Evaluation of biomarkers of response in patients with breast cancer with BRCA 1/2 pathogenic variants treated with PARP inhibitors.

e15188 Background: Several prospective studies highlight the pivotal role of PARP inhibitors (PARPi) in treating BRCA-associated breast cancers (BC), demonstrating a significant overall response rate (ORR) of up to 60% in advanced BC patients with BRCA 1/2 germline pathogenic variants (PV). Despite their clinical efficacy, the factors predicting PARPi response, the mechanisms of resistance to PARPi, and the clinical strategies to overcome these challenges are still unknown. The aim of this study is to establish tissue microarrays (TMA) from primary breast tumor tissue from BC patients with BRCA1/2 PVs who were treated with PARPi and identify biomarkers of PARPi resistance. Methods: Stage 4 BC patients with BRCA 1/2 PVs who presented to the University of Texas MD Anderson Cancer Center Breast Medical Oncology clinics and were treated with PARPi between 2008-2023 were identified using our database. Available patient tissue samples were obtained from institutional tumor banks. TMA from breast tumor tissue created and evaluated by our collabrating pathologists for expression of biomarkers:Wee-1, MRE, BP1,and c-Myc. Clinical and tumor characteristics analyzed included:Age at diagnosis, treatment details, tumor characteristics including histological type, hormonal receptor status, Her-2/Neu status, grade, Ki-67 and clinical outcome. Descriptive statistics, the Fisher exact test, and the log rank test were used to report patient characteristics and outcomes. Here we report on initial TMA construct and biomarker analysis. Results: So far TMA from 12 patients treated with PARPi were succesfully constructed. Patient characteristics include: Median age of 36.5 years (29-57). Nine (75%) and 3 (25%) patients had BRCA1 and BRCA2 mutations, respectively. All patients had invasive ductal carcinoma and 9 (75%) had triple negative BC. Most patient (75%) had high nuclear grade BC and median Ki-67 was 75 (15-90). In 11 evaluable patients, the ORR was 72.7%. The median OS was 383.6 months (95% CI 62.5-383.6) and median PFS was 31.5 months (95% CI 10.8-NE). No significant difference in biomarker expression was observed between PARPi-responders and non-responders (Table1). Conclusions: This report presents unique data on novel predictive biomarkers of response and evaluates the evidence for potential biomarkers. Further analysis on a larger TMA cohort is ongoing. Table: see text