(P 01) ANTITUMOR ACTIVITY OF CYTOTOXIC AND TARGETED AGENTS IN A PATIENT-DERIVED XENOGRAFT (PDX) OF DESMOPLASTIC SMALL ROUND CELL TUMOR (DSRCT) HIGHLIGHTS THE IRINOTECAN-TRABECTEDIN COMBINATION

Objective: DSRCT is an ultra-rare and very aggressive mesenchymal tumor characterized by the chromosomal translocation t(11:22)(p13;q12) that results in the oncogenic EWSR1-WT1 transcription factor. Identification of effective therapeutic options has been limited by the lack of preclinical models and disease rarity. We generated a new PDX of DSRCT and we comparatively assessed the effectiveness of cytotoxic and targeted agents that are currently available for treating patients with sarcomas that may also be relevant for patients with DSRCT.

Methods: We engrafted 2 DSRCT surgical samples into SCID mice and we successfully established a PDX (DSRCT-1) from a 16 years old patient affected by an EWSR1-WT1 translocated DSRCT after three passages in mice (take rate: 50%). This PDX model and the original clinical tumors were characterized and compared by histology, FISH and RNA sequencing. We analyzed the antitumor activity of the following drugs as single-agent: doxorubicin (2.5mg/Kg), pazopanib (100mg/Kg), larotrectenib (50mg/Kg), vinorelbine (5mg/Kg), eribulin (1mg/Kg), irinotecan (2.5mg/Kg) and trabectedin (0.15mg/Kg). Also, we combined irinotecan with either eribuIin or trabectedin. Drug activity was evaluated as maximum tumor volume inhibition percentage (max TVI%) and complete tumor regression. The effects of different treatments on cell proliferation (Ki67 index) and proteins involved in cell cycle regulation and apoptotic/necroptotic cell death were assessed by immunohistochemistry and western blotting, respectively.

Results: Histomorphological and molecular assessment of DSRCT-1 PDX, including the EWSR1-WT1 fusion and overall transcriptomic profile, confirmed consistency with the originating clinical tumor. A moderate antitumor effect was achieved with single-agent doxorubicin, pazopanib and larotrectenib (max TVI: 55-66%), while single agent trabectedin resulted in a higher effectiveness (max TVI: 82%). Single-agent vinorelbine, irinotecan and eribulin showed the highest activity as they almost completely inhibited tumor growth (max TVI: 96-98%). However, tumors showed a re-growth after the end of treatment. When irinotecan was combined with either eribulin or trabectedin, these combinations led to a complete response at the end of treatment. PDX treated with irinotecan plus trabectedin maintained the response even at the end of the experiment, after treatment discontinuation. Consistently, Ki67 index was markedly reduced by irinotecan-based combinations compared to other treatments. Western blot analysis showed that irinotecan-based combinations these combinations nearly abrogated the expression of proteins involved in the G2/M checkpoint, prevented cell entrance in mitosis as well as induced apoptotic and necroptotic cell death.

Conclusion: Findings of this preclinical study strongly support to further investigate irinotecan in combination with either trabectedin or eribulin in DSRCT. The irinotecan-trabectedin combination appears particularly promising, and deserves to be investigated in prospective clinical studies. We plan to test effectiveness of lurbinectedn, a novel synthetic agent derived from trabectedin with a similar mechanism of action, which is currently under investigation in combination with irinotecan in sarcomas.