ABCB1 and ABCG2 restrict the brain penetration of a panel of novel EZH2-Inhibitors

A specific form of cPRC1 containing CBX4 is co-opted to mediate oncogenic gene repression in diffuse midline glioma

Diffuse midline glioma (DMG) is a fatal childhood brain tumor characterized primarily by mutant histone H3 (H3K27M). H3K27M causes a global reduction in Polycomb repressive complex 2 (PRC2)-mediated H3K27 trimethylation (H3K27me3). Paradoxically, PRC2 is essential in DMG cells, although the downstream molecular mechanisms are poorly understood. Here, we have discovered a specific form of canonical PRC1 (cPRC1) containing CBX4 and PCGF4 that drives oncogenic gene repression downstream of H3K27me3 in DMG cells. Via a novel functional region, CBX4 preferentially associates with PCGF4-containing cPRC1. The characteristic H3K27me3 landscape in DMG rewires the distribution of cPRC1 complexes, with CBX4/PCGF4-cPRC1 accumulating at H3K27me3-enriched CpG islands. Despite comprising <5% of cPRC1 in DMG cells, the unique repressive functions of CBX4/PCGF4-cPRC1 are essential for DMG growth. Our findings link the altered distribution of H3K27me3 to imbalanced cPRC1 function, which drives oncogenic gene repression in DMG, highlighting potential therapeutic opportunities for this incurable childhood brain cancer.

Validation of an LC-MS/MS Method for the Simultaneous Intracellular Quantification of the CDK4/6 Inhibitor Abemaciclib and the EZH2 Inhibitors GSK126 and Tazemetostat

Background: Inhibitors of cyclin-dependent kinases (CDKs) and epigenetic modifier enhancer of zeste homolog 2 (EZH2) have emerged as promising options in the pharmacotherapy of malignant tumors. Recently, we demonstrated synergistic antitumor effects of the CDK4/6 inhibitor abemaciclib and the EZH2 inhibitors GSK126 or tazemetostat in patient-derived glioblastoma (GBM) models. Importantly, all three drugs are substrates of the two most important plasma membrane multidrug transporters ABCB1 and ABCG2, with abemaciclib and tazemetostat also being inhibitors of these proteins. Methods: To investigate whether increased intracellular accumulation of either of the two drugs used in combination could have contributed to corresponding synergisms, we developed a simple LC-MS/MS method for simultaneous detection of the three substances in cell culture lysates. The method was validated in accordance with the current International Council for Harmonization of Technical Requirements for Pharmaceuticals for Human Use (ICH) guideline M10 on bioanalytical method validation and study sample analysis. Results: All acceptance criteria were met. Subsequent analysis of intracellular drug concentrations confirmed increased cellular uptake of tazemetostat in the presence of abemaciclib in both GBM cell lines studied compared to single agent treatment. A comparable pattern was also observed for GSK126, but in only one of the two cell lines used. Conclusions: In conclusion, the observed synergistic antitumor effect could be partly due to increased intracellular accumulation, although this alone is certainly not sufficient to explain it. Overall, the developed method provides a valuable approach for characterizing interactions at the transport level and for predicting the efficiency of both anticancer substance classes in different cell lines.

EZH2 functional dichotomy in reactive oxygen species-stratified glioblastoma

BACKGROUND

Enhancer of zeste homolog 2 (EZH2), well known for its canonical methyltransferase activity in transcriptional repression in many cancers including glioblastoma (GBM), has an understudied noncanonical function critical for sustained tumor growth. Recent GBM consortial efforts reveal complex molecular heterogeneity for which therapeutic vulnerabilities correlated with subtype stratification remain relatively unexplored. Current enzymatic EZH2 inhibitors (EZH2inh) targeting its canonical su(var)3-9, enhancer-of-zeste and trithorax domain show limited efficacy and lack durable response, suggesting that underlying differences in the noncanonical pathway may yield new knowledge. Here, we unveiled dual roles of the EZH2 CXC domain in therapeutically distinct, reactive oxygen species (ROS)-stratified tumors.

METHODS

We analyzed differentially expressed genes between ROS classes by examining cis-regulatory elements as well as clustering of activities and pathways to identify EZH2 as the key mediator in ROS-stratified cohorts. Pull-down assays and CRISPR knockout of EZH2 domains were used to dissect the distinct functions of EZH2 in ROS-stratified GBM cells. The efficacy of NF-κB-inducing kinase inhibitor (NIKinh) and standard-of-care temozolomide was evaluated using orthotopic patient-derived GBM xenografts.

RESULTS

In ROS(+) tumors, CXC-mediated co-interaction with RelB drives constitutive activation of noncanonical NF-κB2 signaling, sustaining the ROS(+) chemoresistant phenotype. In contrast, in ROS(-) subtypes, Polycomb Repressive Complex 2 methyltransferase activity represses canonical NF-κB. Addressing the lack of EZH2inh targeting its nonmethyltransferase roles, we utilized a brain-penetrant NIKinh that disrupts EZH2-RelB binding, consequently prolonging survival in orthotopic ROS(+)-implanted mice.

CONCLUSIONS

Our findings highlight the functional dichotomy of the EZH2 CXC domain in governing ROS-stratified therapeutic resistance, thereby advocating for the development of therapeutic approaches targeting its noncanonical activities and underscoring the significance of patient stratification methodologies.

miR-217-5p NanomiRs Inhibit Glioblastoma Growth and Enhance Effects of Ionizing Radiation via EZH2 Inhibition and Epigenetic Reprogramming

Background/Objectives: CSCs are critical drivers of the tumor and stem cell phenotypes of glioblastoma (GBM) cells. Chromatin modifications play a fundamental role in driving a GBM CSC phenotype. The goal of this study is to further our understanding of how stem cell-driving events control changes in chromatin architecture that contribute to the tumor-propagating phenotype of GBM. Methods: We utilized computational analyses to identify a subset of clinically relevant genes that were predicted to be repressed in a Polycomb repressive complex 2 (PRC2)-dependent manner in GBM upon induction of stem cell-driving events. These associations were validated in patient-derived GBM neurosphere models using state-of-the-art molecular techniques to express, silence, and measure microRNA (miRNA) and gene expression changes. Advanced Poly(β-amino ester) nanoparticle formulations (PBAEs) were used to deliver miRNAs in vivo to orthotopic human GBM tumor models. Results: We show that glioma stem cell (GSC) formation and tumor propagation involve the crosstalk between multiple epigenetic mechanisms, resulting in the repression of the miRNAs that regulate PRC2 function and histone H3 lysine 27 tri-methylation (H3K27me3). We also identified miR-217-5p as an EZH2 regulator repressed in GSCs and showed that miR-217-5p reconstitution using advanced nanoparticle formulations re-activates the PRC2-repressed genes, inhibits GSC formation, impairs tumor growth, and enhances the effects of ionizing radiation in an orthotopic model of GBM. Conclusions: These findings suggest that inhibiting PRC2 function by targeting EZH2 with miR-217-5p advanced nanoparticle formulations could have a therapeutic benefit in GBM.

Dual-target EZH2 inhibitor: latest advances in medicinal chemistry

Enhancer of zeste homolog 2 (EZH2), a histone methyltransferase, plays a crucial role in tumor progression by regulating gene expression. EZH2 inhibitors have emerged as promising anti-tumor agents due to their potential in cancer treatment strategies. However, single-target inhibitors often face limitations such as drug resistance and side effects. Dual-target inhibitors, exemplified by EZH1/2 inhibitor HH-2853(28), offer enhanced efficacy and reduced adverse effects. This review highlights recent advancements in dual inhibitors targeting EZH2 and other proteins like BRD4, PARP1, and EHMT2, emphasizing rational design, structure-activity relationships, and safety profiles, suggesting their potential in clinical applications.

