HERPLEX
Clinical safety rating: caution
Comprehensive clinical and safety monograph for HERPLEX (HERPLEX).
Inhibits viral DNA polymerase after phosphorylation to acyclovir triphosphate, leading to chain termination and inhibition of herpes simplex virus replication.
| Metabolism | Renal excretion of unchanged drug and metabolite (9-carboxymethoxymethylguanine) via glomerular filtration and tubular secretion. Minimal hepatic metabolism. |
| Excretion | Renal: ~90% as unchanged drug via glomerular filtration and tubular secretion; minor biliary/fecal elimination (<2%) |
| Half-life | 2.5–3.3 hours in adults with normal renal function; prolonged to 10–20 hours in anuria (CrCl <10 mL/min); requires dose adjustment in renal impairment |
| Protein binding | 9–33% (concentration-dependent; primarily albumin) |
| Volume of Distribution | 0.8–1.6 L/kg (extensive distribution into body water; penetrates most tissues including brain, skin, and vesicles) |
| Bioavailability | Oral: 15–30% (200 mg dose; decreases with higher doses due to saturable absorption); topical: negligible systemic absorption |
| Onset of Action | IV: immediate (antiviral effect within 1 hour); oral: 1–2 hours (peak plasma concentration at 1.5–2 hours for 200 mg; topical: minimal systemic absorption, local effect onset within hours |
| Duration of Action | Dosing interval typically 8 hours due to short half-life; requires multiple daily doses (e.g., 5 times/day for HSV, 3 times/day for VZV); IV infusion over 1 hour maintains therapeutic levels for 8 hours |
| Molecular Weight | 225.21 |
Acyclovir 200 mg orally 5 times daily for 10 days for initial genital herpes; 400 mg orally twice daily for suppressive therapy; 5-10 mg/kg IV every 8 hours for severe infections.
| Dosage form | SOLUTION/DROPS |
| Renal impairment | CrCl 25-50 mL/min: 200 mg every 4 hours (5 times daily) orally; CrCl 10-24 mL/min: 200 mg every 8 hours orally; CrCl <10 mL/min: 200 mg every 12 hours orally. IV: CrCl 25-50 mL/min: 5-10 mg/kg every 12 hours; CrCl 10-24 mL/min: 5-10 mg/kg every 24 hours; CrCl <10 mL/min: 2.5-5 mg/kg every 24 hours. |
| Liver impairment | No dosage adjustment required for mild to moderate hepatic impairment. Severe hepatic impairment (Child-Pugh C): consider dose reduction or alternative therapy; specific guidelines not established. |
| Pediatric use | Neonates: 20 mg/kg IV every 8 hours for 14-21 days. Children (1-12 years): 20 mg/kg (max 800 mg) orally 4 times daily for varicella; 10-20 mg/kg IV every 8 hours for severe herpes. Adolescents: same as adult dosing. |
| Geriatric use | Dose adjustment based on renal function; use lower end of dosing range due to decreased renal clearance; monitor for neurotoxicity (confusion, hallucinations) at higher doses. |
| 1st trimester | Acyclovir is generally avoided in the first trimester unless benefit outweighs risk. Data suggests no increased risk of major birth defects, but caution is advised. |
| 2nd trimester | Acyclovir use is considered relatively safe. Human studies show no evidence of fetal harm, but use only if clearly needed. |
| 3rd trimester | Acyclovir is considered safe in late pregnancy, especially for maternal herpes infections near term to reduce neonatal transmission. |
Clinical note
Comprehensive clinical and safety monograph for HERPLEX (HERPLEX).
| Placental transfer | Acyclovir crosses the placenta. Concentrations in cord blood are about 1-2 times maternal plasma levels after oral dosing. |
| Breastfeeding | Acyclovir is excreted into breast milk in small amounts. After oral administration, the infant dose is approximately 0.3-0.6 mg/kg/day, which is less than the neonatal therapeutic dose. However, caution is recommended, especially with high-dose intravenous therapy. |
■ FDA Black Box Warning
No FDA black box warning.
| Serious Effects |
Hypersensitivity to acyclovir or valacyclovirHypersensitivity to any component of the formulation
| Precautions | Renal impairment: dose adjustment required. Hemolytic uremic syndrome/thrombotic thrombocytopenic purpura (HUS/TTP) reported in immunocompromised patients. CNS adverse reactions including agitation, hallucinations, seizures, and coma, especially in elderly or renally impaired. Use caution with nephrotoxic drugs. Adequate hydration recommended. |
| Food/Dietary | No known food interactions. Maintain adequate hydration. No specific dietary restrictions. |
| Clinical Pearls |
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| Lactation Rating | L2 (Safer) |
| Teratogenic Risk | Pregnancy Category B. First trimester: No evidence of increased malformations in human studies, but animal studies show no fetal harm. Second/Third trimester: No known risks; use only if clearly needed. |
| Fetal Monitoring | Monitor renal function (BUN, creatinine) and liver enzymes periodically; observe for neurological effects (confusion, hallucinations). |
| Fertility Effects | No adverse effects on fertility reported in human or animal studies. |
| Apply with a gloved finger to avoid autoinoculation. Start within 72 hours of symptom onset for maximum efficacy. Do not exceed 5 applications daily. Avoid use in immunocompromised patients or those with severe herpes infections. |
| Patient Advice | Wash hands before and after application. · Apply thin layer to affected area only, not on healthy skin. · Use for 5 days; if no improvement, consult healthcare provider. · Avoid contact with eyes, mouth, or open wounds. · Do not share towel or makeup to prevent spreading. · May cause temporary stinging or burning sensation. |