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On the basis of the structure-activity relationships of delta-opioid-selective peptide ligands and on a model of the proposed bioactive conformation for a potent and selective, conformationally constrained delta-opioid peptide ligand [(2S, 3R)-TMT1]DPDPE, a series of small organic peptide mimetic compounds targeted for the delta-opioid receptor have been designed, synthesized, and evaluated in radiolabeled ligand binding assays and in vitro bioassays. The new non-peptide ligands use piperazine as a template to present the most important pharmacophore groups, including phenol and phenyl groups and a hydrophobic moiety. This hydrophobic group was designed to mimic the hydrophobic character of the D-Pen residues in DPDPE, which has been found to be extremely important for increasing the binding affinity and selectivity of these non-peptide ligands for the delta-opioid receptor over the mu-opioid receptor. Compound 6f (SL-3111) showed 8 nM binding affinity and over 2000-fold selectivity for the delta-opioid receptor over the mu-opioid receptor. Both enantiomers of SL-3111 were separated, and the (-)-isomer was shown to be the compound with the highest affinity for the delta-opioid receptor found in our study (IC50 = 4.1 nM), with a selectivity very similar to that observed for the racemic compound. The phenol hydroxyl group of SL-3111 turned out to be essential to maintain high affinity for the delta-opioid receptor, which also was observed in the case of the delta-opioid-selective peptide ligand DPDPE. Binding studies of SL-3111 and [p-ClPhe4]DPDPE on the cloned wild-type and mutated human delta-opioid receptors suggested that the new non-peptide ligand has a binding profile similar to that of DPDPE but different from that of (+)-4-[((alphaR)-alpha(2S,5R)-4-allyl-2, 5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC-80), another delta-opioid-selective non-peptide ligand.

Adenylyl cyclase (AC) superactivation is thought to play an important role in opioid tolerance, dependence, and withdrawal. In the present study, we investigated the involvement of protein kinases in chronic ␦-opioid agonist-mediated AC superactivation in Chinese hamster ovary (CHO) cells stably expressing the human ␦-opioid receptor (hDOR/CHO). Maximal forskolin-stimulated cAMP formation in hDOR/CHO cells increased by 472 Ϯ 91, 399 Ϯ 2, and 433 Ϯ 73% after chronic treatment with the ␦-opioid-enkephalin, and deltorphin II, respectively. Concurrently, chronic SNC 80 (1 M, 4-h) treatment augmented 32 P incorporation into a 200-kDa protein immunoreactive with the ACV/VI antibody by 300 Ϯ 60% in hDOR/CHO cell lysates. The calmodulin antagonist calmidazolium significantly attenuated chronic deltorphin II-mediated AC superactivation. Tyrosine kinase (genistein) and protein kinase C (chelerythrine) inhibitors individually had minimal effect on chronic ␦-opioid agonist-mediated AC superactivation. Conversely, simultaneous treatment with both genistein and chelerythrine significantly attenuated AC superactivation. Because we showed previously that the Raf-1 inhibitor 3-(3,5-dibromo-4-hydroxybenzylidene-5-iodo-1,3-dihydro-indol-2-one (GW5074) attenuates AC superactivation, we hypothesize that parallel calmidazolium-, chelerythrine-, and genistein-sensitive pathways converge at Raf-1 to mediate AC superactivation by phosphorylating AC VI in hDOR/CHO cells.Chronic opioid receptor activation frequently leads to the sensitization of adenylyl cyclase to stimulators after the inhibitory agonist has been removed (AC superactivation). AC superactivation after chronic opioid agonist exposure is thought to contribute to the development of opioid tolerance, dependence, and withdrawal (Williams et al., 2001). A better understanding of the molecular mechanisms of chronic ␦-opioid agonist treatment-mediated AC superactivation should aid in the development of longer acting analgesics with fewer side effects.We have reported previously that in Chinese hamster ovary (CHO) cells stably expressing the human ␦-opioid receptor (hDOR/CHO), chronic ␦-opioid agonist treatment gives rise to AC superactivation (Malatynska et al., 1996). In addition, we also demonstrated that chronic ␦-opioid agonist treatment of the hDOR/CHO cells augments 32 P incorporation into proteins immunoreactive with an AC V/VI-specific antibody (Varga et al., 1999). The identity of protein kinase(s) involved in chronic ␦-opioid agonist-mediated phosphorylation in hDOR/CHO cells, and the role of the phosphorylation in AC superactivation, however, have not been investigated previously. In the present work, we studied the effect of protein kinase inhibitors on chronic ␦-opioid agonistmediated AC superactivation.Recent data (Tan et al., 2001) indicate the important role of the protein kinase p74Raf-1 in phosphorylation of adenylyl cyclase VI in transfected human embryonic kidney 293 cells. Moreover, Raf-1-mediated phosphorylation led to the sensitization of AC VI to s...

We previously have reported four possible binding conformation of dynorphin A (Dyn A) for the central kappa opioid receptors, induced by the address sequence, using a molecular mechanics energy minimization approach. The lowest energy conformation was found to exhibit an alpha-helical conformation in the cyclized address sequence. It was suggested that an alpha-helical conformation in the cyclized address sequence or a helical conformation induced by the conformational characteristics of the message sequence may be important for binding potency and kappa opioid receptor selectivity. Side chain to side chain lactam bridges between the i and i + 4 positions have been shown to stabilize alpha-helical conformation. Thus, a series of cyclic lactam analogues of dynorphin A(1-11)-NH2 have been designed, synthesized and evaluated by the guinea pig brain (GPB) binding assay and guinea pig ileum (GPI) bioassay to evaluate the conformational analysis prediction and, further, to investigate the conformational requirements for high potency and selectivity for kappa opioid receptors. Positions 2-6, 3-7, and 5-9 were chosen as the sites for incorporating cyclic conformational constraints. Cyclization between D-Asp(2) and Lys(6) in c[D-Asp(2),Lys(6)]Dyn A(1-11)-NH2 led to an analogue with pronounced potency and selectivity enhancement for the mu opioid receptor, whereas cyclization between D-Asp(3) and Lys(7) in c[D-Asp(3),Lys(7)]Dyn A(1-11)-NH2 led to a potent ligand (IC(50) 4.9 nM) with kappa receptor selectivity. The other analogues in the series proved to be less selective. The biological results led to the suggestion that the binding conformation for the kappa receptor may have structural requirements that are distinct from those of mu and delta receptors. Interestingly, analogues with a D-Asp at position 2, 3, or 9 were found to be more potent for the kappa receptor than analogues with an L-Asp at the same positions. It is suggested that the incorporation of D-Asp into position 2, 3, or 9 of Dyn A(1-11)-NH2 may have stereochemical and conformational effects on the nearby amino acids which can help discriminate the preference between kappa, mu, and delta receptors.

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