Pharmacokinetic and pharmacodynamic characteristics of emtricitabine support its once daily dosing for the treatment of HIV infection

Antiretroviral drug exposure in the female genital tract: implications for oral pre- and post-exposure prophylaxis

OBJECTIVES

To describe first dose and steady state antiretroviral drug exposure in the female genital tract.

DESIGN

Non-blinded, single center, open-label pharmacokinetic study in HIV-infected women.

METHOD

Twenty-seven women initiating combination antiretroviral therapy underwent comprehensive blood plasma and cervicovaginal fluid sampling for drug concentrations during the first dose of antiretroviral therapy and at steady-state. Drug concentrations were measured by validated HPLC/UV or HPLC-MS/MS methods. Pharmacokinetic parameters were estimated for 11 drugs by non-compartmental analysis. Descriptive statistics and 95% confidence intervals were generated using Intercooled STATA Release 8.0 (Stata Corporation, College Station, Texas, USA).

RESULTS

For all antiretroviral drugs, genital tract concentrations were detected rapidly after the first dose. Drugs were stratified according to the genital tract concentrations achieved relative to blood plasma. Median rank order of highest to lowest genital tract concentrations relative to blood plasma at steady state were: lamivudine (concentrations achieved were 411% greater than blood plasma), emtricitabine (395%), zidovudine (235%) tenofovir (75%), ritonavir (26%), didanosine (21%), atazanavir (18%), lopinavir (8%), abacavir (8%), stavudine (5%), and efavirenz (0.4%).

CONCLUSIONS

This is the first study to comprehensively evaluate antiretroviral drug exposure in the female genital tract. These findings support the use of lamivudine, zidovudine, tenofovir and emtricitabine as excellent pre-exposure/post-exposure prophylaxis (PrEP/PEP) candidates. Atazanavir and lopinavir might be useful agents for these applications due to favorable therapeutic indices, despite lower genital tract concentrations. Agents such as stavudine, abacavir, and efavirenz that achieve genital tract exposures less than 10% of blood plasma are less attractive PrEP/PEP candidates.

Steady-state pharmacokinetics of emtricitabine and tenofovir disoproxil fumarate administered alone and in combination in healthy volunteers

The approved antiretroviral drugs, emtricitabine and tenofovir disoproxil fumarate, were considered good candidates for a fixed-dose combination product that could be administered as a single pill once daily (qd), thereby simplifying existing treatment regimens and promoting patient adherence. As both drugs are extensively renally eliminated, a randomized, 3-way crossover study was conducted in 19 healthy volunteers to formally evaluate the potential pharmacokinetic interaction when the drugs are administered alone and together (ie, 200 mg emtricitabine qd for 7 days, 300 mg tenofovir disoproxil fumarate qd for 7 days, and 200 mg emtricitabine plus 300 mg tenofovir disoproxil fumarate qd for 7 days) with no washout between treatments. Steady-state pharmacokinetic parameters (AUC(tau), C(max), and C(min)) of emtricitabine and tenofovir (as tenofovir disoproxil fumarate) in combination were essentially equivalent versus each drug alone, providing a pharmacokinetic rationale for combining these products in emtricitabine/tenofovir disoproxil fumarate fixed-dose tablets.

Evidence and possible consequences of the phosphorylation of nucleoside reverse transcriptase inhibitors in human red blood cells

The intracellular metabolism of nucleoside reverse transcriptase inhibitors (NRTI) in mononuclear cells has been thoroughly studied, but that in red blood cells (RBC) has been disregarded. However, the phosphorylation of other analogous nucleosides (in particular, ribavirin) has been described previously. In this study, we investigated for the first time the phosphorylation of NRTI in human RBC. The presence of intracellular zidovudine (AZT) monophosphate, AZT triphosphate, lamivudine (3TC) triphosphate, and tenofovir (TFV) diphosphate, as well as endogenous dATP, dGTP, and dTTP, in RBC collected from human immunodeficiency virus-infected patients was examined. We observed evidence of a selective phosphorylation of 3TC, TFV, and endogenous purine deoxynucleosides to generate their triphosphate moieties. Conversely, no trace of AZT phosphate metabolites was found, and only faint dTTP signals were visible. A comparison of intracellular TFV diphosphate and 3TC triphosphate levels in RBC and peripheral blood mononuclear cells (PBMC) further highlighted the specificity of NRTI metabolism in each cell type. These findings raise the issue of RBC involvement in drug-drug interaction, drug pharmacokinetics, and drug-induced toxicity. Moreover, the typical preparation of PBMC samples by gradient density centrifugation does not prevent their contamination with RBC. We demonstrated that the presence of RBC within PBMC hampers an accurate determination of intracellular TFV diphosphate and dATP levels in clinical PBMC samples. Thus, we recommend removing RBC during PBMC preparation by using an ammonium chloride solution to enhance both the accuracy and the precision of intracellular drug monitoring.

