CerS2 is a druggable target in triple-negative breast cancer.

Triple-negative breast cancer (TNBC) poses a significant therapeutic challenge due to the lack of defined molecular targets. While ceramide synthase 2 (CerS2) has a complex role in oncology, enhancing its enzymatic activity to produce pro-apoptotic, very long-chain ceramides (VLCCs) is a potential anti-cancer strategy. Here, we identify and characterize DH20931, a novel biisoquinoline derivative, as a newly identified small-molecule activator of CerS2. We provide genetic and biochemical evidence that CerS2 is the direct target of DH20931, which shows an effective, receptor-independent cytotoxicity across diverse breast cancer cell lines while sparing normal cells. In vivo, DH20931 demonstrates consistent tumor growth inhibition in both orthotopic xenograft and clinically relevant TNBC patient-derived xenograft (PDX) models, supported by a favorable safety and pharmacokinetic profile. Mechanistically, DH20931 triggers an effective dual mechanism of apoptosis. First, the accumulation of VLCCs induces lipotoxic endoplasmic reticulum (ER) stress, activating the pro-apoptotic ATF4-CHOP pathway. Second, we uncovered a previously unknown physical interaction between CerS2 and the ER calcium channel IP3R1. DH20931 promotes this interaction, enhancing ER-mitochondria proximity and facilitating a marked flux of Ca²⁺ into the mitochondria, which serves as an effective, secondary apoptotic signal. These findings validate CerS2 as a bona fide druggable target and present DH20931 as a promising clinical candidate. This unique synergistic mechanism, coupling lipotoxicity with calcium dysregulation, offers a convincing new strategy for treating aggressive and therapy-resistant breast cancers.