Discovery and characterization of YSA64, a RBM39 degrader with in vivo efficacy and potent cellular activity in pediatric Ewing sarcoma A673.

Depletion of the splicing factor RBM39 disrupts spliceosome function and induces widespread RNA splicing defects, leading to antiproliferative effects in susceptible cancer cells. Here, we report the discovery and characterization of a new series of biphenyl-containing RBM39 degraders. The lead compound 42 promotes RBM39 degradation through formation of a ternary complex with RBM39 and DCAF15/DDB1 in a Cullin-RING E3 ligase- and proteasome-dependent manner, consistent with a molecular glue mechanism. Transcriptomic analyses in HCT-116 and K562 cells revealed extensive alternative splicing alterations and suppression of cell-cycle-associated pathways, resulting in G2/M-phase arrest without apoptosis. Comparative cellular profiling identified 41 (YSA64) as a potent analog in acute myeloid leukemia MV4-11 cells and Ewing sarcoma A673 cells, disease contexts that have been minimally explored for RBM39 degraders. Notably, 41 exhibited favorable oral pharmacokinetics and significant antitumor efficacy in MV4-11 xenograft models. Collectively, this work expands the chemical space of RBM39 degraders and supports their continued development as RNA splicing-targeted anticancer agents.