Investigation of the Interaction Between the Atypical Agonist c[YpwFG] and MOR

Endogenous and exogenous opiates are currently considered the drug of choice for treating different kind of pain. However, their prolonged use produces several adverse symptoms, and in addition many forms of pain resist to any kind of therapy. Therefore, the discovery of compounds active towards MORs by alternative pharmacological mechanisms could be of value for developing novel classes of analgesics. There is evidence that some unusual molecules can bind ORs albeit lacking some of the typical opioid pharmacophoric features. In particular, the recent discovery of a few compounds which showed an agonist behaviour even in the absence of the primary pharmacophore, namely a protonable amine, lead to the re-discussion of the importance of ionic interactions in stabilizing the ligand-receptor complex and in activating signal transduction. Very recently, we synthesized a library of cyclic analogues of the endogenous, MOR-selective agonist endomorphin-1 (EM-1, YPWF-NH2), containing a Gly5 bridge between Tyr1 and Phe4. The cyclopeptide c[YpwFG] showed a good affinity and an agonist behaviour. This atypical MOR-agonist is deprived of the protonable Tyr-amine. In order to gain more information about plausible mechanisms of interaction between c[YpwFG] and the OR, we synthesized a selected set of derivatives containing different bridges between Tyr1 and Phe4, and we tested their affinities towards -opioid receptors. We performed the conformational analysis of the cyclopeptides by NMR spectroscopy and MD, and we investigated plausible, unprecedented modes of interaction with the MOR by Molecular Docking. The successive QM/MM investigation of the complexes obtained by the Molecular Docking procedure furnished a more detailed description of the binding mode and the electronic properties of the ligands. The comparison with the binding mode of the potent agonist JOM-6 seems to indicate that the cyclic EM-1 analogues interact the receptor by way of an alternative mechanism, still maintaining the ability to activate the receptor.