Profiles of opioid analgesia in humans after intravenous bolus administration: alfentanil, fentanyl and morphine compared on experimental pain

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Abstract
This report examines the relationship of plasma drug concentration to analgesic effect following bolus doses of alfentanil, fentanyl and morphine and assesses individual differences in analgesic response among volunteers. We predicted that the 3 opioids would yield disparate analgesic profiles because their physicochemical and pharmacokinetic characteristics differ. Ten healthy volunteers received intravenous bolus doses of either alfentanil, fentanyl, morphine or normal saline on different days. We stimulated their teeth electrically and measured brain evoked potential (EP) and pain report (PR) repeatedly over 2 h to assess analgesic effect. Concurrently, we drew 18 blood samples to assess opioid plasma concentrations during the test period. The relationship between opioid plasma concentration and analgesic effect was well defined for alfentanil but ambiguous for morphine. Fentanyl exhibited a marked hysteresis. We observed noteworthy individual differences in analgesic response with all 3 drugs but these differences were greatest for morphine and least for alfentanil. Inter- and intrasubject variability in analgesic response across drugs is related to the physicochemical properties of the drugs tested.