Tenofovir alafenamide (TAF) has replaced tenofovir disoproxil fumarate (TDF) in HIV regimens because TAF achieves higher intracellular drug concentrations with 10-fold lower plasma levels. What accounts for this pharmacokinetic advantage?
- A TAF is a lipophilic phosphonamidate prodrug that is concentrated in lymphoid cells by cathepsin-mediated intracellular hydrolysis to tenofovir-DP, while plasma exposure is low due to first-pass lymphocyte uptake ✓
- B TAF is absorbed by PEPT2 transporter in the small intestine, enabling more efficient absorption than TDF
- C TAF inhibits renal OAT1/OAT3 transporters, reducing tenofovir elimination and increasing tissue concentrations
- D TAF has higher plasma protein binding than TDF, preventing renal filtration and increasing tissue retention
Correct answer: A. TAF is a lipophilic phosphonamidate prodrug that is concentrated in lymphoid cells by cathepsin-mediated intracellular hydrolysis to tenofovir-DP, while plasma exposure is low due to first-pass lymphocyte uptake
Explanation
TAF is a prodrug with a phosphonamidate moiety that confers greater lipophilicity than TDF. After oral absorption, TAF is concentrated in lymphoid cells/macrophages (key HIV reservoirs) where intracellular cathepsin A hydrolyses it to tenofovir; this is then phosphorylated to the active tenofovir diphosphate (TFV-DP). Because most TAF is activated intracellularly before significant systemic exposure, plasma levels are approximately 10 times lower than TDF — substantially reducing nephrotoxicity and bone toxicity while maintaining efficacy.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.