Structure-activity relationships of arodyn, a novel acetylated kappa opioid receptor antagonist

We previously reported that the novel dynorphin A (Dyn A, Tyr-Gly-Gly-Phe-Leu-Arg-Arg-Ile-Arg-Pro-Lys-Leu-Lys-Trp-Asp-Asn-Gln) analog arodyn (Ac[Phe(1,2,3), Arg(4), D-Ala(8)] Dyn A-(1-11)NH2, Bennett, M. A., Murray, T. F. & Aldrich, J.V. (2002) J. Med. Chem. vol. 45, pp. 5617-5619) is a kappa opioid receptor-selective peptide [K-i(kappa) = 10 nM, K-i ratio (kappa/mu/delta) = 1/174/583] which exhibits antagonist activity at kappa opioid receptors. In this study, a series of arodyn analogs was prepared and evaluated to explore the structure activity relationships (SAR) of this peptide; this included an alanine scan of the entire arodyn sequence, sequential isomeric D-amino acid substitution in the N-terminal 'message' sequence, NMePhe substitution individually in positions 1-3, and modifications in position 1. The results for the Ala-substituted derivatives indicated that Arg(6) and Arg(7) are the most important residues for arodyn's nanomolar binding affinity for kappa opioid receptors. Ala substitution of the other basic residues (Arg(4), Arg(9) and Lys(11)) resulted in lower decreases in affinity for kappa opioid receptors (three- to fivefold compared with arodyn). Of particular interest, while [Ala(10)] arodyn exhibits similar kappa opioid receptor binding as arodyn, it displays higher kappa vs. mu opioid receptor selectivity [K-i ratio (kappa/mu) = 1/350] than arodyn because of a twofold loss in affinity at mu opioid receptors. Surprisingly, the Tyr(1) analog exhibits a sevenfold decrease in kappa opioid receptor affinity, indicating that arodyn displays significantly different SAR than Dyn A; [Tyr(1)] arodyn also unexpectedly exhibits inverse agonist activity in the adenylyl cyclase assay using Chinese hamster ovary cells stably expressing kappa opioid receptors. Substitution of NMePhe in position 1 gave [NMePhe(1)] arodyn which exhibits high affinity [K-i(kappa) = 4.56 nM] and exceptional selectivity for kappa opioid receptors [ K-i ratio (kappa/mu/delta) = 1/1100/> 2170]. This peptide exhibits antagonistic activity in the adenylyl cyclase assay, reversing the agonism of 10 nM Dyn A-(1-13)NH2. Thus [NMePhe(1)] arodyn is a highly kappa opioid receptor-selective antagonist that could be a useful pharmacological tool to study kappa opioid receptor-mediated activities.