Abstract P2-05-07: Pan-cancer analysis of PARP inhibition reveals a suite of biomarkers that correlate with PARP1/2 activity in breast cancer

The development of multiple PARP inhibitors in BRCA mutant cancers has been one of the most successful implementations of molecularly targeted therapies in oncology. The recent approval of niraparib in BRCA WT Ovarian cancers demonstrates the potential for expansion of PARP inhibitors to other BRCA WT indications. The successful development of PARP inhibitors into new cancer indications will depend on using molecular biomarkers to identify tumors with a unique dependency on PARP1/2 activity. We screened a large (N=523), diverse panel of human cancer cell lines for response to three approved PARP inhibitors: olaparib, niraparib, and talazoparib. Despite differences in relative potency, there was a strong correlation between sensitive and resistant cell lines to all three compounds. In vitro experimentation revealed the inhibitors share a similar biologic mechanism of response, namely a global decrease in PARylation, as well as PARP-trapping at DNA nicks leading to G2 arrest and ultimately cell death. A suite of baseline genomic and proteomic characteristics were found to be strongly associated with response to PARP1/2 inhibition, and were largely consistent between different cancer types. Special attention was paid to BRCA1/2 variants, which represent the only approved patient selection biomarkers. Notably, neither mutations in BRCA1 nor BRCA2 were found to be associated with response to PARP1/2 inhibition in our panel, even when restricting to those with confirmed deleterious ClinVar scores. The molecular predictors of response identified in our screen may ultimately be used to develop diagnostic tools for enrollment into biomarker-enriched clinical trials to expand the use of this promising class of drug into areas of high unmet need. Triple-negative breast cancer represents one of the most intriguing spaces for development. The clinical failures in this space may be partially explained by the lack of empirical predictive biomarkers in the trial design. Our diagnostics may finally allow for the efficacious deployment of PARP inhibitors to triple-negative breast cancer. In particular we have identified a panel of biomarkers that can predict for sensitivity to PARP inhibitors in breast cancer that could be used as patient selection criteria for the expanded clinical development of these compounds into PARP-naive patient populations with high unmet need. Citation Format: Martina SJ McDermott, Dylan F Conklin, Neil A O9Brien, Kevin Chau, Dennis J Slamon. Pan-cancer analysis of PARP inhibition reveals a suite of biomarkers that correlate with PARP1/2 activity in breast cancer [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P2-05-07.