Biological indicators of response and resistance to PARP inhibition in BRCA wild-type breast cancer.

Abstract

125 Background: Although PARP inhibitors are currently being tested in clinical trials in breast cancer patients, it is not well understood which subgroup of patients will best respond to such therapy or how to combine anti-PARP therapy to improve therapeutic response. Methods: We assessed the response of a panel of 8 HER2-negative breast cancer cell lines against three PARP inhibitors: veliparib, olaparib, and BMN 673. Efficacy of carboplatin as a single agent and in combination with PARP inhibitors was determined. Therapeutic response and cellular phenotype were elucidated using live-cell imaging, high-content imaging and immunofluorescence. We computed IC50 as the measure of sensitivity to each PARP inhibitor across the panel of cell lines. Cellular phenotype was ascertained by quantifying endpoints for apoptosis and DNA damage. Results: BMN 673 demonstrated the highest potency, with IC50 values in the nanomolar range, while the IC50 values of olaparib and veliparib were in the micromolar range. Drug sensitivity was independent of BRCA mutational status. Biomarkers of apoptosis and DNA damage varied with the PARP inhibitor and across different cell lines. The resistant cell lines could be classified into two groups that varied in DNA damage response and cell death due to DNA damage. A combined DNA damage – cell death (DDD) Response Score was developed and correlated strongly with IC50 values in BRCA wild-type breast cancer cell lines. The DDD Response Score identified cell lines that were highly sensitive to single-agent PARP inhibitors or carboplatin. A Response Score that identified intermediate sensitivity may predict which cell lines may benefit from the combination of anti-PARP therapy and carboplatin. Conclusions: Overall, a spectrum of response to three different PARP inhibitors was identified in a panel of eight breast cancer lines. Cellular phenotype can help classify resistant cancer cell lines. We have developed an experimental approach that may help to inform which breast cancer subtypes may benefit from PARP inhibitors and carboplatin, both as single agents, and in combination.