Tenofovir Alafenamide (TAF) Intracellular Activation and Tenofovir Distribution to HIV-1 Infected Cells

Abstract

Tenofovir alafenamide undergoes intracellular activation primarily through the lysosomal protease cathepsin A, which cleaves TAF to form intermediates that ultimately convert to the active metabolite tenofovir diphosphate (TFV-DP). Liver carboxylesterase 1 provides an alternative hepatic activation pathway. This intracellular activation mechanism enables TAF to achieve 4- to 25-fold higher TFV-DP concentrations in peripheral blood mononuclear cells compared to tenofovir disoproxil fumarate (TDF), while reducing systemic tenofovir exposure by 86-97%. TAF demonstrates preferential distribution to lymphatic tissues and accumulates in epithelial cells and fibroblasts of the female reproductive tract at 10- to 100-fold higher concentrations than in CD4+ T cells, with these cells releasing antiretroviral activity over multiple days to protect CD4+ T cells.

Methods

We analyzed 10 sources from an initial pool of 200, using 9 screening criteria. Each paper was reviewed for 7 key aspects that mattered most to the research question.

• Papers screened using: TAF as Primary Intervention, Intracellular Mechanisms Focus, HIV-1 Infected Cell Models, In Vitro Study Design, Intracellular Concentration Measurements, Mechanistic Data Inclusion, HIV-1 Context Requirement, Intracellular Measurement Focus, Peer-Reviewed Research
• Papers included for extraction: n = 10

TAF Activation Mechanisms

Enzymatic Pathways

TAF undergoes intracellular activation through the lysosomal protease cathepsin A (CatA). The activation occurs in the lysosomal compartment, where CatA cleaves the carboxyester bond in TAF to form a metastable metabolite. This metabolite converts to TFV-Ala, which spontaneously converts to TFV due to the acidic pH in lysosomes. Subsequently, TFV undergoes two sequential phosphorylation reactions to form the active metabolite TFV-DP.

Alternative Activation Pathways

Liver carboxylesterase 1 (Ces1) represents an alternative pathway for TAF activation. Overexpression of Ces1 in HEK293T cells increased intracellular TAF hydrolysis five-fold.

Intracellular Drug Concentrations and Distribution

Comparative TFV-DP Levels

TAF achieves substantially higher intracellular concentrations of the active metabolite TFV-DP compared to its predecessor TDF.

Comparison with Tenofovir Disoproxil Fumarate (TDF)

Potency and Activation Efficiency

TAF demonstrated markedly superior potency compared to parent TFV and the alternative prodrug TDF across multiple dimensions.

Factors Affecting TAF Activation

TAF activation is influenced by various factors including drug interactions and hormonal contraceptive effects. For example, the covalent hepatitis C virus protease inhibitors telaprevir and boceprevir potently inhibited CatA-mediated TAF activation.

Activity Against Resistant HIV-1 Strains

TAF activity was evaluated against multiple resistance mutations, including K65R and thymidine analog-associated mutations (TAMs).

Clinical Relevance

The phase 1b monotherapy study established a dose-response relationship for viral load decrease up to 25 mg TAF. At steady state, TAF showed significantly lower plasma HIV-1 RNA compared to TDF, indicating greater antiviral efficacy and improved safety profile.

Knowledge Gaps and Future Directions

Further research is needed on TAF distribution and activation in genital tissues, particularly under conditions of inflammation and sexually-transmitted infections. Long-term durability and safety profiles require additional study in larger and longer-term clinical trials.