FAMVIR
Clinical safety rating: caution
Comprehensive clinical and safety monograph for FAMVIR (FAMVIR).
Famciclovir is a prodrug that is rapidly converted to penciclovir, which inhibits viral DNA polymerase by competing with deoxyguanosine triphosphate, thereby inhibiting viral DNA synthesis and replication.
| Metabolism | Rapidly deacetylated and oxidized to the active metabolite penciclovir; primarily metabolized by aldehyde oxidase; penciclovir is phosphorylated by viral thymidine kinase. |
| Excretion | Renal: 60–70% as penciclovir via tubular secretion and glomerular filtration; fecal: <10%; biliary: <1%. |
| Half-life | Terminal elimination half-life of penciclovir is approximately 2–3 hours in patients with normal renal function; extends to 9–18 hours in severe renal impairment (CrCl <30 mL/min). |
| Protein binding | <20% bound to plasma proteins (primarily albumin). |
| Volume of Distribution | Approximately 1.5 L/kg, indicating extensive distribution into tissues, including skin and mucous membranes. |
| Bioavailability | Oral bioavailability of famciclovir as penciclovir is 77% (range 70–85%). |
| Onset of Action | Oral: clinical effect on viral replication within 1–2 hours of administration. |
| Duration of Action | Duration of antiviral effect correlates with intracellular penciclovir triphosphate half-life (7–20 hours in herpes-infected cells); dosing interval is 8 hours for normal renal function. |
250 mg orally three times daily for 7 days for herpes zoster; 125 mg orally twice daily for 5 days for recurrent genital herpes; 250 mg orally twice daily for 7 days for first-episode genital herpes; 500 mg orally twice daily for 7 days for herpes zoster in immunocompromised patients; 500 mg orally twice daily for 7 days for recurrent mucocutaneous herpes in HIV patients.
| Dosage form | TABLET |
| Renal impairment | For CrCl >= 60 mL/min: standard dosing; CrCl 40-59: 250 mg q12h (zoster) or 125 mg q12h (genital); CrCl 20-39: 250 mg q24h (zoster) or 125 mg q24h (genital); CrCl < 20: 250 mg q48h (zoster) or 125 mg q48h (genital); hemodialysis: same as CrCl < 20, given after dialysis on dialysis days. |
| Liver impairment | Mild to moderate hepatic impairment (Child-Pugh A or B): no adjustment required. Severe hepatic impairment (Child-Pugh C): No specific studies; use with caution, potential for decreased clearance. |
| Pediatric use | Not approved for pediatric use. Safety and efficacy not established; no standard weight-based dosing available. |
| Geriatric use | Elderly patients may have reduced renal function; initiate dosing based on CrCl as per renal adjustment. Monitor renal function regularly. No specific dose adjustment beyond renal-based modification. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for FAMVIR (FAMVIR).
| Breastfeeding | Excreted in human milk; M/P ratio not reported. Use with caution in nursing mothers. Consider risk-benefit as systemic exposure in infants is likely low. |
| Teratogenic Risk | Pregnancy Category B. No evidence of teratogenicity in animal studies. Insufficient human data; use only if clearly needed. First trimester: limited data, no increased malformation risk. Second/third trimester: no known fetal risks. |
| Fetal Monitoring |
■ FDA Black Box Warning
None
| Serious Effects |
["Hypersensitivity to famciclovir, penciclovir, or any component of the formulation"]
| Precautions | ["Acute renal failure due to precipitation in renal tubules, especially in elderly or dehydrated patients; dose adjustment required in renal impairment","Thrombotic thrombocytopenic purpura/hemolytic uremic syndrome (TTP/HUS) reported in immunocompromised patients","Potential for seizures, hallucinations, or confusion, particularly with high doses or renal impairment"] |
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| Monitor renal function (creatinine clearance) in pregnancy due to altered pharmacokinetics. Assess liver function if prolonged therapy. Observe for fetal growth abnormalities via ultrasound if used long-term. |
| Fertility Effects | No evidence of impaired fertility in animal studies. No human data on reproductive function. Theoretical risk of reversible spermatogenesis suppression with high doses. |