-3,14-DIHYDROXY-17-METHYL-4,5-ALPHA-EPOXYMORPHINAN ([I-125]IOXY-AGO) - A POTENT AND SELECTIVE RADIOLIGAND FOR OPIOID MU-RECEPTORS

The recent cloning and expression of an opioid mu receptor has opened up new opportunities for research in opioid pharmacology. The relative ly low level of transient receptor expression in COS cells emphasizes the need for radioligands with high specific activity and low nonspeci fic binding with which to label receptors. In addition, recent data in dicating that agonists and antagonists bind to different domains on th e same receptor protein indicate the utility of having both agonist an d antagonist radioligands available for the study of opioid receptor m echanisms. Previous studies characterized the binding of the opioid an tagonist 6 125)iodo]-3,14-dihydroxy-17-cyclopropyl-methyl-4,5 alpha-ep oxymorphinan ([I-125]IOXY) and showed that this naltrexone analog labe ls mu and kappa(2) receptors in rat and guinea pig brain with high aff inity and low nonspecific binding. In the present study, we synthesize d the agonist congener of IOXY, 6 beta-iodo-3,14-dihydroxy-17-methyl-4 ,5 alpha-epoxymorphinan. We named this novel agent IOXY-AGO for IOXY-a gonist. Competition binding studies showed that IOXY-AGO has high affi nity for mu receptors (Ki = 0.28 nM) and lower affinity for delta (Ki = 18.7 nM) and kappa(1) (Ki = 33.9 nM), kappa(2a) (Ki = 38.4 nM) and k appa(2b) (Ki = 58.2 nM) binding sites. IOXY-AGO was radioiodinated to a specific activity of 2,200 Ci/mmol. [I-125]IOXY-AGO binding was rapi d, readily reversible, and characterized by low nonspecific binding. E quilibrium binding studies showed that it labeled a single class of bi nding sites (Kd = 1.46 nM, B-max = 112 fmol/mg protein) with the chara cteristics of an opioid mu, receptor. Receptor autoradiography experim ents showed that [I-125]IOXY-AGO labeled binding sites with the anatom ical distribution of mu receptors. Viewed collectively, these studies suggest that [I-125]IOXY-AGO will be a useful radioligand for characte rizing opioid mu,receptors. (C) 1995 Wiley-Liss, Inc.