Metabolism and antiretroviral activity of tenofovir alafenamide in CD4+ T-cells and macrophages from demographically diverse donors

Broadening access to tenofovir alafenamide for the treatment and prevention of HIV-1 infection

INTRODUCTION

Tenofovir alafenamide (TAF), a prodrug of tenofovir, achieves higher intracellular concentrations of tenofovir-diphosphate and 90% lower plasma concentrations of tenofovir compared to tenofovir disoproxil fumarate (TDF). TAF is associated with improved renal and bone safety outcomes.

AREAS COVERED

We review the efficacy and safety of TAF-containing regimens in adults and pediatrics. We highlight safety data during pregnancy, drug interactions during co-administration with tuberculosis treatment, and critical knowledge gaps to be addressed for the successful implementation of TAF in low- and middle-income countries. We performed a search on MEDLINE PubMed and conference websites for relevant articles published from January 2010 to March 2023.

EXPERT OPINION

Current evidence demonstrates that TAF has similar efficacy and tolerability, superior bone and renal safety, and higher rates of dyslipidemia and weight gain, compared with TDF. However, there are several knowledge gaps, in specific sub-populations, that require action. Emerging data suggests that TAF is safe during pregnancy, although fuller safety data to support TAF use in pregnancy is needed. Similarly, there is a lack of evidence that TAF can be used in combination with rifamycin-based tuberculosis treatment in PWH and TB. Further studies are needed to fill knowledge gaps and support the wider rollout of TAF.

Tenofovir alafenamide fumarate

Tenofovir alafenamide fumarate is a lipophilic prodrug of tenofovir which is preferentially metabolized in lymphatic tissue resulting in high concentrations of tenofovir (TFV) and its active diphosphate metabolite inside the cells that replicate HIV. Due to its selectivity for these tissues, lower total doses of TAF can be administered relative to tenofovir disoproxil fumarate (TDF) which results in improved bone and renal biomarkers. Tenofovir alafenamide fumarate has become the “backbone” of multiple combination products for the treatment of HIV, combined with emtricitabine for PreP and as a monotherapy for the treatment or HBV.

Pharmacokinetics of Tenofovir Alafenamide Fumarate and Tenofovir in the Chinese People: Effects of Non-Genetic Factors and Genetic Variations

Background

Tenofovir alafenamide fumarate (TAF) was approved for HBV treatment in China in 2018. Despite higher antiviral efficacy and less impact on renal function and bone mineral density, the pharmacokinetic profiles of TAF are highly variable. The objectives of this study were to investigate the pharmacokinetics of TAF in the Chinese population and explore the associations between TAF and genetic polymorphisms and non-genetic factors.

Patients and Methods

A total of 64 healthy Chinese subjects aged 18~65 years old were planned to enroll. According to the dietary intake status, the subjects were divided into two groups (n = 32 per group). The concentrations of TAF and tenofovir were measured by HPLC-MS/MS, and the single-nucleotide polymorphisms were analyzed by MALDI-TOF MS.

Results

All the enrolled participants (18–35 years) completed the clinical trial study. Similar to the results reported in other ethnic populations, the pharmacokinetic profiles of TAF and tenofovir were highly variable in the Chinese people, and the HFHC diet can significantly increase the systemic exposure of TAF. We determined both HFHC diet and rs7311358 (SLCO1B3) genotypes were independently associated with TAF AUC_0-t, while HFHC diet, age and rs3740066 (ABCC2) variants were predictive of t_1/2 of tenofovir (P < 0.05). The subjects with the AA genotype in rs7311358 had significantly higher TAF AUC_0-t values (1.15 times) than those with a G allele, and the t_1/2 of tenofovir in the rs3740066 TT genotype group was 1.23 times longer than that of CC genotype group. Furthermore, there was a trend of higher TAF AUC and shorter tenofovir t_1/2 for the rs2032582 (ABCB1) T allele and rs3742106 (ABCC4) CC variant, respectively, although not statistically significant in the multiple linear regression analysis.

Conclusion

This study provided new evidence to suggest a critical link between both genetic and non-genetic factors and TAF pharmacokinetics in the Chinese people.

A New Approach to Developing Long-Acting Injectable Formulations of Anti-HIV Drugs: Poly(Ethylene Phosphoric Acid) Block Copolymers Increase the Efficiency of Tenofovir against HIV-1 in MT-4 Cells

