Rearranged during transfection (RET) receptor tyrosine kinase was first identified over thirty years ago as a novel transforming gene. Since its discovery and subsequent pathway characterization, RET alterations have been identified in numerous cancer types and are most prevalent in thyroid carcinomas and non-small cell lung cancer (NSCLC). In other tumor types such as breast cancer and salivary gland carcinomas, RET alterations can be found at lower frequencies. Aberrant RET activity is associated with poor prognosis of thyroid and lung carcinoma patients, and is strongly correlated with increased risk of distant metastases. RET aberrations encompass a variety of genomic or proteomic alterations, most of which confer constitutive activation of RET. Activating RET alterations, such as point mutations or gene fusions, enhance activity of signaling pathways downstream of RET, namely PI3K/AKT, RAS/RAF, MAPK, and PLCγ pathways, to promote cell proliferation, growth, and survival. Given the important role that mutant RET plays in metastatic cancers, significant efforts have been made in developing inhibitors against RET kinase activity. These efforts have led to FDA approval of Selpercatinib and Pralsetinib for NSCLC, as well as, additional selective RET inhibitors in preclinical and clinical testing. This review covers the current biological understanding of RET signaling, the impact of RET hyperactivity on tumor progression in multiple tumor types, and RET inhibitors with promising preclinical and clinical efficacy.

Human epidermal growth factor receptor 2 (HER2) receptor tyrosine kinase is overexpressed in 20–30% of breast cancers and is associated with poor prognosis and worse overall patient survival. Most women with HER2-positive breast cancer receive neoadjuvant chemotherapy plus HER2-targeted therapies. The development of HER2-directed therapeutics is an important advancement in targeting invasive breast cancer. Despite the efficacy of anti-HER2 monoclonal antibodies, they are still being combined with adjuvant chemotherapy to improve overall patient outcomes. Recently, significant progress has been made towards the development of a class of therapeutics known as antibody-drug conjugates (ADCs), which leverage the high specificity of HER2-targeted monoclonal antibodies with the potent cytotoxic effects of various small molecules, such as tubulin inhibitors and topoisomerase inhibitors. To date, two HER2-targeting ADCs have been approved by the FDA for the treatment of HER2-positive breast cancer: Ado-trastuzumab emtansine (T-DM1; Kadcyla®) and fam-trastuzumab deruxtecan-nxki (T-Dxd; Enhertu®). Kadcyla and Enhertu are approved for use as a second-line treatment after trastuzumab-taxane-based therapy in patients with HER2-positive breast cancer. The success of ADCs in the treatment of HER2-positive breast cancer provides novel therapeutic advancements in the management of the disease. In this review, we discuss the basic biology of HER2, its downstream signaling pathways, currently available anti-HER2 therapeutic modalities and their mechanisms of action, and the latest clinical and safety characteristics of ADCs used for the treatment of HER2-positive breast cancer.

HER2-enriched breast cancer and triple-negative breast cancer (TNBC) have the highest propensity to metastasize to the brain; patients with breast cancer brain metastasis (BCBM) survive only 6-18 months after diagnosis. Mechanisms that drive brain metastasis remain unclear, contributing to limited effective treatments and poor prognoses for HER2-enriched breast cancer and TNBC patients. Our lab recently reported that breast cancer with truncated glioma-associated oncogene homolog 1, a BCBM-promoting transcription factor, secretes high levels of extracellular vesicle (EV)-derived miR-1290 (Cancer Letters 540:215726, 2022). EV-miR-1290 activates astrocytes in the brain microenvironment. Using mammosphere assays, which enrich breast cancer stem cells (BCSCs), we found conditioned media from miR-1290-activated astrocytes promotes mammosphere formation of breast cancer cells. Furthermore, miR-1290-activated astrocytes secrete high levels of ciliary neurotrophic factor (CNTF) to promote the progression of brain metastases through the novel EV-miR-1290-FOXA2-CNTF signaling axis. However, it remains unknown whether intratumoral miR-1290 promotes BCSCs and BCBM. To help fill this knowledge gap, we first examined whether a miR-1290 mimic enhanced expression of known BCSC markers in SKBR3, HER2-enriched breast cancer, and CN34, TNBC, cells. Our results showed that overexpression of miR-1290 significantly increased the expression of stemness genes, CD44, Nanog, and OCT4, in both cell lines. Furthermore, we determined whether miR-1290 promotes formation of mammospheres that are enriched with BCSCs, and the results indicated that ectopic miR-1290 expression significantly enriched BCSCs in SKBR3 and CN34 cells. Conversely, inhibition of miR-1290 suppressed mammosphere-forming ability of SKBRM and CN34-BRM cells, two brain metastatic breast cancer cell lines derived from SKBR3 and CN34 cells, respectively. We further observed that miR-1290 overexpression in SKBR3 cells significantly increased the percentage of CD44+/CD24− cells, indicative of the BCSCs. Analysis of Gene Expression Omnibus breast cancer patient datasets revealed that miR-1290 expression is significantly increased in HER2-positive and basal subtypes of breast cancer patient tumors. Using the publicly available miR-1290 gene signature, we performed Gene Set Enrichment Analysis and found that high miR-1290 gene activation signature is positively enriched with multiple pathway gene signatures that are known to be upregulated in breast cancer, such as EGFR, MAPK, PI3K, and STAT3 pathways. Furthermore, the miR-1290 gene signature is upregulated in HER2-positive breast cancer and TNBC tumors, and is correlated with worse metastasis-free survival (MFS) and brain-MFS in breast cancer patients. In summary, our study suggests an important role for intratumoral miR-1290 in BCSCs and BCBM. Citation Format: Grace L. Wong, Mariana Najjar, Yoshua Esquenazi, Nitin Tandon, Angelina T. Regua, Hui-Wen Lo. Intracellular miR-1290 promotes breast cancer stemness in HER2-enriched and 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 3769.

The goal of this study is to identify pharmacological inhibitors that target a recently identified novel mediator of breast cancer brain metastasis (BCBM), truncated glioma-associated oncogene homolog 1 (tGLI1). Inhibitors of tGLI1 are not yet available. To identify compounds that selectively kill tGLI1-expressing breast cancer, we screened 1527 compounds using two sets of isogenic breast cancer and brain-tropic breast cancer cell lines engineered to stably express the control, GLI1, or tGLI1 vector, and identified the FDA-approved antifungal ketoconazole (KCZ) to selectively target tGLI1-positive breast cancer cells and breast cancer stem cells, but not tGLI1-negative breast cancer and normal cells. KCZ’s effects are dependent on tGLI1. Two experimental mouse metastasis studies have demonstrated that systemic KCZ administration prevented the preferential brain metastasis of tGLI1-positive breast cancer and suppressed the progression of established tGLI1-positive BCBM without liver toxicities. We further developed six KCZ derivatives, two of which (KCZ-5 and KCZ-7) retained tGLI1-selectivity in vitro. KCZ-7 exhibited higher blood–brain barrier penetration than KCZ/KCZ-5 and more effectively reduced the BCBM frequency. In contrast, itraconazole, another FDA-approved antifungal, failed to suppress BCBM. The mechanistic studies suggest that KCZ and KCZ-7 inhibit tGLI1’s ability to bind to DNA, activate its target stemness genes Nanog and OCT4, and promote tumor proliferation and angiogenesis. Our study establishes the rationale for using KCZ and KCZ-7 for treating and preventing BCBM and identifies their mechanism of action.