Implications of Efficient Hepatic Delivery by Tenofovir Alafenamide (GS-7340) for Hepatitis B Virus Therapy

Abstract

Tenofovir alafenamide (TAF) is a prodrug of tenofovir (TFV) currently in clinical evaluation for treatment for HIV and hepatitis B virus (HBV) infections. Since the target tissue for HBV is the liver, the hepatic delivery and metabolism of TAF in primary human hepatocytes in vitro and in dogs in vivo were evaluated here. Incubation of primary human hepatocytes with TAF resulted in high levels of the pharmacologically active metabolite tenofovir diphosphate (TFV-DP), which persisted in the cell with a halflife of >24 h. In addition to passive permeability, studies of transfected cell lines suggest that the hepatic uptake of TAF is also facilitated by the organic anion-transporting polypeptides 1B1 and 1B3 (OATP1B1 and OATP1B3, respectively). In order to inhibit HBV reverse transcriptase, TAF must be converted to the pharmacologically active form, TFV-DP. While cathepsin A is known to be the major enzyme hydrolyzing TAF in cells targeted by HIV, including lymphocytes and macrophages, TAF was primarily hydrolyzed by carboxylesterase 1 (CES1) in primary human hepatocytes, with cathepsin A making a small contribution. Following oral administration of TAF to dogs for 7 days, TAF was rapidly absorbed. The appearance of the major metabolite TFV in plasma was accompanied by a rapid decline in circulating TAF. Consistent with the in vitro data, high and persistent levels of TFV-DP were observed in dog livers. Notably, higher liver TFV-DP levels were observed after administration of TAF than those given TDF. These results support the clinical testing of once-daily low-dose TAF for the treatment of HBV infection. Chronic hepatitis B (CHB) is a major global health problem, and the World Health Organization (WHO) estimates that ϳ240 million people worldwide are chronically infected by hepatitis B virus (HBV) (1) (http://www.who.int/mediacentre /factsheets/fs204/en/). The small-molecule anti-HBV agents currently approved by U.S. Food and Drug Administration (FDA) are all nucleoside/nucleotide analogs targeting HBV reverse transcriptase. Tenofovir disoproxil fumarate (TDF), a prodrug of tenofovir (TFV), is a current first-line treatment for CHB (2) and has demonstrated efficacy in both hepatitis B e antigen (HBeAg)-positive and HBeAg-negative patients (3-5). TDF is also widely used as the backbone of current anti-HIV combination regimens (6, 7). For both HIV and HBV, the mechanism of action of TDF is intracellular metabolism to its pharmacologically active form, tenofovir diphosphate (TFV-DP), followed by competition with endogenous 2=-dATP for incorporation by the viral reverse transcriptase and subsequent chain termination of viral DNA replication (8, 9). TFV-DP is an effective inhibitor of HBV reverse transcriptase, with an inhibitor constant (K i ) of 0.18 M in vitro (9).A new prodrug of tenofovir, tenofovir alafenamide (TAF or GS-7340), was selected to more efficiently load HIV target cells while lowering circulating levels of TFV, resulting in reduced offtarget exposure (10-13). The administration of 25 mg of TAF in HIV-i...

Brief Report: Efficacy and Safety of Switching to a Single-Tablet Regimen of Elvitegravir/Cobicistat/Emtricitabine/Tenofovir Alafenamide in HIV-1/Hepatitis B–Coinfected Adults

“…15 In vitro and in dogs, TAF had high intrahepatic extraction and enhanced levels of TFV-DP compared to TDF. 16 TAF 25 mg compared with TDF 300 mg has been studied in 2 global phase 3 HBV-monoinfected studies—one in treatment-naive or -experienced HBV e antigen (HBeAg)–negative participants (GS-US-320-0108) and one in monoinfected HBeAg-positive participants (GS-US-320-0110). Week 48 results were recently reported.…”

Primary resistance of hepatitis B virus to nucleoside and nucleotide analogues

“…In addition, tenofovir alafenamide (TAF) has been approved for the treatment of chronic HBV infection with compensated liver disease by the FDA in the US and has received a positive opinion from the Committee for Medicinal Products for Human Use, the scientific committee of the European Medicines Agency. This prodrug of tenofovir has demonstrated similar antiviral efficacy with improved renal and bone safety as compared to TDF, due to greater plasma stability and more efficient delivery of tenofovir to hepatocytes . Thus, in spite of our findings showing a high prevalence of HBV RASs at baseline in patients never treated with nucleoside/nucleotide analogues, it is possible to broadly achieve high rates of virological response, provided that the cascade of care is optimized.…”

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

“…While CatA is the main TAF-activating enzyme in most tissues, Ces1 may play an important role in metabolism of TAF in the liver. Indeed, Ces1 has recently been shown to be the main hydrolase responsible for TAF activation in the liver, with a minor contribution coming from CatA (35). Those authors showed an impaired metabolism of TAF in primary human hepatocytes in the presence of bis(4-nitrophenyl)phosphate (BNPP), a selective inhibitor of human carboxylesterase 1.…”