Alfentanil and placebo analgesia: no sex differences detected in models of experimental pain.
BACKGROUND
To assess whether patient sex contributes to the interindividual variability in alfentanil analgesic sensitivity, the authors compared male and female subjects for pain sensitivity after alfentanil using a pharmacokinetic-pharmacodynamic modeling approach.


METHODS
Healthy volunteers received a 30-min alfentanil or placebo infusion on two occasions. Analgesia was measured during the subsequent 6 h by assaying tolerance to transcutaneous electrical stimulation (eight men and eight women) of increasing intensity or using visual analog scale scores during treatment with noxious thermal heat (five men and five women). Sedation was concomitantly measured. Population pharmacokinetic-pharmacodynamic models were applied to the analgesia and sedation data using NONMEM. For electrical pain, the placebo and alfentanil models were combined post hoc.


RESULTS
Alfentanil and placebo analgesic responses did not differ between sexes. The placebo effect was successfully incorporated into the alfentanil pharmacokinetic-pharmacodynamic model and was responsible for 20% of the potency of alfentanil. However, the placebo effect did not contribute to the analgesic response variability. The pharmacokinetic-pharmacodynamic analysis of the electrical and heat pain data yielded similar values for the potency parameter, but the blood-effect site equilibration half-life was significantly longer for electrical pain (7-9 min) than for heat pain (0.2 min) or sedation (2 min).


CONCLUSIONS
In contrast to the ample literature demonstrating sex differences in morphine analgesia, neither sex nor subject expectation (i.e., placebo) contributes to the large between-subject response variability with alfentanil analgesia. The difference in alfentanil analgesia onset and offset between pain tests is discussed.
Olofsen
Erik
E
Department of Anesthesiology, Leiden University Medical Center, Leiden, The Netherlands.
Romberg
Raymonda
R
Bijl
Hans
H
Mooren
René
R
Engbers
Frank
F
Kest
Benjamin
B
Dahan
Albert
A
D000293:Adolescent; D000328:Adult; D015760:Alfentanil / Q000008:administration & dosage*; D000698:Analgesia / Q000379:methods*; D018592:Cross-Over Studies; D004305:Dose-Response Relationship, Drug; D004311:Double-Blind Method; D004558:Electric Stimulation / Q000379:methods; D005260:Female; D006801:Humans; D008297:Male; D010146:Pain / Q000517:prevention & control*; D010147:Pain Measurement / Q000187:drug effects* / Q000379:methods; D015990:Placebo Effect; D012727:Sex Characteristics
D015760:Alfentanil