabine, a synthetic nucleoside analog of cytidine, is phosphorylated by cellular enzymes to form emtricitabine 5'-triphosphate. Emtricitabine 5'-triphosphate inhibits the activity of the HIV-1 reverse transcriptase (RT) by competing with the natural substrate deoxycytidine 5'-triphosphate and by being incorporated into nascent viral DNA which results in chain termination. Emtricitabine 5′-triphosphate is a weak inhibitor of mammalian DNA polymerase α, β, ε and mitochondrial DNA polymerase γ.

Tenofovir Disoproxil Fumarate: Tenofovir disoproxil fumarate is an acyclic nucleoside phosphonate diester analog of adenosine monophosphate. Tenofovir disoproxil fumarate requires initial diester hydrolysis for conversion to tenofovir and subsequent phosphorylations by cellular enzymes to form tenofovir diphosphate. Tenofovir diphosphate inhibits the activity of HIV-1 RT by competing with the natural substrate deoxyadenosine 5′-triphosphate and, after incorporation into DNA, by DNA chain termination. Tenofovir diphosphate is a weak inhibitor of mammalian DNA polymerases α, β, and mitochondrial DNA polymerase γ.

Antiviral Activity
Emtricitabine and Tenofovir Disoproxil Fumarate: In combination studies evaluating the cell culture antiviral activity of emtricitabine and tenofovir together, synergistic antiviral effects were observed.

Emtricitabine: The antiviral activity of emtricitabine against laboratory and clinical isolates of HIV-1 was assessed in lymphoblastoid cell lines, the MAGI-CCR5 cell line, and peripheral blood mononuclear cells. The 50% effective concentration (EC₅₀) values for emtricitabine were in the range of 0.0013–0.64 μM (0.0003–0.158 μg/mL). In drug combination studies of emtricitabine with nucleoside reverse transcriptase inhibitors (abacavir, lamivudine, stavudine, zalcitabine, zidovudine), non-nucleoside reverse transcriptase inhibitors (delavirdine, efavirenz, nevirapine), and protease inhibitors (amprenavir, nelfinavir, ritonavir, saquinavir), additive to synergistic effects were observed. Emtricitabine displayed antiviral activity in cell culture against HIV-1 clades A, B, C, D, E, F, and G (EC₅₀ values ranged from 0.007–0.075 μM) and showed strain specific activity against HIV-2 (EC₅₀ values ranged from 0.007–1.5 μM).

Tenofovir Disoproxil Fumarate: The antiviral activity of tenofovir against laboratory and clinical isolates of HIV-1 was assessed in lymphoblastoid cell lines, primary monocyte/macrophage cells and peripheral blood lymphocytes. The EC₅₀ values for tenofovir were in the range of 0.04–8.5 μM. In drug combination studies of tenofovir with nucleoside reverse transcriptase inhibitors (abacavir, didanosine, lamivudine, stavudine, zalcitabine, zidovudine), non-nucleoside reverse transcriptase inhibitors (delavirdine, efavirenz, nevirapine), and protease inhibitors (amprenavir, indinavir, nelfinavir, ritonavir, saquinavir), additive to synergistic effects were observed. Tenofovir displayed antiviral activity in cell culture against HIV-1 clades A, B, C, D, E, F, G and O (EC₅₀ values ranged from 0.5–2.2 μM) and showed strain specific activity against HIV-2 (EC₅₀ values ranged from 1.6 μM to 5.5 μM).

Resistance
Emtricitabine and Tenofovir Disoproxil Fumarate: HIV-1 isolates with reduced susceptibility to the combination of emtricitabine and tenofovir have been selected in cell culture. Genotypic analysis of these isolates identified the M184V/I and/or K65R amino acid substitutions in the viral RT.

In a clinical trial of treatment-naive subjects [Study 934], resistance analysis was performed on HIV-1 isolates from all confirmed virologic failure subjects with greater than 400 copies/mL of HIV-1 RNA at Week 144 or early discontinuation. Development of efavirenz resistance-associated substitutions occurred most frequently and was similar between the treatment arms. The M184V amino acid substitution, associated with resistance to EMTRIVA and lamivudine, was observed in 2/19 analyzed subjects isolates in the EMTRIVA + VIREAD group and in 10/29 analyzed subjects isolates in the zidovudine/lamivudine group. Through 144 weeks of Study 934, no subjects have developed a detectable K65R substitution in their HIV-1 as analyzed through standard genotypic analysis.

Emtricitabine: Emtricitabine-resistant isolates of HIV-1 have been selected in cell culture and in vivo. Genotypic analysis of these isolates showed that the reduced susceptibility to emtricitabine was associated with a substitution in the HIV-1 RT gene at codon 184 which resulted in an amino acid substitution of methionine by valine or isoleucine (M184V/I).

