Buprenorphine kinetics.

Buprenorphine kinetics was determined in surgical patients using radioimmunoassay. Buprenorphine was measured in the plasma of 24 patients who had received 0.3 mg buprenorphine intraoperatively. After 3 hr 10 of these patients then received a further 0.3 mg buprenorphine intravenously for postoperative pain relief, and 11 patients were given 0.3 mg intramuscularly; again, plasma levels were measured for 3 hr. The data fitted closely to a triexponential decay curve. There was a very fast initial phase, with a half-life (t1/2) of 2 min. The terminal t1/2 was slow, approximately 3 hr. Comparison of the kinetics of the same patient, awake and anesthetized, showed that the clearance was significantly lower in the anesthetized state. A notable feature of the drug given intramuscularly is rapid systemic availability, so that peaks are obtained in 2 to 5 min, and in 10 min the resulting levels are the same as for the intravenous and intramuscular routes.