Structure-activity Relationships Of Novel Endomorphin Analogs With Multi-locus Modification

The opioids were among the earliest neuropeptides identified in the nervous system, opioid receptors are most abundant in the central nervous system, but have also been localized in many peripheral tissues of the mammalian organism. Centrally acting plant opiates, such as morphine, are usually used analgesics for relief of the severe pain, but their usefulness is confined by a number of well-known side effects, including respiratory depression, constipation, bradycardia, hypotension, tolerance and physical dependence and so on. Theμ-opioid receptor system is a significant target in the search for novel analgesics. Endomorphin-1 and 2 are two potent and hinghly selectiveμ-opioid receptor agonists, simultaneously they could show a strong ananlgesic activity in acute pain similar to that of morphine even at low doses, but endomorphins are thought to inhibit pain without some of the undesirde side effects of plant opiates. It is possible that endomorphins instead of morphine in the development of novel analgesics. However, endomorphins still suffers from serious limitations including short duration of action, lack of activity after oral administration, relative inability to cross the blood-brain barrier (BBB) into the central nervous system (CNS) and rapidly degradation by individual enzymes (such as dipeptidyl peptidase IV and aminopeptidases). Therefore, we hope to search novel analgesics which possess long duration of action, good metabolic stability without side effects, based various modification on endomorphins.In this paper, we introduced unnatural amino aid and dipeptide fragment (Sar, D-Ala-Gly, Phg, Hfe) into posotion 2 and 3 of endomorphin-1, which C-terminal was substituted by D-Val4-NH-Bzl. Four analogs of EM-1 were synthesized by classic solution-phase method, and their physiological and pharmacological activities were determined, including receptor binding affinity and bioassay activity, isolated tissue assays and in vivo assays. We found that all of the analogs exhibited increased in vitro metabolic stability to EM-1 in mouse brain homogenate. Only analogs 1 ([Sar2, D-Val4-NH-Bzl](EM-1) showed higher affinity and agonist activity toμ-opioid receptor and increased in antinociceptive activity to that of EM-1. Analogs 2 had a less lower affinity and agonist activity toμ-opioid receptor compared with that of EM-1, however, Analogs 2 exhibited aδ-opioid receptor affinity 36-fold higher than that of EM-1, it suggested that Tyr-D-Ala-Gly is important toδ-opioid receptor ligand as "message sequence". We supposed that analogs 2 isμ/δ-opioid receptor agonist with bifunctional activities. The other two analogs all decreased in affinity and antinociceptive activity. Particularly analogs 4 exhibited significantly low affinity and agonist activity toμ-opioid receptor.