For additional information on Mechanism of Action, Antiviral Activity, Resistance and Cross Resistance, consult the EMTRIVA and VIREAD prescribing information.
12.1 Mechanism of Action
TRUVADA is a fixed-dose combination of antiviral drugs emtricitabine and tenofovir disoproxil fumarate [see MICROBIOLOGY (12.4)].
12.3 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 in Table 7. 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 in Table 7. 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.
Table 7 Single Dose Pharmacokinetic Parameters for Emtricitabine and Tenofovir in Adults*
Emtricitabine Tenofovir
*
NC=Not calculated
†
Median (range)
‡
Mean (± SD)
§
Data presented as steady state values
Fasted Oral Bioavailability† (%) 92 (83.1–106.4) 25 (NC–45.0)
Plasma Terminal Elimination Half-Life† (hr) 10 (7.4–18.0) 17 (12.0–25.7)
Cmax‡ (μg/mL) 1.8±0.72§ 0.30±0.09
AUC‡ (μg∙hr/mL) 10.0±3.12§ 2.29±0.69
CL/F‡ (mL/min) 302±94 1043±115
CLrenal‡ (mL/min) 213±89 243±33
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 Cmax by approximately 0.75 hour. The mean increases in tenofovir AUC and Cmax 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 Cmax) 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
The pharmacokinetic data for tenofovir and emtricitabine following administration of TRUVADA in pediatric subjects weighing 17 kg and above are not available. The dosing recommendations of TRUVADA in this population are based on the dosing recommendations of EMTRIVA and VIREAD in this population. Refer to the EMTRIVA and VIREAD prescribing information for pharmacokinetic information on the individual products in pediatric patients.
TRUVADA should not be administered to HIV-1 infected pediatric patients weighing less than 17 kg.
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 [see WARNINGS AND PRECAUTIONS (5.3)]. In adult subjects with creatinine clearance below 50 mL/min, Cmax, and AUC0–∞ of emtricitabine and tenofovir were increased. It is recommended that the dosing interval for TRUVADA be modified in HIV-infected adult patients with estimated creatinine clearance 30–49 mL/min. No data are available to make dose recommendations in pediatric patients with renal impairment. TRUVADA should not be used in patients with estimated creatinine clearance below 30 mL/min and in patients with end-stage renal disease requiring dialysis [see DOSAGE AND ADMINISTRATION (2.4)].
TRUVADA for a PrEP indication should not be used in HIV-1 uninfected individuals with estimated creatinine clearance below 60 mL/min. If a decrease in estimated creatinine clearance is observed in uninfected individuals while using TRUVADA for PrEP, evaluate potential causes and re-assess potential risks and benefits of continued use [see DOSAGE AND ADMINISTRATION (2.4)].
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 (see TABLES 8 and 9). Similarly, no clinically significant drug interactions have been observed between tenofovir disoproxil fumarate and efavirenz, methadone, nelfinavir, oral contraceptives, ribavirin, or sofosbuvir in trials conducted in healthy volunteers (see TABLES 10 and 11).
Table 8 Drug Interactions: Changes in Pharmacokinetic Parameters for Emtricitabine in the Presence of the Coadministered Drug*
Coadministered Drug Dose of Coadministered Drug (mg) Emtricitabine Dose (mg) N % Change of Emtricitabine Pharmacokinetic
Parameters† (90% CI)
Cmax AUC Cmin
*
All interaction trials conducted in healthy volunteers
†
↑ = Increase; ↓ = Decrease; ⇔ = No Effect; NA = Not Applicable
Tenofovir DF 300 once daily × 7 days 200 once daily × 7 days 17 ⇔ ⇔ ↑ 20
(↑ 12 to ↑ 29)
Zidovudine 300 twice daily × 7 days 200 once daily × 7 days 27 ⇔ ⇔ ⇔
Indinavir 800 × 1 200 × 1 12 ⇔ ⇔ NA
Famciclovir 500 × 1 200 × 1 12 ⇔ ⇔ NA
Stavudine 40 × 1 200 × 1 6 ⇔ ⇔ NA
Table 9 Drug Interactions: Changes in Pharmacokinetic Parameters for Coadministered Drug in the Presence of Emtricitabine*
Coadministered Drug Dose of Coadministered
Drug (mg) Emtricitabine Dose (mg) N % Change of Coadministered Drug Pharmacokinetic Parameters† (90% CI)
Cmax AUC Cmin
*
All interaction trials conducted in healthy volunteers
†
↑ = Increase; ↓ = Decrease; ⇔ = No Effect; NA = Not Applicable
Tenofovir DF 300 once daily × 7 days 200 once daily × 7 days 17 ⇔ ⇔ ⇔
Zidovudine 300 twice daily × 7 days 200 once daily × 7 days 27 ↑ 17
(↑ 0 to ↑ 38) ↑ 13
(↑ 5 to ↑ 20) ⇔
Indinavir 800 × 1 200 × 1 12 ⇔ ⇔ NA
Famciclovir 500 × 1 200 × 1 12 ⇔ ⇔ NA
Stavudine 40 × 1 200 × 1 6 ⇔ ⇔ NA
Table 10 Drug Interactions: Changes in Pharmacokinetic Parameters for Tenofovir* in the Presence of the Coadministered Drug
Coadministered Drug Dose of Coadministered Drug (mg) N % Change of Tenofovir Pharmacokinetic Parameters†
(90% CI)
Cmax AUC Cmin
*
Subjects received VIREAD 300 mg once daily
†
Increase = ↑; Decrease = ↓; No Effect = ⇔; NC = Not Calculated
‡
Reyataz Prescribing Information
§
Prezista Prescribing Information
¶
Data generated from simultaneous dosing with HARVONI (ledipasvir/sofosbuvir). Staggered administration (12 hours apart) provided similar results.
