Cys(O 2NH 2) 2]enkephalin analogues and dalargin: Selectivity for δ-opioid receptors

Abstract

To investigate the structure-activity relationships for potent and selective action of enkephalins at the δ-opioid receptors, two newly synthesized analogues, [Cys(O 2NH 2) 2,Leu 5]enkephalin and [Cys(O 2NH 2) 2,Met 5]enkephalin and the hexapeptide [ d-Ala 2,Leu 5]enkephalyl-Arg (dalargin) were tested and compared with [Leu 5]enkephalin and [Met 5]enkephalin, for their effectiveness to inhibit electrically evoked contractions of the mouse vas deferens (predominantly enkephalin-selective δ-opioid receptors) and the guinea pig ileum (μ- and κ-opioid receptors). The mouse vas deferens assays included evaluation of the effects of opioid agonists on the first, purinergic, and the second, adrenergic, components of electrically evoked biphasic responses (10 Hz and 20 Hz) and on ATP- or noradrenaline-evoked, tetrodotoxin-resistant responses. The opioids tested inhibited in a similar manner: (i) the purinergic and the adrenergic components of the electrically evoked contractions; and (ii) the ATP- and noradrenaline-induced postjunctional responses of the mouse vas deferens. Extremely low IC 50 values (of 2–5 orders) were found for [Cys(O 2NH 2) 2,Leu 5]enkephalin, whose relative potency was between 239 and 1316 times higher than that of [Leu 5]enkephalin. The order of potency for the other peptides in this tissue was: [Cys(O 2NH 2) 2,Met 5]enkephalin > [Leu 5]enkephalin > dalargin > [Met 5]enkephalin. The highest IC 50 values in the guinea pig ileum assays, indicating the lowest affinity for μ-/κ-opioid receptors, were obtained for the cysteine sulfonamide analogues, while dalargin showed a potency four times higher than that of [Met 5]enkephalin. The order of potency in this tissue was: dalargin > [Met 5]enkephalin > [Leu 5]enkephalin > [Cys(O 2NH 2) 2,Met 5]enkephalin > [Cys(O 2NH 2) 2,Leu 5]enkephalin. The ratio, IC 50 in guinea pig ileum: IC 50 in mouse vas deferens, indicating selectivity of the respective peptide for δ-opioid receptors, was extremely high for [Cys(O 2NH 2) 2,Leu 5]enkephalin and especially for the adrenergic component of the responses. This ratio for [Cys(O 2NH 2) 2,Met 5]enkephalin was higher than the ratios for dalargin, [Leu 5]enkephalin and [Met 5]enkephalin, which were about 3 orders of magnitude lower. The results suggest that incorporation of hydrophilic Cys(O 2NH 2) in the enkephalin molecule greatly increases the potency and selectivity of the analogues at δ-opioid receptors, while both d-Ala 2 substitution and lengthening of the peptide chain by Arg 6 in the molecule of [Leu 5]enkephalin decrease them.