Abstract 393: Determinants of birinapant efficacy in triple negative breast cancer

Abstract Background: Triple-Negative Breast Cancer (TNBC) is an aggressive subtype of breast cancer with few targeted therapies, limiting patients to harsh systemic treatments and resulting in the lowest 5-year survival rate of all breast cancer subtypes. This survival rate plummets after metastasis to another site, making it crucial to identify new therapeutic options for TNBC patients to prevent recurrence and metastasis. The small molecule therapy birinapant, a secondary mitochondrial activator of caspases (SMAC) mimetic, represents a potential new avenue for TNBC treatment. Birinapant initiates apoptotic signaling through targeting of Inhibitor of Apoptosis (IAP) proteins in the Tumor Necrosis Factor Receptor (TNFR) extrinsic apoptosis signaling pathway, leading to the formation of a death complex, caspase activity, and apoptosis. We have found that approximately 25% of TNBC patient-derived xenograft (PDX) and xenograft-derived organoid (PDxO) models are exquisitely sensitive to birinapant treatment. However, the majority of our TNBC patient-derived lines remain resistant through unknown means. We seek to understand the parameters for birinapant response and the mechanism of inherent resistance in TNBC using our patient-derived TNBC models. Approach and Results: Preliminary Western blot experiments revealed that treatment of birinapant-sensitive and resistant PDX tumors in vivo and PDxOs in vitro with birinapant causes degradation of cIAP1 and cIAP2, but only causes apoptosis in birinapant-sensitive lines. Therefore, there may be a disruption of necessary downstream apoptotic signaling in the birinapant-resistant lines. Through follow-up Western blot experiments we determined that at baseline, birinapant-resistant PDxOs do not lack the TNFR extrinsic apoptosis proteins required for apoptosis signaling after IAP degradation, nor is there an abundance of these proteins in the birinapant-sensitive PDxOs. Furthermore, to determine if TNFR extrinsic apoptosis signaling is impaired in birinapant-resistant PDxOs, we treated birinapant-resistant and -sensitive PDxOs with the TNF-a cytokine, which is known to enhance extrinsic apoptosis signaling in the presence of birinapant. Treatment with TNF-a and birinapant did not induce apoptosis in the birinapant-resistant PDxOs, suggesting that the extrinsic apoptosis signaling pathway may be the culprit of resistance in these patient-derived lines. Discussion: Follow-up experiments will address whether an inability to execute extrinsic apoptosis through other death receptor pathways explains birinapant resistance in these models. This work will allow us to uncover what predisposes TNBC patients to be resistant or sensitive to birinapant, and determine how this may be harnessed therapeutically to improve outcomes for TNBC patients. This project was funded in part with federal funds under HHSN261201500003I and U54CA224076. Citation Format: Elisabeth A. Brown, Chieh-Hsiang Yang, Emilio Cortes-Sanchez, Zhengtao Chu, Jeevan Govindharajulu, Ralph Parchment, James Doroshow, Apurva Srivastava, Alana Welm. Determinants of birinapant efficacy in triple negative breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 393.