Identification and characterization of the rat in-vivo and in-vitro metabolites of tazemetostat using LC-QTOF-MS

In drug discovery, metabolite profiling unveils biotransformation pathways and potential toxicant formation, guiding selection of candidates with optimal pharmacokinetics and safety profiles. Tazemetostat (TAZ) is employed in treating locally advanced or metastatic epithelioid sarcoma. Identification of drug metabolites are of significant importance in improving safety, efficacy and reduced toxicity of drugs. The current study aimed to investigate the comprehensive metabolic fate of TAZ using different in vivo (rat) and in vitro (RLM, HLM, HS9) models. For in vivo studies, drug was orally administered to Sprague-Dawley rats with subsequent analysis of plasma, feces and urine samples. A total of 21 new metabolites were detected across various matrices and were separated on Phenomenex kinetex C18 (2.5 μm; 150 × 4.6 mm) column using acetonitrile and 0.1% formic acid in water as mobile phase. LC-QTOF-MS/MS and NMR techniques were employed to identify and characterize the metabolites from extracted samples. The major metabolic routes found in biotransformation of TAZ were hydroxylation, N-dealkylation, N-oxidation, hydrogenation, hydrolysis and N-acetylation. In silico toxicity revealed potential immunotoxicity for TAZ and few of its metabolites. This research article is the first time to discuss the complete metabolite profiling including identification and characterization of TAZ metabolites as well as its biotransformation mechanism.

Quantitation of tazemetostat in human plasma using liquid chromatography-tandem mass spectrometry

To support a phase 1 trial in patients with lymphomas, we developed a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for tazemetostat quantitation in 20 μL of human plasma. After protein precipitation, chromatographic separation employed a Kinetex C18 column and a gradient of 0.1% formic acid in both water and acetonitrile, during a 3-min run time. Detection was achieved using a SCIEX 6500+ tandem mass spectrometer with electrospray positive-mode ionization. Validation was based on the latest Food and Drug Administration guidance. With a stable isotopic internal standard, the assay was linear within the range of 10-5000 ng/mL and proved to be accurate (91.9%-103.7%) and precise (<4.4% imprecision). Recovery varied between 93.3% and 121.1%, and matrix effect ranged from -25.5% to -4.9%. Hemolysis, lipemia, and dilution did not impact quantitation. Plasma stability was confirmed after three freeze-thaw cycles, 24 h at room temperature, and 4 months at -80°C. Incurred sample reanalysis yielded 94.4% samples within 20% difference (n = 36). External validation showed a mean bias of -11.1%. Pharmacokinetic (PK) data obtained from three patients suggested variable concentration time profiles, warranting collection of further data. The assay proved to be suitable for tazemetostat quantitation in human plasma and will support clinical studies by defining tazemetostat PKs.

Pharmacology and pharmacokinetics of tazemetostat

Tazemetostat, a novel oral selective inhibitor of enhancer of zeste homolog 2 (EZH2), was approved by the Food and Drug Administration (FDA) in 2020 for use in patients with advanced epithelioid sarcoma or relapsed/refractory (R/R) EZH2-mutated follicular lymphoma. These indications were approved by the FDA trough accelerated approval based on objective response rate and duration of response that resulted from phase 2 clinical trials. Tazemetostat competes with S-adenosylmethionine (SAM) cofactor to inhibit EZH2, reducing the levels of trimethylated lysine 27 of histone 3 (H3K27me3), considered as pharmacodynamic marker. Tazemetostat is orally bioavailable, characterized by rapid absorption and dose-proportional exposure, which is not influenced by coadministration with food or gastric acid reducing agents. It highly distributes in tissues, but with limited access to central nervous system. Tazemetostat is metabolized by CYP3A in the liver to 3 major inactive metabolites (M1, M3, and M5), has a short half-life and is mainly excreted in feces. Drug-drug interactions were shown with moderate CYP3A inhibitors as fluconazole, leading the FDA to recommend a 50% dose reduction, while studies investigating coadministration of tazemetostat with strong inhibitors/inducers are ongoing. No dosage modifications are recommended based on renal or hepatic dysfunctions. Overall, tazemetostat is the first-in-class EZH2 inhibitor approved by the FDA for cancer treatment. Current clinical studies are evaluating combination therapies in patients with several malignancies.

EZH2 inhibition reactivates epigenetically silenced FMR1 and normalizes molecular and electrophysiological abnormalities in fragile X syndrome neurons

Fragile X Syndrome (FXS) is a neurological disorder caused by epigenetic silencing of the FMR1 gene. Reactivation of FMR1 is a potential therapeutic approach for FXS that would correct the root cause of the disease. Here, using a candidate-based shRNA screen, we identify nine epigenetic repressors that promote silencing of FMR1 in FXS cells (called FMR1 Silencing Factors, or FMR1- SFs). Inhibition of FMR1-SFs with shRNAs or small molecules reactivates FMR1 in cultured undifferentiated induced pluripotent stem cells, neural progenitor cells (NPCs) and post-mitotic neurons derived from FXS patients. One of the FMR1-SFs is the histone methyltransferase EZH2, for which an FDA-approved small molecule inhibitor, EPZ6438 (also known as tazemetostat), is available. We show that EPZ6438 substantially corrects the characteristic molecular and electrophysiological abnormalities of cultured FXS neurons. Unfortunately, EZH2 inhibitors do not efficiently cross the blood-brain barrier, limiting their therapeutic use for FXS. Recently, antisense oligonucleotide (ASO)-based approaches have been developed as effective treatment options for certain central nervous system disorders. We therefore derived efficacious ASOs targeting EZH2 and demonstrate that they reactivate FMR1 expression and correct molecular and electrophysiological abnormalities in cultured FXS neurons, and reactivate FMR1 expression in human FXS NPCs engrafted within the brains of mice. Collectively, our results establish EZH2 inhibition in general, and EZH2 ASOs in particular, as a therapeutic approach for FXS.

Cyclohexene oxide CA, a derivative of zeylenone, exhibits anti-cancer activity in glioblastoma by inducing G0/G1 phase arrest through interference with EZH2

Introduction: Due to its highly aggressiveness and malignancy, glioblastoma (GBM) urgently requires a safe and effective treatment strategy. Zeylenone, a natural polyoxygenated cyclohexenes compound isolated from Uvaria grandiflora, has exhibited potential biological activities in various human diseases, including tumors. Methods: We designed and synthesized a series of (+)-Zeylenone analogues and evaluated their anti-GBM roles through structural-activity analysis. Cell Counting Kit-8, TUNEL, transwell and flow cytometry were employed for investigating the anticancer effects of CA on GBM cells. Western blotting, molecular docking, qRT-PCR and ChIP assays were performed to reveal the underlying mechanisms by which CA regulates the GBM cell cycle. The nude mouse xenograft model, HE staining, immunohistochemistry and was used to evaluate the anticancer effect of CA in vivo. Results: We identified CA ((1R, 2R, 3S)-3-p-fluorobenzoyl-zeylenone) as having the lowest IC50 value in GBM cells. CA treatment significantly inhibited the malignant behaviors of GBM cells and induced G0/G1 phase arrest in vitro. Furthermore, we validated the molecular mechanism by which CA interferes with EZH2, attenuating the down-regulation of cyclin-dependent kinase inhibitors p27 and p16 by the PRC2 complex. By establishing orthotopic nude mice models, we further validated the inhibitory role of CA on tumorigenesis of GBM cells in vivo and its potential values to synergistically potentiate the anti-tumor effects of EZH2 inhibitors. Conclusion: Overall, this paper elucidated the anti-GBM effects and potential mechanisms of CA, and may provide a therapeutic drug candidate for GBM treatment.

