Concise review: bullseye: targeting cancer stem cells to improve the treatment of gliomas by repurposing disulfiram.

Cancer stem cells (CSCs) are thought to be at the root of cancer recurrence because they resist conventional therapies and subsequently reinitiate tumor cell growth. Thus, targeting CSCs could be the bullseye to successful cancer therapeutics in the future. Brain tumors are some of the most challenging types of cancer to treat and the median survival following the initial diagnosis is 12-18 months. Among the different types of brain tumors, glioblastoma (GBM) is considered the most aggressive and remains extremely difficult to treat. Despite surgery, radiation, and chemotherapy, most patients develop refractory disease. Temozolomide (TMZ) is a chemotherapy used to treat GBM however resistance develops in most patients. The underlying mechanisms for TMZ resistance (TMZ-resistant) involve the expression of DNA repair gene O(6)-methylguanine-DNA methyltransferase. CSC genes such as Sox-2, BMI-1, and more recently Y-box binding protein-1 also play a role in resistance. In order to develop novel therapies for GBM, libraries of small interfering RNAs and off-patent drugs have been screened. Over the past few years, several independent laboratories identified disulfiram (DSF) as an off-patent drug that kills GBM CSCs. Reportedly DSF has several modes of action including its ability to inhibit aldehyde dehydrogenases, E3 ligase, polo-like kinase 1, and NFkB. Due to the fact that GBM is a disease of heterogeneity, chemotherapy with multitargeting properties may be the way of the future. In broader terms, DSF kills CSCs from a range of different cancer types further supporting the idea of repurposing it for "target practice."

YB-1 transforms human mammary epithelial cells through chromatin remodeling leading to the development of basal-like breast cancer.

There is growing evidence that cancer-initiation could result from epigenetic changes. Y-box binding protein-1 (YB-1) is a transcription/translation factor that promotes the formation of tumors in transgenic mice; however, the underlying molecular events are not understood. To explore this in a human model system, YB-1 was expressed in mammary epithelial cells under the control of a tetracycline-inducible promoter. The induction of YB-1 promoted phenotypes associated with malignancy in three-dimensional breast acini cultures. This was attributed to YB-1 enhancing the expression and activity of the histone acetyltransferase p300 leading to chromatin remodeling. Specifically, this relaxation of chromatin allowed YB-1 to bind to the BMI1 promoter. The induction of BMI1 engaged the Polycomb complex resulting in histone H2A ubiquitylation and repression of the CDKN2A locus. These events manifested functionally as enhanced self-renewal capacity that occurred in a BMI1-dependent manner. Conversely, p300 inhibition with anacardic acid prevented YB-1 from binding to the BMI1 promoter and thereby subverted self-renewal. Despite these early changes, full malignant transformation was not achieved until RSK2 became overexpressed concomitant with elevated human telomerase reverse transcriptase (hTERT) activity. The YB-1/RSK2/hTERT expressing cells formed tumors in mice that were molecularly subtyped as basal-like breast cancer. We conclude that YB-1 cooperates with p300 to allow BMI1 to over-ride p16(INK4a) -mediated cell cycle arrest enabling self-renewal and the development of aggressive breast tumors.

Overcoming resistance to Sonic Hedgehog inhibition by targeting p90 ribosomal S6 kinase in pediatric medulloblastoma.

BACKGROUND: Molecular subtyping has allowed for the beginning of personalized treatment in children suffering from medulloblastoma (MB). However, resistance inevitably emerges against these therapies, particularly in the Sonic Hedgehog (SHH) subtype. We found that children with SHH subtype have the worst outcome underscoring the need to identify new therapeutic targets. PROCEDURE: High content screening of a 129 compound library identified agents that inhibited SHH MB growth. Lead molecular target levels, p90 ribosomal S6 kinase (RSK) were characterized by immunoblotting and qRT-PCR. Comparisons were made to human neural stem cells (hNSC). Impact of inhibiting RSK with the small molecule BI-D1870 or siRNA was assessed in growth assays (monolayer, neurosphere, and soft agar). NanoString was used to detect RSK in a cohort of 66 patients with MB. To determine BI-D1870 pharmacokinetics/pharmacodynamics, 100 mg/kg was I.P. injected into mice and tissues were collected at various time points. RESULTS: Daoy, ONS76, UW228, and UW426 MB cells were exquisitely sensitive to BI-D1870 but unresponsive to SHH inhibitors. Anti-tumor growth corresponded with inactivation of RSK in MB cells. BI-D1870 had no effect on hNSCs. Inhibiting RSK with siRNA or BI-D1870 suppressed growth, induced apoptosis, and sensitized cells to SHH agents. Notably, RSK expression is correlated with SHH patients. In mice, BI-D1870 was well-tolerated and crossed the blood-brain barrier (BBB). CONCLUSIONS: RSK inhibitors are promising because they target RSK which is correlated with SHH patients as well as cause high levels of apoptosis to only MB cells. Importantly, BI-D1870 crosses the BBB, acting as a scaffold for development of more long-lived RSK inhibitors.

