Abstract 2866: Truncated glioma-associated oncogene homolog 1 (tGLI1) is an actionable therapeutic target in breast cancer brain metastases

Breast cancer is the second leading cause of brain metastases in women; patients with breast cancer brain metastasis (BCBM) survive only 6-18 months following diagnosis. Current standard of care options for BCBM include stereotactic radiosurgery, whole-brain radiotherapy, and surgical resection. Novel agents that act on targetable driver mutations (EGFR, ALK, PI3K, etc) and penetrate the blood-brain barrier have recently shown increasing promise as systemic options for patients with distant lung and melanoma metastases. Nevertheless, brain metastases represent an area of unmet clinical need for which new pathways, novel mechanisms, and innovative therapies are needed. Cancer stem cells are thought to be one of the driving forces behind not only distant metastasis, but also late-stage recurrence. The hedgehog pathway has been identified as an important mediator of breast cancer stem cells (BCSC); however, FDA-approved therapies targeting this pathway have demonstrated limited efficacy in breast cancer clinical trials. Despite advances made in understanding BCSC, it is still challenging to effectively target BCSC underscoring the need to identify and inhibit novel mediators of BCSC for treating BCBM patients. Our laboratory recently reported that truncated glioma-associated oncogene homolog 1 (tGLI1) promotes preferential metastasis to the brain in breast cancer by activating BCSC and astrocytes in the tumor microenvironment (Oncogene 39:64-78, 2020). tGLI1 is an alternatively spliced GLI1 variant that functions as a tumor-specific gain-of-function transcription factor and terminal effector of the hedgehog pathway. We found that tGLI1 knockdown abrogated BCBM, providing the rationale to therapeutically target tGLI1. Cell-based chemical screens followed by validations demonstrated that ketoconazole, an FDA-approved azole antifungal, specifically inhibits tGLI1 leading to suppression of BCSC in vitro and BCBM in vivo. Modification of KCZ side chains produced derivative compounds that retained tGLI1-selectivity in both in vitro models of BCSC and in vivo models of BCBM with increased blood-brain barrier penetrance. Mechanistic studies suggest that KCZ-dependent cell kill is, in part, mediated through downregulation of tGLI1 target genes OCT4, Nanog, and VEGFA. Based on these data, we opened a window-of-opportunity study in patients with BCBM to determine if ketoconazole penetrates the blood-brain barrier and alters tGLI1 signaling in humans (NCT03796273). Preliminary sample analysis has confirmed tGLI1 expression in collected BCBM and glioma samples. To help identify more effective tGLI1 inhibitors, we screened 63 azole compounds for tGLI1-selectivity and identified four additional compounds as potential tGLI1 inhibitors. Collectively, these data establish tGLI1 as an actionable target for BCBM. Citation Format: Daniel L. Doheny, Sherona R. Sirkisoon, Tadas Rimkus, Dongqin Zhu, Noah R. Aguayo, Marlyn Anguelov, Sara Manore, Fei Xing, Linda Metheny-Barlow, Kounosuke Watabe, Alexandra Thomas, Adrianna Henson Masters, Roy E. Strowd, Hui-Wen Lo. Truncated glioma-associated oncogene homolog 1 (tGLI1) is an actionable therapeutic target in breast cancer brain metastases [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2866.