DISPOSITION OF PARATHION AFTER DERMAL APPLICATION IN PIGS

Pharmacokinetic parameters of parathion were studied in pigs after intravenous (i.v.) and dermal administration of unlabelled and C-14-parathion. Plasma concentration-time data were subjected to non-compartmental analysis. Intravenous injection studies showed a mean residence time (MRT) of 2.15 h, a body clearance (Cl(B)) of 4.4 l/kg/h and a volume of distribution (V(ss)) of 9.8 1/kg. Dermal application led to a mean absorption time (MAT) of 78 h, and a bioavailability of 9.9%. Plasma levels of C-14-parathion (parathion + metabolites) were much higher and more persistent than those of parathion itself. After i.v. administration, recovery of C-14-parathion from urine plus faeces was almost 100% within 3 d, while it was less than 20% after dermal application. Ten days after dermal application high C-14 concentrations remained in the back skin, i.e. the application area. In skin samples from areas where contamination from the application area could not have occurred, the C-14-parathion concentration was as low as 2 mug/g. It is concluded that in view of the low dermal bioavailability for organophosphorus insecticides it is unlikely that pour-on preparations containing these insecticides reach the ectoparasites through absorption and systemic distribution, but rather that this happens after spreading on the surface of the skin.