The pharmacokinetics of alfentanil (R39209): a new opioid analgesic.

The pharmacokinetics of alfentanil (R39209), a new short-acting opioid analgesic, have been studied in eleven patients. Six patients were given 50 micrograms/kg alfentanil and five patients 125 micrograms/kg as an intravenous bolus injection. Plasma concentrations were measured at intervals up to 6 h (50 micrograms/kg) or 8-10 h (125 micrograms/kg), using a specific radioimmunoassay technique. Plasma concentrations declined triexponentially in both groups. The initial elimination of alfentanil from the plasma was very rapid with 90% of the administered dose leaving the plasma within 30 min. The average half-lives for the three phases were similar for both groups. The combined mean (+/- SEM) half-lives for the 11 patients for the rapid and slow distribution phases were short (t 1/2 pi = 1.2 +/- 0.26 min, t 1/2 alpha = 11.6 +/- 1.63 min). The elimination half-life, t 1/2 beta was 94 +/- 5.87 min which is considerably shorter than that of other opioids. The mean (+/- SEM) total body clearance was 6.4 +/- 1.39 ml . kg-1 . min-1 and the volume of distribution (Vd) was 0.86 +/- 0.194 l/kg. The latter is considerably less than reported values for the chemically related drug, fentanyl, and suggests that alfentanil may have a lower tissue binding affinity than fentanyl. The rapid elimination and short duration of clinical action suggests the feasibility of repeated administration of alfentanil and its use by continuous intravenous infusion.