Opiate-like peptides. Part VIII. Methylamides and dimethylamides of [D-Leu5]-enkephalin and [D-Ala2, D-Leu5]-enkephalin. Synthesis and analgesic activity.

Syntheses of [D-Ala2, D-Leu5]-enkephalin, methyl ester of [D-Ala2, D-Leu3]-enkephalin, methylamides and dimethylamides of [D-Leu5]-enkephalin and [D-Ala2, D-Leu5]-enkephalin are described together with their analgesic activity determined on the basis of four analgesic tests: the hot-plate method, the reaction to electric stimulus, the tail immersion test and the frequency of writhing syndrome test. The neurotoxicity was estimated by the rota-rod test. The most pronounced analgesic effect was induced by compound: [D-Ala2, D-Leu5]-enkephalin, [D-Ala2, D-Leu5-OMe]-enkephalin and [D-Ala2, D-Leu5-NMe2]-enkephalin. In the tail immersion test all analogs did not exhibit analgesic activity.