IN-VIVO BIOAVAILABILITY AND METABOLISM OF TOPICAL DICLOFENAC LOTION IN HUMAN VOLUNTEERS

Purpose. The primary objective of this study was to determine the rate and extent of transdermal absorption for systemic delivery of diclofe nac from Pennsaid (Dimethaid Research, Inc.) topical lotion into the s ystemic circulation after the lotion was applied to human volunteers, in an open treatment, non-blinded, non-vehicle controlled study. In ad dition, the in vivo metabolism of this topical diclofenac lotion has a lso been studied. Methods. Human volunteers were dosed with topical [C -14]-diclofenac sodium 1.5% lotion on the knee for 24 h. Sequential ti me blood and urine samples were taken to determine pharmacokinetics, b ioavailability and metabolism. Results. Topical absorption was 6.6% of applied dose. Peak plasma C-14 occurred at 30 h after dosing, and pea k urinary C-14 excretion was at 24-48 h. The urinary C-14 excretion pa ttern exhibits more elimination towards 24 h and beyond, as opposed to early urinary C-14 excretion. This suggests a continuous delivery of [C-14]-diclofenac sodium from the lotion into and through skin which o nly ceased when the dosing site was washed. Skin surface residue at 24 h was 26 +/-: 9.5% dose (remainder assumed lost to clothing and beddi ng). Extraction of metabolites from urine amounted to 7.4-22.7% in unt reated urine, suggesting substantial diclofenac metabolism to more wat er soluble metabolites, probably conjugates, which could not be extrac ted by the method employed. Two Dimensional TLC analysis of untreated urine showed minimal or no diclofenac, again emphasizing the extensive in vivo metabolism of this drug. Treatment of the same urine samples with the enzymes sulfatase and beta-glucuronidase showed a substantial increase in the extractable material. Three spots were consistently p resent in each sample run, namely diclofenac, 3'hydroxy diclofenac and an intermediate polar metabolite (probably a hydroxylated metabolite) . Therefore, there was significant sulfation and glucuronidation of bo th diclofenac and numerous hydroxy metabolites of diclofenac, but many of the metabolites/conjugates remain unidentified. Conclusions. There was a continuous delivery of diclofenac sodium from the lotion into a nd through the skin, which ceased after the dosing site was washed. Th e majority of the material excreted in the urine were conjugates of hy droxylated metabolites, and not the parent chemical, although further identification is required.