Pharmacokinetics of alfentanil in man

The distribution and elimination kinetics of alfentanil, a new short-acting analgesic, were studied in five surgical patients. Its behavior, following a bolus injection of 120 micrograms/ml, was compatible with a three-compartment open-model distribution. The disappearance of the drug from plasma was rapid (t 1/2 pi = 3.5 +/- 1.3 minutes, t 1/2 alpha = 16.8 +/- 6.4 minutes) with 96.4% of the drug eliminated from plasma in 1 hour, indicating extensive transfer to the remote peripheral compartment. This was followed by a slower elimination phase with a t 1/2 beta of 94 +/- 38 minutes. Total volume of distribution was 1.03 +/- 0.50 L/kg. Total plasma clearance was 456 +/- 155 ml/min. The short analgesic effect of this drug might be attributed to the rapid displacement of the drug from the central and intermediate compartments to the remote peripheral compartment. Approximately 25% of the injected dose was present in the remote peripheral compartment 30 to 60 minutes after alfentanil administration. As the return of drug from this peripheral to the central compartment is slower than the elimination rate of the drug, it could be the rate-limiting step in the elimination of alfentanil from the body.