Receptor-selective enkephalin analogs

We investigated the opioid profiles of [Cys(2)]-enkephalin (enk) analogs and [D-Ala(2),Leu(5)]-enkephalyl-Arg (dalargin) in in vitro bioassay systems. The results showed that incorporation of hydrophilic Cys(O2NH2)(2) into the enk molecule greatly increases the potency and selectivity of the analogs to delta-opioid receptors, while substitution of D-Ala(2) and Arg(6) into the molecule of [Leu(5)]-enk, to mu-opioid receptors. To elucidate the structural features ensuring action of the receptor-selective enk analogs, a series of [Cys(2)]-enk analogs and of dalargin analogs were tested for their effectiveness to inhibit electrically evoked contractions of guinea-pig ileum and the vas deferens of the hamster and mouse with and without peptidase inhibitors. D-Configuration of Cys(O2NH2)(2) in the molecule of [Leu(5)]- and [Met(5)]-enk increases the resistance to peptidases of the analogs, but is harmful for their potency and selectivity to delta-opioid receptors. Incorporation of Cys(Bzl)(2) into the molecule of [Leu(5)]- and [Met(5)]-enk greatly increases their potency and selectivity to delta-opioid receptors and their resistance to peptidases. We characterized further the structural features of dalargin opioid profile which strongly suggest that: (i) D-configuration of Ala(2) is of importance for conferring resistance to peptidases; (ii) Leu(5) and the interdependence of Leu(5) and D-Ala(2) are of importance for the selectivity of dalargin to mu-opioid receptors; (iii) L-configuration of Phe(4) makes a major contribution to the pharmacological potency of dalargin and (iv) the CONH2 terminal and its association with N-methylation of L-Phe(4) increases the potency, selectivity and efficacy of dalargin to mu-opioid receptors.