KSQ-4279, an Inhibitor of Ubiquitin Specific Peptidase 1, Enhanced the Chemotherapeutic Efficacy in ABCB1/ABCG2/ABCC1-Mediated Multidrug Resistant Cancers.

Multidrug resistance (MDR) remarkably hinders the success of clinical chemotherapy in carcinomas. The ATP-binding cassette (ABC) transporter as ABCB1, ABCG2, and ABCC1 are most crucial to drive MDR. Regrettably, no MDR modulators have been accepted in clinic. Herein, KSQ-4279, a first-in-class ubiquitin-specific peptidase 1 (USP1) inhibitor in clinical development, was found as a pan-MDR modulator. Our study showed that KSQ-4279 strikingly intensified the cytotoxicity of multiple classical chemotherapeutic drugs in ABCB1/ABCG2/ABCC1-induced MDR cancers independent of its own cytotoxicity in vitro, and remarkably improved chemotherapeutic efficacy not only in ABCB1/ABCG2/ABCC1-overexpressing tumor xenografts in vivo, but also in ABCB1-overexpressing clinical lung cancers ex vivo. Mechanistically, KSQ-4279 weakened ABCB1/ABCG2/ABCC1 efflux function, thus increasing drugs' reservation in cells; more specifically, it was achieved by KSQ-4279 activating the ATPase activity and competing for the substrate-binding pockets of ABCB1/ABCG2/ABCC1. Besides, at the effective reversal concentrations, KSQ-4279 neither altered expression and localization of ABCB1/ABCG2/ABCC1, nor affected USP1's potential downstream AKT or ERK1/2 signaling. This is the first study to investigate the combination of USP1 inhibitor (KSQ-4279) with traditional chemotherapeutic drugs in reversing MDR, which surprisingly hinted ABCB1, ABCG2, and ABCC1 as the new targets of KSQ-4279, and advocated this promising combination therapy in clinical refractory MDR cancers.