Abstract 2849: Palbociclib as a novel therapeutic for desmoplastic small round cell tumor

Abstract Desmoplastic small round cell tumor (DSRCT) is an aggressive, pediatric cancer caused by the EWSR1-WT1 fusion oncoprotein. Targeted therapies are lacking and the current standard of care, multimodal therapy, is insufficient, leading to a 5-year survival rate of less than 25%. While the EWSR1-WT1 oncoprotein is critical to growth in vitro, its importance has yet to be examined in vivo. Further, the set of EWSR1-WT1 downstream targets critical to oncogenesis and the mechanism by which the two EWSR1-WT1 isoforms regulate them remain underinvestigated. Here we generate a toolkit of DSRCT cell lines that deplete EWSR1-WT1 in a doxycycline (dox) -inducible manner. This toolkit enabled us to establish the essentiality of EWSR1-WT1 not only in vitro but also for the first time in vivo. Using this toolkit, we performed RNA sequencing on four DSRCT cell lines and established the most comprehensive analysis of EWSR1-WT1 downstream targets to date. We discovered novel mechanistic insights into EWSR1-WT1 functionality including the uniqueness of its transcriptional alterations as compared to native WT1, the direct role of EWSR1-WT1 binding in gene upregulation, and the dominant role of the E-KTS isoform in transcription. Given the rarity of DSRCT, we focused on downstream targets with existing inhibitors and perform a drug screen that identified the CDK4/6 inhibitor palbociclib as a novel DSRCT therapeutic. We found that the E-KTS isoform of EWSR1-WT1 binds to the CCND1 promoter, leads to CCND1 expression, and stimulates DSRCT growth through the CCND-CDK4/6-RB axis. Palbociclib treatment conversely reduced DSRCT growth by preventing retinoblastoma phosphorylation and blocking the transition from G1 to S phase. Palbociclib treatment was effective not only in vitro but also in vivo where it significantly reduced tumor growth in two xenograft models of DSRCT. Given these novel findings and palbociclib’s previous approval by the FDA for the treatment of breast cancer, we advance palbociclib as an exciting DSRCT therapy that warrants urgent clinical investigation. Citation Format: Justin W. Magrath, Shruthi Sanjitha Sampath, Dane A. Flinchum, Alifiani B. Hartono, Ilon N. Goldberg, Julia R. Boehling, Suzana D. Savkovic, Sean B. Lee. Palbociclib as a novel therapeutic for desmoplastic small round cell tumor abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 2849.