The Apparent Affinity of Morphine-3-Glucuronide At Mu(1)-Opioid Receptors Results From Morphine Contamination - Demonstration Using Hplc and Radioligand Binding

Equilibrium binding studies in sheep thalamic homogenates indicated that morphine- 3-glucuronide (M3G) had an apparent affinity for mu(1)-opioid binding sites (IC50 = 1781 +/- 40 nM, K-i = 116 +/- 25 nM, mean +/- s.e.m., n=4) similar to that reported by Pasternak and co-workers (1). However, when the chemical purity of M3G Was investigated using high-performance-liquid-chromatography (HPLC) with electrochemical detection, it was found to be contaminated with 0.5% (molar basis) of morphine. Reduction of the morphine contamination of M3G to 0.08% resulted in a 7.2-fold decrease in apparent binding affinity (IC50 = 1279 +/- 287 nM, K-i = 766 +/- 30 nM, mean +/- s.e.m., n=4), indicating that the small percentage of morphine present in the M3G raw material drug is the likely explanation for M3G's apparent binding to mu(1)-opioid receptors