Metabolic clearance rate in sheep of a met-enkephalin analogue estimated by radioimmunoassay.

A sensitive and specific radioimmunoassay developed for measuring the met-enkephalin analogue d-ala2-met(0)5-ol-enkephalin (DAMME) was used to study the pharmacokinetics of DAMME in the circulation of sheep. Plasma concentrations of DAMME were measured at varying time-intervals after an intravenous bolus injection or following a constant intravenous infusion of the analogue. The mean metabolic clearance rate of DAMME was 2.8 ml/min per kg, the mean circulating half-life was 52 min and the mean volume of distribution was 190 ml/kg. The longer circulating time of the analogue when compared with that of naturally occurring met-enkephalin would appear to explain its prolonged analgesic effect.