Tenofovir Disoproxil Fumarate: HIV-1 isolates with reduced susceptibility to tenofovir have been selected in cell culture. These viruses expressed a K65R substitution in RT and showed a 2–4 fold reduction in susceptibility to tenofovir.

In treatment-naive subjects, isolates from 8/47 (17%) analyzed subjects developed the K65R substitution in the VIREAD arm through 144 weeks; 7 occurred in the first 48 weeks of treatment and 1 at Week 96. In treatment-experienced subjects, 14/304 (5%) isolates from subjects failing VIREAD through Week 96 showed greater than 1.4 fold (median 2.7) reduced susceptibility to tenofovir. Genotypic analysis of the resistant isolates showed a substitution in the HIV-1 RT gene resulting in the K65R amino acid substitution.

Cross Resistance
Emtricitabine and Tenofovir Disoproxil Fumarate: Cross-resistance among certain nucleoside reverse transcriptase inhibitors (NRTIs) has been recognized. The M184V/I and/or K65R substitutions selected in cell culture by the combination of emtricitabine and tenofovir are also observed in some HIV-1 isolates from subjects failing treatment with tenofovir in combination with either lamivudine or emtricitabine, and either abacavir or didanosine. Therefore, cross-resistance among these drugs may occur in patients whose virus harbors either or both of these amino acid substitutions.

Emtricitabine: Emtricitabine-resistant isolates (M184V/I) were cross-resistant to lamivudine and zalcitabine but retained susceptibility in cell culture to didanosine, stavudine, tenofovir, zidovudine, and NNRTIs (delavirdine, efavirenz, and nevirapine). HIV-1 isolates containing the K65R substitution, selected in vivo by abacavir, didanosine, tenofovir, and zalcitabine, demonstrated reduced susceptibility to inhibition by emtricitabine. Viruses harboring substitutions conferring reduced susceptibility to stavudine and zidovudine (M41L, D67N, K70R, L210W, T215Y/F, K219Q/E), or didanosine (L74V) remained sensitive to emtricitabine. HIV-1 containing the K103N substitution associated with resistance to NNRTIs was susceptible to emtricitabine.

Tenofovir Disoproxil Fumarate: HIV-1 isolates from subjects (N=20) whose HIV-1 expressed a mean of 3 zidovudine-associated RT amino acid substitutions (M41L, D67N, K70R, L210W, T215Y/F, or K219Q/E/N) showed a 3.1-fold decrease in the susceptibility to tenofovir. Subjects whose virus expressed an L74V substitution without zidovudine resistance associated substitutions (N=8) had reduced response to VIREAD. Limited data are available for patients whose virus expressed a Y115F substitution (N=3), Q151M substitution (N=2), or T69 insertion (N=4), all of whom had a reduced response.

USE IN SPECIFIC POPULATIONS

Pregnancy
Pregnancy Category B

Emtricitabine: The incidence of fetal variations and malformations was not increased in embryofetal toxicity studies performed with emtricitabine in mice at exposures (AUC) approximately 60-fold higher and in rabbits at approximately 120-fold higher than human exposures at the recommended daily dose.

Tenofovir Disoproxil Fumarate: Reproduction studies have been performed in rats and rabbits at doses up to 14 and 19 times the human dose based on body surface area comparisons and revealed no evidence of impaired fertility or harm to the fetus due to tenofovir.

There are, however, no adequate and well-controlled trials in pregnant women. Because animal reproduction studies are not always predictive of human response, TRUVADA should be used during pregnancy only if clearly needed.

Antiretroviral Pregnancy Registry: To monitor fetal outcomes of pregnant women exposed to TRUVADA, an Antiretroviral Pregnancy Registry has been established. Healthcare providers are encouraged to register patients by calling 1-800-258-4263.

Nursing Mothers
Nursing Mothers: The Centers for Disease Control and Prevention recommend that HIV-1 infected mothers not breast-feed their infants to avoid risking postnatal transmission of HIV-1. Studies in rats have demonstrated that tenofovir is secreted in milk. It is not known whether tenofovir is excreted in human milk. It is not known whether emtricitabine is excreted in human milk. Because of both the potential for HIV-1 transmission and the potential for serious adverse reactions in nursing infants, mothers should be instructed not to breast-feed if they are receiving TRUVADA.

(For additional information, consult the Truvada complete prescribing information).

