Parathion metabolism during percutaneous absorption in perfused porcine skin

Abstract

The metabolism and disposition of topical parathion was examined in the isolated perfused porcine skin flap (IPPSF), a novel organ perfusion method developed for in vitro studies of the pharmacology and toxicology of skin. Ethanol solutions of 14C-radiolabeled parathion (1.0 mg/ml, 0.05 mCi/ml) were applied to the surface at a dose of 40 μg/cm 2 on skin flaps representing three treatment groups: control ( N = 5), occluded ( N = 4), and 1-aminobenzotriazole (ABT)-pretreated ( N = 2). Radiolabel uptake in the perfusion medium from 0–8 hr postapplication indicated that total chemical flux and peak rates of absorption in occluded preparations were 59% and 47% lower than in the controls ( P < 0.05), and were both > 75% lower in the ABT-pretreated IPPSF's. Thin-layer chromatographs of ethyl acetate extracts demonstrated that most of the absorbed radiolabel recovered in the perfusion medium of the controls was paraoxon (67.8 ± 2.3%, mean ± SE), with a lesser amount of p-nitrophenol (14.5 ± 5.2%) and unmetabolized parathion (16.9 ± 4.1%). Occlusion of the application site increased the fraction of p-nitrophenol in the perfusion medium (43.0 ± 7.8%, P < 0.05), at the apparent expense of paraoxon (39.6 ± 17.8%, P < 0.05), without altering the mean percentage of parent compound recovered (17.4 + 10.1%). Pretreatment of the IPPSF by addition of 50 μg ABT/g wet tissue weight into the perfusion medium blocked most of the paraoxon formation (6.6%), but not that of p-nitrophenol (11.9%), while allowing 78.5% of the parathion absorbed to penetrate intact. Total flux assessments for parathion plus its metabolites (total radiolabel absorption) in vitro varied in conjunction with its fraction metabolized within the skin. Net permeation of unchanged parathion was similar in the control (580 ng/cm 2) and ABT-pretreated skin flaps (660 ng/cm 2), suggesting that the diffusivity of parathion molecules through skin was not altered by this metabolic inhibitor. In contrast, occlusion reduced the net flux of intact parathion molecules to approximately 240 ng/cm 2. These findings show that parathion undergoes significant biotransformation following topical application to porcine skin and that the resultant cutaneous metabolite profiles can be altered by both physical (occlusion) and chemical (ABT) means.