BET Inhibitors Research Articles

IMiDs Synergize with EP300 Inhibition to Disrupt the Ikzf/MYC/IRF4 Axis in Multiple Myeloma

Multiple myeloma (MM) is a heterogeneous plasma cell malignancy for which current therapies eventually fail. In myeloma, endogenous and translocated superenhancers drive the expression of the lineage-defining transcription factor IRF4 and oncogene MYC, respectively - both considered "undruggable". Successful standard-of-care immunomodulatory imide drugs (IMiDs) degrade superenhancer-binding pioneer factors IKAROS and AIOLOS; however, depletion of IKAROS and AIOLOS does not always correlate with a successful IMiD response, and patients treated with IMiDs ultimately relapse. We show responses to the IMiD Pomalidomide (POM) are directly correlated with downregulation of IRF4 and MYC. In IMiD-resistant cells and tumors, IRF4 and MYC downregulation could be achieved with novel coactivator-targeting drugs including the BET inhibitor JQ1 and the CBP/EP300 inhibitor GNE-781, but these were toxic and lacked a therapeutic window in vivo. We show how POM combined with GNE781 synergize and downregulate MYC and IRF4 levels, killing myeloma cells in vitro and in vivo. However, resistance to this combination was induced by expression of the AP-1 factor BATF, which directly maintained IRF4 expression and myeloma cell survival. These results identify a potent therapeutic combination and a hereto unrealized mechanism of IMiD resistance in multiple myeloma.

Super-Enhancer-Driven PIM Kinase Upregulation in Multiple Myeloma Maintains the Plasma Cell-Specific Oncogenic and Microenvironmental Circuits, and Can be Efficiently Targeted By the Pan-PIM Inhibitor MEN1703

The three constitutively active PIM serine/threonine kinases have been previously reported to initiate and sustain lymphoid oncogenesis. In subsequent CRISPR/Cas9 drop-out screens, PIM kinases were highlighted as critical oncogenes in MM, which led to clinical trials of small-molecule PIM kinase inhibitors. Surprisingly, PIM inhibitors in MM patients exhibited limited activity due to their cytostatic, rather cytotoxic effects. These clinical trial results prompted us to further investigate the consequences of PIM kinase inhibition in MM using recently developed pan-PIM/FLT3 inhibitor MEN1703. Immunohistochemical staining revealed a markedly increased PIM1/2/3 levels in malignant plasma cells in comparison with normal plasma cells from healthy donors, with particularly abundant PIM2 expression. Through the integration of CRISPR/Cas9 dropout screen and genome-wide H3K27Ac profiling in MM cells, we identified PIM2 as a super-enhancer-driven oncogene in MM. BET inhibitor JQ1, which disrupts super-enhancer assembly, downregulated PIM kinase expression. Since super-enhancer associated genes are often the most essential nodes of key cellular pathways to be potentially targeted, these results further underscore the critical role of PIMs in MM. Next, we compared the activity of MEN1703 with all available clinical-stage PIM inhibitors in a panel of >20 different cell lines and found almost universal resistance to 4 out of 5 pan-PIM inhibitors administered at submicromolar concentrations achievable in vivo (Fig. 1A). In contrast, MEN1703 exhibited universal cytotoxic activity. We confirmed these results using patient-derived multiple myeloma cells cultured ex vivo and genetic knockdowns of PIM kinases. Mechanistically, MEN1703 decreased the activity of mTOR, JAK/STAT, and NFκB pathways essential for supporting myeloma cell survival. Consequently, we observed diminished expression of MYC and IRF4, two transcription factors crucial for supporting plasma cell-specific oncogenic program. In the IHC analysis of PIMs expression in MM tissue microarray, we also noted a distinctly high expression of PIM3 in myeloma-associated endothelial cells (ECs), but not in healthy bone marrow ECs. Since ECs play a significant role in shaping the bone marrow microenvironment and directly support the dissemination and survival of plasma cells, we studied the mechanism of PIM3 overexpression, PIM3 function, and the consequences of its inhibition in MM ECs. We found that myeloma-secreted VEGFA is responsible for PIM induction in ECs and that endothelial cells reciprocally induce PIMs expression in MM cells in a paracrine manner, thus increasing their proliferation and inhibiting cell death. PIM inhibition blocked these effects, which was confirmed using cocultures with primary MM endothelial cells. PIM inhibition decreased the MM EC vessel formation by decreasing the activity of endothelial GTPase RhoA, which is responsible for actin cytoskeleton formation during angiogenesis and cytokine secretion. Collectively, our data demonstrate that PIM inhibition with MEN1703 induces MM cell death and abolishes important tumor cell-ECs interactions (Fig. 1B), thus representing a promising therapeutic target in this disease. The clinical utility of next generation/more potent PIM inhibitor in MM should be re-investigated. Study supported by National Science Centre Grants PRELUDIUM 2018/31/N/NZ5/03214, ETIUDA 2020/36/T/NZ5/00610 and Ministry of Science and Higher Education Diamond Grant DI2015 007145. Figure 1View largeDownload PPTFigure 1View largeDownload PPT Close modal

