Synthesis and biological evaluation of new biphalin analogues with non-hydrazine linkers.

Biphalin is a potent opioid peptide agonist, with a palandromic structure, composed of two enkephalin-like active fragments connected tail to tail by a hydrazine linker (Tyr-D-Ala-Gly-Phe-NH-NH<-Phe<-Gly<-D-Ala<-Tyr). This study presents the synthesis and in vitro bioassays of six new biphalin analogues with three different non-hydrazine linkers, some of which have higher binding affinity and bioactivity than biphalin.