Opioid profiles of Cys2-containing enkephalin analogues

To elucidate the structural features determining delta-opioid receptor properties of enkephalin analogues containing Cys(O2NH2) in position 2, a series of Cys(2)-containing derivatives were synthesized and tested for their effectiveness in depressing electrically evoked contractions of the mouse vas deferens (predominantly enkephalin-selective delta-opioid receptors) and the guinea-pig ileum (mu- and kappa-opioid receptors). The peptidase resistance of the compounds was also tested. The ratio IC50 in the guinea-pig ileum/IC50 in the mouse vas deferens, indicating selectivity for delta-opioid receptors, was high for Cys(O2NH2)(2)-containing analogues and especially for [Cys(O2NH2)(2), Leu(5)]enkephalin, which was about seven times more selective than delta-opioid receptor selective ligand cyclic [D-Pen(2), D-Pen(5)]enkephalin (DPDPE). The dissociation constant (K-A) and relative efficacy (e(rel)) of the compounds in the mouse-isolated vas deferens were determined using explicit formulae derived by fitting of the data points with two-parametric hyperbolic function. The obtained values for K-A and e(rel) suggest that: (i) incorporation of Cys(O2NH2)(2) in the molecule of [Leu(5)]enkephalin highly increases the efficacy and does not change significantly the affinity of the respective analogues to delta-opioid receptors; [Cys(O2NH2)(2), Leu(5)]enkephalin has higher affinity than DPDPE, but is less resistant to enzyme degradation; the effect of this modification on the efficacy is decreased when methionine is in position 5; (ii) D-configuration of Cys(O2NH2)(2)-containing analogues increases their peptidase resistance, but reduces efficacy and affinity of the peptides towards delta-opioid receptors; (iii) the substitution of Cys(O2NH2) with Hcy(O2NH2) reduces the efficacy, affinity and potency of the respective analogues and maintains their sensitivity to endogenous peptidases; (iv) the substitution of the sulfonamide group with benzyl group in the molecule of Cys in position 2 decreases their efficacy and affinity toward delta-opioid receptors, but attaches resistance to enzyme degradation. The results obtained in this study allow: (i) to involve the receptor affinity and agonist efficacy as drug-design consideration for delta-opioid receptor properties of newly synthesized compounds and (ii) to characterize some of the structural features, which set the pattern for their opioid profiles. (C) 2004 Elsevier B.V. All rights reserved.