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 Cys2-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, Leu5]enkephalin, which was about seven times more selective than delta-opioid receptor selective ligand cyclic [D-Pen2, D-Pen5]enkephalin (DPDPE). The dissociation constant (KA) 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 KA and e(rel) suggest that: (i) incorporation of Cys(O2NH2)2 in the molecule of [Leu5]enkephalin highly increases the efficacy and does not change significantly the affinity of the respective analogues to delta-opioid receptors; [Cys(O2NH2)2, Leu5]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.
Pencheva
Nevena
N
Department of Kinezitherapy, South-West University, 66 Ivan Mihailov Str., 2700 Blagoevgrad, Bulgaria. nevena_pencheva@hotmail.com
Milanov
Peter
P
Vezenkov
Lubomir
L
Pajpanova
Tamara
T
Naydenova
Emilia
E
D000818:Animals; D003545:Cysteine / Q000737:chemistry; D004305:Dose-Response Relationship, Drug; D004745:Enkephalins / Q000737:chemistry / Q000494:pharmacology*; D006168:Guinea Pigs; D007082:Ileum / Q000187:drug effects* / Q000502:physiology; D066298:In Vitro Techniques; D007700:Kinetics; D008297:Male; D051379:Mice; D009119:Muscle Contraction / Q000187:drug effects*; D011957:Receptors, Opioid / Q000819:agonists / Q000502:physiology*; D017465:Receptors, Opioid, delta / Q000819:agonists / Q000502:physiology; D017473:Receptors, Opioid, kappa / Q000819:agonists / Q000502:physiology; D017450:Receptors, Opioid, mu / Q000819:agonists / Q000502:physiology; D013329:Structure-Activity Relationship; D014649:Vas Deferens / Q000187:drug effects* / Q000502:physiology
D004745:Enkephalins; D011957:Receptors, Opioid; D017465:Receptors, Opioid, delta; D017473:Receptors, Opioid, kappa; D017450:Receptors, Opioid, mu; D003545:Cysteine