Acyclovir

# Acyclovir ## Overview - **Classification**: Antiviral, synthetic purine nucleoside analogue. - **Mechanism**: Selectively inhibits viral DNA replication by interfering with viral DNA polymerase and terminating viral DNA chain elongation. Requires viral thymidine kinase for activation. ## Primary Indications 1. **Herpes Simplex Virus (HSV)** - Treatment and suppression of genital herpes, mucocutaneous infections (e.g., cold sores). 2. **Varicella-Zoster Virus (VZV)** - Treatment of herpes zoster (shingles) and varicella (chickenpox). ## Adult Dosing ### Standard Oral Dosing **Genital Herpes (Initial Episode)** - **Dose**: **200 mg** - **Frequency**: 5 times/day (q4h while awake) - **Route**: Oral - **Duration**: 7-10 days **Genital Herpes (Recurrent Episode)** - **Dose**: **200 mg** - **Frequency**: 5 times/day (q4h while awake) - **Route**: Oral - **Duration**: 5 days - **Alternative**: **400 mg** PO TID for 5 days OR **800 mg** PO BID for 5 days. **Genital Herpes (Chronic Suppression)** - **Dose**: **400 mg** - **Frequency**: BID - **Route**: Oral - **Duration**: Up to 12 months, then re-evaluate. **Herpes Zoster (Shingles)** - **Dose**: **800 mg** - **Frequency**: 5 times/day - **Route**: Oral - **Duration**: 7-10 days - **Special Considerations**: Initiate within **72 hours** of rash onset for optimal efficacy. **Varicella (Chickenpox)** - **Dose**: **800 mg** - **Frequency**: QID - **Route**: Oral - **Duration**: 5 days - **Special Considerations**: Initiate within **24 hours** of rash onset for optimal efficacy. ### Dose Adjustments - **Renal Impairment**: Adjust frequency based on creatinine clearance (CrCl). - CrCl 25-50 mL/min: Dose q8h for 200 mg, q12h for 400 mg, q8h for 800 mg. - CrCl 10-25 mL/min: Dose q12h for 200 mg, q24h for 400 mg, q12h for 800 mg. - CrCl <10 mL/min: Dose q24h for 200 mg, q24h for 400 mg, q24h for 800 mg. - Hemodialysis: Administer dose after dialysis. - **Hepatic Impairment**: No specific dose adjustment required. - **Elderly Patients**: Monitor renal function closely; increased risk of CNS adverse effects. ## Pediatric Dosing (Oral) ### Neonates (0-28 days) - **Primary Treatment**: IV acyclovir is preferred for active HSV infection. - **HSV Suppression (after IV treatment)**: **300 mg/m²** PO BID-TID. - **Duration**: Typically 6-12 months as directed by specialist. - **Special Notes**: Dosing is highly specialized; consult with infectious disease. ### Infants (1-12 months) - **Varicella (Chickenpox)**: **20 mg/kg** PO QID. - **Maximum**: **800 mg** per dose. - **Duration**: 5 days. - **Special Notes**: Initiate within **24 hours** of rash onset. ### Children (1-12 years) - **Varicella (Chickenpox)**: **20 mg/kg** PO QID. - **Maximum**: **800 mg** per dose. - **Duration**: 5 days. - **Special Notes**: Initiate within **24 hours** of rash onset. - **Oral HSV (Mucocutaneous)**: **20 mg/kg** PO TID-QID. - **Maximum**: **400 mg** per dose (TID-QID) or **1000 mg** daily. - **Duration**: 5-10 days. ### Adolescents (13-18 years) - **Dose**: Generally follows **adult oral dosing** for specific indications. - **Maximum**: Refer to adult maximum doses for each indication. - **Special Notes**: Adjust for renal function if impaired. ## Safety Information ### Contraindications - **Absolute**: Hypersensitivity to acyclovir or valacyclovir. ### Common Adverse Effects - **Common (1-10%)**: Nausea, vomiting, diarrhea, headache, malaise. - **Serious but Rare**: - Acute renal failure (with rapid IV infusion, dehydration, or concurrent nephrotoxins). - Neurotoxicity (confusion, hallucinations, seizures, tremors, coma), especially in elderly or renally impaired. - Thrombotic Thrombocytopenic Purpura/Hemolytic Uremic Syndrome (TTP/HUS) in immunocompromised. ### Key Drug Interactions - **Nephrotoxic drugs (e.g., NSAIDs, aminoglycosides, cyclosporine)**: Increased risk of renal dysfunction. Monitor renal function closely. - **Probenecid, Cimetidine**: Increase acyclovir AUC and half-life. May require acyclovir dose reduction. - **Mycophenolate Mofetil**: Acyclovir can increase mycophenolate levels. Monitor for mycophenolate toxicity. ## Monitoring & Follow-up - **Before Treatment**: Assess baseline renal function (CrCl). - **During Treatment**: - Monitor renal function periodically, especially in patients with renal impairment, elderly, or on nephrotoxic drugs. - Monitor hydration status (crucial with IV acyclovir). - Observe for signs of CNS toxicity (confusion, lethargy, seizures). - **Clinical Signs**: Assess for reduction in lesion count, pain, and progression of viral symptoms. ## Clinical Pearls - 💡 **Early Initiation**: Acyclovir is most effective when initiated as early as possible after symptom onset (within 24h for varicella, 72h for zoster). - 💡 **Hydration**: Advise patients to maintain adequate hydration, especially when taking high doses, to minimize the risk of renal effects. - 💡 **Not a Cure**: Acyclovir helps manage symptoms and reduce the frequency/severity of herpes outbreaks but does not cure the infection. - 💡 **Formulations**: Available as oral tablets, capsules, and suspension; IV formulation for severe infections. > **⚠️ Important**: This information is for educational purposes only. Always consult current prescribing information, local guidelines, and clinical judgment before prescribing.