Therapeutic vulnerabilities exposed by the 9p21 loss identified through multiparametric drug screening inform rational combination strategies

: Homozygous loss of the 9p21 locus encompassing CDKN2A, CDKN2B, and MTAP is the most frequent copy number alteration across tumor types, making it a promising target for precision medicine strategies. To explore drug vulnerabilities exposed by this loss, we generated 9p21 locus isogenic bladder cancer (BLCA) cell models to perform a multiparametric drug screen, testing 2,349 compounds. We identified cytarabine and methotrexate as significantly more effective in the 9p21 compromised BLCA cells. Analysis of morphological alterations further supported a genotype-specific activity of nucleoside analogs, nominating gemcitabine as a drug with greater efficacy in this context. To further exploit MTAP loss, we explored drug combinations targeting MTAP synthetic lethal partners, PRMT5 and MAT2A. Synergy between cytarabine and inhibitors of PRMT5 (MRTX1719) and MAT2A (AG-270) was mediated by a differential activation of DNA damage and replication stress markers, suggesting an exploitable vulnerability. In fact, rational drug combinations with ATR/CHK1 pathway inhibitors increased efficacy while maintaining 9p21-specificity. Finally, we confirmed the effectiveness of these combinations in cell models of pancreatic adenocarcinoma and pleural mesothelioma, two tumor types with high prevalence of MTAP loss and, most notably, in bladder cancer patient-derived organoids, underscoring the strong translational potential of our findings.