DNAR-07. BET PROTEIN INHIBITION SENSITIZES GLIOBLASTOMA CELLS TO TEMOZOLOMIDE TREATMENT BY ATTENUATING MGMT EXPRESSION

Abstract Bromodomain and extra-terminal tail (BET) proteins have been identified as potential epigenetic targets in cancer, including glioblastoma. These epigenetic modifiers link the histone code to gene transcription that can be disrupted with small molecule BET inhibitors (BETi). With the aim of identifying rational combination treatments for glioblastoma, we analyzed BETi-induced differential gene expression in glioblastoma derived-spheres. This identified 6 distinct expression patterns. To uncover emerging actionable vulnerabilities that can be targeted with a second drug, we extracted the significantly disturbed DNA Damage Response genes. Among the constantly downregulated candidates was the O-6-methylguanine-DNA methyltransferase (MGMT) gene, a known resistance factor for alkylating agent therapy in glioblastoma. BETi not only reduced MGMT expression in GBM cell lines, but also inhibited its induction, typically observed upon temozolomide treatment. To determine the potential clinical relevance, we evaluated the specificity of the effect on MGMT expression and MGMT mediated treatment resistance to temozolomide. BETi-mediated attenuation of MGMT expression was associated with reduction of BRD4- and Pol II-binding at the MGMT promoter. On the functional level, we demonstrated that ectopic expression of MGMT under an unrelated promoter was not affected by BETi, while under the same conditions, pharmacologic inhibition of MGMT restored the sensitivity to temozolomide, reflected in an increased level of gammaH2AX. Importantly, members of the mismatch repair pathway, required for sensitivity to unrepaired O6G-lesions, namely MSH6 and MSH2, were only transiently affected by BETi. Taken together, the addition of BET-inhibitors to the current standard of care, comprising temozolomide treatment, may improve the outcome of the 50% of patients whose GBM exert an unmethylated MGMT promoter.

A novel dual epigenetic approach targeting BET proteins and HDACs in Group 3 (MYC-driven) Medulloblastoma

BackgroundMedulloblastoma (MB) patients with MYC oncogene amplification or overexpression exhibit extremely poor clinical outcomes and respond poorly to current therapies. Epigenetic deregulation is very common in MYC-driven MB. The bromodomain extra-terminal (BET) proteins and histone deacetylases (HDACs) are epigenetic regulators of MYC transcription and its associated tumorigenic programs. This study aimed to investigate the therapeutic potential of inhibiting the BET proteins and HDACs together in MB.MethodsUsing clinically relevant BET inhibitors (JQ1 or OTX015) and a pan-HDAC inhibitor (panobinostat), we evaluated the effects of combined inhibition on cell growth/survival in MYC-amplified MB cell lines and xenografts and examined underlying molecular mechanism(s).ResultsCo-treatment of JQ1 or OTX015 with panobinostat synergistically suppressed growth/survival of MYC-amplified MB cells by inducing G2 cell cycle arrest and apoptosis. Mechanistic investigation using RNA-seq revealed that co-treatment of JQ1 with panobinostat synergistically modulated global gene expression including MYC/HDAC targets. SYK and MSI1 oncogenes were among the top 50 genes synergistically downregulated by JQ1 and panobinostat. RT-PCR and western blot analyses confirmed that JQ1 and panobinostat synergistically inhibited the mRNA and protein expression of MSI1/SYK along with MYC expression. Reduced SYK/MSI expression after BET (specifically, BRD4) gene-knockdown further confirmed the epigenetic regulation of SYK and MSI1 genes. In addition, the combination of OTX015 and panobinostat significantly inhibited tumor growth in MYC-amplified MB xenografted mice by downregulating expression of MYC, compared to single-agent therapy.ConclusionsTogether, our findings demonstrated that dual-inhibition of BET and HDAC proteins of the epigenetic pathway can be a novel therapeutic approach against MYC-driven MB.

