Desmoplastic small round cell tumor (DSRCT) is a rare pediatric sarcoma with poor overall survival. This tumor is absolutely dependent on the continued expression and activity of its defining molecular lesion, the EWS–WT1 transcription factor. Unfortunately, the therapeutic targeting of transcription factors is challenging, and there is a critical need to identify compounds that inhibit EWS–WT1. Here we show that the compound lurbinectedin inhibits EWS–WT1 by redistributing the protein within the nucleus to the nucleolus. This nucleolar redistribution interferes with the activity of EWS–WT1 to reverse the expression of over 70% of the transcriptome. In addition, the compound blocks the expression of the EWS–WT1 fusion protein to inhibit cell proliferation at the lowest GI50 ever reported for this compound in any cell type. The effects occur at concentrations that are easily achievable in the clinic and translate to the in vivo setting to cause tumor regressions in multiple mice in a xenograft and PDX model of DSRCT. Importantly, this mechanism of nucleolar redistribution is also seen with wild-type EWSR1 and the related fusion protein EWS–FLI1. This provides evidence for a “class effect” for the more than 18 tumors driven by EWSR1 fusion proteins. More importantly, the data establish lurbinectedin as a promising clinical candidate for DSRCT.

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