Nwopeprrdes (1988) zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 12, Ml-187 0 Lon@xm Group UK Ltd 1988 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA A Brief Study of the Selectivity of Norbinaltorphimine, (-)-Cyclofoxy, and (+)-Cyclofoxy Among Opioid Receptor Subtypes In Vitro R. B. ROTHMAN’, V. BYKOV’, A. REID’, B. R. DE COSTA’, A-H. NEWMAN*, A. E. JACOBSON* and K. C. RICE’ zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA ‘Unit on Receptor Studies, LCS, NIMH, Bethesda, MD 20892. (Correspondence to R. B. R) ‘Section on Drug Design and Synthesis, LN, NIDDK, Bethesda, MD, 20892. Abstract-Norbinaltorphimine (nor-BNI) is a bifunctional reagent developed as a selective antagonist of the kappa opioid receptor. In this paper we examined the in vitro selectivity of nor-BNI, 6-desoxy-6P-fluoronaltrexone (cycloFOXY), and the enantiomer of cycloFOXY, among opioid receptor subtypes. Nor BNI exhibited the highest affinity for kappa binding sites labeled by 3H-U69593 (Ki = 1.8nM), and was 27- to 29-fold less potent at mu and delta binding sites. In contrast, cycloFOXY had the highest affinity for mu binding sites (Ki = 2,62nM), and bound to kappa and delta binding sites with Ki’s of 9.3nM and 89nM, respectively. The enantiomer of cycloFOXY, did not inhibit binding even at concentrations greater than lOuM, validating in part the use of ‘*F-labeled (+I-cycloFOXY to estimate “non-specific binding” in positron emission tomography. Additionally, we report that (S,S)- U50488 and (/?.I?)450488 bind to kappa binding sites labeled by 3H-U69593 with Ki’s of 0.89 nM and 299 nM, respectively. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Introduction Norbinaltorphimine (nor-BNI) is a bifunctional reagent developed by Portoghese and associates as a selective antagonist of the opioid kappa receptor (1.2). In this study, using ligand binding methods, we examine the selectivity of nor-BNI among multiple opioid receptor subtypes. Parallel exper- iments were conducted using the opiate antagonist 6-desoxy-6p-fluoronaltrexone (cycloFOXY) (3) Date received 9 June 1988 Date accepted 16 June 1988 and its enantiomer, (+)-cycloFOXY, not only to provide comparative data obtained using an antagonist expected to be less selective for kappa binding sites, but also to provide data of interest to investigators using 18F-labeled (+)-cyc1oFOXY as a ligand for positron emission tomography (4). Methods Preparation of zyxwvutsrqponmlkjihgfedcbaZYXWVU membranes Frozen lysed-P2 membranes, and BIT/FIT pre- treated membranes were prepared from rat brain as described (5). For the “H-U69593 binding 181