Which antiretroviral drug class acts as a nucleotide reverse transcriptase inhibitor (NtRTI) and requires intracellular activation by adenylate kinase?

Explanation

## Nucleotide Reverse Transcriptase Inhibitors (NtRTIs) **Key Point:** Tenofovir is the only clinically used NtRTI and differs from NRTIs by already containing a phosphonate group, requiring only one phosphorylation step by adenylate kinase for activation. ### Mechanism of Action Tenofovir is a nucleotide analog of adenosine monophosphate (AMP). Unlike nucleoside RTIs (NRTIs) which require three phosphorylation steps, tenofovir requires only one activation step, making it active in non-dividing cells and tissues with low deoxyribonucleoside kinase activity. ### Key Characteristics of Tenofovir | Feature | Tenofovir | NRTIs (e.g., Lamivudine) | |---------|-----------|------------------------| | Structure | Nucleotide (phosphonate) | Nucleoside | | Phosphorylation steps | 1 (adenylate kinase) | 3 (multiple kinases) | | Active in non-dividing cells | Yes | No | | Renal clearance | High (active secretion) | Variable | | Major adverse effect | Nephrotoxicity, bone loss | Mitochondrial toxicity | **High-Yield:** Tenofovir is preferred in first-line ART regimens but requires monitoring of renal function and bone mineral density due to its nephrotoxic and osteotoxic potential. **Clinical Pearl:** Tenofovir alafenamide (TAF) is a newer prodrug formulation with better renal and bone safety profile compared to tenofovir disoproxil fumarate (TDF).