The Effect of Intrinsic Efficacy on Opioid Tolerance

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Abstract
Background: The intrinsic efficacy of opioid analgesics has been suggested to play a role in the development of tolerance to these agents. However, the effect of differences in dosing protocol on tolerance to opioid analgesics of high or low efficacy has not been addressed. Therefore, the effect of opioid intrinsic efficacy on tolerance in mice was determined in protocols of continuous and intermittent administration of equieffective doses of opioid agonists. Methods: Initial antinociceptive median effective doses (ED50s) for five opioid agonists that vary in intrinsic efficacy were estimated in untreated mice. Groups of mice received continuous infusions of morphine, fentanyl, or etorphine for 72 h or 7 days from osmotic minipumps implanted subcutaneously. The infusion doses were calculated as multiples of the initial antinociceptive ED50. An inert placebo was implanted subcutaneously in controls. At the end of treatment, the pumps and placebos were removed, and 4-24 h later, mice were tested in dose-response studies (tail flick) using the same drug that had been chronically administered. In another study using intermittent dosing, mice received subcutaneous injections every 24 h for 3 days of saline or morphine, etorphine, fentanyl, oxycodone, or meperidine, or received subcutaneous injections every 24 h for 7 days of saline or morphine, etorphine, or fentanyl. Daily doses were calculated as multiples of the initial antinociceptive ED50. Twenty-four hours after the last injection, mice were tested in dose-response studies. Results: High-intrinsic-efficacy compounds (e.g., etorphine and fentanyl) produced less tolerance than a lower-intrinsic-efficacy drug (morphine) in 72-h and 7-day infusion studies. Tolerance for all compounds after intermittent treatment with equieffective doses was similar, and intrinsic efficacy had no effect on the magnitude of tolerance after intermittent dosing. Conclusions: These results indicate that the intrinsic efficacy of opioid analgesics is inversely related to the degree of tolerance after continuous infusion, but that intrinsic efficacy does not significantly affect tolerance after once-daily intermittent administration of these agents. These findings may be of clinical utility in understanding the development of tolerance to the antinociceptive effects of opioids.