Hybrid Minimal Physiological-Based Pharmacokinetic Model of Tenofovir Alafenamide and Its Metabolites Disposition in Humans
Authors: Jiang Weihua , Woo Sukyung 
Published: February 2025

Objectives: Tenofovir alafenamide (TAF) holds promise in HIV prevention and treatment. TAF functions as a prodrug, rapidly hydrolyzed by specific esterases—CES1 in hepatocytes and CatA in lymphoid cells—to generate tenofovir (TFV). This activation process triggers its target-specific pharmacodynamic effect, facilitating potent antiviral activity with low systemic toxicity. TAF exhibits a complex disposition, characterized by transient plasma pharmacokinetics (PK) that render it undetectable approximately 4 hours post oral administration, while its active metabolite, tenofovir diphosphate (TFVdp), sustains prolonged activity for 6-7 days within peripheral blood mononuclear cells (PBMCs). However, existing models primarily focus on the PK of its metabolite TFV alone. This study aims to develop a minimal physiologically based pharmacokinetic (mPBPK) model for TAF and its metabolites in humans. Methods: The PK model was developed using human PK data of TAF, TFV, and TFVd obtained from 4 clinical trials. These clinical trials encompassed diverse drug regimens, including intravenous TFV (1 & 3 mg/kg) administered to HIV-infected adults and oral TAF (5, 10, 25, 40, 120 mg) administered to both HIV-infected adults and healthy seronegative women. PK profiles included measurements of both TAF and TFV plasma concentrations, as well as TFVdp levels in PBMCs. The model integrates mechanisms for the cellular uptake of TAF, the conversion of TAF to TFV, and subsequent metabolism to TFVdp in both PBMCs and hepatocytes. In-vitro studies were incorporated to determine TAF's conversion rates in both hepatocytes and PBMCs. Model verification was conducted using data from 3 additional clinical studies, evaluating predictive performance across diverse populations, including healthy seronegative women, non-cirrhotic adults with chronic hepatitis B, and HIV-infected adults in an African population. Results: The final model integrates an mPBPK model for TAF with a two-compartmental model for TFV, utilizing a permeability-limited structure for TAF tissue distribution. The hybrid mPBPK model effectively characterized the plasma profiles of TAF and TFV, along with the PBMC profiles of TFVdp, demonstrating good agreement between predicted and observed data. Additionally, the model also successfully predicted the validation datasets, aligning 95% of the steady-state concentrations of TFV in an HIV-infected African population.Conclusions: This hybrid mPBPK model represents a valuable tool for predicting the plasma profiles of TAF, TFV, and the PBMC profile of TFVdp. It has the potential to optimize dosing regimens and improve the efficacy and safety of TAF-based therapies for HIV prevention and treatment, such as the development of long-acting formulations.Citations: [1] DESCOVY® [package insert]. Foster City, CA: Gilead Sciences, Inc.; 2015.[2] Deeks SG, Barditch-Crovo P, Lietman PS, et al. Safety, Pharmacokinetics, and Antiretroviral Activity of Intravenous 9-[2-(R)-(Phosphonomethoxy)propyl]adenine, a Novel Anti-Human Immunodeficiency Virus (HIV) Therapy, in HIV-Infected Adults. Antimicrobial Agents and Chemotherapy. 1998;42(9):2380-2384. doi:10.1128/aac.42.9.2380[3] Markowitz M, Zolopa A, Squires K, et al. Phase I/II study of the pharmacokinetics, safety and antiretroviral activity of tenofovir alafenamide, a new prodrug of the HIV reverse transcriptase inhibitor tenofovir, in HIV-infected adults. Journal of antimicrobial chemotherapy. 2014;69(5):1362-1369. doi:10.1093/jac/dkt532[4] Cottrell ML, Garrett KL, Prince HMA, et al. Single-dose pharmacokinetics of tenofovir alafenamide and its active metabolite in the mucosal tissues. Journal of antimicrobial chemotherapy. 2017;72(6):1731-1740. doi:10.1093/jac/dkx064[5] Thurman AR, Schwartz JL, Cottrell ML, et al. Safety and Pharmacokinetics of a Tenofovir Alafenamide Fumarate-Emtricitabine based Oral Antiretroviral Regimen for Prevention of HIV Acquisition in Women: A Randomized Controlled Trial. EClinicalMedicine. 2021;36:100893. doi:10.1016/j.eclinm.2021.100893[6] Birkus G, Kutty N, He G-X, et al. Activation of 9-[(R)-2-[[(S)-[[(S)-1-(Isopropoxycarbonyl)ethyl]amino] phenoxyphosphinyl]-methoxy]propyl]adenine (GS-7340) and Other Tenofovir Phosphonoamidate Prodrugs by Human Proteases. Molecular pharmacology. 2008;74(1):92-100. doi:10.1124/mol.108.045526[7] Ouyang B, Zhou F, Zhen L, et al. Simultaneous determination of tenofovir alafenamide and its active metabolites tenofovir and tenofovir diphosphate in HBV-infected hepatocyte with a sensitive LC–MS/MS method. Journal of pharmaceutical and biomedical analysis. 2017;146:147-153. doi:10.1016/j.jpba.2017.08.028[8] Agarwal K, Fung SK, Nguyen TT, et al. Twenty-eight day safety, antiviral activity, and pharmacokinetics of tenofovir alafenamide for treatment of chronic hepatitis B infection. Journal of hepatology. 2015;62(3):533-540. doi:10.1016/j.jhep.2014.10.035[9] Garrett KL, Chen J, Maas BM, et al. A Pharmacokinetic/Pharmacodynamic Model to Predict Effective HIV Prophylaxis Dosing Strategies for People Who Inject Drugs. The Journal of pharmacology and experimental therapeutics. 2018;367(2):245-251. doi:10.1124/jpet.118.251009[10] Jullien V, Treluyer J-M, Rey E, et al. Population pharmacokinetics of tenofovir in human immunodeficiency virus-infected patients taking highly active antiretroviral therapy (vol 49, pg 3361, 2005). Antimicrobial agents and chemotherapy. 2008;52(2):808-808. doi:10.1128/AAC.01423-07