Differential Mechanisms of Tenofovir and Tenofovir Disoproxil Fumarate Cellular Transport and Implications for Topical Preexposure Prophylaxis

Intravaginal rings releasing tenofovir (TFV) or its prodrug, tenofovir disoproxil fumarate (TDF), are being evaluated for HIV and herpes simplex virus (HSV) prevention. The current studies were designed to determine the mechanisms of drug accumulation in human vaginal and immune cells. The exposure of vaginal epithelial or T cells to equimolar concentrations of radiolabeled TDF resulted in over 10-fold higher intracellular drug levels than exposure to TFV. Permeability studies demonstrated that TDF, but not TFV, entered cells by passive diffusion. TDF uptake was energy independent but its accumulation followed nonlinear kinetics, and excess unlabeled TDF inhibited radiolabeled TDF uptake in competition studies. The carboxylesterase inhibitor bis-nitrophenyl phosphate reduced TDF uptake, suggesting saturability of intracellular carboxylesterases. In contrast, although TFV uptake was energy dependent, no competition between unlabeled and radiolabeled TFV was observed, and the previously identified transporters, organic anion transporters (OATs) 1 and 3, were not expressed in human vaginal or T cells. The intracellular accumulation of TFV was reduced by the addition of endocytosis inhibitors, and this resulted in the loss of TFV antiviral activity. Kinetics of drug transport and metabolism were monitored by quantifying the parent drugs and their metabolites by high-performance liquid chromatography tandem mass spectrometry (HPLC-MS/MS). Results were consistent with the identified mechanisms of transport, and the exposure of vaginal epithelial cells to equimolar concentrations of TDF compared to TFV resulted in ∼40-fold higher levels of the active metabolite, tenofovir diphosphate. Together, these findings indicate that substantially lower concentrations of TDF than TFV are needed to protect cells from HIV and HSV-2.     

Tenofovir disoproxil fumarate-emtricitabine coformulation for once-daily dual NRTI backbone

Truvada is the coformulation of tenofovir disoproxil fumarate (TDF; 300 mg) and emtricitabine (FTC; 200 mg) in a single tablet, providing the nucleotide backbone for once-daily dosing, as a component of highly active antiretroviral therapy (HAART). TDF (the bioavailable prodrug of tenofovir) is hydrolyzed to tenofovir intracellularly and phosphorylated to the active metabolite, tenofovir diphosphate. Tenofovir is a nucleotide analog of deoxyadenosine monophosphate, with activity against HIV-1, -2 and hepatitis B virus. FTC, the fluorinated derivative of lamivudine, is an analog of deoxycitidine, active against HIV-1, -2 and hepatitis B virus. Their long half-lives in plasma and in peripheral blood mononuclear cells allow once-daily dosing. Both are eliminated renally. Resistance mutation K65R is selected for by tenofovir and confers a two- to fourfold reduced susceptibility to this drug. The incidence of K65R is low (3%) and has not been observed in clinical trials with the concomitant use of tenofovir and FTC. FTC selects for M184V mutation less frequently than lamivudine. Tenofovir drug interactions include increased exposure to didanosine and inferior immune recovery that preclude their concomitant use. Boosted protease inhibitors increase exposure to tenofovir without dose adjustment required. FTC has no significant drug interactions. They are not metabolized by cytochrome P450, which confers little potential for interactions with drugs metabolized by these enzymes. As tenofovir and FTC are renally eliminated, drugs eliminated by tubular secretion must be avoided. Both antiretrovirals, as individual agents and in coadministration have evidenced antiviral potency in clinical trials. Pivotal study 934 evidenced superior efficacy of the combination TDF/FTC/efavirenz (EFV) versus zidovudine/FTC/EFV. The toxicity profile of tenofovir and FTC has been extensively studied. Lipid profile is more favorable with tenofovir than thymidine analog. Tenofovir requires surveillance of glomerular filtration rate and dosing interval adjustment when creatinine clearance is less than 50 ml/min and avoidance less than 30 ml/min. Fat loss is less likely with tenofovir than with thymidine analog. Clinical trials have assessed the performance of the coformulation of TDF and FTC.     