Despite the world’s combined efforts, human immunodeficiency virus (HIV), the causative agent of AIDS, remains one of the world’s most serious public health challenges. High genetic variability of HIV complicates the development of anti-HIV vaccine, and there is an actual clinical need for increasing the efficiency of anti-HIV drugs in terms of targeted delivery and controlled release. Tenofovir (TFV), a nucleotide-analog reverse transcriptase inhibitor, has gained wide acceptance as a drug for pre-exposure prophylaxis or treatment of HIV infection. In our study, we explored the potential of tenofovir disoproxil (TFD) adducts with block copolymers of poly(ethylene glycol) monomethyl ether and poly(ethylene phosphoric acid) (mPEG-b-PEPA) as candidates for developing a long-acting/controlled-release formulation of TFV. Two types of mPEG-b-PEPA with numbers of ethylene phosphoric acid (EPA) fragments of 13 and 49 were synthesized by catalytic ring-opening polymerization, and used for preparing four types of adducts with TFD. Antiviral activity of [mPEG-b-PEPA]TFD or tenofovir disoproxil fumarate (TDF) was evaluated using the model of experimental HIV infection in vitro (MT-4/HIV-1_IIIB). Judging by the values of the selectivity index (SI), TFD exhibited an up to 14-fold higher anti-HIV activity in the form of mPEG-b-PEPA adducts, thus demonstrating significant promise for further development of long-acting/controlled-release injectable TFV formulations.

Design of a Drug-Eluting Subcutaneous Implant of the Antiretroviral Tenofovir Alafenamide Fumarate

PURPOSE

Sexual transmission of HIV has been clinically proven to be preventable with a once-daily oral tablet; however, missed doses dramatically increase the risk of HIV infection. Long-acting subcutaneous implants do not allow the user to miss a dose. A desirable long-acting drug-eluting implant can deliver a constant amount of drug, adjust the delivered dose, and be readily manufactured. We present a long-acting, subcutaneous implant design composed of tenofovir alafenamide hemifumarate (TAF) pellets loaded in a sealed polyether urethane tube for the prevention of HIV transmission.

METHODS

Implants were prepared with pressed drug pellets and extruded polyurethane tubing. In vitro release rate of implants using different pellet formulations, rate-controlling membranes, and geometries were measured.

RESULTS

Tenofovir alafenamide release appeared to be governed by a pseudo-steady state and followed a mass transport model of release from a cylindrical drug reservoir. Implant seal integrity was tested and confirmed using mechanical testing. The inclusion of sodium chloride in the pellet increased the release rate and reduced initial lag. The release was sustained for 100 days.

CONCLUSIONS

The release rate of tenofovir alafenamide mechanistically varied with geometry and rate controlling membrane composition. The polyether urethane implant presented herein is modular and tunable to adjust the release rate and duration of the TAF release.

A Subcutaneous Implant of Tenofovir Alafenamide Fumarate Causes Local Inflammation and Tissue Necrosis in Rabbits and Macaques

We describe the in vitro and in vivo evaluation of a subcutaneous reservoir implant delivering tenofovir alafenamide hemifumarate (TAF) for the prevention of HIV infection. These long-acting reservoir implants were able to deliver antiretroviral drug for over 90 days in vitro and in vivo. We evaluated the implants for implantation site histopathology and pharmacokinetics in plasma and tissues for up to 12 weeks in New Zealand White rabbit and rhesus macaque models.

We describe the in vitro and in vivo evaluation of a subcutaneous reservoir implant delivering tenofovir alafenamide hemifumarate (TAF) for the prevention of HIV infection. These long-acting reservoir implants were able to deliver antiretroviral drug for over 90 days in vitro and in vivo. We evaluated the implants for implantation site histopathology and pharmacokinetics in plasma and tissues for up to 12 weeks in New Zealand White rabbit and rhesus macaque models. A dose-ranging study in rabbits demonstrated dose-dependent pharmacokinetics and local inflammation up to severe necrosis around the active implants. The matched placebos showed normal wound healing and fibrous tissue encapsulation of the implant. We designed a second implant with a lower release rate and flux of TAF and achieved a median cellular level of tenofovir diphosphate of 42 fmol per 10⁶ rhesus macaque peripheral blood mononuclear cells at a TAF dose of 10 μg/kg/day. This dose and flux of TAF also resulted in adverse local inflammation and necrosis near the implant in rhesus macaques. The level of inflammation in the primates was markedly lower in the placebo group than in the active-implant group. The histological inflammatory response to the TAF implant at 4 and 12 weeks in primates was graded as a severe reaction. Thus, while we were able to achieve a sustained target dose, we observed an unacceptable inflammatory response locally at the implant tissue interface.

Plasma concentration of neurofilament light chain protein decreases after switching from tenofovir disoproxil fumarate to tenofovir alafenamide fumarate

Background

Because tenofovir alafenamide (TAF) leads to significantly lower plasma tenofovir concentrations than tenofovir disoproxil fumarate (TDF) and is a stronger substrate for P-glycoprotein (P-gp) than TDF, TAF could lead to decreased central nervous system (CNS) tenofovir exposure than TDF. We aimed to determine if switching from TDF to TAF increases the risk of neuronal injury, by quantifying plasma levels of neurofilament light protein (NfL), a sensitive marker of neuronal injury in HIV CNS infection.