Following oral dosing, lamivudine is rapidly absorbed and extensively distributed. Binding to plasma protein is low. Approximately 70% of an intravenous dose of lamivudine is recovered as unchanged drug in the urine. Metabolism of lamivudine is a minor route of elimination. In humans, the only known metabolite is the trans-sulfoxide metabolite (approximately 5% of an oral dose after 12 hours). The mean oral bioavailability, mean apparent volume of distribution, plasma protein binding, median CSF:plasma ratio, mean systemic clearance, mean renal clearance, and elimination half-life (approximate range) of lamivudine in fasting patients are 86%, 1.3 L/kg, less than 36%, 0.12, 0.33 L/hr/kg, 0.22 L/hr/kg and 5 to 7 hrs, respectively. [#]

Zidovudine is also a synthetic nucleoside analogue that is phosphorylated intracellularly to its active 5'-triphosphate metabolite, zidovudine triphosphate (ZDV-TP). The principal mode of action of ZDV-TP is inhibition of RT via DNA chain termination after incorporation of the nucleotide analogue. ZDV-TP is a weak inhibitor of the cellular DNA polymerases α and γ and has been reported to be incorporated into the DNA of cells in culture. In cell culture drug combination studies, zidovudine demonstrates synergistic activity with the nucleoside reverse transcriptase inhibitors (NRTIs) abacavir, didanosine, lamivudine, and zalcitabine; the non-nucleoside reverse transcriptase inhibitors (NNRTIs) delavirdine and nevirapine; and the protease inhibitors (PIs) indinavir, nelfinavir, ritonavir, and saquinavir; and additive activity with interferon alfa. Ribavirin has been found to inhibit the phosphorylation of zidovudine in cell culture. [#]

Following oral administration, zidovudine is rapidly absorbed and extensively distributed. Binding to plasma protein is low. Zidovudine is eliminated primarily by hepatic metabolism. The major metabolite of zidovudine is 3′-azido-3′-deoxy-5′-O-β-D-glucopyranuronosylthymidine (GZDV). GZDV area under the curve (AUC) is about 3-fold greater than the zidovudine AUC. Urinary recovery of zidovudine and GZDV accounts for 14% and 74% of the dose following oral administration, respectively. A second metabolite, 3′-amino-3′-deoxythymidine (AMT), has been identified in plasma. The AMT AUC was one fifth of the zidovudine AUC. The pharmacokinetic properties (mean oral bioavailability, mean apparent volume of distribution, plasma protein binding, median CSF:plasma ratio, mean systemic clearance, mean renal clearance, and elimination half-life [approximate range]) of zidovudine in fasting patients are 64%, 1.6 L/kg, less than 38%, 0.60, 1.6 L/hr/kg, 0.34 L/hr/kg and 0.5 to 3 hrs, respectively. [#]

One lamivudine/zidovudine tablet is bioequivalent to one lamivudine tablet (150 mg) plus one zidovudine tablet (300 mg) following single-dose administration to fasting healthy adults (n = 24). Lamivudine/zidovudine may be administered with or without food. The lamivudine and zidovudine AUC following administration of lamivudine/zidovudine (Combivir) with food was similar when compared to fasting healthy subjects (n = 24). [#]

Lamivudine/zidovudine is in FDA Pregnancy Category C. No adequate or well-controlled studies of the combination drug have been done in pregnant women. Clinical trial data demonstrate that maternal zidovudine treatment during pregnancy reduces vertical transmission of HIV-1 infection to the fetus. Animal reproduction studies performed with lamivudine and zidovudine showed increased embryotoxicity and fetal malformations (zidovudine), and increased embryolethality (lamivudine). Lamivudine/zidovudine should be used during pregnancy only if the potential benefit justifies the potential risk to the fetus. To monitor maternal-fetal outcomes of pregnant women exposed to lamivudine/zidovudine and other antiretroviral agents, an Antiretroviral Pregnancy Registry (APR) has been established. Physicians are encouraged to register patients by calling 1-800-258-4263 or online at http://www.APRegistry.com. [#]

Lamivudine pharmacokinetics were studied in pregnant women during 2 clinical studies conducted in South Africa. The study assessed pharmacokinetics in: 16 women at 36 weeks gestation using 150 mg lamivudine twice daily with zidovudine, 10 women at 38 weeks gestation using 150 mg lamivudine twice daily with zidovudine, and 10 women at 38 weeks gestation using lamivudine 300 mg twice daily without other antiretrovirals. Lamivudine pharmacokinetics in pregnant women were similar to those seen in nonpregnant adults and in postpartum women. Lamivudine concentrations were generally similar in maternal, neonatal, and umbilical cord serum samples. [#]

A randomized, double-blind, placebo-controlled trial was conducted in HIV-1-infected pregnant women to determine the utility of zidovudine for the prevention of maternal-fetal HIV-1 transmission. Zidovudine treatment during pregnancy reduced the rate of maternal-fetal HIV-1 transmission from 24.9% for infants born to placebo-treated mothers to 7.8% for infants born to mothers treated with zidovudine. There were no differences in pregnancy-related adverse events between the treatment groups. Congenital abnormalities occurred with similar frequency between neonates born to mothers who received zidovudine and neonates born to mothers who received placebo. The observed abnormalities included problems in embryogenesis (prior to 14 weeks) or were recognized on ultrasound before or immediately after initiation of study drug. [#]

