Cell‐Active, Irreversible Covalent Inhibitors Targeting a Surface‐Exposed Non‐Catalytic Lysine on Aurora a Kinase by Using Squarate Chemistry

Targeting surface-exposed lysines in kinases through covalent modification presents a major challenge due to their high pK_a and inherently low reactivity. While current research primarily targets more reactive catalytic lysines buried in the ATP-binding pocket, no systematic rational strategy has yet been developed for selectively engaging surface-exposed lysines. Herein, we present a versatile strategy for developing cell-active covalent kinase inhibitors (CKIs) by selectively targeting unique surface-exposed lysines using squarate chemistry. By using Aurora A (AURKA) as a proof-of-concept, we systematically evaluated this approach against other well-known lysine-reactive warheads (e.g., EBA, SO₂F, and OSO₂F) and demonstrated, for the first time, squarates' superior efficacy in engaging these challenging low-reactivity lysines. Amongst various AURKA CKIs, AL8 emerged as the first-in-class squarate-based, cell-active inhibitor, exhibiting excellent selectivity in both biochemical and cellular assays with long-residence time in covalently engaging endogenous AURKA. Detailed investigation of effects of leaving groups on squaric esters provided valuable insights for future development of lysine-reactive CKIs. Our finding has established squarate-containing ligands as a unique and readily tunable platform for covalent modification of surface-exposed, non-catalytic lysines in targeted kinase drug discovery.