Abstract 5327: Simultaneous inhibition of ATR and PARP greatly sensitizes colon cancer cell lines to irinotecan

Proceedings: AACR 106th Annual Meeting 2015; April 18-22, 2015; Philadelphia, PA Enhanced DNA damage repair is one mechanism behind colon cancer drug resistance. Thus, targeting molecular components of repair pathways with specific small molecule inhibitors may improve the efficacy of chemotherapy. ABT-888 and VE-821, inhibitors of poly-ADP-ribose-polymerase (PARP) and the serine/threonine-kinase Ataxia telangiectasia related (ATR), respectively, were used to treat colon cancer cell lines in combination with the topoisomerase-I inhibitor irinotecan (SN38). Our findings show that each of these molecules at nontoxic single agent concentrations synergized with SN38 to produce a 2.2 to 3 fold reduction in the 50% inhibitory concentration (IC50) of SN38 (Table 1). When combined, nontoxic concentrations of ABT-888 and VE-821 produced a 4.5 to 27 fold reduction in the IC50 of SN38. Furthermore, the combination of all three agents was associated with maximal G2-M arrest and enhanced DNA-damage (γH2AX). The mechanism of this enhanced synergy was associated with maximal suppression of SN38 induced PARP activity in the presence of both inhibitors. Furthermore, VE-821 enhancement of SN38 induced DNA-PK phosphorylation was abrogated by ABT-888, resulting in more unrepaired DNA damage. This novel combination of DNA repair inhibitors may be useful to enhance the activity of DNA damaging chemotherapies such as irinotecan and help circumvent resistance to this drug in colon cancer. | Cell Line | VE-821 (μM) | ABT-888 (μM) | SN38 (IC50) ± SE (nM) | I value | P value | |:--------- | ----------- | ------------ | --------------------- | ------- | ------- | | LoVo | | | 13.5±1.2 | | | | | 1 | | 4.5±1.9 | 0.3 | 0.02 | | | 0.5 | | 5.8±2.0 | 0.4 | 0.03 | | | | 0.5 | 9.4±1.0 | 0.7 | 0.06 | | | 1 | 0.5 | 3.0±1.2 | 0.2 | 0.01 | | | 0.5 | 0.5 | 4.7±1.9 | 0.4 | 0.02 | | HCT-116 | | | 8.1±0.7 | | | | | 1 | | 3.5±0.5 | 0.5 | 0.01 | | | 0.5 | | 5.2±0.6 | 0.7 | 0.03 | | | | 0.5 | 3.9±1.1 | 0.5 | 0.03 | | | 1 | 0.5 | 0.3±0.03 | 0.1 | 0.01 | | | 0.5 | 0.5 | 2.1±0.4 | 0.3 | 0.01 | | HT-29 | | | 20.5±1.8 | | | | | 1 | | 9.3±0.6 | 0.6 | 0.01 | | | 0.5 | | 12.2±1.3 | 0.7 | 0.02 | | | | 0.5 | 12.9±0.7 | 0.6 | 0.02 | | | 1 | 0.5 | 3.9±0.5 | 0.3 | 0.01 | | | 0.5 | 0.5 | 6.1±0.2 | 0.4 | 0.01 | Table 1 Colon cancer cell lines LoVo, HCT-116 and HT-29 were treated with SN38 alone or in combination with various concentrations of the ATR inhibitor VE-821 and/or the PARP inhibitor ABT-888. The IC50 values of SN38 were significantly reduced in the presence of VE-821 and/or ABT-888 for all cell lines. Drug synergy (I) values were determined, according to Berenbaum (1978), by calculating the ratio: (IC50 SN38 with inhibitors/IC50 SN38 alone) + (inhibitor concentration used/IC50 inhibitor alone); I less than 1 = synergism, I equal to 1 = additive effect and I greater than 1 = antogonism. Citation Format: David Davidson, Atlal Abu-Sanad, Yunzhe Wang, Fatemeh Hasheminasab, Justin Panasci, Raquel Aloyz, Lawrence Panasci. Simultaneous inhibition of ATR and PARP greatly sensitizes colon cancer cell lines to irinotecan. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 5327. doi:10.1158/1538-7445.AM2015-5327