Inhibition of BRD4 enhanced the tumor suppression effect of dasatinib in gastric cancer

BRD4, a member of the bromodomain and extraterminal (BET) family, is elevated in multiple cancer tissues, including gastric cancer (GC). Targeted therapy with BRD4 may help improve the overall survival of patients with GC. Meanwhile, the approved multi-target kinase inhibitor, dasatinib, was recently reported to show varied tumor-suppressive effects in GC cells. This study investigated BRD4 expression in vivo and in vitro using immunohistochemistry and western blotting, respectively. We discussed the relationship between BRD4 expression and patient prognosis. Next, the antitumor efficacy of dasatinib was measured in BRD4-knockdown GC cells to determine the role of BRD4 blockage in dasatinib treatment. Finally, molibresib, a BET inhibitor, was used to measure the cooperative function of BRD4 inhibition and dasatinib treatment in three GC cell lines. Epithelial BRD4 expression was higher in tumoral and metastatic tissues and was strongly associated with unfavorable tumor, node, and metastasis stages and survival. BRD4 expression was heterogeneous in the three GC cell lines tested in vitro. In SGC7901, a BRD4-high GC cell line, knockdown of BRD4 using specific siRNAs suppressed cell growth individually and cooperatively with dasatinib. Moreover, molibresib and dasatinib showed a cooperative effect in suppressing the proliferation of BRD4-high GC cells. In conclusion, we confirmed that increased epithelial BRD4 expression is associated with poor disease stage and prognosis in GC and BRD4 blockage might be a valuable strategy to improve the sensitivity of dasatinib and other drugs in the chemotherapy of advanced GC.

Combination of Ribociclib with BET-Bromodomain and PI3K/mTOR Inhibitors for Medulloblastoma Treatment In Vitro and In Vivo.

Despite improvement in the treatment of medulloblastoma over the last years, numerous patients with MYC- and MYCN-driven tumors still fail current therapies. Medulloblastomas have an intact retinoblastoma protein RB, suggesting that CDK4/6 inhibition might represent a therapeutic strategy for which drug combination remains understudied. We conducted high-throughput drug combination screens in a Group3 (G3) medulloblastoma line using the CDK4/6 inhibitor (CDK4/6i) ribociclib at IC20, referred to as an anchor, and 87 oncology drugs approved by FDA or in clinical trials. Bromodomain and extra terminal (BET) and PI3K/mTOR inhibitors potentiated ribociclib inhibition of proliferation in an established cell line and freshly dissociated tumor cells from intracranial xenografts of G3 and Sonic hedgehog (SHH) medulloblastomas in vitro. A reverse combination screen using the BET inhibitor JQ1 as anchor, revealed CDK4/6i as the most potentiating drugs. In vivo, ribociclib showed single-agent activity in medulloblastoma models whereas JQ1 failed to show efficacy due to high clearance and insufficient free brain concentration. Despite in vitro synergy, combination of ribociclib with the PI3K/mTOR inhibitor paxalisib did not significantly improve the survival of G3 and SHH medulloblastoma-bearing mice compared with ribociclib alone. Molecular analysis of ribociclib and paxalisib-treated tumors revealed that E2F targets and PI3K/AKT/MTORC1 signaling genes were depleted, as expected. Importantly, in one untreated G3MB model HD-MB03, the PI3K/AKT/MTORC1 gene set was enriched in vitro compared with in vivo suggesting that the pathway displayed increased activity in vitro. Our data illustrate the difficulty in translating in vitro findings in vivo. See related article in Mol Cancer Ther (2022) 21(8):1306-1317.

Retinoblastoma protein as an intrinsic BRD4 inhibitor modulates small molecule BET inhibitor sensitivity in cancer

Bromodomain and extraterminal (BET) proteins including BRD4 play important roles in oncogenesis and immune inflammation. Here we demonstrate that cancer cells with loss of the retinoblastoma (RB) tumor suppressor became resistant to small molecule bromodomain inhibitors of BET proteins. We find that RB binds to bromodomain-1 (BD1) of BRD4, but binding is impeded by CDK4/6-mediated RB phosphorylation at serine-249/threonine-252 (S249/T252). ChIP-seq analysis shows RB knockdown increases BRD4 occupancy at genomic loci of genes enriched in cancer-related pathways including the GPCR-GNBIL-CREB axis. S249/T252-phosphorylated RB positively correlates with GNBIL protein level in prostate cancer patient samples. BET inhibitor resistance in RB-deficient cells is abolished by co-administration of CREB inhibitor. Our study identifies RB protein as a bona fide intrinsic inhibitor of BRD4 and demonstrates that RB inactivation confers resistance to small molecule BET inhibitors, thereby revealing a regulatory hub that converges RB upstream signaling onto BRD4 functions in diseases such as cancer.

