Sebbag L, Tomasy SM, Woodward AP, et al. Pharmacokinetic modeling of penciclovir and BRL42359 in the plasma and tears of healthy cats to optimize dosage recommendations for oral administration of famciclovir. Am J Vet Res. 2016 Aug;77(8):833-45.
Medication dosages and concentrations when used in cats are often translated from use in other species, usually humans, dogs, rats and mice. Drugs are frequently not FDA approved for use in cats and they are considered off label when prescribed. Most cat owners would likely be surprised how many medications used to treat cats are off label. Drug company investment into acquiring FDA approval of drugs in cats is simply not cost effective. Therefore, determination of safety and efficacy of off-label use of drugs in cats is often reliant on studies such as this study by Sebbag at University of California at Davis and collaborators.
Famciclovir is a potent antiviral drug that is effective against many herpesviruses, including feline herpes virus 1 (FHV-1). FHV-1 is perhaps the most common cause of upper respiratory infections in cats. In most species, famciclovir is converted in vivo to penciclovir, the active antiviral metabolite. However, cats are obligate carnivores and have unique and often deficient metabolic pathways regarding drug absorption and metabolic conversion. In cats, famciclovir is variably absorbed and it is metabolized first to BRL42359 then to the active metabolite penciclovir. Penciclovir becomes saturated at increasing famciclovir doses and pharmokinetics is more complex than in other species.
Researchers performed a cross over study in 6 healthy male cats in which each of 3 doses of famciclovir (30, 40, or 90 mg/kg) was administered every 8 or 12 hours for 3 days. Plasma and tear samples were obtained at predetermined times after famciclovir administration. Pharmacokinetic parameters were then determined for BRL42359 and penciclovir by compartmental and noncompartmental methods. Conversion of BRL42359 to penciclovir was found to be a rate-limiting.
In conclusion, in order to acquire therapeutic levels of panciclovir in serum and tears, the researchers recommend dosage and dosage frequency of famciclovir 90 mg/kg every 12 hours for cats infected with feline herpes virus. (GO)
Thomasy SM, Covert SC, et al. Pharmacokinetics of famciclovir and penciclovir in tears following oral administration of famciclovir to cats: a pilot study. Vet Ophthalmol. 2012 Sep;15(5):299-306.