Abstract 425: Overcoming PARP inhibitor resistance in triple-negative breast cancer models through olaparib and RAD51 inhibition with TGF-beta 2 downregulation

Abstract Background: Recent studies have shown that RAD51 involves resistance to the PARP inhibitor in triple-negative breast cancer (TNBC) cells with BRCA1 mutations. RAD51 protein has been reported to be recruited to the double-stranded DNA cleavage site to form homo-polymeric filaments that function as templates for double strand break repair. This study aimed to characterize the mechanisms of PARP inhibitor resistance and the therapeutic relevance of RAD51 using a BRCA1-mutant triple-negative breast cancer (TNBC) patient-derived xenograft (PDX) model. Method: We used three PDX models derived from tumor tissues of TNBC patients with BRCA1 c.5339T>C(L1780P) and c3627 dupA mutation, as well as wild-type BRCA1 status. We administered Olaparib (PARP inhibitor), RI-1 (RAD51 inhibitor), and their combination for 6 weeks to PDX models for tumor growth assay. After 6 weeks, tumor tissues (n=3, each) was collected and subjected to RNA sequencing and RAD51 immunohistochemistry (IHC) staining. To investigate the role of TGF-β in RAD51-mediated resistance, we assessed TGF-β2 expression levels after RAD51 inhibitor treatment. We also established acquired Olaparib resistance model by chronic drug treatment in TNBC cell lines with mutant BRCA1 (L1780P) stable transfection. Combination treatment of RI-1 and LY2109761 (TGF-β receptor inhibitor) was performed in these Olaparib-resistant TNBC cells. Results: In the three TNBC PDX models, BRCA1 c.5339T>C model showed sensitivity to Olaparib treatment, while BRCA1 wild-type and BRCA1 L1780P-mutant PDX models showed resistance to Olaparib treatment. Notably, combination treatment with RI-1 and Olaparib synergistically inhibited growth of BRCA1 wild-type and L1780P mutant PDX model. In the IHC assay of tumor tissues, we observed that RAD51 expression was downregulated in tumors after PARP and RAD51 inhibitor combination treatment. RNA sequencing analysis of tumors identified TGFB2 as downregulated in the RAD51 inhibitor-treated tumors, suggesting TGF-β2 suppression may involves overcoming Olaparib resistance mechanisms. We subsequently found RI-1 and LY2109761 combination treatment induces upregulation of P53 protein expression and decreased RAD51 expression in BRCA1-mutant TNBC cells. RI-1 and LY2109761 combination treatment also decreased cell growth Olaparib-resistant TNBC cells. Conclusion: RAD51 and PARP combinatory inhibition effectively overcomes Olaparib resistance in BRCA1-mutatnt and wild-type TNBC PDX models. RAD51 inhibition decreased expression of TGF-β2 in TNBC tumors, suggesting targeting RAD51 and TGF-β2 may be a potential therapeutic strategy to overcome Olaparib resistance in triple-negative breast cancer. Citation Format: TaeYeong Kim, Jeong Dong Lee, Won-Ji Ryu, Cheol-Hwa Hong, Hyun Myoung Yun, Yumi Hwang, Hyunju Han, Gun Min Kim, Joohyuk Sohn, Min Hwan Kim. Overcoming PARP inhibitor resistance in triple-negative breast cancer models through olaparib and RAD51 inhibition with TGF-beta 2 downregulation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 425.