Bictegravir/emtricitabine/tenofovir alafenamide (BIC/FTC/TAF) is the preferred first-line ART regimen. Tenofovir alafenamide (TAF) has improved renal and bone safety compared to tenofovir disoproxil fumarate (TDF) because:

• A TAF is converted to tenofovir exclusively in T-lymphocytes by CD4-associated enzymes, preventing proximal tubule or osteoblast exposure
• B TAF binds to P-glycoprotein in renal tubule cells acting as a competitive inhibitor protecting renal mitochondria from tenofovir accumulation
• C TAF has a different nucleotide backbone (adenosine rather than adenine) reducing interaction with renal organic anion transporters OAT1/OAT3
• D TAF is a phosphonoamidate prodrug that is concentrated intracellularly via plasma protein binding, delivering higher intracellular tenofovir-DP to lymphocytes at 10-fold lower plasma tenofovir concentrations than TDF ✓

Explanation

TAF is a prodrug of tenofovir that is more stable in plasma due to plasma protein binding and phosphonoamidate masking of the phosphonate charge. It is taken up by lymphocytes (via a different mechanism than TDF's gut/plasma hydrolysis route) and converted intracellularly by cathepsin A to tenofovir monophosphate, then to active tenofovir diphosphate (TFV-DP). Because most TAF conversion occurs intracellularly within target cells, plasma tenofovir concentrations are approximately 90% lower than with TDF at equivalent antiviral efficacy. This reduced systemic tenofovir exposure substantially decreases uptake by renal proximal tubule cells (via OAT1/OAT3) and osteoblasts/osteoclasts, reducing nephrotoxicity and bone mineral density loss.

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

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Written and medically reviewed by the StethoPrep medical team.