Discovery of Novel Selective Opioid Receptor Ligands: G Protein-Biased Agonist and β-Arrestin Inverse Agonist for Mu and Kappa Opioid Receptors

Abstract

In Part 1, we conducted discovery of novel μ-Opioid receptor (MOR) ligand for pain relief agents. MOR Gi-biased agonists with no recruitment of β-arrestin were introduced as a new analgesic strategy to overcome the conventional undesirable side effects of opioid receptor-targeted drugs, such as tolerance, addiction, respiratory depression, and constipation. For the development of novel Gi-biased MOR agonists, the design, synthesis, and SAR analysis of the amino-pyrazole core skeleton were conducted according to the current SAR data of PZM21 (2a) and its derivatives. New derivatives were biologically evaluated for their agonistic effects on cAMP levels for Gi pathway and β-arrestin recruitment in μ/κ/δ opioid receptors. An optimized selective Gi-biased agonist, compound 17a, was discovered with potent cAMP inhibitory activities, with an EC50 value of 87.1 nM and no activity in the MOR β-arrestin pathway and other subtypes. The in vivo pain relief efficacy of compound 17a was confirmed in a dose-dependent manner with spinal nerve ligation and cisplatin-induced peripheral neuropathy rodent neuropathic pain models.

In Part 2, we conducted discovery of novel β-Opioid receptor (KOR) ligand for psychiatric disorder agents. Chronic exposure of stress or unwanted stimuli has been known to activate kappa opioid receptor/dynorphin (KOR/DYN) systems, which could induce aversive and dysphoric depressive state and develop some psychiatric disorders. Here, we report the first discovery of pyrazoloisoquinoline-based novel KOR β-arrestin inverse agonists through synthesis, structure-activity relationships, optimization, and the biological evaluations of μ/κ/δ opioid receptor activities with cAMP and β-arrestin recruitment assays. The optimized compound 7q shows potent and selective β-arrestin inverse agonism at KOR with an EC50 value of 9.33 nM in contrast to lower activities at DOR and no activity at MOR. Moreover, we use molecular dynamics simulations to predict the binding mode of the inverse agonist and propose a mechanism for the inverse agonism. We find that transmembrane helix 6 position of the activated state is different for the OR subtypes, leading to significantly different interactions between the receptor and β-arrestin.