Pharmacological properties of R-84760, a novel κ-opioid receptor agonist

The pharmacological properties of a structurally novel κ-opioid receptor agonist, (3R)-3-(1-pyrrolidinylmethyl)-4-[(1S)-5,6-dichloro-1- indancarbonyl]-tetrahydro-1,4-thiazine hydrochloride (R-84760), were examined. In an opioid receptor binding assay with a guinea pig brain membrane fraction, R-84760 showed a 5 times higher affinity for the κ-opioid receptor than CI-977, the most potent reference κ-opioid receptor agonist, and selectivity of R-84760 for the κ-opioid receptor was higher than that of U-69593, a highly selective κ-opioid receptor agonist. R-84760 was functionally 2.5 times more potent than CI-977 as a κ-opioid receptor agonist in rabbit vas deferens. Subcutaneously administered R-84760 had an antinociceptive effect in the phenylquinone writhing test in mice and its potency was 5–846 times higher than those of CI-977, U-50488, morphine, pentazocine and butorphanol. On oral administration, R-84760 was 20–2077 times more potent than the same reference drugs. The antinociceptive effect of R-84760 was not antagonized by naloxone in a dose at which naloxone antagonized the effect of morphine, but on the other hand nor-binaltorphimine antagonized the effect of R-84760 at a dose which did not affect the effect of morphine. R-84760 and the reference κ-opioid receptor agonists produced sedative and diuretic effects in mice with the same order of potency as for the antinociceptive effect. Tolerance to the antinociceptive effect of R-84760 barely developed, and naloxone-induced jumping tests for morphine-like physical dependence of R-84760 gave negative results.