Zidovudine pharmacokinetics were studied in a Phase 1 study of 8 women during the last trimester of pregnancy. As pregnancy progressed, there was no evidence of drug accumulation. The pharmacokinetics of zidovudine were similar to that of nonpregnant adults. Consistent with passive transmission of the drug across the placenta, zidovudine concentrations in neonatal plasma at birth were essentially equal to those in maternal plasma at delivery. [#]

Animal reproduction studies performed at oral doses up to 130 and 60 times the adult dose in rats and rabbits, respectively, revealed no evidence of teratogenicity due to lamivudine. Increased early embryolethality occurred in rabbits at exposure levels similar to those in humans. However, there was no indication of this effect in rats at exposure levels up to 35 times those in humans. Based on animal studies, lamivudine crosses the placenta and is transferred to the fetus. [#]

Increased fetal resorptions occurred in pregnant rats and rabbits treated with doses of zidovudine that produced drug plasma concentrations 66 to 226 times (rats) and 12 to 87 times (rabbits) the mean steady-state peak human plasma concentration following a single 100-mg dose of zidovudine. There were no other reported developmental anomalies. In another developmental toxicity study, pregnant rats received zidovudine up to near-lethal doses that produced peak plasma concentrations 350 times peak human plasma concentrations (300 times the daily exposure [AUC] in humans given 600 mg/day zidovudine). This dose was associated with marked maternal toxicity and an increased incidence of fetal malformations. However, there were no signs of teratogenicity at doses up to one fifth the lethal dose. [#]

The Centers for Disease Control and Prevention recommend that HIV-1-infected mothers in the United States not breastfeed their infants to avoid risking postnatal transmission of HIV-1 infection. Because of both the potential for HIV-1 transmission and serious adverse reactions in nursing infants, mothers should be instructed not to breastfeed if they are receiving lamivudine/zidovudine. Although no studies of lamivudine/zidovudine excretion in breast milk have been performed, lactation studies performed with lamivudine and zidovudine show that both drugs are excreted in human breast milk. Samples of breast milk obtained from 20 mothers receiving lamivudine monotherapy (300 mg twice daily) or combination therapy (150 mg lamivudine twice daily and 300 mg zidovudine twice daily) had measurable concentrations of lamivudine. In another study, after administration of a single dose of 200 mg zidovudine to 13 HIV-1-infected women, the mean concentration of zidovudine was similar in human milk and serum. [#]

In patients receiving lamivudine monotherapy or combination therapy with lamivudine plus zidovudine, HIV-1 isolates from most patients became phenotypically and genotypically resistant to lamivudine within 12 weeks. In some patients harboring zidovudine-resistant virus at baseline, phenotypic sensitivity to zidovudine was restored by 12 weeks of treatment with zidovudine and lamivudine. Lamivudine/zidovudine combination therapy delayed the emergence of mutations conferring zidovudine resistance. HIV strains resistant to both lamivudine and zidovudine have been isolated from patients after prolonged lamivudine/zidovudine therapy. Dual resistance required the presence of multiple amino acid substitutions, the most essential of which may be G333E. The incidence of dual resistance and the duration of combination therapy required before dual resistance occurs are unknown. [#]

Lamivudine-resistant isolates of HIV-1 have been selected in cell culture and have also been recovered from patients treated with lamivudine or lamivudine plus zidovudine. Genotypic analysis of isolates selected in cell culture and recovered from lamivudine-treated patients showed that the resistance was due to a specific amino acid substitution in the HIV-1 reverse transcriptase at codon 184 changing the methionine to either isoleucine or valine (M184V/I). [#]

HIV-1 isolates with reduced susceptibility to zidovudine have been selected in cell culture and were also recovered from patients treated with zidovudine. Genotypic analyses of the isolates selected in cell culture and recovered from zidovudine-treated patients showed substitutions in the HIV-1 RT gene resulting in 6 amino acid substitutions (M41L, D67N, K70R, L210W, T215Y or F, and K219Q) that confer zidovudine resistance. In general, higher levels of resistance were associated with greater number of amino acid substitutions. [#]

Cross-resistance between lamivudine and zidovudine has not been reported. In some patients treated with lamivudine alone or in combination with zidovudine, isolates have emerged with a substitution at codon 184, which confers resistance to lamivudine. Cross-resistance to abacavir, didanosine, tenofovir, and zalcitabine has been observed in some patients harboring lamivudine-resistant HIV-1 isolates. In some patients treated with zidovudine plus didanosine or zalcitabine, isolates resistant to multiple drugs, including lamivudine, have emerged. [#]

In a study of 167 HIV-1-infected patients, isolates (n = 2) with multi-drug resistance to didanosine, lamivudine, stavudine, zalcitabine, and zidovudine were recovered from patients treated for ≥1 year with zidovudine plus didanosine or zidovudine plus zalcitabine. The pattern of resistance-associated amino acid substitutions with such combination therapies was different (A62V, V75I, F77L, F116Y, Q151M) from the pattern with zidovudine monotherapy, with the Q151M substitution being most commonly associated with multi-drug resistance. The substitution at codon 151 in combination with substitutions at 62, 75, 77, and 116 results in a virus with reduced susceptibility to didanosine, lamivudine, stavudine, zalcitabine, and zidovudine. Thymidine analogue mutations (TAMs) are selected by zidovudine and confer cross-resistance to abacavir, didanosine, stavudine, tenofovir, and zalcitabine. [#]

Pharmacokinetics
The pharmacokinetics of tenofovir disoproxil fumarate have been evaluated in healthy volunteers and HIV-1 infected individuals. Tenofovir pharmacokinetics are similar between these populations.

