Bioavailability and metabolism of cidofovir following topical administration to rabbits

The bioavailability and metabolism of the antiviral nucleotide analog cidofovir (HPMPC) were examined in New Zealand white rabbits following topical administration to normal and abraded skin. Male rabbits (four per group) received 14C-cidofovir (100 microCi/kg) intravenously (1 mg/kg) as a solution or topically (2 mg/animal) as a 1% w/w gel containing hydroxyethylcellulose (HEC) with or without propylene glycol (PG). The same PG/HEC formulation was applied topically to an abraded skin site in a fourth group of animals. All radioactivity detected in plasma and skin was accounted for by cidofovir. Plasma concentrations of radioactivity declined multiexponentially following intravenous administration, with a terminal half-life of 5.4 h. For intact skin, the absolute bioavailabilities of the HEC and PG/HEC formulations were 0.2 and 2.1%, respectively. For abraded skin, the bioavailability for the PG/HEC gel was 41%. Radioactivity in kidneys was attributed to cidofovir ( > 95%) and cyclic HPMPC. Concentrations in kidney following topical administration of cidofovir to normal skin were < 4% of those following intravenous dosing. Topical application of cidofovir to intact skin led to negligible systemic exposure to the drug. The topical bioavailability and hence the flux of cidofovir through intact skin was enhanced by the presence of PG in the formulation. Abrasion of the skin removed the principal barrier to absorption and led to significant systemic exposure to cidofovir.