Ocular tolerance and tear film pharmacokinetics of 1 % penciclovir cream in cats

Feline herpesvirus type 1 (FHV-1) is a leading cause of ocular surface disease in cats, often requiring prolonged antiviral therapy. While oral famciclovir is effective, its use is limited by poor bioavailability and compliance challenges. This study evaluated the ocular tolerance and tear film pharmacokinetics of topical 1 % penciclovir cream (Fenlips®), an FDA-approved dermatologic formulation repurposed for ophthalmic use in cats. A retrospective survey of owners whose cats were treated with Fenlips® revealed high levels of satisfaction, with 85 % reporting no adverse effects and 80 % rating the treatment as effective or highly effective. To further assess safety and pharmacokinetics, two prospective studies were conducted in seven healthy domestic cats. In the first, Fenlips® was applied to one eye and artificial tears (DuraTears®) to the other eye for 10 days. No significant differences were observed between eyes in clinical scores, tear production, or tear film stability. Corneal sensitivity increased significantly in Fenlips®-treated eyes. In the second study, tear fluid samples collected over 24 h revealed that penciclovir concentrations remained above the minimal inhibitory concentration (0.30 μg/ml) for FHV-1 for over 8 h on average, with a terminal half-life of 8.75 h. These findings indicate that Fenlips® is well tolerated and achieves pharmacologically relevant concentrations in the tear film, supporting its potential use as a twice-daily topical treatment for herpetic keratoconjunctivitis in cats.