Active ingredients: Penciclovir

Pharmacodynamic properties

Pharmacotherapeutic group: Topical antiviral agent
ATC code: D06BB06

Mechanism of action

Penciclovir has demonstrated in vivo and in vitro activity against herpes simplex viruses (types 1 and 2) and varicella zoster virus. In virus-infected cells penciclovir is rapidly and efficiently converted into a triphosphate (mediated via virus-induced thymidine kinase).Penciclovir triphosphate persists in infected cells for more than 12 hours where it inhibits replication of viral DNA and has a half-life of 9, 10 and 20 hours in cells infected with varicella zoster virus, herpes simplex virus type 1 and herpes simplex virus type 2 respectively. In uninfected cells treated with penciclovir, concentrations of penciclovir triphosphate are only barely detectable. Accordingly, uninfected cells are unlikely to be affected by therapeutic concentrations of penciclovir.

Clinical efficacy and safety

In clinical studies, Fenistil treated patients healed 30% faster than placebo (up to one day earlier), pain resolution was 25-30% faster (median improvement of up to one day) and infectivity resolved up to 40% faster (one day earlier) than placebo.

Pharmacokinetic properties

General characteristics

Following application of Fenistil Cold Sore Cream in a human volunteer study at a daily dose of 180mg penciclovir (approximately 67 times the proposed daily clinical dose), to occluded and abraded skin for 4 days, penciclovir was not quantifiable in plasma and urine.

Preclinical safety data

General toxicology

Topical application of 5% Fenistil Cold Sore Cream for 4 weeks to rats and rabbits was well tolerated. There was no evidence of contact sensitisation in guinea pigs.

A full programme of studies has been completed using intravenous penciclovir. These studies did not raise any safety concerns regarding topical use of Fenistil Cold Sore Cream. There is a minimal systemic absorption of penciclovir following topical administration.

Genotoxicity and Reproductive toxicity

Animal studies have not shown any embryotoxic or teratogenic effects with penciclovir given intravenously (at doses greater than 1200 times those recommended for clinical use via topical application), nor were there any effects on male and female fertility and general reproductive performance (at doses greater than 1600 times those recommended for clinical use via topical application). Studies in rats show that penciclovir is excreted in the breast milk of lactating females given oral famciclovir (famciclovir; the oral form of penciclovir, is converted in vivo to penciclovir).

The results of a wide range of mutagenicity studies in vitro and in vivo indicates that penciclovir does not pose a genotoxic risk to man.