Tenofovir alafenamide (TAF) has replaced tenofovir disoproxil fumarate (TDF) in HIV regimens primarily because:

• A TAF achieves higher plasma drug levels allowing once-weekly dosing
• B TAF inhibits both HIV-1 and HIV-2 reverse transcriptase while TDF is selective for HIV-1
• C TAF bypasses the need for host kinase activation, directly inhibiting RT as the active compound
• D TAF is a phosphonamidate prodrug preferentially activated inside lymphocytes, achieving higher intracellular concentrations at a 10-fold lower plasma dose, reducing renal and bone toxicity ✓

Explanation

TAF is a phosphonamidate prodrug of tenofovir that is stable in plasma and cleaved preferentially by cathepsin A inside lymphocytes (target cells), generating high intracellular tenofovir diphosphate (the active moiety). This results in 4–5x higher intracellular concentrations and 90% lower plasma tenofovir levels compared to TDF. Lower systemic tenofovir exposure reduces the tubular nephrotoxicity and bone mineral density loss associated with TDF. The dose of TAF is 25 mg vs TDF 300 mg daily. Both require intracellular phosphorylation to the diphosphate active form.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.