Absorption
VIREAD is a water soluble diester prodrug of the active ingredient tenofovir. The oral bioavailability of tenofovir from VIREAD in fasted subjects is approximately 25%. Following oral administration of a single dose of VIREAD 300 mg to HIV-1 infected subjects in the fasted state, maximum serum concentrations (C_max) are achieved in 1.0 ± 0.4 hrs. C_max and AUC values are 0.30 ± 0.09 μg/mL and 2.29 ± 0.69 μg•hr/mL, respectively. The pharmacokinetics of tenofovir are dose proportional over a VIREAD dose range of 75 to 600 mg and are not affected by repeated dosing.

Distribution
In vitro binding of tenofovir to human plasma or serum proteins is less than 0.7 and 7.2%, respectively, over the tenofovir concentration range 0.01 to 25 μg/mL. The volume of distribution at steady-state is 1.3 ± 0.6 L/kg and 1.2 ± 0.4 L/kg, following intravenous administration of tenofovir 1.0 mg/kg and 3.0 mg/kg.

Metabolism and Elimination
In vitro studies indicate that neither tenofovir disoproxil nor tenofovir are substrates of CYP enzymes. Following IV administration of tenofovir, approximately 70–80% of the dose is recovered in the urine as unchanged tenofovir within 72 hours of dosing. Following single dose, oral administration of VIREAD, the terminal elimination half-life of tenofovir is approximately 17 hours. After multiple oral doses of VIREAD 300 mg once daily (under fed conditions), 32 ± 10% of the administered dose is recovered in urine over 24 hours. Tenofovir is eliminated by a combination of glomerular filtration and active tubular secretion. There may be competition for elimination with other compounds that are also renally eliminated.

Effects of Food on Oral Absorption
Administration of VIREAD following a high-fat meal (~700 to 1000 kcal containing 40 to 50% fat) increases the oral bioavailability, with an increase in tenofovir AUC_0-∞ of approximately 40% and an increase in C_max of approximately 14%. However, administration of VIREAD with a light meal did not have a significant effect on the pharmacokinetics of tenofovir when compared to fasted administration of the drug. Food delays the time to tenofovir C_max by approximately 1 hour. C_max and AUC of tenofovir are 0.33 ± 0.12 μg/mL and 3.32 ± 1.37 μg•hr/mL following multiple doses of VIREAD 300 mg once daily in the fed state, when meal content was not controlled.

Special Populations

Race: There were insufficient numbers from racial and ethnic groups other than Caucasian to adequately determine potential pharmacokinetic differences among these populations.

Gender: Tenofovir pharmacokinetics are similar in male and female subjects.

Pediatric Patients 12 Years of Age and Older: Steady-state pharmacokinetics of tenofovir were evaluated in 8 HIV-1 infected pediatric subjects (12 to <18 years). Mean (± SD) C_max and AUC_tau are 0.38 ± 0.13 μg/mL and 3.39 ± 1.22 μg•hr/mL, respectively. Tenofovir exposure achieved in these pediatric subjects receiving oral daily doses of VIREAD 300 mg was similar to exposures achieved in adults receiving once-daily doses of VIREAD 300 mg. Pharmacokinetic studies have not been performed in pediatric subjects <12 years of age.

Geriatric Patients: Pharmacokinetic studies have not been performed in the elderly (>65 years).

Patients with Impaired Renal Function: The pharmacokinetics of tenofovir are altered in subjects with renal impairment. In subjects with creatinine clearance <50 mL/min or with end-stage renal disease (ESRD) requiring dialysis, C_max, and AUC_0-∞ of tenofovir were increased (see below). It is recommended that the dosing interval for VIREAD be modified in patients with creatinine clearance <50 mL/min or in patients with ESRD who require dialysis.

Pharmacokinetic Parameters (Mean ± SD) of Tenofovir^a in Subjects with Varying Degrees of Renal Function

Baseline Creatinine Clearance >80 mL/min (N=3) – C_max: 0.34 ± 0.03 μg/mL; AUC_0-∞: 2.18 ± 0.26 μg•hr/mL; CL/F: 1043.7 ± 115.4 mL/min; CL_renal: 243.5 ± 33.3 mL/min.