Dissecting super-enhancer driven transcriptional dependencies reveals novel therapeutic strategies and targets for group 3 subtype medulloblastoma

BackgroundMedulloblastoma is the most common malignant pediatric brain tumor and group 3 subtype medulloblastoma (G3-MB) exhibits the worst prognosis. Super enhancers (SEs) are large clusters of enhancers that play important roles in cancer through transcriptional control of cell identity genes, oncogenes and tumor-dependent genes. Dissecting SE-driven transcriptional dependencies of cancer leads to identification of novel oncogenic mechanisms, therapeutic strategies and targets.MethodsIntegrative SE analyses of primary tissues and patient-derived tumor cell lines of G3-MB were performed to extract the conserved SE-associated gene signatures and their oncogenic potentials were evaluated by gene expression, tumor-dependency and patient prognosis analyses. SE-associated subtype-specific upregulated tumor-dependent genes, which were revealed as members of SE-driven core transcriptional regulatory network of G3-MB, were then subjected to functional validation and mechanistic investigation. SE-associated therapeutic potential was further explored by genetic or pharmaceutical targeting of SE complex components or SE-associated subtype-specific upregulated tumor-dependent genes individually or in combination, and the underlying therapeutic mechanisms were also examined.ResultsThe identified conserved SE-associated transcripts of G3-MB tissues and cell lines were enriched of subtype-specifically upregulated tumor-dependent genes and MB patients harboring enrichment of those transcripts exhibited worse prognosis. Fourteen such conserved SE-associated G3-MB-specific upregulated tumor-dependent genes were identified to be members of SE-driven core transcriptional regulatory network of G3-MB, including three well-recognized TFs (MYC, OTX2 and CRX) and eleven newly identified downstream effector genes (ARL4D, AUTS2, BMF, IGF2BP3, KIF21B, KLHL29, LRP8, MARS1, PSMB5, SDK2 and SSBP3). An OTX2-SE-ARL4D regulatory axis was further revealed to represent a subtype-specific tumor dependency and therapeutic target of G3-MB via contributing to maintaining cell cycle progression and inhibiting neural differentiation of tumor cells. Moreover, BET inhibition with CDK7 inhibition or proteasome inhibition, two combinatory strategies of targeting SE complex components (BRD4, CDK7) or SE-associated effector gene (PSMB5), were shown to exhibit synergistic therapeutic effects against G3-MB via stronger suppression of SE-associated transcription or higher induction of ER stress, respectively.ConclusionsOur study verifies the oncogenic role and therapeutic potential of SE-driven transcriptional dependencies of G3-MB, resulting in better understanding of its tumor biology and identification of novel SE-associated therapeutic strategies and targets.

Viral Hijacking of BET Proteins.

Proteins of the bromodomain and exterminal domain (BET) family mediate critical host functions such as cell proliferation, transcriptional regulation, and the innate immune response, which makes them preferred targets for viruses. These multidomain proteins are best known as transcriptional effectors able to read acetylated histone and non-histone proteins through their tandem bromodomains. They also contain other short motif-binding domains such as the extraterminal domain, which recognizes transcriptional regulatory proteins. Here, we describe how different viruses have evolved to hijack or disrupt host BET protein function through direct interactions with BET family members to support their own propagation. The network of virus-BET interactions emerges as highly intricate, which may complicate the use of small-molecule BET inhibitors–currently in clinical development for the treatment of cancer and cardiovascular diseases–to treat viral infections.

