Abstract 3708: Targeting synthetic lethality with PARP inhibition in colorectal cancer

Abstract

Abstract Background: Small molecule poly ADP-ribose (PAR) polymerase (PARP) inhibitors (PARPi) purportedly result in DNA damage and are promising agents in the treatment of cancer. In a quest to identify best “synthetic lethality” for treatment, we have screened various combination therapies in colorectal cancer (CRC) cell lines. In addition, we have interrogated if the presence of microsatellite instability (MSI) status favor susceptibility of cells to synthetic lethality and thus acts as a prognostic marker of combination therapy. Methods: Using the SRB assay, the 12 combinations of the PARPi, veliparib (V), rucaparib (R), olaparib (Ol) and PJ34, and the cytotoxic drugs, irinotecan (I), 5-fluorouracil (5-FU) and oxaliplatin (Ox) were screened in HCT116 and LIM2405 cell lines. We assessed the effect of the combination using the CalcuSyn software for combination index (CI). We next evaluated the effectiveness and sequence dependence of combination in an additional 2 cell lines that had MSI status (RKO and HCT15) and 2 that had microsatellite stable (MSS) status (HT29 and SW837) and in 7 HCT116 isogenic lines (mutants of Bax, p21, p53, PTEN, Kras and DNMT and wild type of PIK). The combination of I and R was confirmed by FACS analysis in HCT116 and LIM2405. Results: Ol and R showed synergy in combination with I, whereas, V and PJ34 showed antagonism with Ox, including in Ox-sensitive cells. Single agent cytotoxicity was observed at 200nM for R, whereas V required more than 30uM. Early PARP-1 protein upregulation was observed upon Ox administration; but the changes were not reversed with either R or V. Nevertheless, R at 200nM and V 1uM significantly reduced PAR formation in Ox-sensitive cell lines. FACS analysis showed S phase arrest in SW837 (MSS) (p = 0.03) whereas G2-M arrest was observed in LIM2405 (MSI) (non-significant) upon treatment with R 400nM and I 200nM. In addition, studies using isogenic cell lines revealed the importance of p21, p53 and PTEN in exerting synergistic effect while combining R and I. Administration of I prior to R (24h each) was more cytotoxic than vice versa and simultaneous treatment in MSI cell lines. Conclusion: Among the various combinations studied, R with I was the most synergistic. The effect appears to be more pronounced when I precedes R, particularly in MSI cell lines. Alteration in cell cycle is MSI status dependent. Further work to identify mechanism of synergy is ongoing. Citation Format: Titto Augustine, Radhashree Maitra, Jinghang Zhang, Jay Nayak, Sanjay Goel. Targeting synthetic lethality with PARP inhibition in colorectal cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3708.