Methods

Plasma NfL concentration was measured at baseline, week 24, and week 84 in stored plasma samples from 416 participants (272 switching to elvitegravir (E)/cobicistat (C)/emtricitabine (F)/TAF and 144 continuing E/C/F/TDF) enrolled in the randomized, active-controlled, multicenter, open-label, noninferiority Gilead GS-US-292-0109 trial.

Results

While plasma NfL levels in both groups were within the normal range, we found a small but significant decrease in the E/C/F/TAF arm after 84 weeks from a geometric mean of 9.3 to 8.8 pg/mL (5.4% decline, 95% CI 2.0–8.4, p = 0.002). This change was significantly different (p = 0.001) from that of the E/C/F/TDF arm, in which plasma NfL concentration changed from 9.7 pg/mL at baseline to 10.2 pg/mL at week 84 (5.8% increase, 95% CI -0.8–12.9, p = 0.085). This increase is in line with what could be expected in normal ageing. Plasma NfL concentrations significantly correlated with age. No correlation was found between plasma NfL and serum creatinine.

Conclusions

We found no biomarker evidence of CNS injury when switching from TDF to TAF. It is unclear whether the small decrease in plasma NfL found after switch to TAF is of any clinical relevance, particularly with plasma NfL levels in both arms remaining within the limits found in HIV-negative controls. These results indicate that switching from TDF to TAF appears safe with regard to neuronal injury.

Switching from a regimen containing abacavir/lamivudine or emtricitabine/tenofovir disoproxil fumarate to emtricitabine/tenofovir alafenamide fumarate does not affect central nervous system HIV-1 infection

Background: Despite suppressive antiretroviral therapy (ART), many HIV-infected individuals have low-level persistent immune activation in the central nervous system (CNS). There have been concerns regarding the CNS efficacy of tenofovir alafenamide fumarate (TAF) because of its low cerebrospinal fluid (CSF) concentrations and because it is a substrate of the active efflux transporter P-glycoprotein. Our aim was to investigate whether switching from emtricitabine (FTC)/tenofovir disoproxil fumarate (TDF) or abacavir (ABC)/lamivudine (3TC) to FTC/TAF would lead to changes in residual intrathecal immune activation, viral load, or neurocognitive function. Methods: Twenty HIV-1-infected neuro-asymptomatic adults (11 on ABC/3TC and 9 on FTC/TDF) were included in this prospective study. At baseline, all participants changed their nucleoside analogues to FTC/TAF without any other changes in their ART regimen. We performed lumbar punctures, venipunctures, and neurocognitive testing at baseline and after three and 12 months. Results: During follow-up, there were no significant changes in CSF or plasma HIV RNA, CSF neopterin, CSF β2-microglobulin, IgG index, albumin ratio, CSF NFL, or neurocognitive function in assessed by Cogstate in any of the groups. Conclusion: This small pilot study indicates that switching to FTC/TAF from ABC/3TC or FTC/TDF has neither a positive, nor a negative effect on the HIV infection in the CNS.

Towards the worldwide eradication of hepatitis B virus infection: A combination of prophylactic and therapeutic factors

Hepatitis B virus (HBV) is still a global health problem, mostly because of the intermediate/high rates of HBV chronic carriers living in most Asian, African and eastern European countries. The universal HBV vaccination of new-borns undertaken in most nations over the last 3 decades and effective HBV antiviral treatments (nucleos(t)ide analogue with high genetic barrier to viral resistance) introduced in the last decade have shown their beneficial effects in inducing a clear reduction of HBV endemicity in the countries where they have been extensively applied. Great hopes are now placed on new antiviral and immunotherapeutic drugs that are now at an advanced stage of study. It is in fact already conceivable that the synergistic use of new drugs targeting more than one HBV-lifecycle steps (covalent closed circular DNA destruction/silencing, HBV entry inhibitors, nucleocapsid assembly modulators targeting viral transcripts) and of some new immunotherapeutic agents might eliminate the intrahepatic covalent closed circular DNA and achieve the eradication of HBV infection. In spite of this, a strong effort should be given to extensive educational and screening programs for the at-risk population and to the implementation of HBV vaccination in developing countries.

Nucleotide Reverse Transcriptase Inhibitors: A Thorough Review, Present Status and Future Perspective as HIV Therapeutics

BACKGROUND

Human immunodeficiency virus type-1 (HIV-1) infection leads to acquired immunodeficiency syndrome (AIDS), a severe viral infection that has claimed approximately 658,507 lives in the US between the years 2010-2014. Antiretroviral (ARV) therapy has proven to inhibit HIV-1, but unlike other viral illness, not cure the infection.

OBJECTIVE

Among various Food and Drug Administration (FDA)-approved ARVs, nucleoside/ nucleotide reverse transcriptase inhibitors (NRTIs) are most effective in limiting HIV-1 infection. This review focuses on NRTIs mechanism of action and metabolism.

METHODS

A search of PubMed (1982-2016) was performed to capture relevant articles regarding NRTI pharmacology.