#
Comparison based on exposures when administered as atazanavir/ritonavir + emtricitabine/tenofovir DF.
Þ
Comparison based on exposures when administered as darunavir/ritonavir + emtricitabine/tenofovir DF.
S
Study conducted with ATRIPLA (efavirenz/emtricitabine/tenofovir DF) coadministered with HARVONI.
À
Study conducted with COMPLERA (emtricitabine/rilpivirine/tenofovir DF) coadministered with HARVONI.
È
Study conducted with ATRIPLA coadministered with SOVALDI ® (sofosbuvir).
Ð
Aptivus Prescribing Information.
Atazanavir‡ 400 once daily × 14 days 33 ↑ 14
(↑ 8 to ↑ 20) ↑ 24
(↑ 21 to ↑ 28) ↑ 22
(↑ 15 to ↑ 30)
Atazanavir/Ritonavir‡ 300/100 once daily 12 ↑ 34
(↑ 20 to ↑ 51) ↑ 37
(↑ 30 to ↑ 45) ↑ 29
(↑ 21 to ↑ 36)
Darunavir/Ritonavir§ 300/100 twice daily 12 ↑ 24
(↑ 8 to ↑ 42) ↑ 22
(↑ 10 to ↑ 35) ↑ 37
(↑ 19 to ↑ 57)
Indinavir 800 three times daily × 7 days 13 ↑ 14
(↓ 3 to ↑ 33) ⇔ ⇔
Ledipasvir/Sofosbuvir¶,# 90/400 once daily ×10 days 24 ↑ 47
(↑ 37 to ↑ 58) ↑ 35
(↑ 29 to ↑42 ) ↑ 47
(↑ 38 to ↑ 57)
Ledipasvir/Sofosbuvir¶,Þ 23 ↑ 64
(↑ 54 to ↑ 74) ↑ 50
(↑ 42 to ↑ 59) ↑ 59
(↑ 49 to ↑ 70)
Ledipasvir/SofosbuvirS 90/400 once daily ×14 days 15 ↑ 79
(↑ 56 to ↑ 104) ↑ 98
(↑ 77 to ↑ 123) ↑ 163
(↑ 132 to ↑ 197)
Ledipasvir/SofosbuvirÀ 90/400 once daily ×10 days 14 ↑ 32
(↑ 25 to ↑ 39 ) ↑ 40
(↑ 31 to ↑ 50 ) ↑ 91
(↑ 74 to ↑ 110)
Lopinavir/Ritonavir 400/100 twice daily × 14 days 24 ⇔ ↑ 32
(↑ 25 to ↑ 38) ↑ 51
(↑ 37 to ↑ 66)
Saquinavir/Ritonavir 1000/100 twice daily × 14 days 35 ⇔ ⇔ ↑ 23
(↑ 16 to ↑ 30)
SofosbuvirÈ 400 single dose 16 ↑ 25
(↑ 8 to ↑ 45) ⇔ ⇔
Tacrolimus 0.05 mg/kg twice daily × 7 days 21 ↑ 13
(↑ 1 to ↑ 27) ⇔ ⇔
Tipranavir/RitonavirÐ 500/100 twice daily 22 ↓ 23
(↓ 32 to ↓ 13) ↓ 2
(↓ 9 to ↑ 5) ↑ 7
(↓ 2 to ↑ 17)
750/200 twice daily (23 doses) 20 ↓ 38
(↓ 46 to ↓ 29) ↑ 2
(↓ 6 to ↑ 10) ↑ 14
(↑ 1 to ↑ 27)
No effect on the pharmacokinetic parameters of the following coadministered drugs was observed with TRUVADA: abacavir, didanosine (buffered tablets), emtricitabine, entecavir and lamivudine.
