Phase II study of a PARP inhibitor, talazoparib, in HER2- metastatic breast cancer (MBC) with a somatic BRCA1/2mutation identified in a cell-free DNA or tumor tissue genotyping assay.

TPS1143 Background: PARP inhibitors are approved for germline BRCA1/2mutant MBC but the applicability of these well-tolerated oral targeted therapies is limited, as germline BRCA1/2mutations account for 5% of breast cancer. We demonstrated that pathogenic somatic BRCA1/2mutations are detectable in cell-free DNA and tumor tissue genotyping assays in a subset of HER2- MBC, in patients who are not known germline BRCA1/2carriers. In a circulating tumor cell culture model derived from a patient with a pathogenic somatic BRCA1 mutation, we demonstrated that the growth inhibitory effect of a PARP inhibitor paralleled germline BRCA1/2mutant cultures. We hypothesize that somatic BRCA1/2mutations may be amenable to PARP inhibition. In this study, we are evaluating the efficacy of a PARP inhibitor in somatic BRCA1/2mutant MBC. The results may expand the population of patients who benefit from PARP inhibitors. Methods: In this phase II investigator-initiated clinical trial, 30 patients with HER2- MBC with pathogenic somatic BRCA1/2mutations (in the absence of a known germline BRCA1/2mutation) identified in a CLIA certified cell-free DNA or tumor tissue genotyping assay are being enrolled at 7 academic centers. Mutations are vetted for pathogenicity by genetic counselors using validated genomic databases such as ClinVar. Patients with metastatic triple-negative or hormone receptor positive (HR)+/HER2- breast cancer who received ≥1 prior therapy in the metastatic setting are eligible. Any number of prior therapies are acceptable, including a prior platinum (in absence of disease progression). Patients must have adequate baseline performance status and organ function. Patients are treated with talazoparib 1 mg/day until disease progression. Disease assessments occur at baseline and every 3 months via CT chest, abdomen, and pelvis, and bone scan. The primary endpoint is progression free survival (PFS) via RECIST 1.1. This study has 81% power to demonstrate the 12-week PFS is ≥53% (4% alpha). Secondary endpoints include objective response rate and toxicity (NCI CTCAE v 5.0). Exploratory endpoints, determined using serial cell-free DNA collected at baseline and monthly, include identifying resistance mutations, and evaluating the impact of mutant allele fraction and BRCA1/2reversion mutations on response. The Cancer Risk B (CR-B) assay, a novel method to identify double-strand break repair mutations in circulating blood cells, will be correlated with response. The study (NCT03990896) is open at Massachusetts General Hospital, University of California San Francisco, MD Anderson, Emory, Northwestern, and Vanderbilt and will open at Cornell, with 17 patients enrolled as of 2/2024. Clinical trial information: NCT03990896 .