Design and Synthesis of a Novel Series of Opioid Dipeptides and Evaluation of Their Analgesic Effect in Vivo

The opioid system is one of the most studied pain relieving systems. Opioid peptides have been studied extensively since their discovery, and many efforts have been dedicated to the determination of their intrinsic nature. However, opioid peptides still suffer from serious limitations including lack of oral activity, short duration of action, poor metabolic stability, and relative inability to cross the blood-brain barrier (BBB). Inspired by the structure-activity studies of opioid peptides, four novel opioid dipeptides were synthesized by introduction of Tyr/Dmt to the N-termius and novel unnatural alpha-methylene-beta-amino acids (Map) to the C-terminus. Their in vitro and in vivo activities were determined and compared. The affinity and selectivity of these dipeptides were evaluated by radioligand binding assay in whole cell preparations from HEK293 cells expressing the delta opioid receptor (MOR) or d opioid receptor (DOR), compound 4 exhibited the highest binding affinity toward MOR, and this compound also showed the most significant MOR selectivity over DOR. Pharmacological activities were evaluated in vitro using isolated guinea pig ileum (GPI) for MOR and mouse vas deferens (MVD) for DOR, and the results of GPI and MVD were in accordance with the binding affinity assays. The antinociceptive activities of the compound 4 were assessed in the mouse tail-flick and writhing test after intracerebroventricular (i.c.v.) administration. In the two tests, compound 4 showed good ability to reduce the thermal pain and the inflammatory pain, respectively. Naloxone significantly reversed the activity of the compound, indicating that opioid receptors were involved in its analgesic effects. The compound 4 was then administered inravenously (i.v.) to determine whether this dipeptide was able to cross the BBB. As a result, compound 4 was effective in both tail-flick and writhing test after i.v. administration. Furthermore, the peripherally restricted opioid antagonist naloxone methiodide did not alter the peptide-induced antinociception. Collectively, these results indicated that compound 4 was able to cross the highly selective BBB and might elicit an analgesic response via a central mechanism. Therefore, compound 4 may be a potential lead compound for future clinical development for pain management.