Morphine, pethidine and buprenorphine disposition in the cat

Pharmacokinetics of morphine, buprenorphine and pethidine were determined in 10 cats. Six cats received morphine (0.2 mg/kg) intravenously and four intramuscularly. Five received buprenorphine (0.01 mg/kg) intravenously and six intramuscularly. Six received pethidine (5 mg/kg) intramuscularly. jugular venous blood samples were collected at time points to 24 h, and plasma morphine concentrations were measured by high performance liquid chromatograpy (HPLC), buprenorphine by radioimmunoassay (RIA) and pethidine by gas chromatography. Our data for morphine show elimination half-life (t(1/2el)) 76.3 min intravenous (i.v.) and 93.6 min intramuscular (i.m.); mean residence time (MRT) 105.0 and 126.5 min; clearance (Cl-p) 24.1 and 13.9 mL/kg/min; and volume of distribution (V-(dss)) 2.6 and 1.7 L/kg, respectively. Comparable data for buprenorphine are t(1/2el) 416.8 and 380.2 min; MRT 417.6 and 409.8 min; Cl-p 16.7 and 23.7 mL/kg/min; and V-(dss) 7.1 and 8.9 L/kg. For i.m. pethidine, t(1/2el) 216.4 min; MRT 307.5 min; Cl-p 20.8 mL/kg/min and V-(dss) 5.2 L/kg. For i.m. dosing, the t(max) for morphine, buprenorphine and pethidine were 15, 3 and 10 min, respectively. The pharmacokinetics of the three opioids in cats are broadly comparable with those of the dog, although there is a suggestion that the cat may clear morphine more slowly.