Abstract PO4-19-06: Phase II study of a PARP inhibitor, talazoparib, in HER2- metastatic breast cancer with a somatic BRCA1/2 mutation present in cell-free DNA or tumor tissue genotyping

Abstract Background: PARP inhibitors improve both progression free survival (PFS) and patient quality of life in germline BRCA1/2 mutant metastatic breast cancer (MBC), which accounts for 5-10% of breast cancer, leading to their approval in this setting. We have previously shown that a subset of patients with MBC who are not germline BRCA1/2 carriers may harbor pathogenic somatic BRCA1/2 mutations that are identified by cell-free DNA and/or tumor tissue genotyping. In our prior work, we developed a circulating tumor cell culture from a patient with MBC harboring a pathogenic somatic BRCA1 mutation, and demonstrated that a PARP inhibitor induced cell growth inhibition similar to germline BRCA1/2 mutant cultures. Thus, we hypothesize that a PARP inhibitor may be effective in treating somatic BRCA1/2 mutant MBC. In this clinical trial, we are studying the efficacy of a PARP inhibitor in somatic BRCA1/2 mutant MBC. Our work may help expand the clinical application of PARP inhibitors in MBC. Trial Design: This phase II investigator-initiated clinical trial is enrolling 30 patients with MBC who have a pathogenic somatic BRCA1/2 mutation identified by a CLIA certified cell-free DNA and/or tumor tissue genotyping assay. Patients are treated with the PARP inhibitor talazoparib 1 mg/day until progression of disease. Imaging (CT chest, abdomen and pelvis, and bone scan) occurs for disease assessment at baseline and every 3 months. Cell-free DNA is collected at baseline and then monthly to evaluate changes in the genomic environment. Patients also undergo the Cancer Risk B (CR-B) assay, a novel flow variant assay to identify double-strand break repair mutations in circulating blood cells, at baseline. Eligibility Criteria: Patients must have MBC with a pathogenic somatic BRCA1/2 mutation identified in a cell-free DNA and/or tumor tissue genotyping assay, with pathogenicity confirmed by a genetics counselor using validated genomic databases such as ClinVar. Patients may have triple-negative (receipt of at least 1 prior chemotherapy) or hormone receptor positive/HER2- MBC (receipt of at least 1 prior hormone therapy). Patients must not be known germline BRCA1/2 carriers. Any number of prior therapies including a prior platinum (in absence of progressive disease on a platinum) are allowed, but patients should not have received a prior PARP inhibitor. Patients must have adequate organ function and performance status. Specific Aims: The primary aim is to determine PFS (RECIST 1.1). Secondary aims include determining the objective response rate and toxicity (NCI CTCAE v 5.0). Exploratory aims include evaluating serial changes in BRCA1/2 mutant allelic frequency in cell-free DNA, understanding the impact of BRCA1/2 reversion mutations in cell-free DNA, comparing pre- and post-treatment cell-free DNA results to understand changes in the genomic environment, studying the CR-B assay positivity rate, and correlating these biomarker analyses with treatment response. Statistical Methods: This study has 81% power to demonstrate that the 12-week PFS is 53% or higher. In contrast, there is a 4% (alpha) probability of concluding that the 12-week PFS is ≥ 53% if the true 12-week PFS is 30% or lower. If 14 or more of 30 total patients achieve PFS 12 weeks, the null hypothesis (12-week PFS ≤ 30%) will be rejected. Present accrual and target accrual: This study (NCT03990896) is open at Massachusetts General Hospital, MD Anderson, University of California San Francisco, Emory, Northwestern, and Vanderbilt. As of 7/2023, 14/30 patients are enrolled. Funding: This study is funded by a Pfizer ASPIRE award and Conquer Cancer Foundation of ASCO–Breast Cancer Research Foundation- Career Development Award. Contact information: Neelima Vidula, MD, Massachusetts General Hospital, nvidula@mgh.harvard.edu. Citation Format: Neelima Vidula, Senthil Damodaran, Manali Bhave, Hope Rugo, Ami N. Shah, Erica Blouch, Nathan Royce Ruffle-Deignan, Ogadinma Ogbenna, Lisa E. Flaum, Massimo Cristofanilli, Joseph Sparano, Harry Ostrer, Vandana Abramson, Nora Horick, Aditya Bardia. Phase II study of a PARP inhibitor, talazoparib, in HER2- metastatic breast cancer with a somatic BRCA1/2 mutation present in cell-free DNA or tumor tissue genotyping abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO4-19-06.