Discovery of Stable Non-opioid Dynorphin A Analogues Interacting at the Bradykinin Receptors for the Treatment of Neuropathic Pain

Dynorphin A (Dyn A) is a unique endogenous ligand that possesses well-known neuroinhibitory effects via opioid receptors with a preference for the kappa receptor but also neuroexcitatory effects, which cause hyperalgesia. We have shown that the neuroexcitatory effects are mediated through bradykinin (BK) receptors and that intrathecal (i.th.) administration of our lead ligand 1, [des-Arg⁷]-Dyn A-(4-11), which shows good binding affinity (IC₅₀ = 150 nM) at the BK receptors, blocks Dyn A-induced hyperalgesia in naïve animals and reverses thermal and tactile hypersensitivities in a dose-dependent manner in nerve-injured animals. However, 1 has a serious drawback as a potential drug candidate for the treatment of neuropathic pain because of its susceptibility to enzymatic degradation. In an effort to increase its stability, we modified ligand 1 using non-natural amino acids and found that analogues substituted at or near the N-terminus with a d-isomer retain binding at the receptor and provide a large increase in stability. In particular when Leu⁵ was modified, with either the d-isomer or N-methylation, there was a large increase in stability (t_1/2 = 0.7-160 h in rat plasma) observed. From these studies, we have developed a very stable Dyn A analogue 16, [d-Leu⁵,des-Arg⁷]-Dyn A-(4-11), that binds to BK receptors (IC₅₀ = 130 nM) in the same range as ligand 1 and shows good antihyperalgesic effects in both naïve rats and L₅/L₆ spinal nerve ligation rats.