Tenofovir alafenamide (TAF) treatment of HBV,what are the unanswered questions?

Introduction: Tenofovir disoproxil fumarate (TDF), an ester prodrug of tenofovir (TFV), is one of the recommended drugs for chronic hepatitis B (CHB) patients. However, reduced kidney function and loss of bone mineral density have been reported in some CHB patients treated with TDF. Consequent to these safety issues, tenofovir alafenamide (TAF) [Vemlidy®], a phosphonate prodrug of TFV, was developed for the treatment of CHB patients.

Areas covered: The favourable pharmacological profile of TAF allows a marked reduction in dosage (25 mg/day) thus reducing systemic exposure to tenofovir and improving the bone and renal safety, keeping however the same virological efficacy, compared to TDF 300 mg/day. In two ongoing 96-week phase III trials in mainly treatment-naive HBeAg-positive or -negative patients, TAF showed similar viral suppression but was associated with significantly higher alanine aminotransferase normalization rates and more favourable renal and bone safety compared to TDF. In a 48-week TAF switch study enrolling patients treated with TDF for 96 weeks, glomerular, tubular and bone safety parameters rapidly improved while virological suppression was maintained.

Expert commentary: Waiting long-term large scale clinical practice studies aimed to confirm these advantages, TAF represents an helpful treatment option for both naïve and TDF-exposed CHB patients.     

Concentrations of tenofovir and emtricitabine in saliva: implications for preexposure prophylaxis of oral HIV acquisition

To prevent acquisition of HIV through oral sex, drugs used for preexposure prophylaxis (Prep) need to diffuse in saliva. We measured tenofovir (TFV) and emtricitabine (FTC) concentrations simultaneously in the plasma and saliva of 41 HIV-infected patients under stable antiretroviral treatment. Mean ratios of saliva/plasma concentration were 3% (±4%) and 86.9% (±124%) for TFV and FTC, respectively. Tenofovir disoproxil fumarate (TDF) should be used in combination with FTC to prevent oral acquisition of HIV.     

No easy way to exterminate ‘superbugs’ at the dawn of the third millennium

Truvada^® is the coformulation of tenofovir disoproxil fumarate (TDF; 300 mg) and emtricitabine (FTC; 200 mg) in a single tablet, providing the nucleotide backbone for once-daily dosing, as a component of highly active antiretroviral therapy (HAART). TDF (the bioavailable prodrug of tenofovir) is hydrolyzed to tenofovir intracellularly and phosphorylated to the active metabolite, tenofovir diphosphate. Tenofovir is a nucleotide analog of deoxyadenosine monophosphate, with activity against HIV-1, -2 and hepatitis B virus. FTC, the fluorinated derivative of lamivudine, is an analog of deoxycitidine, active against HIV-1, -2 and hepatitis B virus. Their long half-lives in plasma and in peripheral blood mononuclear cells allow once-daily dosing. Both are eliminated renally. Resistance mutation K65R is selected for by tenofovir and confers a two- to fourfold reduced susceptibility to this drug. The incidence of K65R is low (3%) and has not been observed in clinical trials with the concomitant use of tenofovir and FTC. FTC selects for M184V mutation less frequently than lamivudine. Tenofovir drug interactions include increased exposure to didanosine and inferior immmune recovery that preclude their concomitant use. Boosted protease inhibitors increase exposure to tenofovir without dose adjustment required. FTC has no significant drug interactions. They are not metabolized by cytochrome P450, which confers little potential for interactions with drugs metabolized by these enzymes. As tenofovir and FTC are renally eliminated, drugs eliminated by tubular secretion must be avoided. Both antiretrovirals, as individual agents and in coadministration have evidenced antiviral potency in clinical trials. Pivotal study 934 evidenced superior efficacy of the combination TDF/FTC/efavirenz (EFV) versus zidovudine/FTC/EFV. The toxicity profile of tenofovir and FTC has been extensively studied. Lipid profile is more favorable with tenofovir than thymidine analog. Tenofovir requires surveillance of glomerular filtration rate and dosing interval adjustment when creatinine clearance is less than 50 ml/min and avoidance less than 30 ml/min. Fat loss is less likely with tenofovir than with thymidine analog. Clinical trials have assessed the performance of the coformulation of TDF and FTC.     

