The effect of repeated intramuscular alfentanil injections on experimental pain and abuse liability indices in healthy males.

OBJECTIVE

Opioid-induced hyperalgesia (OIH), increased sensitivity to noxious stimuli after repeated opioid exposures, has been demonstrated in preclinical studies. However, there is no accepted, prospective model of OIH after repeated opioid exposures currently available in humans. This study assessed a potential prospective OIH model.

METHODS

Double-blind intramuscular injections of a short-acting opioid (alfentanil 15 mcg/kg; N=8) were compared to active placebo (diphenhydramine 25 mg; N=3) on cold and pressure pain testing and standard abuse liability measures in eight 10-hour sessions (1 injection/session) over 4 to 5 weeks in healthy, pain-free males. Decreases from session baseline pain threshold (PThr) and tolerance (PTol) were calculated to represent hyperalgesia, and were assessed both within and across sessions.

RESULTS

Mean decreases in cold PTol were seen in the alfentanil group at 180 minutes (-3.8 s, ±26.5) and 480 minutes (-1.63 s, ±31.5) after drug administration. There was a trend for differences between conditions on cold PThr hyperalgesia but not for pressure PThr. Alfentanil participants had greater mean ratings on Liking and High visual analog scales at peak effects (30 min), but these scores did not change across sessions.

DISCUSSION

Repeated alfentanil exposures over 4 to 5 weeks resulted in within session decreases in cold pain tolerance from baseline but these differences were not substantially different from diphenhydramine controls. The results did not support the phenomenon of OIH in this model, although definitive conclusions regarding the existence of OIH in humans likely requires a larger sample size or an alternative model.