Lack of Analgesic Activity of Morphine-6-glucuronide after Short-term Intravenous Administration in Healthy Volunteers

Background: The analgesic activity of morphine-6-glucuronide (M-6-G) is well recognized for its contribution to the effects of morphine and its possible use as an opioid analgesic with a wider therapeutic range than morphine. The present study attempted to quantify the relative contribution of M-6-G to analgesia observed after systemic administration of morphine.

Methods: In a placebo-controlled, sixfold crossover study in 20 healthy men, the effects of M-6-G were assessed at steady-state plasma concentrations of M-6-G identical to and two and three times higher than those measured after administration of morphine. Morphine and M-6-G were administered as an intravenous bolus followed by infusion over 4 h. Dosage A was M-6-G-bolus of 0.015 mg/kg plus infusion of 0.0072 mg [center dot] kg sup -1 [center dot] h sup -1. Dosage B was M-6-G-bolus of 0.029 mg/kg plus infusion of 0.014 mg [center dot] kg sup -1 [center dot] h sup -1. Dosage C was M-6-G-bolus of 0.044 mg/kg plus infusion of 0.022 mg [center dot] kg sup -1 [center dot] h sup -1. Dosage D was a morphine bolus of 0.14 mg/kg plus infusion of 0.05 mg [center dot] kg sup -1 [center dot] h sup -1 for 4 h. Dosage E was M-6-G combined with morphine (doses A + D). Dosage F was a placebo. The analgesic effects of M-6-G and morphine were measured before administration of the bolus and after 3.5 h using an experimental pain model based on pain-related cortical potentials and pain ratings after specific stimulation of the nasal nociceptor with short pulses of gaseous carbon dioxide.

Results: Morphine significantly reduced subjective and objective pain correlates compared with placebo. In contrast, M-6-G produced no statistically significant effects. The addition of M-6-G to morphine did not increase the effects of morphine. Morphine produced significantly more side effects than M-6-G.