Abstract PS5-08: Comparison of PD-L1 protein expression between primary tumors and metastatic lesions in triple negative breast cancers

Background: Several recent studies that compared small cohorts of metastatic and primary lesions, suggested substantial heterogeneity in tumor infiltrating lymphocyte count, immune gene expression and PD-L1 protein expression across different metastatic sites and between primary breast cancers and metastasis. Understanding the frequency of PD-L1 positivity rates across different tissue sites can indicate differences in the immune microenvironment and may also guide biopsy site selection. We compared PD-L1 positivity on immune cells and tumor cells in primary and metastatic triple negative breast cancer tumors (TNBC).Methods: A retrospective data analysis of the Foundation Medicine PD-L1 IHC database was conducted on 340 cases of TNBC. PD-L1 positivity was determined by IHC using SP142CDx. Results are reported as percent of PD-L1 stained immune cells (IC) in the tumor area. A tumor was considered PD-L1 positive if ≥ 1% IC stained positive with PD-L1. As an exploratory analysis, PD-L1 positivity of tumor cells (TC) was also assessed. PD-L1 percent positive staining results are reported as means with 95% CI. The proportion of PD-L1 positive and negative IC and TC in primary tumors vs metastatic sites was compared using Chi-Square test. Prism 8 was used for all data analysis.Results: All patients were female, with median age 56 years (range 26-89); 179 samples were from primary tumors and 161 from metastatic lesions, representing 15 different tissue sites. Overall, PD-L1 expression on immune cells was statistically significantly more frequent in primary tumors compared to metastatic sites (63.7% [n=114] vs 42.9% [n=69]), p Citation Format: Mariya Rozenblit, Richard Huang, Natalie Danziger, Brian Alexander, Shakti Ramkissoon, Kim Blenman, Jeffrey Ross, David Rimm, Lajos Pusztai. Comparison of PD-L1 protein expression between primary tumors and metastatic lesions in triple negative breast cancers [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS5-08.