Tazemetostat synergistically combats multidrug resistance by the unique triple inhibition of ABCB1, ABCC1, and ABCG2 efflux transporters in vitro and ex vivo

ATP-binding cassette (ABC) drug efflux transporters and drug metabolizing enzymes play crucial roles in pharmacokinetic drug-drug interactions and multidrug tumor resistance (MDR). Tazemetostat (EPZ-6438, Tazverik) is a novel epigenetic drug that has been recently approved for the therapy of advanced epithelioid sarcoma and follicular lymphoma. Additionally, this medication is currently being clinically tested to treat several other cancers such as non-small cell lung cancer (NSCLC). This study aimed to investigate the inhibitory effects of tazemetostat on selected ABC transporters/cytochrome P450 3A4 (CYP3A4) enzyme to comprehensively explore its role in MDR. First, our accumulation and molecular docking studies showed that tazemetostat is a unique triple inhibitor of ABCB1, ABCC1, and ABCG2 transporters. In contrast, tazemetostat exhibited only low level of interaction with the CYP3A4 isozyme. Drug combination assays confirmed that tazemetostat is a multipotent MDR modulator able to synergize with various conventional chemotherapeutics in vitro. Subsequent caspase activity assays and microscopic staining of apoptotic nuclei proved that the effective induction of apoptosis is behind the observed synergies. Notably, a potent MDR-modulatory capacity of tazemetostat was recorded in primary ex vivo NSCLC explants generated from patients' biopsies. On the contrary, its possible position of pharmacokinetic MDR's victim was excluded in comparative proliferation assays. Finally, tested drug has not been identified as an inducer of resistant phenotype in NSCLC cell lines. In conclusion, we demonstrated that tazemetostat is a unique multispecific chemosensitizer, which has strong potential to overcome limitations seen in the era of traditional MDR modulators.

Sevoflurane Preconditioning Alleviates Posttraumatic Stress Disorder-Induced Apoptosis in the Hippocampus via the EZH2-Regulated Akt/mTOR Axis and Improves Synaptic Plasticity

Posttraumatic stress disorder (PTSD) is a persistent and severe psychological and mental disorder resulting from experiences of serious trauma or stress and is suffered by many individuals. Previous studies have shown that pretreatment with sevoflurane is efficient in reducing the incidence of PTSD. However, we require a more comprehensive understanding of the specific mechanisms by which sevoflurane works. Enhancer of zeste homolog 2 (EZH2) has been reported to be regulated by sevoflurane, and to improve patient cognition. In this study, we aimed to explore the mechanisms of sevoflurane and the role of EZH2 in PTSD cases. We explored the effects of sevoflurane and EPZ-6438 (inhibitor of EZH2) on rat behavior, followed by an investigation of EZH2 mRNA and protein expression. The effects of sevoflurane and EZH2 on neuronal survival were assessed by western blotting and TUNEL staining, while western blotting was used to examine the expression of PSD95 and the AKT/mTOR proteins. Sevoflurane preconditioning restored EZH2 expression and significantly inhibited apoptosis by regulating phosphorylation of the AKT/mTOR pathway. Synaptic plasticity was also significantly improved. These results suggest that pretreatment with sevoflurane could play an important role in PTSD prevention by regulating EZH2 expression.

H3K27me3 in Diffuse Midline Glioma and Epithelial Ovarian Cancer: Opposing Epigenetic Changes Leading to the Same Poor Outcomes

Histone post-translational modifications modulate gene expression through epigenetic gene regulation. The core histone H3 family members, H3.1, H3.2, and H3.3, play a central role in epigenetics. H3 histones can acquire many post-translational modifications, including the trimethylation of H3K27 (H3K27me3), which represses transcription. Triple methylation of H3K27 is performed by the histone methyltransferase Enhancer of Zeste Homologue 2 (EZH2), a component of the Polycomb Repressive Complex 2. Both global increases and decreases in H3K27me3 have been implicated in a wide range of cancer types. Here, we explore how opposing changes in H3K27me3 contribute to cancer by highlighting its role in two vastly different cancer types; (1) a form of glioma known as diffuse midline glioma H3K27-altered and (2) epithelial ovarian cancer. These two cancers vary widely in the age of onset, sex, associated mutations, and cell and organ type. However, both diffuse midline glioma and ovarian cancer have dysregulation of H3K27 methylation, triggering changes to the cancer cell transcriptome. In diffuse midline glioma, the loss of H3K27 methylation is a primary driving factor in tumorigenesis that promotes glial cell stemness and silences tumor suppressor genes. Conversely, hypermethylation of H3K27 occurs in late-stage epithelial ovarian cancer, which promotes tumor vascularization and tumor cell migration. By using each cancer type as a case study, this review emphasizes the importance of H3K27me3 in cancer while demonstrating that the mechanisms of histone H3 modification and subsequent gene expression changes are not a one-size-fits-all across cancer types.

A tumor suppressor role for EZH2 in diffuse midline glioma pathogenesis

Pediatric high-grade gliomas, specifically diffuse midline gliomas, account for only 20% of clinical cases but are 100% fatal. A majority of the DMG cases are characterized by the signature K27M mutation in histone H3. The H3K27M mutation opposes the function of enhancer of zeste homolog 2 (EZH2), the methyltransferase enzyme of the polycomb repressor complex 2. However, the role of EZH2 in DMG pathogenesis is unclear. In this study, we demonstrate a tumor suppressor function for EZH2 using Ezh2 loss- and gain-of-function studies in H3WT DMG mouse models. Genetic ablation of Ezh2 increased cell proliferation and tumor grade while expression of an Ezh2 gain-of-function mutation significantly reduced tumor incidence and increased tumor latency. Transcriptomic analysis revealed that Ezh2 deletion upregulates an inflammatory response with upregulation of immunoproteasome genes such as Psmb8, Psmb9, and Psmb10. Ezh2 gain-of-function resulted in enrichment of the oxidative phosphorylation/mitochondrial metabolic pathway namely the isocitrate dehydrogenase Idh1/2/3 genes. Pharmacological inhibition of EZH2 augmented neural progenitor cell proliferation, supporting the tumor suppressive role of EZH2. In vivo 7-day treatment of H3K27M DMG tumor bearing mice with an EZH2 inhibitor, Tazemetostat, did not alter proliferation or significantly impact survival. Together our results suggest that EZH2 has a tumor suppressor function in DMG and warrants caution in clinical translation of EZH2 inhibitors to treat patients with DMG.

Glioma-derived exosomes hijack the blood-brain barrier to facilitate nanocapsule delivery via LCN2

Exosomes are small extracellular vehicles which could transport genetic materials and proteins between cells. Although there are reports about exosomes crossing the blood-brain barrier (BBB), the underlying mechanisms still need further study. We found that exosomes from primary brain tumors could upregulate the expression of Lipocalin-2 (LCN2) in bEnd.3 brain microvascular endothelial cells (BMVECs). Furthermore, exosomes increased the membrane fluidity of bEnd.3 cells in an LCN2 dependent manner. Both intraperitoneal injection and caudal vein injection of LCN2 increased the number of nanocapsules crossing the BBB. Evans Blue staining revealed that LCN2 does not interrupt the integrity of the BBB, as observed in the traumatic brain injury model. Tandem mass tags quantitative proteomics and bioinformatics analysis revealed that LCN2 is upregulated by exosomes via the JAK-STAT3 pathway, but not delivered from exosomes. Knocking down LCN2 in bEnd.3 cells significantly abrogated the effect of exosomes on BMVEC membrane fluidity. Previously, we have reported that 2-methacryloyloxyethyl phosphorylcholine (MPC) and a peptide crosslinker could encapsulate mAbs to achieve nanocapsules. The nanocapsules containing choline analogs could effectively penetrate the BBB to deliver therapeutic monoclonal antibodies (tAbs) to the glioma. However, the delivered tAbs could be significantly reduced by blocking the release of exosomes from the gliomas. Application of tAb nanocapsules prior to treatment with MK2206, an AKT pathway inhibitor that has been shown to inhibit the production of exosomes, resulted in a better combination. Insights from this study provide a mechanistic framework with regard to how glioblastomas hijack BMVECs using exosomes. In addition, we provide a strategy for maximizing the effect of the choline-containing nanocapsules and MK2206 combination. These results also demonstrate the therapeutic role of tAbs in glioblastoma and brain tumor metastasis, by shedding new light on strategies that can be used for BBB-penetrating therapies.

