Abstract 6615: The smac mimetic Xevinapant increases the efficacy of radiotherapy in triple negative breast cancer but only in the presence of tumor necrosis factor alpha.

Abstract Background: There is an unmet need in the treatment of triple negative breast cancer (TNBC) as recurrence rates remain a challenge. Indeed, the treatment of TNBC remains a challenge due to the paucity of targetable pathways as opposed to estrogen receptor-positive or Her2-positive breast cancers, thus novel treatment strategies are warranted to improve outcomes for patients. Smac mimetics (SMs) are FDA-approved small compound molecules that inhibit the inhibitors of apoptosis. In this study, we investigated the radiosensitizing properties of the SM compound Xevinapant (AT406) in a triple negative breast cancer model and potential therapeutic applications. Methods: A panel of TNBC cell lines were treated with increasing concentrations of Xevinapant (AT406) in the presence or absence of ionizing RT and cell viability and apoptosis measured in vitro using a live cell imaging system. Clonogenic assays were conducted in MDA-MB-231 with different Xevinapant and RT combinations. TNFα has been previously shown to enhance SM-induced apoptosis, thus we also evaluated the radiosensitizing properties of Xevinapant in combination with recombinant TNFα by clonogenic assays and evaluated apoptosis-mediated cell death by flow cytometry. Finally, a gene expression analysis was done to identify a potential TNFα gene signature Results: Xevinapant increased cell cytotoxicity in a panel of in irradiated TNBC cell lines. When assessed with the gold standard of clonogenic assays, we found that Xevinapant on its own is not a radiosensitizing agent in TNBC. However, when combined with a low dose of recombinant human TNFα, Xevinapant increased the sensitivity of MDA-MB-231 cells to RT with a dose enhancing factor (DEF) of 1.57. We also show that this radiosensitization effect is mediated by a cell cycle arrest in G2/M and an increase in apoptosis as evidenced by an increase in caspase-3/7 activity in irradiated cells treated with Xevinapant + TNFα and abrogation of the DEF by the pan-caspase inhibitor zVAD-fmk. Finally, a gene expression analysis of TNBC tumor samples suggests an immunogenic profile with a potential TNFα gene response signature. Conclusion: Xevinapant (AT406) synergizes with RT in the presence of TNFα to induce the death of triple-negative breast cancer cells. TNFα is central to this radiosensitization through activation of the programmed cell death. These results are promising, and investigations are underway to characterize a potential TNFα gene signature that could be used to identify patients that could respond to such a combination therapy. Citation Format: Mame Daro Faye, Julie Lafontaine, Philip Wong, Cynthia Menard, . The smac mimetic Xevinapant increases the efficacy of radiotherapy in triple negative breast cancer but only in the presence of tumor necrosis factor alpha [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 6615.