585P Trabectedin induces apoptosis regardless of acquired resistance to PARP inhibitors in BRCA2 mutant high grade ovarian cancer cell lines

Retrospective evidence suggests that patients with high grade ovarian cancer (HGOC) progressing to PARP inhibitors (PARPi) might be less likely to response to platinum therapy. The aim of our study was to assess in preclinical models if trabectedin will be an active agent in carboplatin-and PARPi-resistant HGOC cell lines. HGOC Olaparib (OLA)/Carboplatin (carbo)-sensitive BRCA2 mutant p.Y1655* cell line PEO1, and carbo-resistant cell line PEO4 were acquired. PEO1 resistant to OLA (PEO1-R) cell line was generated by pulse method. Cells were exposed for 24 hours to the PEO1 IC75 OLA and recovered for two weeks. Pulses were repeated during a 6-month period. IC50 of OLA, carbo and trabectedin were calculated for PEO1, PEO4 and PEO1-R cell lines. DNA damage was assessed by gamma-H2AX expression by western-blot (WB). BRCA and TP53 mutations were explored by NGS. NGS confirmed that reversion of BRCA2 mutation was present in PEO4 but not in the PEO1-R cell line in which BRCA2 mutation p.Y1655* was detectable. TP53 mutations were invariable. We confirmed that PEO4 was platinum-resistant vs PEO1 parental line (IC50 for carbo 63.54uM vs 99.1uM; p<0.0001). Moreover PEO4 showed a cross-resistance to OLA (IC50 5.65 nM PEO1 vs 55.70 nM PEO4; p<0.0001). PEO1-R cell line was resistant to OLA compared with parental PEO1 (3.18 nM for PEO1 vs 48.31 nM PEO1R; p<0.0001) and PEO1-R showed a cross-resistance to carbo (42.47nM PEO1 s 71.05nM for PEO1R; p< 0.0001). In a second step we assessed if trabectedin could be a potential agent to overcome carbo or OLA resistance in cell lines. PEO4 carbo-resistant cell line was also resistant to trabectedin (trabectedin IC50 PEO1 378.7 nM vs PEO4 1231 nM; p=0.0021). However, OLA resistant PEO1-R cell line showed similar values for trabectedin IC50 compared to PEO1 (454nM vs 397nM for PEO1R; p=0.645). Increased gamma-H2AX expression in WB vs control was seen in all three lines PEO1, PEO1-R and PEO4 when treated with trabectedin. This preclinical in vitro model suggests that some HGOC cell lines resistant to PARPi could potentially be still sensitive to trabectedin.