Analogs of the κ opioid receptor antagonist arodyn cyclized by ring-closing metathesis retain κ opioid receptor affinity, selectivity and κ opioid receptor antagonism

The dynorphin (Dyn) A analog arodyn (Ac[Phe^1-3,Arg⁴,d-Ala⁸]Dyn A-(1–11)NH₂) is a κ opioid receptor-selective antagonist, but as a linear peptide it is conformationally flexible and subject to metabolism by proteases. To restrict its conformational flexibility both short- and long-range cyclizations via ring-closing metathesis (RCM) involving residues in both the N-terminal “message” and C-terminal “address” sequences were explored. Cyclization between allyglycine (AllGly) residues in positions 5 and 8 yielded peptides with the highest κ opioid receptor affinity (K_i = 54 and 63 nM) and selectivity for κ over µ receptors (185- and 149-fold) comparable to arodyn (175-fold), with similar results for the peptides with the cis and trans double bond configurations; both isomers exhibited competitive antagonism of κ opioid receptors with potencies similar to arodyn. The minor cis isomer cyclized between AllGly residues in positions 2 and 8 exhibited similar κ receptor affinity to arodyn, but lacked selectivity for κ over µ opioid receptors. These results provide important structure-activity relationship information for cyclization of arodyn that we are using in the design of the next generation of cyclic arodyn analogs. [Figure not available: see fulltext.]