Morphine-like activity of substituted amides & imides of [D-Ala2, Met5]-enkephalin

Six enkephalin analogues (N-substituted amides and imides of [D-Ala2, Met5]-enkephalin) were synthesized and tested for opioid activity. All the compounds, except one i.e., compound IV, showed analgesic activity which was much higher than Met-enkephalin and morphine in mice and inhibited electrically induced contractions of isolated guineapig ileum, [D-Ala2, Met5]-enkephalin-morpholide and [D-Ala2, Met5]-enkephalin-beta-Ala-amide were the most potent analgesics and nearly 6 and 500 times as active as morphine and Met-enkephalin respectively. Both the compounds were equipotent on the guineapig ileum preparation, whereas the beta-Ala-amide was about twice as active as the morpholide in the electrically stimulated mouse vas deferens preparation.