RESULTS

The current classical NRTIs pharmacology for HIV-1 prevention and treatment are presented. Finally, various novel strategies are proposed to improve the efficacy of NRTIs, which will increase therapeutic efficiency of present-day HIV-1 prevention/treatment regimen.

CONCLUSION

Use of NRTIs will continue to be critical for successful treatment and prevention of HIV-1.

Darunavir/Cobicistat/Emtricitabine/Tenofovir Alafenamide: A Review in HIV-1 Infection

Darunavir/cobicistat/emtricitabine/tenofovir AF (Symtuza^®) is the first protease inhibitor (PI)-based single-tablet regimen (STR) available for the treatment of adults and adolescents (aged ≥ 12 years) with HIV-1 infection. It combines the PI darunavir (which has a high genetic barrier to resistance) with the pharmacokinetic booster cobicistat and the nucleos(t)ide reverse transcriptase inhibitors emtricitabine and tenofovir alafenamide (tenofovir AF), the latter being associated with less off-target tenofovir exposure than its predecessor tenofovir disoproxil fumarate (tenofovir DF). Over 48 weeks in phase 3 trials, darunavir/cobicistat/emtricitabine/tenofovir AF was noninferior to darunavir/cobicistat plus emtricitabine/tenofovir DF in establishing virological suppression in antiretroviral therapy (ART)-naïve adults and, likewise, was noninferior to an ongoing boosted PI, emtricitabine plus tenofovir DF regimen in preventing virological rebound in virologically-suppressed, ART-experienced adults. Resistance did not emerge to the STR components, with the exception being an emtricitabine resistance-associated mutation (RAM) [M184I/V] in one of seven recipients who experienced virological failure (although M184V was a minority variant at screening in this patient). Darunavir/cobicistat/emtricitabine/tenofovir AF was generally well tolerated, with renal and bone profile improvements but less favourable effects on some lipids versus tenofovir DF-based regimens. Thus, although longer-term and cost-effectiveness data would be beneficial, darunavir/cobicistat/emtricitabine/tenofovir AF is a welcome addition to the STRs available for the treatment of adults and adolescents with HIV-1 infection, being the first to combine the high genetic resistance barrier of darunavir with the renal/bone profile of tenofovir AF, thus expanding the patient population for whom an STR may be suitable.

Pharmacokinetics of Tenofovir Alafenamide, Tenofovir, and Emtricitabine Following Administration of Coformulated Emtricitabine/Tenofovir Alafenamide in Healthy Japanese Subjects

A fixed-dose combination of tenofovir alafenamide (TAF) and emtricitabine (FTC) is available in 2 tablet strengths in Japan (FTC/TAF 200/10 mg and FTC/TAF 200/25 mg). These are used once daily in combination with other antiretroviral agents for the treatment of human immunodeficiency virus type 1 infection. The primary objective of this study was to investigate if there is any clinically relevant pharmacokinetic difference for TAF, tenofovir (TFV), and FTC between Japanese and non-Japanese with historical data. Three treatment groups were set in the study; FTC/TAF 200/10 mg in combination with darunavir (DRV) 800 mg + ritonavir (RTV) 100 mg (treatment A) or DRV/cobicistat (COBI) 800/150 mg (treatment B) and FTC/TAF 200/25 mg alone (treatment C). Especially for treatment C, it was designated for another purpose to evaluate the pharmacokinetic boosting effects of RTV and COBI on TAF bioavailability. As a result, the mean exposure of TAF among treatment groups was 125 to 154 ng/mL for C_max and 119 to 179 ng·h/mL for AUC_inf , which were comparable with the historical data in non-Japanese. The exposures of TFV and FTC were also consistent with the historical data. Therefore, no clinically relevant pharmacokinetic differences for TAF, TFV, and FTC were observed between Japanese and non-Japanese. Boosting effects of RTV and COBI on TAF bioavailability were slightly lower than we expected, less than a 2.5-fold increase, but it was within the range of exposures associated with efficacy and safety in phase 3 studies. Therefore, it was not considered clinically relevant.

Current and future directions of management of hepatitis B: steps toward a cure

Universal hepatitis B virus vaccination has been effective in reducing incident chronic hepatitis B but will not have the requisite effect on the prevalence of end-stage liver disease in chronically infected persons. The natural history and immunological stages of hepatitis B virus infection are still being defined. Over three decades, current therapies have reduced morbidity from chronic hepatitis B. The majority require nucleoside analog maintenance therapy. The preferential preservation of covalently closed circular DNA (cccDNA), and capsid reverse transcriptase–cccDNA interactions currently precludes cure in most. A functional cure in  the host may require several synergistic antiviral and immunological intercessions. The correct sequencing and combinations of treatment with either host or viral targeting agents have yet to be determined. Proven surrogates for cccDNA for clinical trials are required. Different strategies may become apparent for patients at different stages of the disease. Curative therapies will require affordability. This review focuses on steps toward a cure.