A Chemical Strategy toward Novel Brain-Penetrant EZH2 Inhibitors

Aberrant gene-silencing through dysregulation of polycomb protein activity has emerged as an important oncogenic mechanism in cancer, implicating polycomb proteins as important therapeutic targets. Recently, an inhibitor targeting EZH2, the methyltransferase component of PRC2, received U.S. Food and Drug Administration approval following promising clinical responses in cancer patients. However, the current array of EZH2 inhibitors have poor brain penetrance, limiting their use in patients with central nervous system malignancies, a number of which have been shown to be sensitive to EZH2 inhibition. To address this need, we have identified a chemical strategy, based on computational modeling of pyridone-containing EZH2 inhibitor scaffolds, to minimize P-glycoprotein activity, and here we report the first brain-penetrant EZH2 inhibitor, TDI-6118 (compound 5). Additionally, in the course of our attempts to optimize this compound, we discovered TDI-11904 (compound 21), a novel, highly potent, and peripherally active EZH2 inhibitor based on a 7 member ring structure.

An EZH2 blocker sensitizes histone mutated diffuse midline glioma to cholesterol metabolism inhibitors through an off-target effect

Background

Diffuse Midline Glioma, H3K27M-mutant (DMG) is a rare, highly aggressive pediatric tumor affecting the brainstem, and is one of the deadliest cancers. Currently available treatment options such as chemotherapy and radiotherapy do only modestly prolong survival. In this pathology, H3K27 mutations deregulate Polycomb Repressive Complex 2 (PRC2), including enzymatic activity of EZH2, which is therefore under investigation as a therapeutic target.

Methods

We used a chemical EZH2 inhibitor, GSK126, small interfering RNAs, and a CRISPR/Cas9 knockout approaches in a series of DMG tumor cell lines to investigate metabolic treatment responses by proteomic analysis. A combination strategy was elaborated and studied in primary and established DMG cells, spheroid 3D cultures, and in vivo in a chick chorio-allantoic membrane DMG assay and an orthotopic intracranial DMG mouse model.

Results

GSK126 shows significant (P < .05–.001) inhibitory effects in in vitro cell proliferation assays and induces apoptosis. Chemical targeting of EZH2 induced expression of proteins implicated in cholesterol metabolism. Low-dose GSK126 treatment together with statins revealed strong growth inhibition in combinatorial treatments, but not in single treatments, both in DMG cells in vitro, in DMG spheroid cultures, and in chick and mouse in vivo models (P < .05). All statistical tests were two-sided.

Conclusions

Our results reveal an unexpected GSK126-inducible sensitivity to cholesterol biosynthesis inhibitors in highly aggressive pediatric glioma that warrants further evaluation as treatment strategy. This combinatorial therapy should have few side effects because of the low doses used to achieve significant anti-tumor activity.

GSK-126 Protects CA1 Neurons from H3K27me3-Mediated Apoptosis in Cerebral Ischemia

Epigenetics, including histone modifications, play a significant role in central nervous system diseases, but the underlying mechanism remains to be elucidated. The aim of this study was to evaluate the role of H3K27me3 in regulating transcriptomic and pathogenic mechanisms following global ischemic stroke. Here, we found that in vivo ischemic/reperfusion (I/R) injury induced marked upregulation of H3K27me3 in the hippocampus. The administration of GSK-126 to rat brains decreased the levels of H3K27me3 in the hippocampus and reduced neuronal apoptosis after experimental stroke. Furthermore, ChIP-seq data demonstrated that the primary role of GSK-126 in the ischemic brain is to reduce H3K27me3 enrichment, mediating negative regulation of the execution phase of apoptosis and the MAPK signaling pathway. Further study suggested that the protective role of GSK-126 in ischemic rats was antagonized by U0126, an inhibitor of ERK1/2. Collectively, we demonstrated the potential of H3K27me3 as a novel stroke therapeutic target, and GSK-126 exerted a neuroprotective function in ischemic brain injury, which might be associated with activation of the MAPK/ERK pathway.

An overview of the development of EED inhibitors to disable the PRC2 function

Polycomb repressive complex 2 (PRC2) catalyzes the methylation of histone H3 lysine 27 (H3K27) and the enrichment of its catalytic product H3K27me3 is responsible for the silencing of tumor suppressor genes and the blocking of transcripts related to immunity and cell terminal differentiation. Aberrations of PRC2 components, such as mutation and overexpression, have been observed in various cancers, which makes PRC2 a potential therapeutic target for cancer. Up to now, targeting the enhancer of zeste homolog 2 (EZH2), the catalytic subunit of PRC2, represents the main strategy in the development of PRC2 inhibitors. Although significant progress has been made, new problems also emerge, e.g. the drug resistance caused by secondary mutations. In recent years, more and more efforts have shifted to another new strategy - targeting embryonic ectoderm development (EED) to disrupt its major interactions with other components, which are necessary to the PRC2 function, and some promising results have been obtained. This review summarizes the recent development of EED inhibitors as possible chemotherapy for cancer treatment, which could help accelerate future related research work.

Therapeutic Targets in Diffuse Midline Gliomas-An Emerging Landscape

Simple Summary

Diffuse midline gliomas (DMGs) remain one of the most devastating childhood brain tumour types, for which there is currently no known cure. In this review we provide a summary of the existing knowledge of the molecular mechanisms underlying the pathogenesis of this disease, highlighting current analyses and novel treatment propositions. Together, the accumulation of these data will aid in the understanding and development of more effective therapeutic options for the treatment of DMGs.

Abstract

Diffuse midline gliomas (DMGs) are invariably fatal pediatric brain tumours that are inherently resistant to conventional therapy. In recent years our understanding of the underlying molecular mechanisms of DMG tumorigenicity has resulted in the identification of novel targets and the development of a range of potential therapies, with multiple agents now being progressed to clinical translation to test their therapeutic efficacy. Here, we provide an overview of the current therapies aimed at epigenetic and mutational drivers, cellular pathway aberrations and tumor microenvironment mechanisms in DMGs in order to aid therapy development and facilitate a holistic approach to patient treatment.

NeuroD1 Dictates Tumor Cell Differentiation in Medulloblastoma

Tumor cells are characterized by unlimited proliferation and perturbed differentiation. Using single-cell RNA sequencing, we demonstrate that tumor cells in medulloblastoma (MB) retain their capacity to differentiate in a similar way as their normal originating cells, cerebellar granule neuron precursors. Once they differentiate, MB cells permanently lose their proliferative capacity and tumorigenic potential. Differentiated MB cells highly express NeuroD1, a helix-loop-helix transcription factor, and forced expression of NeuroD1 promotes the differentiation of MB cells. The expression of NeuroD1 in bulk MB cells is repressed by trimethylation of histone 3 lysine-27 (H3K27me3). Inhibition of the histone lysine methyltransferase EZH2 prevents H3K27 trimethylation, resulting in increased NeuroD1 expression and enhanced differentiation in MB cells, which consequently reduces tumor growth. These studies reveal the mechanisms underlying MB cell differentiation and provide rationales to treat MB (potentially other malignancies) by stimulating tumor cell differentiation.

