Abstract P69: Immune Landscape of BRCA1/2 -mutated Breast and Ovarian Cancers in an Asian Cohort

Abstract Background: The majority of hereditary breast and ovarian cancers (HBOC) harbor defects in the homologous recombination (HR) DNA repair pathway, such as BRCA1 and BRCA2 mutations, which may influence the response to immune checkpoint blockade (ICB) therapy. Recent clinical trials combining poly-ADP ribose inhibitors and ICB therapy in advanced breast and ovarian cancer have shown promising results. This Asian cohort study set out to determine if immune phenotype, specifically the PD-1/PD-L1 axis, was associated with germline BRCA status in advanced breast and ovarian cancer patients and to develop novel biomarkers of HR deficiency to stratify patients for clinical management. Methods: Tissue microarrays of ovarian tumors and breast tumors were analyzed by multiplex immunohistochemistry (mIHC) to detect the expression of immune markers (CD68, CD8, PD-L1, PD-1) and HR markers (DNA polymerase theta, FANCD2, RAD51, γH2AX, phospho-ATR). The results were used to build a prediction model for BRCA using backward stepwise logistic regression analysis. In addition, we investigated immune infiltration in tumors and performed immune phenotyping on peripheral blood mononuclear cells in a subset of patients. Results: We identified potential BRCA1/2 predictors by mIHC, namely RAD51 and PD-L1 263 for ovarian tumors and FANCD2 and phospho-ATR for breast tumors. The area under the curve was 0. 770 (95% confidence interval CI: 0. 564, 0. 977) in ovarian cancer patients and 0. 841 (95% CI: 0. 682, 0. 999) in breast cancer patients. In addition, the proportion of tumor-infiltrating CD68+PD-L1+ macrophages was significantly upregulated in BRCA1-mutated tumors, while the levels of PD-1 in circulating central memory CD8+ T cells were higher in breast cancer patients with germline BRCA2 mutations, compared with non-BRCA. Conclusion: This study provides evidence of the association of the PD-1/PD-L1 axis with germline BRCA status in HBOC, which may aid clinical management. Citation Format: Siao Ting Chong, Felicia Wee, Jiang Feng Ye, Craig Joseph, Xinru Lim, Zhen Wei Neo, Justina Nadia Lee, Nur Diana Binte Ishak, Ming Ren Toh, Ee Ling Chew, Claresta Yeo, Sock Hoai Chan, Jeffrey Lim, Joe Yeong, Joanne Ngeow. Immune Landscape of BRCA1/2 -mutated Breast and Ovarian Cancers in an Asian Cohort abstract. In: Proceedings of Frontiers in Cancer Science; 2023 Nov 6-8; Singapore. Philadelphia (PA): AACR; Cancer Res 2024;84 (8Suppl): Abstract nr P69.