Abstract PS2-11-22: Understanding resistance to the SMAC mimetic birinapant in triple-negative breast cancer

Abstract Title: Understanding resistance to the SMAC mimetic birinapant in triple-negative breast cancer Background: Triple-negative breast cancer (TNBC) is the most aggressive subtype of breast cancer, with limited targeted therapies and high metastatic potential. Thus, it is crucial to identify new therapies for TNBC patients in both the primary and metastatic settings. SMAC (Secondary Mitochondrial Activator of Caspases) mimetics represent a potential new therapeutic avenue for TNBC patients. Previously, we found that approximately twenty-five percent of our TNBC patient-derived xenograft (PDX) and xenograft-derived organoid (PDxO) models are exquisitely sensitive to the SMAC mimetic birinapant; response profiles also remained consistent across multiple SMAC mimetics. However, the majority of our TNBC patient-derived models are resistant to birinapant through unknown means. Despite promising pre-clinical results in several cancers, SMAC mimetics have not reached FDA approval. This may be explained by a dichotomy of patient responses, as observed in our birinapant-treated preclinical models, and highlights the need to understand the mechanism of resistance to SMAC mimetics to better stratify patients for treatment. The goal of this work is to uncover this mechanism of resistance and identify biomarkers of response and co-treatment strategies to sensitize more TNBC patients to SMAC mimetics. Results: SMAC mimetics, such as birinapant, function by targeting and degrading Inhibitor of Apoptosis (IAP) proteins in the Tumor Necrosis Factor Receptor (TNFR) and intrinsic apoptosis pathways, leading to apoptosis signaling and tumor cell death. We found that both birinapant-sensitive (BS) and –resistant (BR) TNBC PDxOs (n=2 lines each) show uniform IAP degradation after birinapant treatment and have similar levels of apoptotic proteins. Therefore, disrupted apoptotic signaling downstream of IAP degradation is likely the culprit of birinapant resistance. To interrogate this further, we tested the functionality of multiple cell death pathways, including intrinsic, TRAIL-mediated and TNFR-mediated apoptosis, as well as TNFR-mediated necroptosis. BR-PDxOs remained resistant to cell death exclusively in TNFR-mediated pathways, whereas BS-PDxOs died via all treatments, indicating a modality of resistance specific to TNFR signaling. Conclusions: Using PDxOs, inability to execute TNFR-mediated apoptosis signaling was identified as a mechanism of birinapant-resistance in TNBC. This regulation protects TNBC tumors from birinapant-induced cell death and is likely protective against all SMAC mimetics. These data suggest that identifying ways to overcome the failure to execute TNFR-mediated apoptosis signaling may expand the patient population that will benefit from birinapant treatment. Citation Format: E. Brown, C. Yang, E. Cortes-Sanchez, Z. Chu, A. Welm. Understanding resistance to the SMAC mimetic birinapant in triple-negative breast cancer [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS2-11-22.