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PH8.10 | PH8.10 | Antiviral Drugs Including HIV Therapy — SDL Guide — SDL Guide (Part 3)

Self-Assessment: Antiviral Pharmacotherapy

Apply your antiviral pharmacology knowledge to these clinical scenarios:

Scenario A: A 65-year-old diabetic man develops painful vesicular rash in a dermatomal distribution over the right thoracic region — herpes zoster (shingles). He presents 36 hours after rash onset. He is otherwise well (no immunosuppression). Which drug, dose, and duration are appropriate? What is the goal of treatment?

Discussion: Valaciclovir 1g TDS × 7 days OR aciclovir 800mg 5 times daily × 7 days. Both are effective for herpes zoster. Valaciclovir is preferred for its simpler dosing (3× vs 5× daily). Treatment started within 72 hours of rash onset reduces: severity of acute neuritis, duration of vesicular eruption, and most importantly — risk of post-herpetic neuralgia (PHN — chronic neuropathic pain that can persist for months in the same dermatomal distribution). At 36 hours, treatment is clearly within the 72-hour window. Consider corticosteroids (prednisolone) as adjunct for severe pain in older patients — reduces acute neuritis (no effect on PHN). Varicella-zoster vaccine given before zoster occurrence (Shingrix — recombinant glycoprotein E vaccine) prevents both zoster and PHN — counsel this patient about vaccination for his other (unaffected) eye in the future.

Scenario B: A 35-year-old HIV-positive patient (CD4 450, VL undetectable on TDF+3TC+DTG) develops chronic HCV genotype 3 co-infection. He is otherwise stable. What is the HCV treatment of choice, and what drug interactions should you check?

Discussion: For HCV genotype 3 (common in South/Southeast Asian settings), sofosbuvir+velpatasvir (pan-genotypic) × 12 weeks is the first-line treatment (WHO 2022); cure rates ~95% for genotype 3. Check drug interactions between DAAs and his ART: TDF+3TC+DTG has minimal interactions with sofosbuvir-based regimens — this combination is safe. However, note: if his regimen contained boosted PIs (lopinavir/ritonavir, darunavir/cobicistat) or certain NNRTIs, significant DAA drug interactions can occur (ritonavir/cobicistat boost the NS5A inhibitor, causing elevated DAA levels — which can cause hepatotoxicity in HCV patients with cirrhosis). The patient should have liver fibrosis staged (FibroScan or APRI score) before treatment, as cirrhotic patients with genotype 3 may need ribavirin addition or longer course.

Scenario C: A 26-year-old HIV-negative woman asks about PrEP (pre-exposure prophylaxis) — she has a regular HIV-positive male partner who is not on ART. She is otherwise healthy with normal renal function. What do you prescribe, and what monitoring is required?

Discussion: TDF 300mg + FTC 200mg once daily (oral PrEP — Truvada or generic equivalent) — the WHO-recommended PrEP regimen for HIV-negative individuals at high risk. Onset of protection: full protection for receptive anal intercourse at 7 days; for vaginal intercourse at 21 days. Monitoring: HIV test at baseline (must confirm HIV-negative before starting; PrEP is contraindicated in HIV-infected individuals — would constitute inadequate monotherapy → resistance). 3-monthly HIV testing + STI screening + renal function (TDF can cause renal tubular toxicity — check creatinine and phosphate 3-monthly). She should also counsel her partner to start ART immediately — a virologically-suppressed partner (Undetectable = Untransmittable, U=U) eliminates transmission risk and is the most effective strategy for the couple.