Synthesis of two enzyme resistant enkephalin analogs possessing enhanced analgesic activity

Two enzyme resistant analogs of methionine enkephalin (H-Try-Gly-Gly-Phe-Met-OH) have been synthesized and tested for morphine-like properties in the stimulated guinea pig ileum, rat tail-flick, mouse tail-flick and for stability in whole brain homogenates. The two analogs are: Nalpha-Methyl-Tyrosyl-Glycyl-Glycyl-Phenylalanyl-des-carboxy-Norleucine (I) and Tyrosyl-D-Alanyl-Glycyl-Phenylalanyl-des-carboxy-Norleucine (II). Both peptides were completely stable for 90 minutes when incubated with whole brain homogenate (rat) while methionine enkephalin was rapidly destroyed (t1/2=4 min at 37 degrees C). In addition each was more potent than methionine enkephalin in the stimulated guinea pig ileum assay and had good activity in the tail-flick test when given intraventricularly in mice or via indwelling catheters into the periaqueductal gray of rats. Each of these activities were blocked by naloxone. Interestingly, the antin-ociceptive activity of these peptides was much reduced in morphine tolerant animals.