Distinct relations among plasma concentrations required for different pharmacological effects in oxycodone, morphine, and fentanyl.

Several clinical reports showed that adverse effect profiles are not the same in morphine, oxycodone, and fentanyl. The authors investigated whether the relationship between plasma concentrations for antinociceptive effect and for various pharmacological effects differed among oxycodone, morphine, and fentanyl under controlled experimental setting using animal models. Oxycodone induced constipation and an antinociceptive effect in a similar concentration-dependent manner, whereas morphine required approximately 9-fold higher plasma concentration for antinociceptive effect compared with that for constipation when 50% effective plasma concentration (EC(50)) levels were compared. The EC(50) values for inhibition of behavioral activity were 2.1-, 2.7-, and 1.3-fold higher than those for antinociceptive effect in oxycodone, morphine, and fentanyl, respectively. Respiratory inhibition was observed even at higher plasma concentrations in all three opioids, and the differences in the EC(50) values compared with those for antinociceptive effects were 234.5-fold (oxycodone), 233.1-fold (morphine), or 104.2-fold (fentanyl). These results showed that oxycodone, morphine, and fentanyl exhibited unique patterns of plasma concentrations required for different pharmacological effects. The different adverse effect profiles observed in a clinical setting appear to be resulted from, at least in part, distinct intrinsic pharmacological profiles among these μ-opioid receptor agonists.