FAMCICLOVIR
Clinical safety rating: safe
Animal studies have demonstrated safety
Famciclovir is a prodrug of penciclovir, which inhibits viral DNA polymerase by competing with deoxyguanosine triphosphate, thus inhibiting viral DNA replication. It has activity against herpes simplex virus (HSV-1, HSV-2), varicella-zoster virus (VZV), and Epstein-Barr virus (EBV).
| Metabolism | Famciclovir is rapidly metabolized to penciclovir by deacetylation and oxidation via aldehyde oxidase. Penciclovir is phosphorylated by viral thymidine kinase to its active triphosphate form. Minimal hepatic CYP450 metabolism. |
| Excretion | Renal elimination: ~60% as penciclovir (active metabolite) and <10% as unchanged famciclovir; biliary/fecal: <5%; the remainder is metabolized to inactive compounds. |
| Half-life | Terminal half-life of penciclovir is 2-3 hours in healthy adults, prolonged to 3-6 hours in hepatic impairment and >20 hours in severe renal impairment (CrCl <30 mL/min), requiring dose adjustment. |
| Protein binding | Penciclovir is <20% bound to plasma proteins (primarily albumin). |
| Volume of Distribution | Apparent Vd of penciclovir is 1.08 L/kg (75 L in a 70 kg adult), indicating extensive distribution into total body water and tissues, including skin and mucous membranes. |
| Bioavailability | Oral bioavailability of penciclovir from famciclovir is 77% (range 70-84%) in healthy adults; food does not significantly affect absorption. |
| Onset of Action | Oral: symptom relief in acute herpes zoster within 24-48 hours; for genital herpes, lesion healing begins within 24-72 hours; time to peak penciclovir concentration is 0.9 hours post-dose. |
| Duration of Action | Intracellular penciclovir triphosphate has a half-life of 7-10 hours in HSV-infected cells and up to 20 hours in VZV-infected cells, supporting twice-daily dosing; clinical effect persists for the duration of treatment (7 days for zoster, 5 days for genital herpes). |
500 mg orally three times daily for 7 days for herpes zoster; 125 mg twice daily for 5 days for recurrent genital herpes; 250 mg three times daily for 7 days for first-episode genital herpes; 500 mg twice daily for 7 days for recurrent herpes labialis.
| Dosage form | TABLET |
| Renal impairment | CrCl >=60 mL/min: no adjustment; CrCl 40-59: same dose, interval every 12h; CrCl 20-39: 50% of dose, interval every 24h; CrCl <20: 50% of dose, interval every 48h. |
| Liver impairment | No specific Child-Pugh based adjustments recommended; caution in severe hepatic impairment. |
| Pediatric use | Not approved for pediatric use; safety and efficacy not established. |
| Geriatric use | Dose adjustment based on renal function; monitor for neuropsychiatric effects. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Probenecid may decrease renal excretion of penciclovir Dosage adjustment is required in renal impairment.
| Breastfeeding | Excreted in breast milk; M/P ratio unknown. Use with caution; monitor infant for adverse effects. |
| Teratogenic Risk | Pregnancy Category B. Animal studies show no fetal risk, but no adequate human studies. Avoid in first trimester unless benefit outweighs risk; limited data on second/third trimester. |
| Fetal Monitoring | Monitor maternal renal function; in neonate, monitor for neutropenia and thrombocytopenia if exposed in utero. |
■ FDA Black Box Warning
No FDA black box warning.
| Common Effects | herpes simplex |
| Serious Effects |
["Hypersensitivity to famciclovir or penciclovir"]
| Precautions | ["Acute renal failure; dose adjustment required for reduced creatinine clearance","Thrombotic thrombocytopenic purpura/hemolytic uremic syndrome (TTP/HUS) reported in immunocompromised patients","Cutaneous reactions including Stevens-Johnson syndrome","Use caution in elderly patients due to age-related renal decline"] |
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| Fertility Effects | No known significant effects on fertility based on animal studies; human data lacking. |