Design and synthesis of highly potent and selective cyclic dynorphin A analogs. 2. New analogs

We have designed and synthesized several cyclic disulfide-containing peptide analogs of dynorphin A (Dyn A) which are conformationally constrained in the putative "address" segment of the opioid ligand. Several of these Dyn A analogs exhibit unexpected apparent selectivities for the kappa and mu opioid receptors(s) of the central vs peripheral nervous systems. Thus, incorporation of conformational constraint in the putative "address" segment of Dyn A analogs has resulted in the kappa/mu opioid receptor ligands [L-Pen5,Cys11]Dyn A1-11-NH2 (4), [Cys5,Cys10]Dyn A1-11-NH2 (5), [Cys5,Cys9]DynA1-11-NH2 (6), and [Cys4,Cys9,Arg10]DynA1-11-NH2(7). All of these analogs possess high kappa and mu opioid receptor affinities for the central receptor (guinea pig brain), but effect only weak potency at peripheral kappa and mu opioid receptors (GPI). In fact cyclic dynorphin A analog 4 shows > 19,000-fold differences between central kappa opioid affinity and potency in the guinea pig ileum (GPI). Additionally analog 4 is not an antagonist in the GPI, suggesting possible receptor differences between these sites. Substitution of Tyr1 by Phe1 in the cyclic 1-11 series gave the analog [Phe1,Cys5,Cys11]Dyn A1-11-NH2 (1) that was surprisingly potent in the guinea pig brain binding assay (IC50 = 15.1 nM) at the kappa receptor, but was inactive in the GPI and mouse vas deferens bioassays. D-Ala2 and Tic4 analogs of 1 had lower affinity at brain kappa receptors and had very weak potencies in the GPI and MVD bioassays. On the other hand, [Cys6,Cys10]DynA1-11-NH2 (8), [Cys8,D-Cys13]DynA1-13-NH2 (9), [D-Cys8,D-Cys12]DynA1-13-NH2 (10), and [D-Pro10,Cys5,Cys13]-Dyn A1-13-NH2 (11) were surprisingly potent in the GPI bioassay, though considerable apparent selectivity for central receptors is still retained. The apparent lack of correlation between the pharmacological profiles observed in smooth muscle and in the brain binding assays, particularly with 1 and 4, may suggest the existence of different subtypes of the kappa and mu opioid receptors in the brain and peripheral systems.