Biological activity of Leu-enkephalin containing hydrophobic moieties.

A series of enkephalin analogues (Tyr-Gly-Gly-Phe-Leu-C-NH-R;R=C6,C8,C10,C12 and C14; C=CH2 groups) with increasing hydrophobic character have been tested behaviourally in the analgesic hot plate and tail flick tests. The analogues were administered intraperitoneally and were found to possess anti-nociceptive activity. The results indicate that apparently there is not a direct relationship between increasing hydrophobicity and activity of the enkephalin-alkyl-amide analogues. The enkephalin analogues were found to behave in the same way as the enkephalins in producing tolerance.