Docking 14 Million Virtual Isoquinuclidines against the μ and κ Opioid Receptors Reveals Dual Antagonists–Inverse Agonists with Reduced Withdrawal Effects

Large library docking of tangible molecules has revealed potent ligands across many targets. While make-on-demand libraries now exceed 75 billion enumerated molecules, their synthetic routes are dominated by a few reaction types, reducing diversity and inevitably leaving many interesting bioactive-like chemotypes unexplored. Here, we investigate the large-scale enumeration and targeted docking of isoquinuclidines. These "natural-product-like" molecules are rare in current libraries and are functionally congested, making them interesting as receptor probes. Using a modular, four-component reaction scheme, we built and docked a virtual library of over 14.6 million isoquinuclidines against both the μ- and κ-opioid receptors (MOR and KOR, respectively). Synthesis and experimental testing of 18 prioritized compounds found nine ligands with low μM affinities. Structure-based optimization revealed low- and sub-nM antagonists and inverse agonists targeting both receptors. Cryo-electron microscopy structures illuminate the origins of activity on each target. In mouse behavioral studies, a potent joint MOR-antagonist and KOR-inverse-agonist reversed morphine-induced analgesia, phenocopying the MOR-selective antioverdose agent naloxone. Encouragingly, the isoquinuclidine induced less severe opioid-withdrawal symptoms versus naloxone and did not induce conditioned-place aversion, reflecting reduced dysphoria, consistent with its KOR-inverse agonism. The strengths and weaknesses of bespoke library docking and of docking for opioid receptor polypharmacology will be considered.