OLAPARIB-MEDIATED ENHANCEMENT OF 5-FLUOROURACIL CYTOTOXICITY IS DEPENDENT ON MMR STATUS IN COLORECTAL CANCER CELLS

Introduction: The advances in colorectal cancer (CRC) treatment include the identification of deficiencies in Mismatch Repair (MMR) pathway to predict the benefit of adjuvant 5-fluorouracil (5-FU) and oxaliplatin for stage II CRC and immunotherapy for advanced disease. Although 5-FU-based chemotherapy regimens remain the cornerstone of CRC therapeutic approaches, it is considered unspecific and suboptimal, drawing attention to the need of new strategies. A growing body of evidence has been suggesting new rationale for the use of PARP inhibitors, such as Olaparib, beyond tumors with intrinsic deficiencies in Homologous Recombination Repair (HRR), particularly in combination to DNA damaging agents. Aim: to evaluate the influence of MMR pathway on the cytotoxicity of 5-FU when combined with Olaparib. Methodology: Cell lines with proficiency (HT29) and deficiency (HCT166) in MMR were treated with 5-FU and Olaparib. Viability was assessed by MMT assay, proliferation was assessed by clonogenic assay, type of cell death and cell cycle profile were analyzed by flow cytometry, DNA damage was assessed by comet assay, adhesion and migration tests were also performed. Results: In the combination, Olaparib increased the cytotoxic effect of 5-FU in HCT116, demonstrating synergistic effect. We observed decrease of 70% of the HCT116 ability to form colonies in relation to the 5-FU monotherapy. Also, Olaparib monotherapy reduced the number of colony formation in both cell lines. Beyond that, the combination enhances apoptosis induction is HCT116, but not in HT29. Interestingly, single administration of Olaparib increased apoptosis in HCT116. No significant cell cycle alterations are detected with the combination in HT29, HCT116 increased the percentage in G2/M phase. We detected a specific cell cycle interference following single Olaparib treatment, as a slight increase in the G2/M population of both cell lines and polyploidy induction only in HCT116 cells. Additional tests showed that Olaparib treatment and the combination reduced adhesion capacity in HT29, no effect was observed for the monotherapy in HCT116. Migration capacity was also reduced in Olaparib monotherapy in HT29. Conclusion: our results suggest benefits of Olaparib inclusion in CRC treatment: (1) in combination with 5-FU for MMR deficient CRC and (2) as monotherapy for MMR proficient CRC.