Abstract P087: Targeting LSD1 protein scaffolding function in FET-rearranged sarcomas with SP-2577
Authors: Rask Galen , Theisen Emily R. , Duncan Aundrietta D. , Santiesteban Daniela Y. 
Published: December 2021

Abstract
The RNA-binding proteins, FUS, EWSR1, and TAF15 (FET protein family) are frequently fused to transcription factors, and the resulting oncogenic fusion proteins characterize various sarcomas. These fusion proteins act as aberrant transcription factors and promote oncogenic gene expression profiles. FET family rearranged sarcomas include Ewing Sarcoma, desmoplastic round cell tumors, myxoid liposarcoma, and clear cell sarcoma, amongst others. A growing body of work suggests that, independently of Lysine Specific Demethylase 1 (LSD1) demethylase activity, LSD1’s association with FET family oncogenic transcription factors is critical for FET-rearranged sarcoma development and progression. Extensive ongoing work aims to characterize the functional relationship of LSD1 and EWS/FLI, as well as the anti-tumor effect of disrupting that interaction. SP-2577 is an oral, first-in-class, small molecule with reversible, noncompetitive inhibition of LSD1 demethylase activity (IC50: 25–50 nM) and, importantly, also disrupts LSD1 protein-protein interactions. In vitro and in vivo data demonstrate SP-2577, or analogs, reverses EWS/ETS-mediated transcriptional regulation at both up- and downregulation target genes, leading to significant tumor growth inhibition in Ewing Sarcoma mouse xenograft studies. In this study, we set out to expand our understanding of SP-2577’s efficacy in diverse FET-rearranged malignancies including desmoplastic small round cell tumor (DSCRT: EWSR1/WT1), myxoid liposarcoma (ML: FUS/DDIT3), and clear cell sarcoma (CCS: EWS/ATF1). To examine whether these additional FET-rearranged sarcomas are sensitive to LSD1 scaffolding inhibition, we measured viability of Ewing sarcoma cells A673, TC32, TTC-446, and SK-N-MC, clear cell sarcoma cell lines SU-CCS-1, DTC-1, myxoid liposarcoma cell lines DL-221, 402-91, and 1765-92, and DSCRT patient derived xenograft organoids from SJDSRCT046151, X0069, SJDSRCT046155 after 96 hours of treatment with the clinical compound SP-2577, a first generation compound SP-2509, an inactive control, SP2513, and, a TCP-scaffold based, irreversible inhibitor of LSD1, OG-L002. While SP-2513 and OG-L002 had no activity against these cells SP-2509 and SP-2577 demonstrated potent activity across cell lines in vitro. Preliminary in vivo assessment of SP-2577 efficacy in DSCRT PDX SJDSRCT046151 resulted in a significant delay in time to event (p&lt;0.02). Together, these data demonstrate that the unique scaffolding inhibition function of SP-2577 is essential for reducing cell viability through LSD1 targeting. Currently, we are elucidating changes to the transcriptome and proteome following SP-2577 treatment, in vitro. These data, in combination with results from the ongoing dose expansion phase 1 trial of SP-2577 in select sarcoma patients, may offer further mechanistic insight into the dependency of FET-rearranged sarcomas on LSD1 scaffolding functions and potentially lead to identifying biomarkers of sensitivity and response.
Citation Format: Galen Rask, Emily R. Theisen, Aundrietta D. Duncan, Daniela Y. Santiesteban. Targeting LSD1 protein scaffolding function in FET-rearranged sarcomas with SP-2577 [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr P087.