Etorphines: mu-opioid receptor-selective antinociception and low physical dependence capacity.

Comparative analgesic studies revealed that dihydroetorphine was more potent than etorphine in the tail-flick and hot-plate tests, respectively and nearly equipotent in the phenylquinone assay. Both compounds were short acting. Studies with selective opioid receptor antagonists beta-funaltrexamine, nor-binaltorphimine and naltrindole revealed that both etorphines were mu-selective agonists. Presumptive evidence for competitive antagonism of these compounds with naloxone was provided by Schild regressions with slopes of near unity. In a suppression test in rhesus monkeys maximally dependent on morphine, dihydroetorphine and etorphine dose-dependently replaced morphine. Drug-naive simians chronically exposed to frequent, intermittent and escalating doses of dihydroetorphine for 42 days showed few withdrawal signs when challenged with large doses of naloxone or were abruptly withdrawn from this drug. The results suggest that these atypical opioids may be useful in the clinical treatment of pain and opiate drug abuse.