Table 11 Drug Interactions: Changes in Pharmacokinetic Parameters for Coadministered Drug in the Presence of Tenofovir
Coadministered Drug Dose of Coadministered Drug (mg) N % Change of Coadministered Drug Pharmacokinetic Parameters*
(90% CI)
Cmax AUC Cmin
*
Increase = ↑; Decrease = ↓; No Effect = ⇔; NA = Not Applicable
†
Reyataz Prescribing Information
‡
In HIV-infected subjects, addition of tenofovir DF to atazanavir 300 mg plus ritonavir 100 mg, resulted in AUC and C min values of atazanavir that were 2.3 and 4-fold higher than the respective values observed for atazanavir 400 mg when given alone.
§
Prezista Prescribing Information.
¶
Videx EC Prescribing Information. Subjects received didanosine enteric-coated capsules.
#
373 kcal, 8.2 g fat
Þ
Compared with didanosine (enteric-coated) 400 mg administered alone under fasting conditions.
S
Increases in AUC and C min are not expected to be clinically relevant; hence no dose adjustments are required when tenofovir DF and ritonavir-boosted saquinavir are coadministered.
À
Aptivus Prescribing Information.
Abacavir 300 once 8 ↑ 12
(↓ 1 to ↑ 26) ⇔ NA
Atazanavir† 400 once daily × 14 days 34 ↓ 21
(↓ 27 to ↓ 14) ↓ 25
(↓ 30 to ↓ 19) ↓ 40
(↓ 48 to ↓ 32)
Atazanavir† Atazanavir/Ritonavir 300/100 once daily × 42 days 10 ↓ 28
(↓ 50 to ↑ 5) ↓ 25‡
(↓ 42 to ↓ 3) ↓ 23‡
(↓ 46 to ↑ 10)
Darunavir§ Darunavir/Ritonavir 300/100 once daily 12 ↑ 16
(↓ 6 to ↑ 42) ↑ 21
(↓ 5 to ↑ 54) ↑ 24
(↓ 10 to ↑ 69)
Didanosine¶ 250 once, simultaneously with tenofovir DF and a light meal# 33 ↓ 20Þ
(↓ 32 to ↓ 7) ⇔Þ NA
Emtricitabine 200 once daily × 7 days 17 ⇔ ⇔ ↑ 20
(↑ 12 to ↑ 29)
Indinavir 800 three times daily × 7 days 12 ↓ 11
(↓ 30 to ↑ 12) ⇔ ⇔
Entecavir 1 once daily × 10 days 28 ⇔ ↑ 13
(↑ 11 to ↑ 15) ⇔
Lamivudine 150 twice daily × 7 days 15 ↓ 24
(↓ 34 to ↓ 12) ⇔ ⇔
Lopinavir
Ritonavir Lopinavir/Ritonavir 400/100 twice daily × 14 days 24 ⇔
⇔ ⇔
⇔ ⇔
⇔
Saquinavir Saquinavir/Ritonavir 1000/100 twice daily × 14 days 32 ↑ 22
(↑ 6 to ↑41) ↑ 29S
(↑ 12 to ↑ 48) ↑ 47S
(↑ 23 to ↑ 76)
Ritonavir ⇔ ⇔ ↑ 23
(↑ 3 to ↑ 46)
Tacrolimus 0.05 mg/kg twice daily × 7 days 21 ⇔ ⇔ ⇔
TipranavirÀ Tipranavir/Ritonavir 500/100 twice daily 22 ↓ 17
(↓ 26 to ↓ 6) ↓ 18
(↓ 25 to ↓ 9) ↓ 21
(↓ 30 to ↓ 10)
Tipranavir/Ritonavir 750/200 twice daily (23 doses) 20 ↓ 11
(↓ 16 to ↓ 4) ↓ 9
(↓ 15 to ↓ 3) ↓ 12
(↓ 22 to 0)
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 enteric-coated capsules significantly increases the Cmax 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. See DRUG INTERACTIONS (7.1) regarding use of didanosine with VIREAD.
12.4 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: No antagonism was observed in combination studies evaluating the cell culture antiviral activity of emtricitabine and tenofovir together.
