Disposition of parathion after dermal application in pigs

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Abstract
Pharmacokinetic parameters of parathion were studied in pigs after intravenous (i.v.) and dermal administration of unlabelled and 14C-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 (ClB) of 4.4 l/kg/h and a volume of distribution (Vss) of 9.8 l/kg. Dermal application led to a mean absorption time (MAT) of 78 h, and a bioavailability of 9.9%. Plasma levels of 14C-parathion (parathion + metabolites) were much higher and more persistent than those of parathion itself. After i.v. administration, recovery of 14C-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 14C 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 14C-parathion concentration was as low as 2 micrograms/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.