Design, synthesis, and biological activities of cyclic lactam peptide analogues of dynorphin A(1-11)-NH₂¹

We previously have reported four possible binding conformations of dynorphin A (Dyn A) for the central κ opioid receptors, induced by the address sequence, using a molecular mechanics energy minimization approach. The lowest energy conformation was found to exhibit an α-helical conformation in the cyclized address sequence. It was suggested that an α-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 κ opioid receptor selectivity. Side chain to side chain lactam bridges between the i and i + 4 positions have been shown to stabilize α-helical conformations. 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 κ opioid receptors. Positions 2-6, 3-7, and 5-9 were chosen as the sites for incorporating cyclic conformational constraints. Cyclization between D-Asp² and Lys⁶ in c[D-Asp²,Lys⁶]Dyn A(1-11)-NH₂ led to an analogue with pronounced potency and selectivity enhancement for the μ opioid receptor, whereas cyclization between D-Asp³ and Lys⁷ in c[D-Asp³,Lys⁷]Dyn A(1-11)-NH₂ led to a potent ligand (IC₅₀ 4.9 nM) with κ 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 κ receptor may have structural requirements that are distinct from those of μ and δ receptors. Interestingly, analogues with a D-Asp at position 2, 3, or 9 were found to be more potent for the κ 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 κ, μ, and δ receptors.