Preclinical study of trabectedin (TR) and poly(ADP-ribose)polymerase 1 (PARP-1) inhibitor combination in soft tissue sarcoma (STS).

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10066 Background: TR is an alkylating agent approved in Europe for the treatment of advanced STS as second/third line therapy. TR binds to the minor groove of DNA and interferes with gene transcription and nucleotide excision repair mechanism, inducing DNA double strand breaks (DSBs), and S/G2 cell cycle arrest. There is a strong clinical interest to increase TR activity combining it with other anti-cancer drugs. PARP-1 inhibitors disable DNA base-excision repair mechanism causing the accumulation of DSBs and look like a reasonable TR partner to be explored. We focused our in vitro studies on the effects of the combination of TR with the PARP-1 inhibitor Olaparib (OL). Methods: We explored the activity of TR-OL combination against a panel of different histotypes of STS cell lines, evaluating cell viability after 72h treatment with escalating doses of TR (0-2 nM), OL (0-20 µM), and their constant combination. Following colony formation, cell cycle, apoptosis (annexin V+/PI+) and DNA damage (phospho-histone H2AX - Ser139) were checked. Results: The TR-OL combination strongly affects STS cell viability, showing synergism (Combination Index < 1, based on Chou–Talalay method) on 8 out of 13 cell lines tested. We observed a strong synergism, as a massive reduction of colony growth (402.91, MES-SA, and DMR-SN-8.4.98 lines) induced by the combination if compared with each single agent (78% vs. ~27% : p<0.05). OL potentiates the S/G2 cell-cycle arrest caused by TR at 48h (Control= 29%, TR= 33.4%, OL= 31.5%, TR-OL= 80.9%), and induces a strong increase of the apoptotic cells at 72h (Control= 17.4%, TR= 28.3%, OL= 33.5%, TR-OL= 56.8%). Furthermore, the TR-OL synergism on DNA damage is confirmed by a significant increase of the DSBs marker (Control= 8.7%, TR= 59.5%, OL= 23.8%, TR-OL= 76.5%). Conclusions: These results validate the biological rationale to combine TR and PARP-1 inhibitors in STS and suggest assessing this drug combination in the clinical setting.