Design,Synthesis and Pharmacological Characterization of Endomorphin Analogues with Non‐Cyclic Amino Acid Residues in Position 2

Abstract: A series of endomorphin‐1 (EM‐1) and endomorphin‐2 (EM‐2) analogues, containing non‐cyclic amino acids (Ala, d ‐Ala, β‐Ala, NMeAla, d ‐NMeAla or Sar) instead of Pro in position 2 was synthesized, where NMeAla = N‐methylalanine and Sar = N‐methylglycine, sarcosine. The opioid activity profiles of these peptides were determined in μ and δ opioid receptor (MOR and DOR)‐representative binding assays and bioassays in vitro, as well as in the mouse hot‐plate test in vivo. Finally, the degradation rates of all analogues in the presence of either rat brain homogenate or selected proteolytic enzymes were determined. Analogues of EM‐2 were generally more potent than the respective analogues of EM‐1. EM‐2 analogues with d ‐Ala or d ‐NMeAla were about twofold more potent than the parent peptide and were least prone to degradation by brain homogenate, dipeptydyl peptidase IV and aminopeptidase M. In the in vivo test, [d ‐Ala²]EM‐2 and [d ‐NMeAla²]EM‐2 showed much higher analgesic potency than EM‐2 which confirmed the usefulness of structural modifications in obtaining new leads for pain‐relief therapeutics.