Hormonal Contraceptives Differentially Suppress TFV and TAF Inhibition of HIV Infection and TFV-DP in Blood and Genital Tract CD4+ T cells

HIV prevention research is focused on combining antiretrovirals (ARV) and progestin contraceptives to prevent HIV infection and pregnancy. The possibility that progestins compromise ARV anti-HIV activity prompted us to evaluate the effects of progestins on tenofovir (TFV) and TFV-alafenamide (TAF) on HIV infection and intracellular TFV-diphosphate (TFV-DP) concentrations in blood and genital CD4+ T cells. Following incubation of blood CD4+ T cells with TFV or TAF, Medroxyprogesterone acetate (MPA), but not Levonorgestrel, Norethisterone or progesterone, suppressed the anti-HIV effect of TFV by reducing intracellular TFV-DP, but had no effect on TAF inhibition of infection or TFV-DP. In contrast, with genital CD4+ T cells, MPA suppressed TAF inhibition of HIV infection and lowered of TFV-DP concentrations without affecting TFV protection. These findings demonstrate that MPA selectively compromises TFV and TAF protection in blood and genital CD4+ T cells and suggests that MPA may decrease ARV protection in individuals who use ARV intermittently for prevention.

Role of tenofovir alafenamide (TAF) in the treatment and prophylaxis of HIV and HBV infections

Tenofovir (TFV) is the cornerstone of the treatment and prophylaxis of HIV infections. It has been routinely used in its prodrug form TDF (tenofovir disoproxil fumarate) combined with emtricitabine ((-)FTC) and other antiretroviral agents. TDF has now been replaced by TAF (tenofovir alafenamide) which allows better uptake by the lymphoid tissue. In combination with elvitegravir (E), cobicistat (C), emtricitabine (F), TAF can be advocated as an STR (single tablet regimen, Genvoya®) for the treatment of HIV infections. In this combination, E and C may in the future be replaced by bictegravir. The prophylaxis of HIV infection is momentarily based upon Truvada®, the combination of F with TDF, which in the future may also be replaced by TAF. TAF (Vemlidy®) has also replaced TDF (Viread®) for the treatment of hepatitis B virus (HBV) infections. Both TDF and TAF offer little or no risk for virus-drug resistance. As compared to TDF, TAF limits the risk for nephrotoxicity and loss of bone mineral density. What remains to be settled, however, before the universal use of TAF could be recommended, is its safety during pregnancy and its applicability in the treatment of tuberculosis, in combination with rifampicin.

Novel multipurpose pod-intravaginal ring for the prevention of HIV, HSV, and unintended pregnancy: Pharmacokinetic evaluation in a macaque model

Globally, women bear an uneven burden for sexual HIV acquisition. Results from two clinical trials evaluating intravaginal rings (IVRs) delivering the antiretroviral agent dapivirine have shown that protection from HIV infection can be achieved with this modality, but high adherence is essential. Multipurpose prevention technologies (MPTs) can potentially increase product adherence by offering protection against multiple vaginally transmitted infections and unintended pregnancy. Here we describe a coitally independent, long-acting pod-IVR MPT that could potentially prevent HIV and HSV infection as well as unintended pregnancy. The pharmacokinetics of MPT pod-IVRs delivering tenofovir alafenamide hemifumarate (TAF2) to prevent HIV, acyclovir (ACV) to prevent HSV, and etonogestrel (ENG) in combination with ethinyl estradiol (EE), FDA-approved hormonal contraceptives, were evaluated in pigtailed macaques (N = 6) over 35 days. Pod IVRs were exchanged at 14 days with the only modification being lower ENG release rates in the second IVR. Plasma progesterone was monitored weekly to determine the effect of ENG/EE on menstrual cycle. The mean in vivo release rates (mg d-1) for the two formulations over 30 days ranged as follows: TAF2 0.35-0.40; ACV 0.56-0.70; EE 0.03-0.08; ENG (high releasing) 0.63; and ENG (low releasing) 0.05. Mean peak progesterone levels were 4.4 ± 1.8 ng mL-1 prior to IVR insertion and 0.075 ± 0.064 ng mL-1 for 5 weeks after insertion, suggesting that systemic EE/ENG levels were sufficient to suppress menstruation. The TAF2 and ACV release rates and resulting vaginal tissue drug concentrations (medians: TFV, 2.4 ng mg-1; ACV, 0.2 ng mg-1) may be sufficient to protect against HIV and HSV infection, respectively. This proof of principle study demonstrates that MPT-pod IVRs could serve as a potent biomedical prevention tool to protect women's sexual and reproductive health and may increase adherence to HIV PrEP even among younger high-risk populations.

