Abstract 3057: Synergistic antiproliferative activity between anticancer drugs and RAD51 inhibitors IBR2 and IBR120

The inherent genetic instability of cancer cells and the dependence of many tumor types on oncogenic drivers contribute selectivity of anticancer agents against tumor cells. However, that selectivity is limited, and toxicity to normal cells remains a major limitation to the success of chemotherapy. Aiming to increase selectivity by exploiting cancer cell genetic instability, we demonstrated that the small molecule IBR2 (an inhibitor of the DNA repair protein RAD51) enhanced cytotoxicity of numerous anticancer drugs, including agents that do not directly target DNA (J Pharmacol Expt Ther, 364: 46-54, 2018). Given the potential importance of combining IBR2 with anticancer treatment to increase therapeutic index, IBR2 was tested in combination with other untested agents against a panel of tumor cell lines. In addition, studies were expanded to include IBR120 [(R)-3-(2-(benzylsulfonyl)isoindolin-1-yl)-1h-indole], a new small molecule RAD51 inhibitor that is reportedly more selective against cancer cell lines (Euro J Med Chem 96: 196-208, 2015). Four-day drug exposures were conducted in 96-well plates. Relative cell density, determined using vital stains (alamarBlue©, neutral red), was reported as a percent of the fluorescence/absorbance of control cultures. Cell lines were representative of tumors from breast (MCF-7, SK-BR-3), prostate (LNCaP, DU145), colon (HT-29), stomach (N87), skin (SK-MEL-5), and lung (A549b, H1650, H1975), including osimertinib-resistant derivatives of the non-small-cell lung cancer (NSCLC) cell lines. The chemotherapy agents included inhibitors of epidermal growth factor receptor (EGFR) family (osimertinib, afatinib, lapatinib), Bcr-Abl (imatinib), B-raf (vemurafenib), multiple tyrosine kinases (regorafenib), sex steroid receptors (4-OH-tamoxifen, enzalutamide), and microtubules (vincristine). Both IBR2 and IBR120 decreased the concentration of drugs that inhibited proliferation by 50% (IC50) by up to 90%, indicating synergy between the agents. However, there were a small number of combinations in which no synergy was observed, indicating selectivity for this interaction. In the NSCLC cell lines, IBR2, IBR120, osimertinib, gefitinib, and afatinib decreased the cellular content of RAD51 by over 80% within 24 h (western blot with antibody 3C10), at concentrations at or lower than their IC50 value. The contribution of this decrease in RAD51 to synergy is being investigated. Studies are ongoing to determine the effect of IBR120 on growth of