A method for the combined measurement of ethiofos and WR-1065 in plasma: Application to pharmacokinetic experiments with ethiofos and its metabolites

Abstract

An analytical method for the combined measurement of ethiofos (WR-2721) and a major metabolite (WR-1065) in plasma is described. Plasma samples were subjected to conditions which quantitatively converted both ethiofos and bound WR-1065 to free WR-1065 which was subsequently separated by HPLC and detected electrochemically using established procedures. Although bound WR-1065 in plasma is thought to exist mainly in the form of mixed disulfides, the symmetrical disulfide, WR-33278, also was quantitatively converted to the free thiol form. Standard curves were linear over the range 0.10 to 25 μg/mL (0.75 to 186 μmol/L). Mean precision over the range was 5.4% (coefficient of variation, CV) and recoveries of various mixtures of ethiofos, WR-1065 and WR-33278 averaged 102% (CV = 6.6%). This analytical procedure and others specific for ethiofos, free WR-1065 and WR-33278 were applied to dosing experiments in which the parent drug and its major metabolites were variously administered to beagle dogs and rhesus monkeys. Following i.v. administration of ethiofos (120–150 mg per kg body weight) to monkeys, plasma concentrations of unchanged drug ranged from 477 μg/mL (2.23 mM) down to the minimum detectable limit of the analytical procedure (0.05 μg/mL, 0.23 μM) 2–3 hours postinfusion. Clearances averaged 43.5 ± 13.4 (SD) mL min −1 kg −1 and half-lives observed in the 20–60 minute postinfusion period were 8–15 min.