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

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 micrograms/mL (0.75 to 186 mumol/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 micrograms/mL (2.23 mM) down to the minimum detectable limit of the analytical procedure (0.05 micrograms/mL, 0.23 microM) 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.