DISPOSITION OF MORPHINE AND ITS GLUCURONIDE METABOLITES AFTER ORAL AND RECTAL ADMINISTRATION - EVIDENCE OF ROUTE SPECIFICITY

Morphine-6-glucuronide is a metabolite of morphine that shows significant analgesic effects in animals and humans. To evaluate route-specific differences in the potential contribution of morphine-6-glucuronide to morphine analgesia, we studied the pharmacokinetics of morphine, morphine-6-glucuronide, and morphine-3-glucuronide after oral and rectal administration of morphine sulfate in a six-subject randomized, single-dose, two-way crossover evaluation. The mean area under the plasma concentration-time curve (AUC) molar ratios of morphine-6-glucuronide (M6G) and morphine-3-glucuronide (M3G) to morphine (M) were greater after oral morphine administration compared with rectal morphine administration (M6G/M ratio, 2.7: 1 versus 1.3: 1, p = 0.025; M3G/M ratio, 18.3: 1 versus 9.3: 1, p = 0.002). Systemic bioavailability and peak plasma concentrations of morphine-6-glucuronide and morphine-3-glucuronide were significantly greater after oral morphine administration compared with the rectal route (AUC: M6G, 377.1 +/- 124.2 versus 236.2 +/- 133.7 nmol . hr/L, p = 0.05; M3G, 2610.1 +/-446.4 versus 1650.2 +/- 309.0 nmol . hr/L, p = 0.004; maximum concentration: M6G, 110.9 +/- 37.5 versus 64.6 +/- 28.8 nmol/L, p = 0.002; M3G, 576.9 +/- 155.8 versus 266.8 +/- 110.5 nmol/L, p = 0.007). Conversely, the systemic availability of morphine was lower after oral administration, although this difference failed to achieve statistical significance (142.3 +/- 17.1 versus 176.6 +/- 69.4 nmol . hr/L, p = 0.14). These data suggest that rectal administration of morphine is associated with significant avoidance of hepatic biotransformation, and they provide a convincing argument for the evaluation of morphine-6-glucuronide concentrations in pharmacokinetic and pharmacodynamic comparisons involving different routes of morphine administration.