LAUSR

Background: Recent data indicate a promising response to immune check point blockade (ICB) in patients with breast cancer. Pembrolizumab, a humanized monoclonal antibody against programmed death 1 (PD-1) receptor and one of several ICB agents in development, was given an FDA approval for all MSI (microsatellite instability)-high solid tumors. MSI incidence in breast cancer is not fully elucidated. Other biomarkers being explored in possible relationship to ICB activity include PD-1 ligand (PD-L1) status and tumor mutational load (TML). In this study, we aimed to explore the incidence of these biomarkers in invasive breast cancers. Methods: A retrospective data analysis of patients profiled by commercial next-generation sequencing (NGS) at Caris Life Sciences was performed. MSI results were either high, stable, or equivocal. MSI was calculated by comparing repeat-insertions or deletions across over 7,000 microsatellite sequences in the patient sample to the hg19 reference genome. Samples with repeat variances in more than 45 microsatellites were classified as MSI-High PD-L1 expression was evaluated using immunohistochemical analysis (IHC), with clone SP-142 (Roche Diagnostics). A sample was considered positive if there was >5% membranous staining of tumor cells. Tumor mutational load was calculated as a total number of non-synonymous somatic mutations identified per megabase of the genome coding area with high being greater than or equal to 17. Results: A total of 9,627 breast cancer cases were queried from the Caris Life Sciences database. The mean age (±SD) was 56.8 ± 12.4 years (range 20-90). The tumor distribution was 60.7% hormone receptor (HR) positive (ER and/or PR) and HER2 negative, 9.5% HER2 positive (with HR negative or positive), and 29.8% triple negative (negative for ER, PR and HER2). Of all cases, there were 5,203 tested for PD-L1 status, 354 (6.8%, 95% CI 6.2-7.5%) were positive. Of 1,440 tumors tested for MSI status, 15 (1.04%, 95% CI 0.58-1.71%) were either high (8) or equivocal (7), the rest were MSI-low. Tumor mutational load (TML) was available on 1,766 tumors, of which 55 (3.1%, 95% CI 2.4-4.0%) were high. Seven out of the 8 MSI-high tumors were also TML-high. Four out of the 8 MSI-high breast cancers were triple negative. Conclusion: In this large dataset of molecularly profiled breast cancer, MSI was observed in about 1% of the breast tumors tested. Overall, modest positivity of TML, PD-L1, and MSI of all invasive breast cancers was observed. The percentage of patients that had at least one of these biomarkers that may confer responsiveness to ICB is planned and will be further stratified by subtype. MSI-high breast cancers mostly overlapped with those that were TML-high. Future research is needed to show the clinical utility of these biomarkers in response to ICB. Updated data will be presented. Citation Format: Obeid E, Ellerbrock A, Handorf E, Goldstein L, Gatalica Z, Arguello D, Swain SM, Isaacs C, Vacirca J, Tan A, Schwartzberg L. Distribution of microsatellite instability, tumor mutational load, and PD-L1 status in molecularly profiled invasive breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr PD6-03.