Design, Synthesis, and Biological Activities of Cyclic Lactam Peptide Analogues of Dynorphin A(1−11)-NH₂

Abstract

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.

Conformation-activity relationships of opioid peptides with selective activities at opioid receptors

“…103 Cyclic lactam analogues substituted at positions 2-6, 3-7, 5-9, and their linear counterparts were synthesized and examined (Table VIII). The cyclic analogue 52 had high selectivity for receptors and with high potency, while analogue 53 had good potency and high selectivity for receptors.…”

Human Growth Hormone-Releasing Hormone hGHRH(1−29)-NH2: Systematic Structure−Activity Relationship Studies

“…yield potent analogues with lactam constraints are those that are proposed to assume a helical conformation in their bioactive state (GHRH, CRF, PTH, and dynorphins, among others). [63][64][65][66][67] Felix et al 61 first synthesized GHRH analogues that contained [i-(i+4)] lactam bridges, formed through a BOP-mediated, side-chain-to-side-chain lactam cyclization reaction. 37 Of five peptides containing the cyclic motif between residues 4-8, 8-12, 12-16, 16-20, and 21-25, all but the cyclic (16)(17)(18)(19)(20) peptide were equipotent or had greater biological potency than hGHRH(1-44)-NH 2 in vitro.…”

A Novel N-Terminal Cyclic Dynorphin A AnaloguecycloN,5[Trp3,Trp4,Glu5] Dynorphin A-(1−11)NH2That Lacks the Basic N-Terminus

“…This inherent conformational flexibility of Dyn A, which permits the peptide to adopt different conformations in different environments (i.e., in different opioid receptor binding sites), may be one of the reasons for the low κ opioid receptor selectivity of Dyn A. 39,40 To restrict the flexibility of Dyn A, various side chainto-side chain cyclic analogues of Dyn A have been synthesized, [41][42][43][44][45][46] but none of these peptides have constrained the critical Tyr 1 residue. Also to date, none of the reported cyclic analogues of Dyn A have shown antagonist activity.…”