Baseline Creatinine Clearance 50–80 mL/min (N=10) – C_max: 0.33 ± 0.06 μg/mL; AUC_0-∞: 3.06 ± 0.93 μg•hr/mL; CL/F: 807.7 ± 279.2 mL/min; CL_renal: 168.6 ± 27.5 mL/min.

Baseline Creatinine Clearance 30–49 mL/min (N=8) – C_max: 0.37 ± 0.16 μg/mL; AUC_0-∞: 6.01 ± 2.50 μg•hr/mL; CL/F: 444.4 ± 209.8 mL/min; CL_renal: 100.6 ± 27.5 mL/min.

Baseline Creatinine Clearance 12–29 mL/min (N=11) – C_max: 0.60 ± 0.19 μg/mL; AUC_0-∞: 15.98 ± 7.22 μg•hr/mL; CL/F: 177.0 ± 97.1 mL/min; CL_renal: 43.0 ± 31.2 mL/min.

a. 300 mg, single dose of VIREAD

Tenofovir is efficiently removed by hemodialysis with an extraction coefficient of approximately 54%. Following a single 300 mg dose of VIREAD, a four-hour hemodialysis session removed approximately 10% of the administered tenofovir dose.

Patients with Hepatic Impairment: The pharmacokinetics of tenofovir following a 300 mg single dose of VIREAD have been studied in non-HIV infected subjects with moderate to severe hepatic impairment. There were no substantial alterations in tenofovir pharmacokinetics in subjects with hepatic impairment compared with unimpaired subjects. No change in VIREAD dosing is required in patients with hepatic impairment.

Assessment of Drug Interactions
At concentrations substantially higher (~300-fold) than those observed in vivo, tenofovir did not inhibit in vitro drug metabolism mediated by any of the following human CYP isoforms: CYP3A4, CYP2D6, CYP2C9, or CYP2E1. However, a small (6%) but statistically significant reduction in metabolism of CYP1A substrate was observed. Based on the results of in vitro experiments and the known elimination pathway of tenofovir, the potential for CYP mediated interactions involving tenofovir with other medicinal products is low.

Microbiology

Mechanism of Action
Tenofovir disoproxil fumarate is an acyclic nucleoside phosphonate diester analog of adenosine monophosphate. Tenofovir disoproxil fumarate requires initial diester hydrolysis for conversion to tenofovir and subsequent phosphorylations by cellular enzymes to form tenofovir diphosphate, an obligate chain terminator. Tenofovir diphosphate inhibits the activity of HIV-1 reverse transcriptase and HBV reverse transcriptase by competing with the natural substrate deoxyadenosine 5’-triphosphate and, after incorporation into DNA, by DNA chain termination. Tenofovir diphosphate is a weak inhibitor of mammalian DNA polymerases α, β, and mitochondrial DNA polymerase γ.

Activity against HIV

Antiviral Activity
The antiviral activity of tenofovir against laboratory and clinical isolates of HIV-1 was assessed in lymphoblastoid cell lines, primary monocyte/macrophage cells and peripheral blood lymphocytes. The EC₅₀ (50% effective concentration) values for tenofovir were in the range of 0.04 μM to 8.5 μM. In drug combination studies of tenofovir with nucleoside reverse transcriptase inhibitors (abacavir, didanosine, lamivudine, stavudine, zalcitabine, zidovudine), non-nucleoside reverse transcriptase inhibitors (delavirdine, efavirenz, nevirapine), and protease inhibitors (amprenavir, indinavir, nelfinavir, ritonavir, saquinavir), additive to synergistic effects were observed. Tenofovir displayed antiviral activity in cell culture against HIV-1 clades A, B, C, D, E, F, G, and O (EC₅₀ values ranged from 0.5 μM to 2.2 μM) and strain specific activity against HIV-2 (EC₅₀ values ranged from 1.6 μM to 5.5 μM).

Resistance
HIV-1 isolates with reduced susceptibility to tenofovir have been selected in cell culture. These viruses expressed a K65R substitution in reverse transcriptase and showed a 2– 4 fold reduction in susceptibility to tenofovir.

In Study 903 of treatment-naïve subjects (VIREAD + lamivudine + efavirenz versus stavudine + lamivudine + efavirenz), genotypic analyses of isolates from subjects with virologic failure through Week 144 showed development of efavirenz and lamivudine resistance-associated substitutions to occur most frequently and with no difference between the treatment arms. The K65R substitution occurred in 8/47 (17%) analyzed patient isolates on the VIREAD arm and in 2/49 (4%) analyzed patient isolates on the stavudine arm. Of the 8 subjects whose virus developed K65R in the VIREAD arm through 144 weeks, 7 of these occurred in the first 48 weeks of treatment and one at Week 96. Other substitutions resulting in resistance to VIREAD were not identified in this study.