Tenofovir Inhibits Wound Healing of Epithelial Cells and Fibroblasts from the Upper and Lower Human Female Reproductive Tract

Disruption of the epithelium in the female reproductive tract (FRT) is hypothesized to increase HIV infection risk by interfering with barrier protection and facilitating HIV-target cell recruitment. Here we determined whether Tenofovir (TFV), used vaginally in HIV prevention trials, and Tenofovir alafenamide (TAF), an improved prodrug of TFV, interfere with wound healing in the human FRT. TFV treatment of primary epithelial cells and fibroblasts from the endometrium (EM), endocervix (CX) and ectocervix (ECX) significantly delayed wound closure. Reestablishment of tight junctions was compromised in EM and CX epithelial cells even after wound closure occurred. In contrast, TAF had no inhibitory effect on wound closure or tight junction formation following injury. TAF accumulated inside genital epithelial cells as TFV-DP, the active drug form. At elevated levels of TAF treatment to match TFV intracellular TFV-DP concentrations, both equally impaired barrier function, while wound closure was more sensitive to TFV. Furthermore, TFV but not TAF increased elafin and MIP3a secretion following injury, molecules known to be chemotactic for HIV-target cells. Our results highlight the need of evaluating antiretroviral effects on genital wound healing in future clinical trials. A possible link between delayed wound healing and increased risk of HIV acquisition deserves further investigation.

Elvitegravir/Cobicistat/Emtricitabine/Tenofovir Alafenamide: A Review in HIV-1 Infection

Tenofovir alafenamide (tenofovir AF) is a novel oral prodrug of the nucleos(t)ide reverse transcriptase inhibitor (NRTI) tenofovir that has several pharmacological advantages over tenofovir disoproxil fumarate (tenofovir DF), including increased plasma stability and reduced tenofovir systemic exposure. Tenofovir AF has been coformulated with elvitegravir, cobicistat and emtricitabine as a once-daily, single-tablet regimen (elvitegravir/cobicistat/emtricitabine/tenofovir AF; Genvoya(®)) for the treatment of adults and adolescents with HIV-1 infection. With regard to establishing and/or maintaining virological suppression over 48 weeks in randomized, phase III trials, elvitegravir/cobicistat/emtricitabine/tenofovir AF was noninferior to elvitegravir/cobicistat/emtricitabine/tenofovir DF in antiretroviral therapy (ART)-naive adults, and statistically superior (subsequent to established noninferiority) to ongoing treatment with tenofovir DF-containing regimens in ART-experienced adults with virological suppression. In single-arm, phase III trials, elvitegravir/cobicistat/emtricitabine/tenofovir AF also provided high rates of virological suppression among ART-naive adolescents and ART-experienced adults with stable renal impairment. In general, elvitegravir/cobicistat/emtricitabine/tenofovir AF was well tolerated and associated with more favourable renal and bone parameters, but a less favourable lipid profile, than tenofovir DF-containing regimens. Thus, elvitegravir/cobicistat/emtricitabine/tenofovir AF is an alternative single-tablet regimen for adults and adolescents with HIV-1 infection, particularly those with an estimated creatinine clearance of ≥30 to <50 mL/min or an increased risk of tenofovir DF-related bone toxicity.

Rapid In Vitro Evaluation of Antiretroviral Barrier to Resistance at Therapeutic Drug Levels

Failure of combination antiretroviral (ARV) therapy in HIV-infected patients is often associated with the emergence of drug resistance-associated mutations (RAMs). To facilitate analysis of the barrier to resistance at therapeutically relevant ARV concentrations, we performed fixed-dose in vitro HIV-1 drug resistance selection assays using the immortalized MT-2 T-cell line and primary human CD4⁺ T cells with a panel of FDA-approved ARVs, each at their respective cell culture equivalent clinical trough concentration (CCE C_min). At high multiples of its CCE C_min, emtricitabine (FTC) selected for the rapid emergence of M184I/V, a result consistent with resistance emergence in vivo. While the rate of viral breakthrough in the presence of rilpivirine or efavirenz was delayed relative to FTC, both inhibitors selected for virus with known clinically relevant RAMs. No viral breakthrough was observed for the protease inhibitor atazanavir even at subtherapeutic drug concentrations, which is consistent with its previously characterized high in vivo barrier to resistance. Depending on assay conditions, treatment with integrase inhibitors elvitegravir and raltegravir resulted in breakthrough of both resistant and wild-type virus. The RAMs observed in drug selections were not detected above a 2% threshold by deep sequencing in the in vitro virus inoculum, and only rarely in isolates from treatment-naive HIV+ patients. These new viral breakthrough assays facilitate the analysis of multiple experimental replicates and conditions in parallel and provide a rapid quantitative means to evaluate drug resistance emergence at therapeutically relevant drug concentrations, which should facilitate the identification of new ARVs with a high barrier to resistance.

