Endomorphin-1Analogues Modified With Novel Unnatural β-amino Acid Shows Superactive Agonist Activity For μ-opioid Receptor

Endomorphin-1(Tyr-Pro-Trp-Phe-NH2) is an endogenous ligand for μ-opioid receptor, which can mediate strong analgesic effect.However,the application of endomorphin-1as available analgesic in clinic is impeded by its side effect,such as acute tolerance and physiological dependence,lower stability against enzymes degradation and inefficient to penetrate the blood-brain barrier (BBB). In order to conquer this defects,we substituted P-amino acid (our group synthesised in chemical method and named it as Amp) for the amino acid in peptide backbone of EM-1,got analogues as follow:Tyr-Amp-Trp-Phe-NH2(1), Tyr-Pro-Amp-Phe-NH2(2), Tyr-Pro-Trp-Amp-NH2(3), Tyr-Pro-Amp-NH2(4) and Tyr-Amp-Phe-NH2(5). We evaluated bioactivity of this analogues by detecting the binding affinities for μ-and8-opioid receptor, detecting the level of cyclic AMP formation, phosphorylation level of extracellular signal-regulated kinase(ERK1/2) and antinociceptive activity test in vivo.Our study discovered that analogues1,2,4and5showed lower activity in both binding affinities and inhibition of fosklin-stimulated cyclic AMP formation,while analogue3displayed five times more powerful than EMI in μ-opioid receptor binding affinity,but ten times lower in8-opioid receptor binding affinity,so that increasing selectivity strongly for μ-opioid receptor. moreover,analogue3displayed higher efficient at inhibiting formation of cAMP and antinociceptive activity in vivo than both the other analogues and EM1.Obviously,substitution the novel β-amino acid Amp for Phe in position4of EMI can improve the affinity and selectivity for μ-opioid receptor,then enhance its bioactivities,which might indicate a new way to study the structure-activity relationship of EM1.