Design and synthesis of cyclic analogs of the kappa opioid receptor antagonist arodyn

Abstract

Opioid receptors are important therapeutic targets for mood disorders and pain. Kappa opioid receptor (KOR) antagonists have recently shown potential for treating drug addiction and depression. Arodyn (Ac[Phe1,2,3,Arg4,D-Ala8]Dyn A(1-11)-NH2), an acetylated dynorphin A (Dyn A) analog, has demonstrated potent and selective KOR antagonism, but can be rapidly metabolized by proteases. Cyclization of arodyn could enhance metabolic stability and potentially stabilize the bioactive conformation to give potent and selective analogs. Accordingly, novel cyclization strategies utilizing ring closing metathesis (RCM) were pursued. However, side reactions involving olefin isomerization of O-allyl groups limited the scope of the RCM reactions, and their use to explore structure-activity relationships of aromatic residues. Here we developed synthetic methodology in a model dipeptide study to facilitate RCM involving Tyr(All) residues. Optimized conditions that included microwave heating and the use of isomerization suppressants were applied to the synthesis of cyclic arodyn analogs. Initial pharmacological data indicates the constraints involving aromatic residues were generally well tolerated at KOR with most of the analogs exhibiting affinities within 3- to 4-fold that of arodyn. RCM was also used in the synthesis of head to side chain cyclized arodyn analogs. Attempted cyclizations involving Tyr(All) residues proceeded in low yields, in contrast to cyclizations involving AllGly residues. However, ring contraction products as a result of olefin isomerization were also observed during the latter cyclizations. The resulting head to side chain cyclized arodyn analogs exhibited a 5-fold decrease in KOR affinity compared to arodyn. We further explored synthesis of arodyn analogs cyclized in both the N-terminal and C-terminal segments resulting in bicyclic arodyn analogs. Here, we present the synthesis of two initial bicyclic peptide KOR ligands with different topologies. The RCM-based bicyclic arodyn analog exhibited KOR affinity within 3-fold that of arodyn, whereas the lactam-based bicyclic analog displayed a substantial loss in affinity for KOR. There are currently no reports of bicyclic opioid peptide ligands and such bicyclic arodyn analogs could be useful as pharmacological tools.