1701P - Inhibition of mTOR signaling enhances trabectedin activity in soft tissue sarcoma

Soft-tissue sarcomas (STS) are life-threatening diseases, for which more efficient therapeutic options are necessary. Trabectedin (T) is an anti-tumor drug approved for the treatment of advanced STS. Synergistic drug combinations could improve T results in STS patients. Rapamycin (R) is an mTOR pathway inhibitor. Synergy with T has been described in ovary clear cell carcinoma. Of note, R suppresses DNA double-strand breaks repair, thus generating the perfect set-up for T activity. Altogether, we hypothesize that T+R combination (T+R) is synergistic in STS, and that mTOR inhibition enhances T activity. Human STS cell lines (n=9) were treated with increasing doses of T (1x10-7M to 1x10-11M) and/ or R (1x10-9 to 1x10-11) to determine IC50 and combination index (CI) values. Cells were initially exposed to R, 2hours later T was added, under the assumption that R pre-treatment favors T cytotoxicity. Cell viability, at 72h, was measured by MTS assay. Apoptosis was determined by Western Blot for PARP and Caspase 3 cleavage. In vivo experiments were performed in immunocompetent 3-methylcolanthrene fibrosarcoma mice: T was administrated via IV (0.15mg/kg; q7dx1) and R via IP (0.50mg/kg; q7dx2). Body weight and tumor volume were measured every 2 days for 15 days of treatment. Tumors were collected for analysis (snap frozen and paraffin embedding). T+R was synergistic in all STS cell lines: CP0024 primary leiomyosarcoma and 93T449 liposarcoma cell lines showed strong synergism with CI at the ED50 of 0.129 and 0.027, respectively. This synergy was followed by an increase of PARP and Caspase 3 cleavage. The synergism was confirmed in vivo: mice treated with R+T showed tumor growth delay in comparison to the drugs alone (p