Re-evaluation of the mechanism of action of α,β-unsaturated carbonyl DUB inhibitors b-AP15 and VLX1570: CIAPIN1 is a major target of VLX1570 for non-selective covalent modification and aggregation in a multiple myeloma model.

Deubiquitinating enzymes are a growing target class across multiple disease states, with several inhibitors now reported. b-AP15 and VLX1570 are two structurally related USP14/UCH-37 inhibitors. Initially taken forward into a phase I/II clinical trial for refractory Multiple Myeloma, VLX1570 has since been put on full clinical hold due to dose limiting toxicity. Though a proteomic approach, here we demonstrate that these compounds target a diverse range of proteins, resulting in the formation of higher molecular weight complexes. Activity-based proteome profiling identified CIAPIN1 as a sub-micromolar covalent target of VLX1570, and further analysis demonstrated that high molecular weight complex formation leads to aggregation of CIAPIN1 in intact cells.