Tenofovir alafenamide (TAF) as the successor of tenofovir disoproxil fumarate (TDF)

Tenofovir alafenamide (TAF) can be considered a new prodrug of tenofovir (TFV), as successor of tenofovir disoproxil fumarate (TDF). It is in vivo as potent against human immunodeficiency virus (HIV) at a 30-fold lower dose (10mg) than TDF (300mg). TAF has been approved in November 2015 (in the US and EU), as a single-tablet regimen (STR) containing 150mg elvitegravir (E), 150mg cobicistat (C), 200mg emtricitabine [(-)FTC] (F) and 10mg TAF, marketed as Genvoya®, on 01 March 2016 in the US as an STR containing 25mg rilpivirine (R), 200mg F and 25mg TAF, marketed as Odefsey®, and on 4 April 2016 in the US, as an STR containing 200mg F and 25mg TAF, marketed as Descovy®, for the treatment of HIV infections. STR combinations containing TAF and emtricitabine could be paired with a range of third agents, for example, darunavir and cobicistat. TAF has a much lower risk of kidney toxicity or bone density changes than TDF, and also offers long-term potential in the pre-exposure prophylaxis (PrEP) of HIV infections. TAF is specifically accumulated in lymphatic tissue, and in the liver, and hence also holds great potential for the treatment of hepatitis B virus (HBV) infections. Akin to TDF, TAF is converted intracellularly to TFV. Its active diphosphate metabolite (TFVpp) is targeted at the RNA-dependent DNA polymerase (reverse transcriptase) of either HIV or HBV.

The current treatment situation and definitions of a cure for chronic HBV infection

HBV vaccination, while effective in reducing incident chronic disease in endemic regions, will not have the desired impact on the rates of end-stage liver disease in chronically infected persons. Over three decades, IFN-α and nucleoside analogs have reduced the morbidity from the disease. A large reservoir of chronic infection remains. The natural history of HBV infection is still being defined. Understanding the interactions between HBV and the host will be fundamental to achieving higher rates of cure. Curing hepatitis B will require several steps for either eradication, or a functional cure in the host. It is unclear whether covently closed circular DNA chromatin would need to be cleared to cure hepatitis B, or whether low threshold levels would slow the disease.

Intracellular Activation of Tenofovir Alafenamide and the Effect of Viral and Host Protease Inhibitors

Tenofovir alafenamide fumarate (TAF) is an oral phosphonoamidate prodrug of the HIV reverse transcriptase nucleotide inhibitor tenofovir (TFV). Previous studies suggested a principal role for the lysosomal serine protease cathepsin A (CatA) in the intracellular activation of TAF. Here we further investigated the role of CatA and other human hydrolases in the metabolism of TAF. Overexpression of CatA or liver carboxylesterase 1 (Ces1) in HEK293T cells increased intracellular TAF hydrolysis 2- and 5-fold, respectively. Knockdown of CatA expression with RNA interference (RNAi) in HeLa cells reduced intracellular TAF metabolism 5-fold. Additionally, the anti-HIV activity and the rate of CatA hydrolysis showed good correlation within a large set of TFV phosphonoamidate prodrugs. The covalent hepatitis C virus (HCV) protease inhibitors (PIs) telaprevir and boceprevir potently inhibited CatA-mediated TAF activation (50% inhibitory concentration [IC50] = 0.27 and 0.16 μM, respectively) in vitro and also reduced its anti-HIV activity in primary human CD4(+) T lymphocytes (21- and 3-fold, respectively) at pharmacologically relevant concentrations. In contrast, there was no inhibition of CatA or any significant effect on anti-HIV activity of TAF observed with cobicistat, noncovalent HIV and HCV PIs, or various prescribed inhibitors of host serine proteases. Collectively, these studies confirm that CatA plays a pivotal role in the intracellular metabolism of TAF, whereas the liver esterase Ces1 likely contributes to the hepatic activation of TAF. Moreover, this work demonstrates that a wide range of viral and host PIs, with the exception of telaprevir and boceprevir, do not interfere with the antiretroviral activity of TAF.

In Vitro Virology Profile of Tenofovir Alafenamide, a Novel Oral Prodrug of Tenofovir with Improved Antiviral Activity Compared to That of Tenofovir Disoproxil Fumarate

Tenofovir alafenamide (TAF) is an investigational oral prodrug of the HIV-1 nucleotide reverse transcriptase inhibitor tenofovir (TFV). Tenofovir disoproxil fumarate (TDF) is another TFV prodrug, widely used for the treatment of HIV-1 infection. TAF is converted mostly intracellularly to TFV and, in comparison to TDF, achieves higher tenofovir diphosphate (TFV-DP) levels in peripheral blood mononuclear cells. As a result, TAF has demonstrated potent anti-HIV-1 activity at lower doses than TDF in monotherapy studies. Here, the in vitro virology profile of TAF was evaluated and compared to that of TDF. TAF displayed potent antiviral activity against all HIV-1 groups/subtypes, as well as HIV-2. TAF exhibited minimal changes in the drug concentration needed to inhibit 50% of viral spread (EC50) upon removal of the prodrug, similar to TDF, demonstrating intracellular antiviral persistence. While TAF and TDF exhibited comparable potencies in the absence of serum pretreatment, TAF maintained activity in the presence of human serum, whereas TDF activity was significantly reduced. This result demonstrates TAF's improved plasma stability over TDF, which is driven by the different metabolic pathways of the two prodrugs and is key to TAF's improved in vivo antiviral activity. The activity of TAF is specific for HIV, as TAF lacked activity against a large panel of human viruses, with the exception of herpes simplex virus 2, where weak TAF antiviral activity was observed, as previously observed with TFV. Finally, in vitro combination studies with antiretroviral drugs from different classes showed additive to synergistic interactions with TAF, consistent with ongoing clinical studies with TAF in fixed-dose combinations with multiple other antiretroviral drugs for the treatment of HIV.

