Discovery and Binding Mechanism of Pyrazoloisoquinoline-Based Novel β‑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 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 the 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.