Combination bromo- and extraterminal domain and poly (ADP-ribose) polymerase inhibition synergistically enhances DNA damage and inhibits neuroblastoma tumorigenesis

PurposeJQ1 is a bromo- and extraterminal (BET) domain inhibitor that downregulates MYC expression and impairs the DNA damage response. Poly (ADP-ribose) polymerase (PARP) inhibitors prevent DNA damage sensing and repair. We hypothesized that JQ1 would promote a DNA repair-deficient phenotype that sensitizes neuroblastoma cells to PARP inhibition.MethodsFour human neuroblastoma cell lines were examined: two MYCN-amplified (BE(2)-C and IMR-32), and two non-MYCN-amplified (SK-N-SH and SH-SY5Y). Cells were treated with JQ1 (BET inhibitor), Olaparib (PARP inhibitor), or in combination to assess for therapeutic synergy of JQ1 and Olaparib. Treated cells were harvested and analyzed. Quantitative assessment of combination treatment synergy was performed using the median effect principle of Chou and Talalay.ResultsCombination treatment with Olaparib decreased the IC50 of JQ1 by 19.9-fold, 2.0-fold, 12.1-fold, and 2.0-fold in the BE(2)-C, IMR-32, SK-N-SH, and SH-SY5Y cell lines, respectively. In the MYCN-amplified cell lines, BE(2)-C and IMR-32, combination treatment decreased gene expression of MYCN relative to single-drug treatment alone or control. Combination treatment decreased protein expression of DNA repair proteins Ku80 and RAD51, led to accumulation of DNA damage marker phospho-histone H2A.X, and increased caspase activity. In the non-MYCN-amplified cell lines, SK-N-SH and SH-SY5Y, combination treatment induced G0/G1 cell cycle arrest.ConclusionsCombination BET and PARP inhibition synergistically inhibited neuroblastoma tumorigenesis in vitro. In MYCN-amplified neuroblastoma cells, this effect may be induced by downregulation of MYCN transcription, defects in DNA repair, accumulation of DNA damage, and apoptosis. In non-MYCN-amplified cell lines, combination treatment induced cell cycle arrest.

Pharmacophore-Model-Based Virtual-Screening Approaches Identified Novel Natural Molecular Candidates for Treating Human Neuroblastoma.

The mortality of cancer patients with neuroblastoma is increasing due to the limited availability of specific treatment options. Few drug candidates for combating neuroblastoma have been developed, and identifying novel therapeutic candidates against the disease is an urgent issue. It has been found that muc-N protein is amplified in one-third of human neuroblastomas and expressed as an attractive drug target against the disease. The myc-N protein interferes with the bromodomain and extraterminal (BET) family proteins. Pharmacologically inhibition of the protein potently depletes MYCN in neuroblastoma cells. BET inhibitors target MYCN transcription and show therapeutic efficacy against neuroblastoma. Therefore, the study aimed to identify potential inhibitors against the BET family protein, specifically Brd4 (brodamine-containing protein 4), to hinder the activity of neuroblastoma cells. To identify effective molecular candidates against the disease, a structure-based pharmacophore model was created for the binding site of the Brd4 protein. The pharmacophore model generated from the protein Brd4 was validated to screen potential natural active compounds. The compounds identified through the pharmacophore-model-based virtual-screening process were further screened through molecular docking, ADME (absorption, distribution, metabolism, and excretion), toxicity, and molecular dynamics (MD) simulation approach. The pharmacophore-model-based screening process initially identified 136 compounds, further evaluated based on molecular docking, ADME analysis, and toxicity approaches, identifying four compounds with good binding affinity and lower side effects. The stability of the selected compounds was also confirmed by dynamic simulation and molecular mechanics with generalized Born and surface area solvation (MM-GBSA) methods. Finally, the study identified four natural lead compounds, ZINC2509501, ZINC2566088, ZINC1615112, and ZINC4104882, that will potentially inhibit the activity of the desired protein and help to fight against neuroblastoma and related diseases. However, further evaluations through in vitro and in vivo assays are suggested to identify their efficacy against the desired protein and disease.

Inhibition of BRD4 decreases fibrous scarring after ischemic stroke in rats by inhibiting the phosphorylation of Smad2/3