Vaginally Delivered Tenofovir Disoproxil Fumarate Provides Greater Protection than Tenofovir against Genital Herpes in a Murine Model of Efficacy and Safety

Increased susceptibility to genital herpes in medroxyprogesterone-treated mice may provide a surrogate of increased HIV risk and a preclinical biomarker of topical preexposure prophylaxis safety. We evaluated tenofovir disoproxil fumarate (TDF) in this murine model because an intravaginal ring eluting this drug is being advanced into clinical trials. To avoid the complications of surgically inserting a ring, hydroxyethylcellulose (HEC)-stable formulations of TDF were prepared. One week of twice-daily 0.3% TDF gel was well tolerated and did not result in any increase in HSV-2 susceptibility but protected mice from herpes simplex virus 2 (HSV-2) disease compared to mice treated with the HEC placebo gel. No significant increase in inflammatory cytokines or chemokines in vaginal washes or change in cytokine, chemokine, or mitochondrial gene expression in RNA extracted from genital tract tissue was detected. To further evaluate efficacy, mice were treated with gel once daily beginning 12 h prior to high-dose HSV-2 challenge or 2 h before and after viral challenge (BAT24 dosing). The 0.3% TDF gel provided significant protection compared to the HEC gel following either daily (in 9/10 versus 1/10 mice, P < 0.01) or BAT24 (in 14/20 versus 4/20 mice, P < 0.01) dosing. In contrast, 1% tenofovir (TFV) gel protected only 4/10 mice treated with either regimen. Significant protection was also observed with daily 0.03% TDF compared to HEC. Protection was associated with greater murine cellular permeability of radiolabeled TDF than of TFV. Together, these findings suggest that TDF is safe, may provide substantially greater protection against HSV than TFV, and support the further clinical development of a TDF ring.     

A 90-Day Tenofovir Reservoir Intravaginal Ring for Mucosal HIV Prophylaxis

A vaginal gel containing the antiretroviral tenofovir (TFV) recently demonstrated 39% protection against HIV infection in women. We designed and evaluated a novel reservoir TFV intravaginal ring (IVR) to potentially improve product effectiveness by providing a more controlled and sustained vaginal dose to maintain cervicovaginal concentrations. Polyurethane tubing of various hydrophilicities was filled with a high-density TFV/glycerol/water semisolid paste and then end-sealed to create IVRs. In vitro, TFV release increased with polyurethane hydrophilicity, with 35 weight percent water-swelling polyurethane IVRs achieving an approximately 10-mg/day release for 90 days with mechanical stiffness similar to that of the commercially available NuvaRing. This design was evaluated in two 90-day in vivo sheep studies for TFV pharmacokinetics and safety. Overall, TFV vaginal tissue, vaginal fluid, and plasma levels were relatively time independent over the 90-day duration at approximately 10⁴ ng/g, 10⁶ ng/g, and 10¹ ng/ml, respectively, near or exceeding the highest observed concentrations in a TFV 1% gel control group. TFV vaginal fluid concentrations were approximately 1,000-fold greater than levels shown to provide significant protection in women using the TFV 1% gel. There were no toxicological findings following placebo and TFV IVR treatment for 28 or 90 days, although slight to moderate increases in inflammatory infiltrates in the vaginal epithelia were observed in these animals compared to naïve animals. In summary, the controlled release of TFV from this reservoir IVR provided elevated sheep vaginal concentrations for 90 days to merit its further evaluation as an HIV prophylactic.     

