PHARMACOKINETIC PROPERTIES OF METHADONE HYDROCHLORIDE AFTER SINGLE INTRAMUSCULAR ADMINISTRATION IN ADULT DAIRY GOATS

Pharmacokinetic parameters of methadone were studied in adult dairy go ats. Five goats were each given methadone hydrochloride as a single 0. 2 mg/kg of body weight dosage by intramuscular (IM) administration. Pl asma methadone concentrations were determined for 96 h after dosing. P lasma methadone concentrations after IM administration were best descr ibed by an open one-compartment model. Overall elimination half-life ( t(1/2)) was 1.38 h. Peak plasma concentrations were reached 0.25 h aft er dosing, and the actual plasma concentration averaged 37.8 ng/ml (SD = 12.76) at that time. The data obtained hom this study suggest that plasma concentrations, similar to those that are analgesic in humans, can be achieved after IM administration of methadone at a dose rate of 0.2 mg/kg of body weight. In addition, these plasma concentrations ca n be maintained for up to 3 h after a single injection and, therefore, may provide satisfactory analgesia for such period,