Cheng et al. demonstrate that medulloblastoma cells retain the capacity to undergo differentiation. The differentiation of tumor cells is regulated by NeuroD1 expression, which is repressed by H3K27me3 in tumor cells. EZH2 inhibitors suppress medulloblastoma growth by stimulating tumor cell differentiation.

A vasculature-centric approach to developing novel treatment options for glioblastoma

Introduction: Glioblastoma is invariably deadly and is characterized by extensive vascularization and macrophage-dominant immunosuppression; nevertheless, anti-angiogenesis has so far failed to prolong overall survival of patients. Regardless of the problems in clinical development, the rationale for the application of anti-angiogenics in glioblastoma remains.Areas covered: Resistance to anti-angiogenics is discussed, including vessel co-option and amplification of hypoxic signaling in response to vessel destruction. The modulation of GSC and tumor-associated macrophages by dysfunctional tumor vessels and by hypoxia are outlined. Pharmacologic approaches to sensitizing glioblastomas to anti-angiogenics and evidence for the cooperation of anti-angiogenics with immunotherapies are summarized. Database search: https://pubmed.ncbi.nlm.nih.gov prior to December 12, 2020.Expert opinion: Despite drawbacks in the clinical development of vascular endothelial growth factor A (VEGF)-targeted agents, there is still rationale for the use of anti-angiogenics. The better understanding of vascular co-option and adverse effects of blood vessel destruction guides to improve strategies for vascular targeting. The pivotal role of the vasculature and of angiogenic factors such as VEGF for the induction and maintenance of immunosuppression in glioblastoma supports the use of anti-angiogenics in combination with immunotherapy. Proinflammatory repolarization of perivascular and perinecrotic tumor-associated macrophages is probably paramount for overcoming treatment resistance to virtually any treatment.

Convection-Enhanced Delivery of Enhancer of Zeste Homolog-2 (EZH2) Inhibitor for the Treatment of Diffuse Intrinsic Pontine Glioma

BACKGROUND

Diffuse intrinsic pontine glioma (DIPG) is a fatal childhood brain tumor and the majority of patients die within 2 yr after initial diagnosis. Factors that contribute to the dismal prognosis of these patients include the infiltrative nature and anatomic location in an eloquent area of the brain, which precludes total surgical resection, and the presence of the blood-brain barrier (BBB), which reduces the distribution of systemically administered agents. Convection-enhanced delivery (CED) is a direct infusion technique to deliver therapeutic agents into a target site in the brain and able to deliver a high concentration drug to the infusion site without systemic toxicities.

OBJECTIVE

To assess the efficacy of enhancer of zeste homolog-2 (EZH2) inhibitor by CED against human DIPG xenograft models.

METHODS

The concentration of EZH2 inhibitor (EPZ-6438) in the brainstem tumor was evaluated by liquid chromatography-mass spectrometry (LC/MS). We treated mice-bearing human DIPG xenografts with EPZ-6438 using systemic (intraperitoneal) or CED administration. Intracranial tumor growth was monitored by bioluminescence image, and the therapeutic response was evaluated by animal survival.

RESULTS

LC/MS analysis showed that the concentration of EPZ-6438 in the brainstem tumor was 3.74% of serum concentration after systemic administration. CED of EPZ-6438 suppressed tumor growth and significantly extended animal survival when compared to systemic administration of EPZ-6438 (P = .0475).

CONCLUSION

Our results indicate that CED of an EZH2 inhibitor is a promising strategy to bypass the BBB and to increase the efficacy of an EZH2 inhibitor for the treatment of DIPG.

EZH2 targeting reduces medulloblastoma growth through epigenetic reactivation of the BAI1/p53 tumor suppressor pathway

Medulloblastoma (MB) is a malignant pediatric brain tumor for which new therapies are urgently needed. We demonstrate that treatment with EPZ-6438 (Tazemetostat), an enhancer of zeste homolog 2 (EZH2) inhibitor approved for clinical trials, blocks MB cell growth in vitro and in vivo, and prolongs survival in orthotopic xenograft models. We show that the therapeutic effect is dependent on epigenetic reactivation of adhesion G-protein-coupled receptor B1 (BAI1/ADGRB1), a tumor suppressor that controls p53 stability by blocking Mdm2. Histone 3 trimethylated on lysine 27 (H3K27me3), a marker of silent chromatin conformation is present at the ADGRB1 promoter, and inhibition of EZH2, the catalytic component of the Polycomb Repressive complex 2 (PRC2) that methylates H3K27, switches the gene into an active chromatin status and reactivates BAI1 expression. Mechanistically, targeting EZH2 promotes transition from H3K27me3 to H3K27ac at the promoter, recruits the C/EBPβ (CREB-binding protein) and CBP transcription factors and activates ADGRB1 gene transcription. Taken together, our results identify key molecular players that regulate ADGRB1 gene expression in MB, demonstrate that reactivation of BAI1 expression underlies EPZ-6438 antitumorigenic action, and provide preclinical proof-of-principle evidence for targeting EZH2 in patients with MB.

3-benzazecine-based cyclic allene derivatives as highly potent P-glycoprotein inhibitors overcoming doxorubicin multidrug resistance

Aim: Enamino 3-benzazecine compounds, incorporating the C6-C8 allene system, were synthesized and evaluated in vitro as inhibitors of P-glycoprotein (P-gp) and/or multidrug resistance-associated protein 1 (MRP1), two efflux pumps mainly connected with multidrug resistance (MDR) in cancer cells. Results & methodology: Most of the synthesized compounds were selective P-gp inhibitors in Calcein-AM uptake assay. Structure–activity relationships (SARs) pointed out that CO2Me derivatives are more potent than acetyl derivatives, and 10,11-dimethoxy compounds are five to tenfold more potent inhibitors than the respective unsubstituted compounds, and that the P-gp inhibition potency is mainly related to volume parameters. Conclusion: Nanomolar P-gp inhibitors, such as 23 (IC50 = 4.2 nM), restored the antiproliferative activity of doxorubicin in multidrug-resistant cells. The observed activities showed that 3-benzazecine-based compounds may be promising MDR reversers.

Pyrazole-based inhibitors of enhancer of zeste homologue 2 induce apoptosis and autophagy in cancer cells

Novel pyrazole-based EZH2 inhibitors have been prepared through a molecular pruning approach from known inhibitors bearing a bicyclic moiety as a central scaffold. The hit compound 1o (N-((4,6-dimethyl-2-oxo-1,2-dihydropyridin-3-yl)methyl)-5-methyl-1-phenyl-1H-pyrazole-4-carboxamide) showed low micromolar EZH2/PRC2 inhibition and high selectivity towards a panel of other methyltransferases. Moreover, 1o displayed cell growth arrest in breast MDA-MB231, leukaemia K562, and neuroblastoma SK-N-BE cancer cells joined to reduction of H3K27me3 levels and induction of apoptosis and autophagy.This article is part of a discussion meeting issue 'Frontiers in epigenetic chemical biology'.

