A 58-year-old man presents with acute herpes zoster (shingles) affecting the thoracic dermatome T5 on the right side. He has severe pain, multiple vesicular lesions, and presented 3 days after rash onset. He is immunocompetent with no contraindications. What is the drug of choice for systemic treatment?

Explanation

## First-Line Treatment of Acute Herpes Zoster **Key Point:** Valacyclovir is the preferred first-line agent for acute herpes zoster in immunocompetent adults due to superior CNS penetration, convenient dosing, and proven efficacy in reducing pain duration and post-herpetic neuralgia (PHN). ### Why Valacyclovir for Zoster? **High-Yield:** For herpes zoster specifically: - **Superior CNS penetration** compared to acyclovir (critical for zoster involving cranial nerves or spinal cord) - **Convenient dosing:** 1 g three times daily (vs acyclovir 800 mg 5 times daily) - **Better bioavailability:** Achieves higher serum acyclovir levels - **Proven to reduce PHN duration** when started within 72 hours of rash onset - **Reduced incidence of disseminated zoster** in immunocompetent hosts ### Comparison of Antivirals for Acute Zoster | Drug | Dose (Zoster) | Frequency | CNS Penetration | Bioavailability | Notes | | --- | --- | --- | --- | --- | --- | | **Valacyclovir** | **1 g** | **TID** | **Superior** | **High (54%)** | **First-line** | | Acyclovir | 800 mg | 5 times daily | Moderate | Low (15–30%) | More frequent dosing; older standard | | Famciclovir | 500 mg | TID | Good | High (77%) | Alternative; preferred for CNS involvement | | Penciclovir | Topical | 5 times daily | None (topical) | N/A | Topical only; adjunctive use | **Clinical Pearl:** Famciclovir is an alternative with excellent CNS penetration and may be preferred if: - Severe ophthalmic zoster (V1 involvement) - Ramsay Hunt syndrome (CN VII involvement) - Meningitis or encephalitis suspected However, valacyclovir remains the standard first-line choice due to convenience and guideline recommendations. ### Timeline for Efficacy **Key Point:** Antivirals are most effective when started **within 72 hours of rash onset**. This patient presented at day 3, so he is at the cutoff but still eligible for treatment. **Mnemonic:** **VVV for Zoster** = **V**alacyclovir, **V**iral suppression, **V**esicles (within 72 hours) ### Mechanism & Zoster-Specific Considerations 1. Valacyclovir is metabolized to acyclovir in vivo 2. Acyclovir is phosphorylated by viral thymidine kinase 3. Inhibits viral DNA polymerase and terminates DNA synthesis 4. **For zoster:** Higher doses (1 g TID) and longer duration (7 days) than HSV suppression 5. Reduces acute pain, accelerates lesion healing, and decreases PHN incidence by ~50% **Warning:** Do NOT use topical antivirals alone for systemic zoster — systemic therapy is mandatory to prevent dissemination and PHN. [cite:Harrison 21e Ch 187; Park 26e Ch 23]