Potent biphalin analogs with µ/δ mixed opioid activity: in vivo and in vitro biological evaluation.

Biphalin [(Tyr-D-Ala-Gly-Phe-NH-)2 ] is an octapeptide with mixed μ/δ opioid activity. Its structure is based on two identical enkephalin-like portions linked "tail-to-tail" by a hydrazine bridge. This study presents the synthesis and in vitro and in vivo bioassays of two biphalin analogs that do not present the toxicity connected with the presence of the hydrazine moiety and are able to elicit a higher antinociceptive effect than biphalin.