A patient with HIV is started on tenofovir alafenamide (TAF) rather than tenofovir disoproxil fumarate (TDF). The switch is made primarily because TAF:

• A Is less potent and requires a lower dose because it is a nucleoside (not nucleotide) reverse transcriptase inhibitor
• B Is a prodrug that delivers higher intracellular tenofovir diphosphate concentrations in lymphocytes at a 10-fold lower plasma dose, reducing renal tubular and bone toxicity ✓
• C Does not require intracellular phosphorylation and directly inhibits HIV reverse transcriptase as an active compound
• D Is eliminated entirely by hepatic glucuronidation without renal clearance, making it safe in advanced CKD without dose adjustment

Explanation

TAF is a phosphonamidate prodrug of tenofovir designed for improved lymphocyte targeting. It is hydrolysed intracellularly to tenofovir and then to the active tenofovir diphosphate (TFV-DP) at concentrations 4-5 fold higher in PBMCs than with TDF, while plasma tenofovir levels are 90% lower. This reduced systemic exposure dramatically decreases renal proximal tubule toxicity (Fanconi syndrome, CKD) and bone mineral density loss compared to TDF. TAF still requires intracellular phosphorylation; it is not administered without dose adjustment in ESRD (eGFR <15).

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.