Abstract. Previous studies implicated PRMT5 as a synthetic lethal target for MTAP deleted (MTAP del) cancers, however, the pharmacological characterization of small molecule inhibitors that recapitulate the synthetic lethal phenotype have not been described. MRTX1719 selectively inhibited PRMT5 in the presence of MTA, which is elevated in MTAP del cancers, and inhibited PRMT5-dependent activity and cell viability with >70-fold selectivity in HCT116 MTAP del compared to HCT116 MTAP WT cells. MRTX1719 demonstrated dose-dependent anti-tumor activity and inhibition of PRMT5-dependent SDMA modification in MTAP del tumors. In contrast, MRTX1719 demonstrated minimal effects on SDMA and viability in MTAP WT tumor xenografts, mouse or human hematopoietic cells. MRTX1719 demonstrated marked anti-tumor activity across a panel of xenograft models at well-tolerated doses. Early signs of clinical activity were observed including objective responses in patients with MTAP del melanoma, gallbladder ade