Liquid chromatography–tandem mass spectrometric assay for the nucleoside reverse transcriptase inhibitor emtricitabine in human plasma

A liquid chromatography-tandem mass spectrometric assay for the determination of the antiretroviral nucleoside emtricitabine in human plasma was developed and validated using a simple sample pre-treatment procedure. After addition of 5'-deoxy-5-fluorocytidine as the internal standard and protein precipitation with acetonitrile, the supernatant was directly injected in the isocratic chromatographic system using a polar embedded reversed-phase column and formic acid in water-methanol as the eluent. The eluate was completely led into an electrospray interface with positive ionization and the analytes were quantified using triple quadrupole mass spectrometry. The assay was validated in the range 5-5000 ng/mL. Intra-day precisions were <or=7% and inter-day precisions were <or=10%. Accuracies between 92 and 99% were found. The analytes were chemically stable under all relevant conditions and the assay was applied in the analysis of plasma samples of HIV-infected patients treated with the drug.

Tenofovir Disoproxil Fumarate, Emtricitabine, and Efavirenz Versus Fixed-Dose Zidovudine/Lamivudine and Efavirenz in Antiretroviral-Naive Patients

BACKGROUND

In antiretroviral-naive patients, tenofovir disoproxil fumarate (TDF), emtricitabine (FTC), and efavirenz (EFV) demonstrated superior outcomes compared with fixed-dose zidovudine (ZDV)/lamivudine (3TC) and EFV through 48 weeks. Results through a 96-week extension phase are presented.

METHODS

In this randomized, open-label, noninferiority trial, 517 antiretroviral-naive HIV-infected patients received TDF, FTC, and EFV (TDF + FTC + EFV) or ZDV/3TC and EFV (ZDV/3TC + EFV). The primary endpoint was the proportion of patients with an HIV RNA level <400 copies/mL in patients without baseline nonnucleoside resistance.

RESULTS

Through week 96, significantly more patients receiving TDF + FTC + EFV achieved and maintained an HIV RNA level <400 copies/mL (75% receiving TDF + FTC + EFV vs. 62% receiving ZDV/3TC + EFV; P = 0.004). There was a trend toward greater virologic suppression to <50 copies/mL in the TDF + FTC + EFV group (67% vs. 61%; P = 0.16). The TDF + FTC + EFV group demonstrated a significantly greater increase in CD4 count (270 vs. 237 cells/mm; P = 0.036). No patient developed the K65R mutation. Limb fat at week 96 was significantly greater in the TDF + FTC + EFV group versus the ZDV/3TC + EFV group (7.7 vs. 5.5 kg; P < 0.001).

CONCLUSION

Over 96 weeks, the combination of TDF, FTC, and EFV was superior to fixed-dose ZDV/3TC + EFV for achieving and maintaining an HIV RNA level <400 copies/mL and an increase in CD4 cells.

Pharmacokinetics and safety of tenofovir disoproxil fumarate on coadministration with lopinavir/ritonavir

OBJECTIVE

Lopinavir/ritonavir (LPV/r) and tenofovir disoproxil fumarate (TDF) are frequently used antiretrovirals. A pharmacokinetic study in healthy volunteers was conducted to assess the potential for a drug interaction between these agents.

METHODS

This was a 36-day, multiple-dose, drug-drug interaction study of TDF and lopinavir/ritonavir (LPV/r). Subjects received TDF alone for 7 days, followed by 14 days each of TDF plus LPV/r and LPV/r alone in a randomized manner. Pharmacokinetic assessments were performed over 24 hours on days 7, 21, and 35. LPV/r and tenofovir plasma/serum concentrations were measured by high-performance liquid chromatography/mass spectometry (MS)/MS. Geometric mean ratios and 90% confidence intervals of pharmacokinetic parameters for tenofovir, LPV, and ritonavir (RTV) were estimated using analysis of variance and compared with the no-effect criterion for pharmacokinetic equivalence.