Inhibition of enhancer of zest homologue 2 is a potential therapeutic target for high-MYC medulloblastoma

MYC amplification is common in Group 3 medulloblastoma and is associated with poor survival. Group 3 and Group 4 medulloblastomas are also known to have elevated levels of histone H3-lysine 27-tri-methylation (H3K27me3), at least in part due to high expression of the H3K27 methyltransferase enhancer of zest homologue 2 (EZH2), which can be regulated by MYC. We therefore examined whether MYC expression is associated with elevated EZH2 and H3K27me3 in medulloblastoma, and if high-MYC medulloblastomas are particularly sensitive to pharmacological EZH2 blockade. Western blot analysis of low (DAOY, UW228, CB SV40) and high (DAOY-MYC, UW228-MYC, CB-MYC, D425) MYC cell lines showed that higher levels of EZH2 and H3K27me3 were associated with elevated MYC. In fixed medulloblastoma samples examined using immunohistochemistry, most MYC positive tumors also had high H3K27me3, but many MYC negative ones did as well, and the correlation was not statistically significant. All high MYC lines tested were sensitive to the EZH2 inhibitor EPZ6438. Many low MYC lines also grew more slowly in the presence of EPZ6438, although DAOY-MYC cells responded more strongly than parent DAOY cultures with lower MYC levels. We find that higher MYC levels are associated with increased EZH2, and pharmacological blockade of EZH2 is a potential therapeutic strategy for aggressive medulloblastoma with elevated MYC.

Six Years (2012-2018) of Researches on Catalytic EZH2 Inhibitors: The Boom of the 2-Pyridone Compounds

Enhancer of zeste homolog 2 (EZH2), the catalytic subunit of the Polycomb repressive complex 2 (PRC2), catalyzes the methylation of lysine 27 of histone H3 (H3K27) up to its trimethylated form (H3K27me), inducing by this way block of transcription and gene silencing. High levels of H3K27me3 have been found in both hematological malignancies and solid cancers, due to EZH2 overexpression and/or EZH2 mutation. From 2012, a number of highly potent and selective catalytic inhibitors of EZH2 have been reported, almost all bearing a 2-pyridone group in their structure. Typically, 2-pyridone inhibitors are selective for EZH2 over other methyltransferases, and some of them are specific for EZH2 over EZH1, others behave as dual EZH2/EZH1 inhibitors. The 2-pyridone moiety was crucial for the enzyme inhibition, as revealed later by crystallographic studies because it occupies partially the site for the co-substrate SAM (or the by-product, SAH) in the binding pocket of the enzyme, accounting for the SAM-competitive mechanism of action displayed by all the 2-pyridone inhibitors. The 2-pyridone warhead is linked to a support substructure, that can be either a bicyclic heteroaromatic ring (such as indazole, see for instance EPZ005687 and UNC1999, or indole, see for instance GSK126, EI1, and the more recent CPI-1205) or a simple monocyclic (hetero) aromatic ring (tazemetostat, MC3629, (R)-OR-S1/2), eventually annulated with the amide chain carrying the 2-pyridone group (3,4-dihydroisoquinoline-1(2H)-ones). Different substitutions at the support moiety influence the pharmacokinetics and pharmacodynamics of the compounds as well as their water solubility. In cancer diseases, the first reported 2-pyridone inhibitors displayed high antiproliferative effects in vitro and in vivo in lymphomas characterized by mutant EZH2 (such as Y641N), but the most recent compounds exert their anticancer activity against tumors with wild-type EZH2 as well. The dual EZH2/1 inhibitors have been recently reported to be more effective than EZH2 selective inhibitors in specific leukemias including leukemias cancer stem cells.

Next-Generation in vivo Modeling of Human Cancers

Animal models of human cancers played a major role in our current understanding of tumor biology. In pre-clinical oncology, animal models empowered drug target and biomarker discovery and validation. In turn, this resulted in improved care for cancer patients. In the quest for understanding and treating a diverse spectrum of cancer types, technological breakthroughs in genetic engineering and single cell "omics" offer tremendous potential to enhance the informative value of pre-clinical models. Here, I review the state-of-the-art in modeling human cancers with focus on animal models for human malignant gliomas. The review highlights the use of glioma models in dissecting mechanisms of tumor initiation, in the retrospective identification of tumor cell-of-origin, in understanding tumor heterogeneity and in testing the potential of immuno-oncology. I build on the deep review of glioma models as a basis for a more general discussion of the potential ways in which transformative technologies may shape the next-generation of pre-clinical models. I argue that refining animal models along the proposed lines will benefit the success rate of translation for pre-clinical research in oncology.

ABCB1 Attenuates the Brain Penetration of the PARP Inhibitor AZD2461

Poly (ADP-ribose) polymerase (PARP) inhibitors are a relatively new class of anticancer agents that have attracted attention for treatment of glioblastoma because of their ability to potentiate temozolomide chemotherapy. Previous studies have demonstrated that sufficient brain penetration is a prerequisite for efficacy of PARP inhibitors in glioma mouse models. Unfortunately, however, most of the PARP inhibitors developed to date have a limited brain penetration due to the presence of P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) at the blood-brain barrier. AZD2461 is a novel PARP inhibitor that is unaffected by P-gp mediated resistance in breast cancer models and thus appears to have promising characteristics for brain penetration. We here use a comprehensive set of in vitro and in vivo models to study the brain penetration and oral bioavailability of AZD2461. We report that AZD2461 has a good membrane permeability. However, it is a substrate of P-gp and BCRP, and P-gp in particular limits its brain penetration in vivo. We show that AZD2461 has a low oral bioavailability, although it is not affected by P-gp and BCRP. Together, these findings are not in favor of further development of AZD2461 for treatment of glioblastoma.

Buparlisib is a brain penetrable pan-PI3K inhibitor

Characterization of the genomic landscapes of intracranial tumours has revealed a clear role for the PI3K-AKT-mTOR pathway in tumorigenesis and tumour maintenance of these malignancies, making phosphatidylinositol 3-kinase (PI3K) inhibition a promising therapeutic strategy for these tumours. Buparlisib is a novel pan-PI3K inhibitor that is currently in clinical development for various cancers, including primary and secondary brain tumours. Importantly however, earlier studies have revealed that sufficient brain penetration is a prerequisite for antitumor efficacy against intracranial tumours. We therefore investigated the brain penetration of buparlisib using a comprehensive set of in vitro and in vivo mouse models. We demonstrate that buparlisib has an excellent brain penetration that is unaffected by efflux transporters at the blood-brain barrier, complete oral bioavailability and efficient intracranial target inhibition at clinically achievable plasma concentrations. Together, these characteristics make buparlisib the ideal candidate for intracranially-targeted therapeutic strategies that involve PI3K inhibition.

A New Class of 1-Aryl-5,6-dihydropyrrolo[2,1-a]isoquinoline Derivatives as Reversers of P-Glycoprotein-Mediated Multidrug Resistance in Tumor Cells

A number of aza-heterocyclic compounds, which share the 5,6-dihydropyrrolo[2,1-a]isoquinoline (DHPIQ) scaffold with members of the lamellarin alkaloid family, were synthesized and evaluated for their ability to reverse in vitro multidrug resistance in cancer cells through inhibition of P-glycoprotein (P-gp) and/or multidrug-resistance-associated protein 1. Most of the investigated DHPIQ compounds proved to be selective P-gp modulators, and the most potent modulator, 8,9-diethoxy-1-(3,4-diethoxyphenyl)-3-(furan-2-yl)-5,6-dihydropyrrolo[2,1-a]isoquinoline-2-carbaldehyde, attained sub-micromolar inhibitory potency (IC₅₀ : 0.19 μm). Schiff bases prepared by the condensation of some 1-aryl-DHPIQ aldehydes with p-aminophenol also proved to be of some interest, and one of them, 4-((1-(4-fluorophenyl)-5,6-dihydro-8,9-dimethoxypyrrolo[2,1-a]isoquinolin-2-yl)methyleneamino)phenol, had an IC₅₀ value of 1.01 μm. In drug combination assays in multidrug-resistant cells, some DHPIQ compounds, at nontoxic concentrations, significantly increased the cytotoxicity of doxorubicin in a concentration-dependent manner. Studies of structure-activity relationships and investigation of the chemical stability of Schiff bases provided physicochemical information useful for molecular optimization of lamellarin-like cytotoxic drugs active toward chemoresistant tumors as well as nontoxic reversers of P-gp-mediated multidrug resistance in tumor cells.

