Discovery and Binding Mechanism of Pyrazoloisoquinoline-Based Novel beta-Arrestin Inverse Agonists of the Kappa-Opioid Receptor

Chronic exposure to stress or unwanted stimuli has been known to activate kappa opioid receptor/dynorphin (KOR/ DYN) systems, which could induce depressive states and develop into some psychiatric disorders. Here, we report the first discovery of pyrazoloisoquinoline-based novel KOR beta-arrestin inverse agonists through synthesis, structure-activity relationships, optimization, and the biological evaluations of mu/kappa/delta opioid receptor activities with cAMP and beta-arrestin recruitment assays. The optimized compound 7q shows potent and selective beta-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 the transmembrane helix 6 position of the activated state is different for the OR subtypes, leading to significantly different interactions between the receptor and beta-arrestin.