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, zidovudine), non-nucleoside reverse transcriptase inhibitors (delavirdine, efavirenz, nevirapine), and protease inhibitors (amprenavir, nelfinavir, ritonavir, saquinavir), no antagonism was 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, zidovudine), non-nucleoside reverse transcriptase inhibitors (delavirdine, efavirenz, nevirapine), and protease inhibitors (amprenavir, indinavir, nelfinavir, ritonavir, saquinavir), no antagonism was 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).
Prophylactic Activity in a Nonhuman Primate Model of HIV Transmission
Emtricitabine and Tenofovir Disoproxil Fumarate: The prophylactic activity of the combination of daily oral emtricitabine (FTC) and tenofovir disoproxil fumarate (TDF) was evaluated in a controlled study of macaques inoculated once weekly for 14 weeks with SIV/HIV-1 chimeric virus (SHIV) applied to the rectal surface. Of the 18 control animals, 17 became infected after a median of 2 weeks. In contrast, 4 of the 6 animals treated daily with oral FTC and TDF remained uninfected and the two infections that did occur were significantly delayed until 9 and 12 weeks and exhibited reduced viremia. An M184I-expressing FTC-resistant variant emerged in 1 of the 2 macaques after 3 weeks of continued drug exposure.
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 addition, a K70E substitution in HIV-1 reverse transcriptase has been selected by tenofovir and results in reduced susceptibility to tenofovir.
In a clinical trial of treatment-naïve subjects [Study 934, see CLINICAL STUDIES (14.1)], 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 subject isolates in the EMTRIVA + VIREAD 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 or K70E 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-naïve 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 K65R amino acid substitution in the HIV-1 RT.
iPrEx Trial: In a clinical study of HIV-1 seronegative subjects [iPrEx Trial, see CLINICAL STUDIES (14.2)], no amino acid substitutions associated with resistance to emtricitabine or tenofovir were detected at the time of seroconversion among 48 subjects in the TRUVADA group and 83 subjects in the placebo group who became infected with HIV-1 during the trial. Ten subjects were observed to be HIV-1 infected at time of enrollment. The M184V/I substitutions associated with resistance to emtricitabine were observed in 3 of the 10 subjects (2 of 2 in the TRUVADA group and 1 of 8 in the placebo group). One of the two subjects in the TRUVADA group harbored wild type virus at enrollment and developed the M184V substitution 4 weeks after enrollment. The other subject had indeterminate resistance at enrollment but was found to have the M184I substitution 4 weeks after enrollment.
Partners PrEP Trial: In a clinical study of HIV-1 seronegative subjects [Partners PrEP Trial, see CLINICAL STUDIES (14.3)], no variants expressing amino acid substitutions associated with resistance to emtricitabine or tenofovir were detected at the time of seroconversion among 12 subjects in the TRUVADA group, 15 subjects in the VIREAD group, and 51 subjects in the placebo group. Fourteen subjects were observed to be HIV-1 infected at the time of enrollment (3 in the TRUVADA group, 5 in the VIREAD group, and 6 in the placebo group). One of the three subjects in the TRUVADA group who was infected with wild type virus at enrollment selected an M184V expressing virus by week 12. Two of the five subjects in the VIREAD group had tenofovir-resistant viruses at the time of seroconversion; one subject infected with wild type virus at enrollment developed a K65R substitution by week 16, while the second subject had virus expressing the combination of D67N and K70R substitutions upon seroconversion at week 60, although baseline virus was not genotyped and it is unclear if the resistance emerged or was transmitted. Following enrollment, 4 subjects (2 in the VIREAD group, 1 in the TRUVADA group, and 1 in the placebo group) had virus expressing K103N or V106A substitutions, which confer high-level resistance to NNRTIs but have not been associated with tenofovir or emtricitabine and may have been present in the infecting virus.
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 emtricitabine or lamivudine, 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 but retained susceptibility in cell culture to the NRTIs didanosine, stavudine, tenofovir, and zidovudine, and to NNRTIs (delavirdine, efavirenz, and nevirapine). 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 K103N substitution associated with resistance to NNRTIs was susceptible to emtricitabine.
Tenofovir Disoproxil Fumarate: The K65R and K70E substitutions selected by tenofovir are also selected in some HIV-1-infected patients treated with abacavir or didanosine. HIV-1 isolates with the K65R and K70E substitutions also showed reduced susceptibility to emtricitabine and lamivudine. Therefore, cross-resistance among these NRTIs may occur in patients whose virus harbors the K65R or K70E substitutions. 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.