Phenylpiperidine opioid antagonists that promote weight loss in rats have high affinity for the κ2B (enkephalin-sensitive) binding site

Certain opioid antagonists of the phenylpiperidine series (PPAs), such as LY255582, seem unique efficacious at producing weight loss in lean and meal-fed obese Zucker rats. Comparison of the pharmacological and receptor binding profile of PPAs that promote marked weight loss with those that do not has failed to find any obvious differences between these two groups of narcotic antagonists, which might explain the differences in their biological activities. The potent stimulatory effect of dynorphin, and other kappa agonists, on feeding behavior suggests that the antagonists that promote weight loss might have high affinity for kappa receptors. The recent demonstration by several laboratories Of kappa receptor heterogeneity prompted us to test the hypothesis that the antagonists that promote weight loss might have high affinity for a subtype of kappa binding sites. In the present study, therefore, we determined the K(i) values of five PPAs, naloxone, and naltrexone at mu, delta, kappa1, kappa2a, and kappa2b binding sites. The data indicate that antagonists having subnanomolar K(i) values and high selectivity for the kappa2b binding site (relative to the kappa2a binding site) are efficacious at promoting weight loss.