Pharmacologic activity of CI-977, a selective kappa opioid agonist, in rhesus monkeys.

CI-977 is a selective, nonpeptide kappa opioid agonist. In rhesus monkeys, CI-977 is a potent antinociceptive agent against thermal stimuli after i.m. administration. Increasing the intensity of the nociceptive stimulus can reduce the analgesic activity of CI-977. Antinociceptive activity also was seen when PD 126212, containing CI-977 as the (-)-enantiomer, was administered sublingually. Naloxone antagonized the antinociceptive action of CI-977, demonstrating opiate receptor involvement in this activity. Monkeys treated with CI-977 also showed sedation at doses close to those required to produce antinociception. As with morphine, the sedative properties of CI-977 were associated with impaired cognitive performance. Aged monkeys appeared more sensitive than young monkeys to the performance-impairing effect of CI-977. Tolerance developed to the antinociceptive and response-suppressing effects. CI-977 was approximately 1000 times more potent than morphine as an analgesic when tested against a moderate (50 degrees C) thermal stimulus but less effective than morphine against a strong (55 degrees C) thermal stimulus.