The dawn of targeted therapies for triple negative breast cancer (TNBC): a snapshot of investigational drugs in phase I and II trials.

INTRODUCTION: Triple negative breast cancer (TNBC) was once thought to be an insurmountable disease marked by a lack of targeted treatments. However, we are now witnessing the dawn of targeted therapies for TNBC in which progress has stemmed from an improved understanding of the components that make TNBC unique. The identification of biomarkers, such as BRCA1/2, PIK3CA and RSK2, have advanced the field remarkably and there is considerable interest in finding novel therapeutics for TNBC that offer durable clinical benefit with fewer adverse events. AREAS COVERED: We discuss phase I/II trials of new and emerging targeted therapies for TNBC, according to ClinicalTrials.gov up to June 2020. Although the emphasis is on ongoing and completed early phase trials, we also highlight pivotal studies that have led to the approval of new targeted classes of drugs for TNBC, with a focus on outcomes and common adverse events of each class of therapy. EXPERT OPINION: The way forward for TNBC treatment is through precision medicine. The use of novel agents matched with biomarkers to identify patients with the best chance of sustainable response offers new hope. We now have great potential for improving the outcomes for patients with TNBC.

Personalizing the treatment of pediatric medulloblastoma: Polo-like kinase 1 as a molecular target in high-risk children.

Medulloblastoma is the most common malignant brain tumor in children. This disease is heterogeneous and is composed of four subtypes of medulloblastoma [WNT, Sonic Hedgehog (SHH), Group 3, and Group 4]. An immediate goal is to identify novel molecular targets for the most aggressive forms of medulloblastoma. Polo-like kinase 1 (PLK1) is an oncogenic kinase that controls cell cycle and proliferation, making it a strong candidate for medulloblastoma treatment. In this study, pediatric medulloblastomas were subtyped in two patient cohorts (discovery cohort, n = 63 patients; validation cohort, n = 57 patients) using NanoString nCounter analysis and PLK1 mRNA was assessed. We determined that the SHH and Group 3 subtypes were independently associated with poor outcomes in children as was PLK1 using Cox regression analyses. Furthermore, we screened a library of 129 compounds in clinical trials using a model of pediatric medulloblastoma and determined that PLK1 inhibitors were the most promising class of agents against the growth of medulloblastoma. In patient-derived primary medulloblastoma isolates, the PLK1 small-molecule inhibitor BI2536 suppressed the self-renewal of cells with high PLK1 but not low PLK1 expression. PLK1 inhibition prevented medulloblastoma cell proliferation, self-renewal, cell-cycle progression, and induced apoptosis. In contrast, the growth of normal neural stem cells was unaffected by BI2536. Finally, BI2536 extended survival in medulloblastoma-bearing mice with efficacy comparable with Headstart, a standard-of-care chemotherapy regimen. We conclude that patients with medulloblastoma expressing high levels of PLK1 are at elevated risk. These preclinical studies pave the way for improving the treatment of medulloblastoma through PLK1 inhibition.

Disulfiram, a drug widely used to control alcoholism, suppresses the self-renewal of glioblastoma and over-rides resistance to temozolomide.

Glioblastomas (GBM) are associated with high rates of relapse. These brain tumors are often resistant to chemotherapies like temozolomide (TMZ) and there are very few treatment options available to patients. We recently reported that polo-like kinase-1 (PLK1) is associated with the proliferative subtype of GBM; which has the worst prognosis. In this study, we addressed the potential of repurposing disulfiram (DSF), a drug widely used to control alcoholism for the past six decades. DSF has good safety profiles and penetrates the blood-brain barrier. Here we report that DSF inhibited the growth of TMZ resistant GBM cells, (IC90=100 nM), but did not affect normal human astrocytes. At similar DSF concentrations, self-renewal was blocked by ~100% using neurosphere growth assays. Likewise the drug completely inhibited the self-renewal of the BT74 and GBM4 primary cell lines. Additionally, DSF suppressed growth and self-renewal of primary cells from two GBM tumors.These cells were resistant to TMZ, had unmethylated MGMT, and expressed high levels of PLK1. Consistent with its role in suppressing GBM growth, DSF inhibited the expression of PLK1 in GBM cells. Likewise, PLK1 inhibition with siRNA, or small molecules (BI-2536 or BI-6727) blocked growth of TMZ resistant cells. Our studies suggest that DSF has the potential to be repurposed for treatment of refractory GBM.