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 th at morphine- 3-glucuronide (M3G) had an apparent affinity for mu(1)-op ioid 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 hi gh-performance-liquid-chromatography (HPLC) with electrochemical detec tion, it was found to be contaminated with 0.5% (molar basis) of morph ine. Reduction of the morphine contamination of M3G to 0.08% resulted in a 7.2-fold decrease in apparent binding affinity (IC50 = 1279 +/- 2 87 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 rece ptors.