Kappa opioid agonists as targets for pharmacotherapies in cocaine abuse

Abstract

Kappa opioid receptors derive their name from the prototype benzomorphan, ketocyclazocine (1a) which was found to produce behavioral effects that were distinct from the behavioral effects of morphine but that were antagonized by the opioid antagonist, naltrexone. Recent evidence suggests that agonists and antagonists at kappa opioid receptors may modulate the activity of dopaminergic neurons and alter the neurochemical and behavioral effects of cocaine. Kappa agonists blocked the effects of cocaine in squirrel monkeys in studies of cocaine discrimination and scheduled-controlled responding. Studies in rhesus monkeys suggested that kappa opioids may antagonize the reinforcing effects of cocaine. These studies prompted the synthesis and evaluation of a series of kappa agonists related to the morphinan, l-cyclorphan (3a) and the benzomorphan, l-cyclazocine (2). We describe the synthesis and preliminary evaluation of a series of morphinans, structural analogs of cyclorphan 3a–c, the 10-keto morphinans 4a and b, and the 8-keto benzomorphan 1b, structurally related to ketocyclazocine (1a). In binding experiments l-cyclorphan (3a), the cyclobutyl (3b), the tetrahydrofurfuryl 3c and the 10-keto 4b analogs had high affinity for mu (μ), delta (δ) and kappa (κ) opioid receptors. Both 3a and 3b were more selective for the κ receptor than the μ receptor. However, 3b was 18-fold more selective for the κ receptor in comparison to the δ receptor, while cyclorphan (3a) had only a 4-fold greater affinity for the κ receptor in comparison to the δ receptor. The cyclobutyl compound 3b was found to have significant μ agonist properties, while 3a was a μ antagonist. All compounds were also examined in the mouse tail flick and writhing assay. Compounds 3a and 3b were κ agonists. Correlating with the binding results, compound 3a had some δ agonist properties, while 3b was devoid of any activity at the δ receptor. In addition, compounds 3a and 3b had opposing properties at the μ opioid receptor. The cyclobutyl compound 3b was found to have significant μ agonist properties, while 3a was a μ antagonist.