Human Immunodeficiency Virus Pre-Exposure Prophylaxis: Use of Emtricitabine/Tenofovir Alafenamide

Pre-exposure prophylaxis (PrEP) involves the use of antiretroviral medication in concert with safer sex practices to significantly reduce the risk of type 1 human immunodeficiency virus infection. In October 2019, daily use of emtricitabine/tenofovir alafenamide (FTC/TAF) 200 mg/25 mg was approved as a newer agent for PrEP to prevent HIV-1 infection in at-risk adults and adolescents. The purpose of this article is to: 1) provide an overview of the FTC/TAF regimen and its safety and efficacy profiles, 2) discuss indications and contraindications of FTC/TAF as PrEP, 3) provide a focused comparison of FTC/TDF with FTC/TAF, and 4) examine issues surrounding cost and accessibility in the United States.     

Effect of switching from tenofovir disoproxil fumarate to tenofovir alafenamide on estimated glomerular filtration rate slope in patients with HIV: A retrospective observational study

IntroductionThere is limited data on the effects of switching from tenofovir disoproxil fumarate (TDF) to tenofovir alafenamide (TAF) on estimated glomerular filtration rates (eGFR) slope in patients with human immunodeficiency virus (HIV) infection. This study aimed to compare the eGFR slope when administering TDF and TAF and to investigate the predictors of improvement in eGFR slope after switching from TDF to TAF.MethodsWe conducted a single-center, retrospective, observational study in Japanese patients with HIV infection who switched the antiretroviral drug from TDF to TAF. eGFR was calculated using serum cystatin C. The eGFR slope was defined as the regression coefficient between eGFR and time. Differences between eGFR slope during TDF and TAF administration were compared using Wilcoxon signed rank test. A stepwise logistic regression model was used to examine the associations between improvement of eGFR slope after switching from TDF to TAF and various parameters.ResultsOverall, 63 patients (656 eGFR) were included in the analysis. The median analyzed durations of TDF and TAF exposures were 1.6 and 1.5 years, respectively. There were no significant differences between eGFR slope during TDF and TAF periods (median: 0.6 vs. 4.0 mL/min/1.73 m²/year, p = 0.165). The eGFR slopes during the TDF period and while switching from TDF to TAF were independent predictors of improvement in eGFR slope after switching from TDF to TAF.ConclusionsThe results suggest that patients with poor renal function and those with progressive worsening during TDF administration would benefit from switching to TAF.     

Evaluation of Single and Combination Therapies with Tenofovir Disoproxil Fumarate and Emtricitabine In Vitro and in a Robust Mouse Model Supporting High Levels of Hepatitis B Virus Replication

Next-generation therapies for chronic hepatitis B virus (HBV) infection will involve combinations of established and/or experimental drugs. The current study investigated the in vitro and in vivo efficacy of tenofovir disoproxil fumarate (TDF) and/or emtricitabine [(−)-FTC] alone and in combination therapy for HBV infection utilizing the HepAD38 system (human hepatoblastoma cells transfected with HBV). Cellular pharmacology studies demonstrated increased levels of (−)-FTC triphosphate with coincubation of increasing concentrations of TDF, while (−)-FTC had no effect on intracellular tenofovir (TFV) diphosphate levels. Quantification of extracellular HBV by real-time PCR from hepatocytes demonstrated the anti-HBV activity with TDF, (−)-FTC, and their combination. Combination of (−)-FTC with TDF or TFV (ratio, 1:1) had a weighted average combination index of 0.7 for both combination sets, indicating synergistic antiviral effects. No cytotoxic effects were observed with any regimens. Using an in vivo murine model which develops robust HBV viremia in nude mice subcutaneously injected with HepAD38 cells, TDF (33 to 300 mg/kg of body weight/day) suppressed virus replication for up to 10 days posttreatment. At 300 mg/kg/day, (−)-FTC strongly suppressed virus titers to up to 14 days posttreatment. Combination therapy (33 mg/kg/day each drug) sustained suppression of virus titer/ml serum (<1 log₁₀ unit from pretreatment levels) at 14 days posttreatment, while single-drug treatments yielded virus titers 1.5 to 2 log units above the initial virus titers. There was no difference in mean alanine aminotransferase values or mean wet tumor weights for any of the groups, suggesting a lack of drug toxicity. TDF–(−)-FTC combination therapy provides more effective HBV suppression than therapy with each drug alone.