Characterization of HIV-1 Resistance to Tenofovir Alafenamide In Vitro

Tenofovir alafenamide (TAF) is an investigational prodrug of the HIV-1 nucleotide reverse transcriptase (RT) inhibitor (NtRTI) tenofovir (TFV), with improved potency and drug delivery properties over the current prodrug, tenofovir disoproxil fumarate (TDF). TAF is currently in phase 3 clinical studies for the treatment of HIV-1 infection, in combination with other antiretroviral agents. Phase 1 and 2 studies have shown that TAF was associated with increased peripheral blood mononuclear cell (PBMC) drug loading and increased suppression of HIV-1 replication compared to treatment with TDF. In this study, selection of in vitro resistance to both TAF and the parent compound, TFV, led to the emergence of HIV-1 with the K65R amino acid substitution in RT with 6.5-fold-reduced susceptibility to TAF. Although TAF is more potent than TFV in vitro, the antiviral susceptibilities to TAF and TFV of a large panel of nucleoside/nucleotide RT inhibitor (NRTI)-resistant mutants were highly correlated (R(2) = 0.97), indicating that the two compounds have virtually the same resistance profile when assessed as fold change from the wild type. TAF showed full antiviral activity in PBMCs against primary HIV-1 isolates with protease inhibitor, nonnucleoside RT inhibitor (NNRTI), or integrase strand transfer inhibitor resistance but reduced activity against isolates with extensive NRTI resistance amino acid substitutions. However, the increased cell loading of TFV with TAF versus TDF observed in vivo suggests that TAF may retain activity against TDF-resistant mutant viruses.

Investigational reverse transcriptase inhibitors for the treatment of HIV

INTRODUCTION

While considerable advances have been made in the development of antiretroviral agents, there is still work to be done. Reverse transcriptase inhibitors are important drugs for the treatment of HIV, and considerable research is currently ongoing to develop new agents and to modify currently existing agents.

AREAS COVERED

Herein, the authors discuss both investigational nucleoside reverse transcriptase inhibitors (NRTIs) and non-nucleoside reverse transcriptase inhibitors (NNRTIs), including agents that are in various stages of development. They also discuss novel formulations that are being investigated for currently available drugs, and discuss the advantages that these new formulations may provide.

EXPERT OPINION

New formulations and co-formulations of currently existing antiretrovirals will represent an important area of development, as a means to improve adherence for HIV-positive individuals. New formulations will continue to be developed, with a focus on allowing for less-frequent administration, as well increasing drug concentrations at local sites such as vaginal tissue, rectal tissue and sites in the immune system.

Tenofovir alafenamide is not a substrate for renal organic anion transporters (OATs) and does not exhibit OAT-dependent cytotoxicity

BACKGROUND

Tenofovir alafenamide (TAF) is a novel prodrug of tenofovir that shows enhanced antiretroviral effect and reduced plasma tenofovir exposures at approximately one-tenth the clinically approved dose of tenofovir disoproxil fumarate (TDF). Tenofovir released from TDF undergoes active renal secretion via organic anion transporters (OAT1 and OAT3), leading to higher exposure of renal proximal tubules to tenofovir and a potential for renal adverse effects in a small subset of TDF-treated patients. Here, we evaluate the interaction of TAF with OAT1 and OAT3 to assess the potential for its active accumulation in proximal tubules.

METHODS

OAT-mediated transport and cytotoxicity (CC50) of TAF and tenofovir were assessed in cells expressing OATs and compared with matched transporter-null cells.

RESULTS

While OAT1 and OAT3 expression increased tenofovir cellular uptake by >70-fold and 8.2-fold, respectively, the expression of either OAT did not significantly change TAF intracellular accumulation under identical conditions. In addition, although tenofovir was significantly more cytotoxic in OAT1- and OAT3-expressing cells (>21 and >3.6 fold change in CC50 values, respectively), TAF in vitro cytotoxicity showed little to no change upon overexpression of either renal transporter (0.5-3.5 fold change in CC50).

CONCLUSIONS

Unlike tenofovir, TAF does not interact with renal transporters OAT1 or OAT3 and exhibits no OAT-dependent cytotoxicity. TAF is thus unlikely to actively accumulate in renal proximal tubules in an OAT-dependent manner, supporting the potential for an improved renal safety profile.