Morphine-3-Glucuronide Has a Minor Effect on Morphine Antinociception. Pharmacodynamic Modeling

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Abstract
The objective of this study was to quantify the influence of morphine-3-glucuronide (M3G) on the morphine antinociceptive effect (ANE) and respiratory effects in the rat. Three groups of rats were pretreated with either saline or M3G at two different rates. Morphine infusion of 10 mg/h/kg (group A) or 20 mg/h/kg (group B) was administered to each pretreatment group for 3 h. The ANE was measured by the electrical stimulation vocalization method, and blood gas parameters (pCO2, pO2, and pH) were assessed. Independent of pretreatment all groups displayed a concurrent increase in the ANE. The maximal effect diverged between pretreatments. Acute tolerance was observed, but no rebound effect was detected. To characterize the ANE, an effect compartment model and an indirect response model were selected, both capable of describing the observed features. In both models incorporation of M3G led to a better explanation of the data. On the basis of the parameters obtained in the fits, naturally occurring M3G would reduce the antinociceptive effect during a morphine infusion (plasma concentration 15 microM) by 15-20%. The exposure of M3G did not significantly change the respiratory response following the morphine treatment.