Famciclovir for veterinary use

Basic Information

Famciclovir is a prodrug that is used to treat herpes virus infections in humans and cats. Famciclovir is converted into penciclovir through a two-step process in the blood and in the liver. Penciclovir inhibits herpes virus DNA synthesis. The half-life of penciclovir in a herpes-infected cell is considerably longer than the half-life in plasma. Famciclovir/penciclovir is virostatic, not virucidal.

Oral famciclovir is well-absorbed by humans. The absorption and blood levels are much more variable in the cat. This is thought to be due to either poor intestinal absorption or a lack of the appropriate hepatic enzyme for the conversion of famciclovir to penciclovir.

Cats

Despite the complex pharmacokinetics of famciclovir/penciclovir, recent studies have demonstrated that the oral administration of famciclovir "improved outcomes for systemic, ophthalmic, clinicopathologic, virologic, and histologic variables in cats experimentally infected with FHV-1." (Thomasy et al. 2011) Earlier work by Malik et al. in 2009 showed improvement in ocular, sinus, and dermatologic manifestations in cats with clinical FHV-1 that were treated with famciclovir.

Side Effects

• Famciclovir appears to be well-tolerated in cats. Side effects in humans are mostly related to the GI tract.

Precautions

• Famciclovir should be used with caution and at a reduced dose in patients with poor renal function.
• Work in laboratory animals has shown that famciclovir is excreted in maternal milk. It is not known if this is of clinical significance in the cat.

Drug Interactions

• Concurrent use of famciclovir and probenecid may increase blood levels of penciclovir.

Overdose

• Overdose with penciclover can be treated with hemodialysis.