In Study 934 of treatment-naïve subjects (VIREAD + EMTRIVA + efavirenz versus zidovudine (AZT)/lamivudine (3TC) + efavirenz), genotypic analysis performed on HIV-1 isolates from all confirmed virologic failure subjects with >400 copies/mL of HIV-1 RNA at Week 144 or early discontinuation showed development of efavirenz resistance-associated substitutions occurred most frequently and was similar between the two treatment arms. The M184V substitution, associated with resistance to EMTRIVA and lamivudine, was observed in 2/19 analyzed subject isolates in the VIREAD + EMTRIVA group and in 10/29 analyzed subject isolates in the zidovudine/lamivudine group. Through 144 weeks of Study 934, no subjects have developed a detectable K65R substitution in their HIV-1 as analyzed through standard genotypic analysis.

Cross Resistance
Cross-resistance among certain reverse transcriptase inhibitors has been recognized. The K65R substitution selected by tenofovir is also selected in some HIV-1 infected subjects treated with abacavir, didanosine, or zalcitabine. HIV-1 isolates with this mutation also show reduced susceptibility to emtricitabine and lamivudine. Therefore, cross-resistance among these drugs may occur in patients whose virus harbors the K65R substitution. HIV-1 isolates from subjects (N=20) whose HIV-1 expressed a mean of 3 zidovudine-associated reverse transcriptase substitutions (M41L, D67N, K70R, L210W, T215Y/F, or K219Q/E/N), showed a 3.1-fold decrease in the susceptibility to tenofovir.

In Studies 902 and 907 conducted in treatment-experienced subjects (VIREAD + Standard Background Therapy (SBT) compared to Placebo + SBT), 14/304 (5%) of the VIREAD-treated subjects with virologic failure through Week 96 had >1.4-fold (median 2.7-fold) reduced susceptibility to tenofovir. Genotypic analysis of the baseline and failure isolates showed the development of the K65R substitution in the HIV-1 reverse transcriptase gene.

The virologic response to VIREAD therapy has been evaluated with respect to baseline viral genotype (N=222) in treatment-experienced subjects participating in Studies 902 and 907. In these clinical studies, 94% of the participants evaluated had baseline HIV-1 isolates expressing at least one NRTI mutation. Virologic responses for subjects in the genotype substudy were similar to the overall study results.

Several exploratory analyses were conducted to evaluate the effect of specific substitutions and substitutional patterns on virologic outcome. Because of the large number of potential comparisons, statistical testing was not conducted. Varying degrees of cross-resistance of VIREAD to pre-existing zidovudine resistance-associated substitutions (M41L, D67N, K70R, L210W, T215Y/F, or K219Q/E/N) were observed and appeared to depend on the type and number of specific substitutions. VIREAD-treated subjects whose HIV-1 expressed 3 or more zidovudine resistance-associated substitutions that included either the M41L or L210W reverse transcriptase substitution showed reduced responses to VIREAD therapy; however, these responses were still improved compared with placebo. The presence of the D67N, K70R, T215Y/F, or K219Q/E/N substitution did not appear to affect responses to VIREAD therapy. Subjects whose virus expressed an L74V substitution without zidovudine resistance associated substitutions (N=8) had reduced response to VIREAD. Limited data are available for subjects whose virus expressed a Y115F substitution (N=3), Q151M substitution (N=2), or T69 insertion (N=4), all of whom had a reduced response.

In the protocol defined analyses, virologic response to VIREAD was not reduced in subjects with HIV-1 that expressed the abacavir/emtricitabine/lamivudine resistance-associated M184V substitution. HIV-1 RNA responses among these subjects were durable through Week 48.

Studies 902 and 907 Phenotypic Analyses
Phenotypic analysis of baseline HIV-1 from treatment-experienced subjects (N=100) demonstrated a correlation between baseline susceptibility to VIREAD and response to VIREAD therapy. Table 13 summarizes the HIV-1 RNA response by baseline VIREAD susceptibility.

HIV-1 RNA Response at Week 24 by Baseline VIREAD Susceptibility (IntentTo-Treat)^a

Baseline VIREAD Susceptibility^b     Change in HIV-1 RNA^c (N)
                 <1                                                       -0.74 (35)
                 >1 and ≤3                                          -0.56 (49)
                 >3 and ≤4                                           -0.3 (7)
                 >4                                                       -0.12 (9)

a. Tenofovir susceptibility was determined by recombinant phenotypic Antivirogram assay (Virco).
b. Fold change in susceptibility from wild-type.
c. Average HIV-1 RNA change from baseline through Week 24 (DAVG₂₄) in log₁₀ copies/mL.

Activity against HBV

Antiviral Activity
The antiviral activity of tenofovir against HBV was assessed in the HepG2 2.2.15 cell line. The EC₅₀ values for tenofovir ranged from 0.14 to 1.5 μM, with CC₅₀ (50% cytotoxicity concentration) values >100 μM. In cell culture combination antiviral activity studies of tenofovir with the nucleoside anti-HBV reverse transcriptase inhibitors emtricitabine, entecavir, lamivudine and telbivudine, no antagonistic activity was observed.