Revisiting the role of ABC transporters in multidrug-resistant cancer

Most patients who die of cancer have disseminated disease that has become resistant to multiple therapeutic modalities. Ample evidence suggests that the expression of ATP-binding cassette (ABC) transporters, especially the multidrug resistance protein 1 (MDR1, also known as P-glycoprotein or P-gp), which is encoded by ABC subfamily B member 1 (ABCB1), can confer resistance to cytotoxic and targeted chemotherapy. However, the development of MDR1 as a therapeutic target has been unsuccessful. At the time of its discovery, appropriate tools for the characterization and clinical development of MDR1 as a therapeutic target were lacking. Thirty years after the initial cloning and characterization of MDR1 and the implication of two additional ABC transporters, the multidrug resistance-associated protein 1 (MRP1; encoded by ABCC1)), and ABCG2, in multidrug resistance, interest in investigating these transporters as therapeutic targets has waned. However, with the emergence of new data and advanced techniques, we propose to re-evaluate whether these transporters play a clinical role in multidrug resistance. With this Opinion article, we present recent evidence indicating that it is time to revisit the investigation into the role of ABC transporters in efficient drug delivery in various cancer types and at the blood-brain barrier.

Therapeutic Potential and Utility of Elacridar with Respect to P-glycoprotein Inhibition: An Insight from the Published In Vitro, Preclinical and Clinical Studies

The occurrence of efflux mechanisms via Permeability-glycoprotein (P-gp) recognized as an important physiological process impedes drug entry or transport across membranes into tissues. In some instances, either low oral bioavailability or lack of brain penetration has been attributed to P-gp mediated efflux activity. Therefore, the objective of development of P-gp inhibitors was to facilitate the attainment of higher drug exposures in tissues. Many third-generation P-gp inhibitors such as elacridar, tariquidar, zosuquidar, etc. have entered clinical development to fulfil the promise. The body of evidence from in vitro and in vivo preclinical and clinical data reviewed in this paper provides the basis for an effective blockade of P-gp efflux mechanism by elacridar. However, clinical translation of the promise has been elusive not just for elacridar but also for other P-gp inhibitors in this class. The review provides introspection and perspectives on the lack of clinical translation of this class of drugs and a broad framework of strategies and considerations in the potential application of elacridar and other P-gp inhibitors in oncology therapeutics.

Improved Brain Penetration and Antitumor Efficacy of Temozolomide by Inhibition of ABCB1 and ABCG2

The anticancer drug temozolomide is the only drug with proven activity against high-grade gliomas and has therefore become a part of the standard treatment of these tumors. P-glycoprotein (P-gp; ABCB1) and breast cancer resistance protein (BCRP; ABCG2) are transport proteins, which are present at the blood-brain barrier and limit the brain uptake of substrate drugs. We have studied the effect of P-gp and BCRP on the pharmacokinetics and pharmacodynamics of temozolomide, making use of a comprehensive set of in vitro transport experiments and in vivo pharmacokinetic and antitumor efficacy experiments using wild-type, Abcg2^−/−, Abcb1a/b^−/−, and Abcb1a/b;Abcg2^−/− mice. We here show that the combined deletion of Abcb1a/b and Abcg2 increases the brain penetration of temozolomide by 1.5-fold compared to wild-type controls (P < .001) without changing the systemic drug exposure. Moreover, the same increase was achieved when temozolomide was given to wild-type mice in combination with the dual P-gp/BCRP inhibitor elacridar (GF120918). The antitumor efficacy of temozolomide against three different intracranial tumor models was significantly enhanced when Abcb1a/b and Abcg2 were genetically deficient or pharmacologically inhibited in recipient mice. These findings call for further clinical testing of temozolomide in combination with elacridar for the treatment of gliomas, as this offers the perspective of further improving the antitumor efficacy of this already active agent.

ABCG1 maintains high-grade glioma survival in vitro and in vivo

The overall survival for adults with malignant glioma (glioblastoma) remains poor despite advances in radiation and chemotherapy. One of the mechanisms by which cancer cells develop relative resistance to treatment is through de-regulation of endoplasmic reticulum (ER) homeostasis. We have recently shown that ABCG1, an ATP-binding cassette transporter, maintains ER homeostasis and suppresses ER stress-induced apoptosis in low-grade glioma. Herein, we demonstrate that ABCG1 expression is increased in human adult glioblastoma, where it correlates with poor survival in individuals with the mesenchymal subtype. Leveraging a mouse model of mesenchymal glioblastoma (NPcis), shRNA-mediated Abcg1 knockdown (KD) increased CHOP ER stress protein expression and resulted in greater NPcis glioma cell death in vitro. Moreover, Abcg1 KD reduced NPcis glioma growth and increased mouse survival in vivo. Collectively, these results demonstrate that ABCG1 is critical for malignant glioma cell survival, and might serve as a future therapeutic target for these deadly brain cancers.

Small molecule epigenetic screen identifies novel EZH2 and HDAC inhibitors that target glioblastoma brain tumor-initiating cells

Glioblastoma (GBM) is the most lethal and aggressive adult brain tumor, requiring the development of efficacious therapeutics. Towards this goal, we screened five genetically distinct patient-derived brain-tumor initiating cell lines (BTIC) with a unique collection of small molecule epigenetic modulators from the Structural Genomics Consortium (SGC). We identified multiple hits that inhibited the growth of BTICs in vitro, and further evaluated the therapeutic potential of EZH2 and HDAC inhibitors due to the high relevance of these targets for GBM. We found that the novel SAM-competitive EZH2 inhibitor UNC1999 exhibited low micromolar cytotoxicity in vitro on a diverse collection of BTIC lines, synergized with dexamethasone (DEX) and suppressed tumor growth in vivo in combination with DEX. In addition, a unique brain-penetrant class I HDAC inhibitor exhibited cytotoxicity in vitro on a panel of BTIC lines and extended survival in combination with TMZ in an orthotopic BTIC model in vivo. Finally, a combination of EZH2 and HDAC inhibitors demonstrated synergy in vitro by augmenting apoptosis and increasing DNA damage. Our findings identify key epigenetic modulators in GBM that regulate BTIC growth and survival and highlight promising combination therapies.

Application of Small Epigenetic Modulators in Pediatric Medulloblastoma

Medulloblastoma is one of the most frequent among pediatric brain tumors, and it has been classified in various subgroups. Some of them already benefit from quite good therapeutic options, whereas others urgently need novel therapeutic approaches. Epigenetic modulators have long been studied in various types of cancer. Within this review, we summarize the main preclinical studies regarding epigenetic targets (such as HDAC, SIRT, BET, EZH2, G9a, LSD1, and DNMT) inhibitors in medulloblastoma. Furthermore, we shed light on the increasing number of applications of drug combinations as well as hybrid compounds involving epigenetic mechanisms. Nevertheless, in the studies published so far, mainly un-specific or old modulators have been used, and the PKs (brain permeability) have not been well-evaluated. Thus, these findings should be considered as a starting point for further improvement and not as a final result.

