## CMV Antiviral Agents and Resistance Mechanisms ### Correct Statements (Options 0, 1, 3) **Option 0 — Ganciclovir activation:** - Ganciclovir is phosphorylated by **CMV-encoded protein kinase (UL97)** in CMV-infected cells - This viral kinase phosphorylates ganciclovir to its monophosphate form; cellular kinases then complete activation to the triphosphate - This selectivity for CMV-infected cells (which express UL97) reduces toxicity to uninfected cells - Mutations in UL97 gene lead to ganciclovir resistance **Option 1 — Cidofovir mechanism and resistance:** - Cidofovir is a **cytidine nucleotide analogue** that does NOT require viral enzyme activation - It is phosphorylated by cellular kinases alone (not dependent on UL97) - Therefore, cidofovir remains effective against ganciclovir-resistant CMV strains with UL97 mutations or DNA polymerase mutations - Used as salvage therapy for resistant CMV, particularly in immunocompromised patients **Option 3 — Valganciclovir:** - Valganciclovir is the L-valine ester prodrug of ganciclovir - Has **10 times higher oral bioavailability** than oral ganciclovir (60% vs 6%) - Rapidly hydrolyzed to ganciclovir in the intestinal wall and liver - Can be used for oral maintenance therapy of CMV retinitis in stable, compliant patients with adequate CD4 count recovery (>100 cells/µL) - Reduces need for IV access and improves quality of life ### Incorrect Statement (Option 2 — THE ANSWER) **Foscarnet CNS penetration — the trap:** - While foscarnet does penetrate the CNS, it does **NOT have superior CNS penetration compared to ganciclovir** - Ganciclovir achieves **10–50% of serum levels in CSF**, which is actually reasonable for CNS infections - Foscarnet achieves **5–10% of serum levels in CSF** — actually LOWER than ganciclovir - For CMV encephalitis or ventriculoencephalitis, **ganciclovir is preferred** over foscarnet for CNS penetration - Foscarnet is reserved for ganciclovir-resistant CMV (UL97 or DNA polymerase mutations) or when renal function is preserved - The statement falsely claims foscarnet has superior CNS penetration — this is incorrect ### High-Yield Comparison Table | Agent | Activation | Resistance Mechanism | CNS Penetration | Primary Use | | --- | --- | --- | --- | --- | | Ganciclovir | CMV UL97 kinase | UL97 mutations, DNA pol mutations | 10–50% of serum | CMV retinitis, colitis, esophagitis | | Valganciclovir | CMV UL97 (after hydrolysis) | UL97 mutations, DNA pol mutations | Similar to IV GCV | Oral maintenance therapy (CD4 >100) | | Cidofovir | Cellular kinases only | **Not dependent on UL97** | Moderate | Ganciclovir-resistant CMV | | Foscarnet | None (direct action) | Not dependent on UL97 | **5–10% of serum** | Ganciclovir-resistant CMV, acyclovir-resistant HSV | **Key Point:** - Ganciclovir has BETTER CNS penetration than foscarnet - For CMV encephalitis or ventriculoencephalitis, ganciclovir is the preferred agent - Foscarnet is used when ganciclovir resistance develops or renal function is severely compromised **Clinical Pearl:** - In a patient with both CMV retinitis and HSV encephalitis, the choice between ganciclovir and foscarnet should be based on: - Ganciclovir resistance status (if UL97 mutant, use cidofovir or foscarnet) - Renal function (foscarnet is nephrotoxic) - CNS penetration (ganciclovir is superior) - The statement's claim that "foscarnet is preferred because of superior CNS penetration" is a classic NEET PG trap **Mnemonic:** - **GCV = Good CNS penetration** (Ganciclovir) - **FOS = Foscarnet Often Spares UL97** (doesn't need it for activation) **High-Yield:** - Ganciclovir-resistant CMV is usually due to mutations in UL97 (protein kinase) or viral DNA polymerase - Cidofovir and foscarnet bypass the need for UL97, making them effective salvage agents - Valganciclovir allows oral maintenance therapy, improving adherence and quality of life in stable patients
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