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.

The enhanced intracellular TFV-DP loading translates to superior antiviral activity, with TAF demonstrating >600-fold greater potency than parent tenofovir in CD4+ T cells and a 3.5-fold higher resistance barrier than TDF. In viral breakthrough assays at physiologically relevant concentrations, TAF inhibited 40 of 42 K65R-containing isolates while TDF inhibited only 32 of 42, and TAF prevented breakthrough in 65 of 68 isolates with thymidine analog-associated mutations compared to 53 of 68 for TDF. The covalent hepatitis C protease inhibitors telaprevir and boceprevir potently inhibit cathepsin A-mediated TAF activation, reducing anti-HIV activity 3- to 21-fold, while medroxyprogesterone acetate differentially suppresses TAF activity in genital versus blood CD4+ T cells. These findings support TAF’s clinical efficacy at lower doses than TDF with improved safety profiles and activity against resistant HIV-1 strains.

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.

Data extraction

We asked a large language model to extract each data column below from each paper.

• Study Methods:

  • Cell types used (primary vs. cell lines, HIV-1 infected vs. uninfected)
  • Experimental approach (enzyme assays, drug uptake studies, resistance selection, etc.)
  • Duration of experiments
  • Technical methods for measuring drug concentrations or enzyme activity
  • Any special conditions or treatments applied

• TAF Activation Mechanisms:

  • Extract all details about how TAF is activated intracellularly.

• Intracellular Drug Concentrations:

  • Extract quantitative data on tenofovir and metabolite levels in cells.

• TAF vs TDF Comparison:

  • Extract comparative data between TAF and TDF regarding intracellular TFV-DP concentrations.

• Factors Affecting Activation:

  • Extract information about factors that enhance or inhibit TAF activation.

• HIV-1 Resistance Context:

  • Extract data specific to TAF activity against HIV-1 resistant strains.

• Clinical Relevance:

  • Extract findings about clinical implications of TAF.

Results

Characteristics of Included Studies

The review included 10 studies published between 2013 and 2023 examining TAF intracellular activation and tenofovir distribution to HIV-1 infected cells. Eight studies had full text available, while two were assessed using abstract-only information.

TAF Activation Mechanisms

Enzymatic Pathways

TAF undergoes intracellular activation primarily 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 then 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.

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. The fold increases in TFV-DP concentrations varied across studies and doses:

Activity Against Resistant HIV-1 Strains

Resistance Mutation Profiles

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

Clinical Implications

Dosing and Efficacy

The phase 1b monotherapy study established a dose-response relationship for viral load decrease up to 25 mg TAF. At steady state, TAF yielded mean TFV plasma exposures significantly lower compared to TDF.

Safety Considerations

The reduced systemic TFV exposure with TAF compared to TDF has important safety implications, including reduced nephrotoxicity.

Monitoring and Resistance Testing

Consideration of cervical vaginal lavage monitoring in association with TAF treatment should involve factors such as local inflammation, microbiome composition, and sexually-transmitted infections.

Knowledge Gaps and Future Directions

Further research on TAF activation and distribution in genital tissues under conditions of inflammation and sexually-transmitted infections is necessary. Long-term durability studies in larger clinical trials are required to fully understand the safety and efficacy of TAF.