Epigenetic regulation in medulloblastoma

Medulloblastoma is the most common malignant childhood brain tumor. The heterogeneous tumors are classified into four subgroups based on transcription profiles. Recent developments in genome-wide sequencing techniques have rapidly advanced the understanding of these tumors. The high percentages of somatic alterations of genes encoding chromatin regulators in all subgroups suggest that epigenetic deregulation is a major driver of medulloblastoma. In this report, we review the current understanding of epigenetic regulation in medulloblastoma with a focus on the functional studies of chromatin regulators in the initiation and progression of specific subgroups of medulloblastoma. We also discuss the potential usage of epigenetic inhibitors for medulloblastoma treatment.

Transgenerational programming of longevity through E(z)-mediated histone H3K27 trimethylation in Drosophila

Transgenerational effects on health and development of early-life nutrition have gained increased attention recently. However, the underlying mechanisms of transgenerational transmission are only starting to emerge, with epigenetics as perhaps the most important mechanism. We recently reported the first animal model to study transgenerational programming of longevity after early-life dietary manipulations, enabling investigations to identify underlying epigenetic mechanisms. We report here that post-eclosion dietary manipulation (PDM) with a low-protein (LP) diet upregulates the protein level of E(z), an H3K27 specific methyltransferase, leading to higher levels of H3K27 trimethylation (H3K27me3). This PDM-mediated change in H3K27me3 corresponded with a shortened longevity of F0 flies as well as their F2 offspring. Specific RNAi-mediated post-eclosion knockdown of E(z) or pharmacological inhibition of its enzymatic function with EPZ-6438 in the F0 parents improved longevity while rendering H3K27me3 low across generations. Importantly, addition of EPZ-6438 to the LP diet fully alleviated the longevity-reducing effect of the LP PDM, supporting the increased level of E(z)-dependent H3K27me3 as the primary cause and immediate early-life period as the critical time to program longevity through epigenetic regulation. These observations establish E(z)-mediated H3K27me3 as one epigenetic mechanism underlying nutritional programming of longevity and support the use of EPZ-6438 to extend lifespan.

Targeting glioma stem cells through combined BMI1 and EZH2 inhibition

Glioblastomas are lethal cancers defined by angiogenesis and pseudopalisading necrosis. Here, we demonstrate that these histological features are associated with distinct transcriptional programs, with vascular regions showing a proneural profile, and hypoxic regions showing a mesenchymal pattern. As these regions harbor glioma stem cells (GSCs), we investigated the epigenetic regulation of these two niches. Proneural, perivascular GSCs activated EZH2, whereas mesenchymal GSCs in hypoxic regions expressed BMI1 protein, which promoted cellular survival under stress due to downregulation of the E3 ligase RNF144A. Using both genetic and pharmacologic inhibition, we found that proneural GSCs are preferentially sensitive to EZH2 disruption, whereas mesenchymal GSCs are more sensitive to BMI1 inhibition. Given that glioblastomas contain both proneural and mesenchymal GSCs, combined EZH2 and BMI1 targeting proved more effective than either agent alone both in culture and in vivo, suggesting that strategies that simultaneously target multiple epigenetic regulators within glioblastomas may be effective in overcoming therapy resistance caused by intratumoral heterogeneity.

The impact of P-glycoprotein and breast cancer resistance protein on the brain pharmacokinetics and pharmacodynamics of a panel of MEK inhibitors

Mitogen/extracellular signal-regulated kinase (MEK) inhibitors have been tested in clinical trials for treatment of intracranial neoplasms, including glioblastoma (GBM), but efficacy of these drugs has not yet been demonstrated. The blood-brain barrier (BBB) is a major impediment to adequate delivery of drugs into the brain and may thereby also limit the successful implementation of MEK inhibitors against intracranial malignancies. The BBB is equipped with a range of ATP-dependent efflux transport proteins, of which P-gp (ABCB1) and BCRP (ABCG2) are the two most dominant for drug efflux from the brain. We investigated their impact on the pharmacokinetics and target engagement of a panel of clinically applied MEK inhibitors, in order to select the most promising candidate for brain cancers in the context of clinical pharmacokinetics and inhibitor characteristics. To this end, we used in vitro drug transport assays and conducted pharmacokinetic and pharmacodynamic studies in wildtype and ABC-transporter knockout mice. PD0325901 displayed more promising characteristics than trametinib (GSK1120212), binimetinib (MEK162), selumetinib (AZD6244), and pimasertib (AS703026): PD0325901 was the weakest substrate of P-gp and BCRP in vitro, its brain penetration was only marginally higher in Abcb1a/b;Abcg2^-/- mice, and efficient target inhibition in the brain could be achieved at clinically relevant plasma levels. Notably, target inhibition could also be demonstrated for selumetinib, but only at plasma levels far above levels in patients receiving the maximum tolerated dose. In summary, our study recommends further development of PD0325901 for the treatment of intracranial neoplasms.

The histone methyltransferase EZH2 as a druggable target in SHH medulloblastoma cancer stem cells

The histone methyltransferase EZH2 plays a role in maintenance of the stem component of cancer, and its overexpression and/or mutation typically drives tumor aggressiveness, drug resistance and patients' poor prognosis. In this study, we use mouse and human medulloblastoma stem-like cells belonging to the Sonic Hedgehog subgroup (SHH MB-SLCs) and demonstrate that genetic suppression of EZH2 reduces the level of its histone mark H3K27me3 and lowers proliferation and self-renewal. We designed an EZH2 inhibitor (EZH2i) as a simplified analog of EPZ005687 and GSK2816126, MC3629, and we tested its biological activity in SHH MB-SLCs. Pharmacological inhibition of EZH2 impairs SHH MB cells proliferation and self-renewal, and induces apoptosis in vitro. Finally, we generated xenograft MB-SLCs orthotopic tumors in nude mice to test MC3629 in vivo. In treated mice, we observed impairment of tumor growth, together with induction of apoptosis and reduction of proliferation and stemness. Overall, these findings describe EZH2 as a druggable target in MB and provide insight into the biological activity of MC3629 as an EZH2i.

Initial testing (stage 1) of tazemetostat (EPZ-6438), a novel EZH2 inhibitor, by the Pediatric Preclinical Testing Program

BACKGROUND

Tazemetostat (EPZ-6438) is a selective inhibitor of the histone methyltransferase EZH2 and currently in clinical development for non-Hodgkin lymphoma and genetically defined tumors.

PROCEDURES

Tazemetostat was tested against the Pediatric Preclinical Testing Program (PPTP) solid tumor xenografts using a dose of 400 mg/kg administered twice daily by oral gavage for 28 days. H3K27me3:H3 ratios were determined in control and treated tumors.

RESULTS

Tazemetostat induced significant differences in event-free survival (EFS) distribution compared with control in nine of 30 (30%) of the xenografts studied. Significant differences in EFS distribution were observed in five of seven (71%) rhabdoid tumor xenograft lines compared with four of 23 (17%) nonrhabdoid xenograft lines (chi-square [χ² ] test P = 0.006). Tazemetostat induced tumor growth inhibition meeting criteria for intermediate and high EFS treated-to-control (T/C) activity in two of 25 (8%) and one of 25 (4%) xenografts, respectively. Intermediate and high activity for the EFS T/C metric was observed exclusively among rhabdoid tumor xenografts (three of five rhabdoid tumor vs 0 of 22 nonrhabdoid tumors (χ² test P < 0.001). One rhabdoid tumor xenograft (G401) showed stable disease. For one rhabdoid tumor (G401), delayed tumor regression to tazemetostat was noted following 1 week of tumor growth. Tazemetostat induced significant reduction of H3K27me3 levels in the majority of tumors compared with controls.

CONCLUSIONS

Tazemetostat demonstrated significant antitumor activity in rhabdoid tumor models but showed no consistent activity against any other histology. Tazemetostat reduced H3K27me3 levels irrespective of tumor response. Further preclinical testing to evaluate tazemetostat in combination with other anticancer agents is warranted.