6 beta-[125iodo]-3, 14-dihydroxy-17-methyl-4, 5 alpha-epoxymorphinan ([125I]IOXY-AGO): a potent and selective radioligand for opioid mu receptors.

The recent cloning and expression of an opioid μ receptor has opened up new opportunities for research in opioid pharmacology. The relatively low level of transient receptor expression in COS cells emphasizes the need for radioligands with high specific activity and low nonspecific binding with which to label receptors. In addition, recent data indicating that agonists and antagonists bind to different domains on the same receptor protein indicate the utility of having both agonist and antagonist radioligands available for the study of opioid receptor mechanisms. Previous studies characterized the binding of the. opioid antagonist 6β-[125iodo]-3,14-dihydroxy-17-cyclopropylmethyl-4,5α-epoxymorphinan ([125I]IOXY) and showed that this naltrexone analog labels μ and K2 receptors in rat and guinea pig brain with high affinity and low nonspecific binding. In the present study, we synthesized the agonist congener of IOXY, 6β-iodo-3,14-dihydroxy-17-methyl-4,5α-epoxymorphinan. We named this novel agent IOXY-AGO for IOXY-agonist. Competition binding studies showed that IOXY-AGO has high affinity for δ receptors (Ki = 0.28 nM) and lower affinity for δ (Ki = 18.7 nM) and K1 (Ki = 33.9 nM), K2a (Ki = 38.4 nM) and K2b (Ki = 58.2 nM) binding sites. IOXY-AGO was radioiodinated to a specific activity of 2,200 Ci/mmol. [125I]IOXY-AGO binding was rapid, readily reversible, and characterized by low nonspecific binding. Equilibrium binding studies showed that it labeled a single class of binding sites (Kd = 1.46 nM, Bmax = 112 fmol/mg protein) with the characteristics of an opioid μ receptor. Receptor autoradiography experiments showed that [125I]IOXY-AGO labeled binding sites with the anatomical distribution of μ receptors. Viewed collectively, these studies suggest that [125I]IOXY-AGO will be a useful radioligand for characterizing opioid μ receptors. © 1995 Wiley-Liss, Inc.