Discovery of Novel Biased Opioid Receptor Ligands through Structure-Based Pharmacophore Virtual Screening and Experiment

G_i-protein-biased agonists with minimal β-arrestin recruitment represent opportunities to overcome the serious adverse effects of human mu opioid receptor (μ-OR) agonists and developing alternative and safe treatments for pain. In order to discover novel non-morphinan opioid receptor agonists, we applied hierarchical virtual screening of our in-house database against a pharmacophore based on modeling the active conformations of opioid receptors. We discovered an initial hit compound, a novel μ-OR agonist with a pyrazoloisoquinoline scaffold. We applied computational R-group screening to this compound and synthesized 14 derivatives predicted to be the best. Of these, a new G_i-protein-biased compound, 1-{5-(3-chlorophenyl)-7,8-dimethoxy-3-[4-(methylsulfonyl)benzyl]-3H-pyrazolo[3,4-c]isoquinolin-1-yl}-N,N-dimethylmethanamine, showed an EC₅₀ value of 179 nm against the μ-OR. This resulted in significant pain relief for mice in the phase II period of formalin response tests. This study provides a new strategy to identify diverse sets of promising compounds that might prove useful for the development of drugs that target other G-protein-coupled receptors.