RESULTS

Tenofovir measurements with an area under the concentration-time curve over the dosing interval, maximum concentration, and concentration at the end of the dosing interval (Ctau) were 32%, 15%, and 51% higher, respectively, when TDF was coadministered with LPV/r (n = 24). LPV and RTV pharmacokinetics, including Ctau, were unaffected by TDF (n = 24). Clinical estimates of renal function were unaffected by administration of TDF alone or with LPV/r.

DISCUSSION

Coadministration of TDF with LPV/r resulted in increased tenofovir exposures at steady state, possibly through increased absorption. This increase is not believed to be clinically relevant based on the safety and efficacy of TDF plus LPV/r-containing regimens in HIV-infected patients in long-term controlled clinical trials.

Use of tenofovir disoproxil fumarate and emtricitabine combination in HIV-infected patients

With the continuing spread of HIV infection, particularly in developing countries, cost-effective treatment for its management is a high priority. Truvada (Gilead Sciences) is a single combination pill of the nucleotide reverse transcriptase inhibitors tenofovir disoproxil fumarate and emtricitabine, which is used once daily. It is anticipated to be a clinically potent combination that is free of short-term irritating toxicity. The drug has recently been licensed but there are currently little clinical efficacy data regarding its use. The limited published data have indicated that emtricitabine and lamivudine have equivalent potency, and randomised controlled trials have produced evidence of the efficacy of lamivudine combined with tenofovir disoproxil fumarate in a regimen containing either the non-nucleoside reverse transcriptase inhibitor efavirenz or a protease inhibitor lopinavir/ritonavir. In these trials, long-term durability data are available for < or = 96 weeks.

Pharmacology of current and promising nucleosides for the treatment of human immunodeficiency viruses

Nucleoside antiretroviral agents are chiral small molecules that have distinct advantages compared to other classes including long intracellular half-lives, low protein binding, sustained antiviral response when a dose is missed, and ease of chemical manufacture. They mimic natural nucleosides and target a unique but complex viral polymerase that is essential for viral replication. They remain the cornerstone of highly active antiretroviral therapy (HAART) and are usually combined with non-nucleoside reverse [corrected] transcriptase and protease inhibitors to provide powerful antiviral responses to prevent or delay the emergence of drug-resistant human immunodeficiency virus (HIV). The pharmacological and virological properties of a selected group of nucleoside analogs are described. Some of the newer nucleoside analogs have a high genetic barrier to resistance development. The lessons learned are that each nucleoside analog should be treated as a unique molecule since any structural modification, including a change in the enantiomeric form, can affect metabolism, pharmacokinetics, efficacy, toxicity and resistance profile.

Pharmacogenetics of plasma efavirenz exposure after treatment discontinuation: an Adult AIDS Clinical Trials Group Study

BACKGROUND

Efavirenz has a long plasma half-life and a low genetic barrier to resistance. Simultaneously stopping treatment with all agents in efavirenz-containing regimens may result in functional efavirenz monotherapy that selects for drug-resistant human immunodeficiency virus type 1. Lower plasma efavirenz clearance is associated with a cytochrome P450 2B6 gene (CYP2B6) polymorphism (516G-->T) that is more frequent among African American individuals than among European American individuals.

METHODS

We characterized relationships between this polymorphism and predicted plasma efavirenz concentration-time profiles after discontinuation of therapy with use of data obtained from subjects receiving therapy. Pharmacokinetic parameters were estimated using population-based methods. Concentrations after discontinuation of therapy were predicted from subject-specific estimates. RESULTS. Median estimated efavirenz half-lives were 23, 27, and 48 h for patients with CYP2B6 position 516 GG (78 patients), GT (60), and TT (14) genotypes, respectively (P<.001). After therapy was stopped, plasma efavirenz concentrations in patients with GG, GT, and TT genotypes were predicted to exceed 46.7 ng/mL (the estimated protein-adjusted 95% inhibitory concentration for wild-type virus) for a median of 5.8 days (interquartile range [IQR], 4.4-8.3 days), 7.0 days (IQR, 5.0-8.0 days), and 14 days (IQR, 11.1-21.2 days), respectively (P<.001). Plasma efavirenz levels were predicted to exceed 46.7 ng/mL for >21 days in 5% of subjects with GG genotype, 5% of subjects with GT genotype, and 29% of subjects with TT genotype.

CONCLUSIONS

The CYP2B6 position 516 TT genotype or a prolonged measured elimination half-life may predict increased risk of developing drug resistance among patients who discontinue efavirenz-containing regimens. This has implications for strategies to safely discontinue antiretroviral regimens while avoiding the emergence of drug resistance.