BACKGROUND: Ewing sarcoma is a pediatric malignancy characterized by the fusion of the EWSR1 and FLI1 genes, which creates a constitutively activated transcription factor, EWS/FLI1. It is widely known that Ewing sarcoma cells depend on the transcriptional program of EWS/FLI1 for cell survival. Therefore, the goal of this research is to develop and clinically translate small molecules that suppress EWS/FLI1 activity. We have previously reported that the natural product trabectedin suppresses EWS/FLI1 activity. In addition, the compound synergizes with irinotecan to suppress Ewing sarcoma tumor growth. However, the compound suffers from a narrow therapeutic index that limits the poisoning of EWS/FLI1 in patients. In this report, we show that the trabectedin analog PM01183 has improved targeting of EWS/FLI1 and preserved synergy with irinotecan. In addition, the compound is known to have a dramatically improved therapeutic index suggesting improved activity of this compound in the clinic.

METHODS: In this report, we characterize the mechanism of suppression of EWS/FLI1 by trabectedin using confocal immunocytochemistry and chromatin immunoprecipitation. We show enhanced suppression of EWS/FLI1 by PM01183 using the Fluidigm platform and confirm the results with a luciferase reporter construct and high-content quantitative PCR. Finally, we tested the ability of PM01183 to suppress tumor growth in xenograft models of Ewing sarcoma both in the absence and presence of irinotecan.

RESULTS: PM01183 more effectively suppressed the gene signature of EWS/FLI1 than the parent compound trabectedin. In addition, the drug re-localizes EWS/FLI1 away from target genes in the nucleus leading to suppression of these EWS/FLI1 targets. These results are consistent with the activity of the parent compound, trabectedin, that blocks binding of EWS/FLI1 to chromatin. More importantly, the drug causes a regression of a TC32 xenograft and markedly suppresses growth of a TC71 xenograft when combined with irinotecan. Finally, PM01183 has a substantially improved pharmacologic profile in patients in comparison to trabectedin suggesting that these effects are bio-achievable in patients.

CONCLUSIONS: In comparison to trabectedin, we have shown that its structural analog, PM01183 shows enhanced suppression of an EWS/FLI1 gene signature and preserves the synergy with irinotecan that we previously reported. We suggest a mechanistic basis for this activity against EWS/FLI1 and show excellent activity against Ewing sarcoma xenografts. Together, these results suggest that the clinical translation of PM01183 as an EWS/FLI1 targeted therapy both alone and in combination with irinotecan is warranted.

Citation Format: Matt Harlow, Nichole Maloney, Maria Jose Guillen Navarro, Maurizio D'Incalci, Carlos Galmarini, Pablo Manuel Aviles Marin, Patrick Grohar. PM01183 shows an improved therapeutic index relative to trabectedin and suppresses EWS/FLI1 activity at clinically achievable concentrations. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3962. doi:10.1158/1538-7445.AM2014-3962