[I-125] IOXY-AGO - A HIGH-AFFINITY, MU-SELECTIVE AGONIST OPIOID RECEPTOR-LIGAND

We previously reported that epimeric 6 beta-iodo-3,14-dihydroxy-17-cyc lopropylmethyl-4,5 alpha-epoxymorphinan (IOXY) was a potent opioid rec eptor antagonist with high affinity mu, kappa(1) and kappa(2) binding sites (1,2). In order to compare the binding of agonist and antagonist ligands at cloned mu opioid receptors (3-5) (see Rothman et al., this meeting), we prepared the agonist analog of IOXY, IOXY-AGO for ''IOXY -agonist'' (6 beta-iodo-3,14-dihydroxy-17-methyl-4,5 alpha epoxymorphi nan). This paper reports on [I-125]IOXY-AGO binding to rat brain membr anes. [I-125]IOXY-AGO was radioiodinated (2200 Ci/mmol) as reported fo r [I-125]IOXY (1) and the product purified by HPLC. High affinity, spe cific (90 %) and reversible binding was demonstrated. NaCl and GppNHp inhibited [I-125]IOXY-AGO binding indicating that it possesses agonist properties. Its Ki values at opioid receptor subtypes were 0.28+/-0.0 4 (mu), 18.7+/-0.9 (delta) and 33.9+/-2.2 nM (kappa(1)), respectively. Competition studies showed that [I-125]IOXY-AGO selectively labeled m u receptors: mu-ligands, morphine, naloxone and DAMGO had nanomolar Ki values while delta- and kappa-ligands had moderate or weak Ki values. Viewed collectively, these data indicate that [I-125]IOXY-AGO is a ve ry useful ligand for studies of opioid receptors.