Resistance
Cumulative VIREAD genotypic resistance was evaluated annually with the paired HBV reverse transcriptase amino acid sequences of the pre-treatment and on-treatment isolates from subjects who received at least 24 weeks of VIREAD monotherapy and remained viremic with HBV DNA ≥400 copies/mL at the end of each study year (or at discontinuation of VIREAD monotherapy) using an as-treated analysis. From four ongoing VIREAD trials (Studies 102, 103, and 106 in subjects with compensated liver disease, and Study 108 in subjects with decompensated liver disease), 10% (69/660) of VIREAD recipients with compensated liver disease receiving up to 144 weeks of VIREAD monotherapy and 18% (7/39) of VIREAD recipients with decompensated liver disease receiving up to 48 weeks of VIREAD monotherapy remained viremic at their last time-point on VIREAD monotherapy. In the HEPSERA-naïve HBeAg+ subject population from Study 103, 74% (17/23) of the subjects with HBV DNA ≥400 copies/mL at their last time-point on VIREAD monotherapy had a baseline viral load of >9 log₁₀ copies/mL. Treatment emergent amino acid substitutions in the HBV reverse transcriptase were identified in 46% (32/69) of those subjects in Studies 102, 103, 106, and 108 with evaluable paired genotypic data; no specific substitutions occurred at a sufficient frequency to be associated with resistance to VIREAD (genotypic or phenotypic analyses).

Cross Resistance
Cross resistance has been observed between HBV nucleoside/nucleotide analogue reverse transcriptase inhibitors.

In cell based assays, HBV strains expressing the rtV173L, rtL180M, and rtM204I/V substitutions associated with resistance to lamivudine and telbivudine showed a susceptibility to tenofovir ranging from 0.7 to 3.4-fold that of wild type virus. The rtL180M and rtM204I/V double substitutions conferred 3.4-fold reduced susceptibility to tenofovir.

HBV strains expressing the rtL180M, rtT184G, rtS202G/I, rtM204V, and rtM250V substitutions associated with resistance to entecavir showed a susceptibility to tenofovir ranging from 0.6 to 6.9-fold that of wild type virus. An HBV strain expressing rtL180M, rtT184G, rtS202I, and rtM204V together had a 6.9-fold reduction in susceptibility to tenofovir.

HBV strains expressing the adefovir resistance-associated substitutions rtA181V and/or rtN236T showed reductions in susceptibility to tenofovir ranging from 2.9- to 10-fold that of wild type virus. Strains containing the rtA181T substitution showed changes in susceptibility to tenofovir ranging from 0.9 to 1.5-fold that of wild type virus.

In the four VIREAD-treatment studies, prior to treatment with VIREAD, 14, 15, and 2 subjects had HBV harboring either adefovir resistance-associated substitutions (rtA181T/V and/or rtN236T) or lamivudine resistance-associated substitutions (rtM204I/V), or both, respectively. Following up to 144 weeks of VIREAD treatment, 11 of the 14 subjects with adefovir-resistant HBV, 12 of the 15 subjects with lamivudineresistant HBV, and 1 of the 2 subjects with both adefovir- and lamivudine-resistant HBV achieved virologic suppression (HBV DNA <400 copies/mL). Two of the 5 subjects whose virus harbored both the rtA181T/V and rtN236T substitutions and one of the 5 subjects whose virus harbored these substitutions and an rtM204I substitution remained viremic following up to 32 weeks of VIREAD monotherapy.

USE IN SPECIFIC POPULATIONS

Pregnancy

Pregnancy Category B
Reproduction studies have been performed in rats and rabbits at doses up to 14 and 19 times the human dose based on body surface area comparisons and revealed no evidence of impaired fertility or harm to the fetus due to tenofovir. There are, however, no adequate and well-controlled studies in pregnant women. Because animal reproduction studies are not always predictive of human response, VIREAD should be used during pregnancy only if clearly needed.

Antiretroviral Pregnancy Registry: To monitor fetal outcomes of pregnant women exposed to VIREAD, an Antiretroviral Pregnancy Registry has been established. Healthcare providers are encouraged to register patients by calling 1-800-258-4263.

Nursing Mothers
Nursing Mothers: The Centers for Disease Control and Prevention recommend that HIV-1-infected mothers not breast-feed their infants to avoid risking postnatal transmission of HIV-1. Studies in rats have demonstrated that tenofovir is secreted in milk. It is not known whether tenofovir is excreted in human milk. Because of both the potential for HIV-1 transmission and the potential for serious adverse reactions in nursing infants, mothers should be instructed not to breast-feed if they are receiving VIREAD.

(For additional information, consult the Viread complete prescribing information).