Abstract B019: Deciphering the role of homologous recombination deficiency mutations in the response to combined immune checkpoint and PARP inhibitors
Authors: Chaudhuri Sibapriya , Thomas Scott , Timmerman Luika , Munster Pamela 
Published: February 2024

Abstract
Background: Over 2 million women develop breast cancer globally every year despite immense advances in disease biology, diagnostic strategies and novel treatment options. The deeper knowledge of disease biology and genomic landscape has fostered a broader mechanistic approach to patients and their therapeutic choices. Many preclinical and clinical studies suggest a benefit of combining PARP inhibitors (PARPi) and immune checkpoint inhibitors (ICPi) in patients with homologous recombination deficiency (HRD) intact and mutated cancers. However, similar to the varied benefits of either drug class based on tissue and mutational context, the benefits of such combination are strongly variable based on tumor type, tissue and mutation context. Methods: To study how select HRD mutations respond to a combination of a PARPi inhibitor with an ICPi, we used CRISPR engineering to functionally impair or repress BRCA1, BRCA2, ATM, CHEK2, PALB2 in different mouse breast cancer cell lines. EMT6 and 4T1 cells were engineered to expressed deactivated cas9 nuclease (dcas9) fused to the repressor domain Kruppel-associated- box (KRAB) and mcherry florescent protein. The dcas9-KRAB-mcherry breast cancer cells were further engineered to express BFP-tagged guide RNAs targeting selected genes under a doxycycline inducible tet-on system. After establishing functional loss/repression of the targeted HRD mutations, cells were then implanted in the mammary fat pad of balb/cJ mice and treated with PARPi and ICPi to determine the ICPi, PARPi and their combination induced changes in inflammatory signals and their association with therapy response. Results: Induced suppression of BRCA1 and PALB2 in these breast cancer cells showed exquisite sensitivity to PARPi (IC50 0.01 uM and 0.098 uM) and diminishing sensitivity to PARPi in BRCA2 and ATM (IC50 0.16 uM and IC50 0.28 uM). In contrast, inducible suppression of CHEK2 showed resistance to PARPi versus parental EMT6 cells (IC50 13.9 uM versus 5.3 uM). Evaluation of changes in expression levels of PDL1 and select chemokines including CCL2, CCL5, CXCL9, CXCL10 and CXCL11 suggest a differential response to PARPi depending on specific HRD mutations. HRD mutations and tissue-context will be further characterized in combination therapy of ICPi and PARPi. Conclusion: Using inducible suppression of HRD function in an immune competent mouse breast cancer model helps to understand the differential role of individual HRD mutations (BRCA1, BRCA2, ATM, CHEK2, PALB2) in altering inflammatory signals in response to PARPi and ICI. This data may guide the clinical development of such combination in breast cancer and other cancers.
Citation Format: Sibapriya Chaudhuri, Scott Thomas, Luika Timmerman, Pamela Munster. Deciphering the role of homologous recombination deficiency mutations in the response to combined immune checkpoint and PARP inhibitors [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Breast Cancer Research; 2023 Oct 19-22; San Diego, California. Philadelphia (PA): AACR; Cancer Res 2024;84(3 Suppl_1):Abstract nr B019.