AimsFibrous scarring may play a much more important role in preventing secondary expansion of tissue damage and hindering repair and regeneration than glial scarring after central nervous system (CNS) injury. However, relatively little is known about how fibrous scars form and how fibrous scar formation is regulated after CNS injury. Bromodomain-containing protein 4 (BRD4) is involved in fibrosis in many tissues, and transforming growth factor-β1 (TGF-β1)/Smad2/3 signaling is one of the critical pathways of fibrosis. However, it is unclear whether and how BRD4 affects fibrous scar formation after ischemicbraininjury. In the present study, whether BRD4 can regulate the formation of fibrous scars after ischemic stroke via TGF-β1/Smad2/3 signaling was assessed. Materials and methodsPrimary meningeal fibroblasts isolated from neonatal SD rats were treated with TGF-β1, SB431542 (a TGF-β1 receptor inhibitor) and JQ1 (a small-molecule BET inhibitor that can also inhibit BRD4). BRD4 was knocked down in adult Sprague–Dawley (SD) rats by using adenovirus before middle cerebral artery occlusion/reperfusion (MCAO/R) injury. The proliferation and migration of meningeal fibroblasts in vitro were evaluated with the Cell Counting Kit-8 (CCK-8) assay and scratch test, respectively. Neurological function was assessed with Longa scores, modified Bederson Scores and modified neurological severity scores (mNSSs). The infarct volume was assessed with TTC staining. The protein expression of synaptophysin (SY), BRD4, Smad2/3, p-Smad2/3, α-smooth muscle actin (α-SMA), collagen-1 (COL1) and fibronectin (FN) in vivo and in vitro was examined with immunocytochemistry, immunofluorescence, and Western blotting. Key findingsBRD4 expression was upregulated in a TGF-β1-induced meningeal fibroblast fibrosis model and was downregulated by the TGF-β1 receptor inhibitor SB431542 in vitro. JQ1, a small-molecule BET inhibitor, inhibited BRD4 and decreased TGF-β1-induced meningeal fibroblast proliferation, migration and activation. Furthermore, MCAO/R injury induced fibrosis and upregulated BRD4 expression in the cerebral infarct center. BRD4 knockdown by adenovirus inhibited fibrous scarring, promoted synaptic survival, decreased the infarct volume, and improved neurological function after MCAO/R injury. Moreover, inhibition of BRD4, either by JQ1 in vitro or adenovirus in vivo, decreased the phosphorylation of Smad2/3. ConclusionsThis study is the first to indicate that inhibition of BRD4 delays fibrous scarring after ischemic stroke through mechanisms involving the phosphorylation of Smad2/3.

Dual inhibition of BET and HAT/p300 suppresses colorectal cancer via DR5- and p53/PUMA-mediated cell death.

BackgroundColorectal cancer (CRC) frequently has a dysregulated epigenome causing aberrant up-regulation of oncogenes such as c-MYC. Bromodomain and extra-terminal domain (BET) proteins and histone acetyltransferases (HAT) are epigenetic regulatory proteins that create and maintain epigenetic states supporting oncogenesis. BET inhibitors and HAT inhibitors are currently being investigated as cancer therapeutics due to their ability to suppress cancer-promoting epigenetic modifiers. Due to the extensive molecular crosstalk between BET proteins and HAT proteins, we hypothesized that dual inhibition of BET and HAT could more potently inhibit CRC cells than inhibition of each individual protein.MethodsWe investigated the activity and mechanisms of a dual BET and HAT inhibitor, NEO2734, in CRC cell lines and mouse xenografts. MTS, flow cytometry, and microscopy were used to assess cell viability. qPCR, Western blotting, and immunofluorescent staining were used to assess mechanisms of action.ResultsWe found that NEO2734 more potently suppresses CRC cell growth than first generation BET inhibitors, regardless of the status of common CRC driver mutations. We previously showed that BET inhibitors upregulate DR5 to induce extrinsic apoptosis. In the current study, we show that NEO2734 treatment induces CRC cell apoptosis via both the intrinsic and extrinsic apoptosis pathways. NEO2734 increases p53 expression and subsequently increased expression of the p53-upregulated mediator of apoptosis (PUMA), which is a critical mechanism for activating intrinsic apoptosis. We demonstrate that inhibition of either the intrinsic or extrinsic branches of apoptosis partially rescues CRC cells from NEO2734 treatment, while the dual inhibition of both branches of apoptosis more strongly rescues CRC cells from NEO2734 treatment. Finally, we show that NEO2734 monotherapy is able to suppress tumor growth in CRC xenografts by inducing apoptosis.ConclusionsOur study demonstrates NEO2734 potently suppresses CRC cells in vitro and in vivo by simultaneously upregulating PUMA and DR5 to induce cell death. Further studies of NEO2734 for treating CRC are warranted.

HDAC inhibitor Vorinostat and BET inhibitor Plx51107 epigenetic agents' combined treatments exert a therapeutic approach upon acute myeloid leukemia cell model.

The process of cancer initiation and development is regulated via the transcriptional expression of cells going under genomic and epigenetic changes. Targeting epigenetic "readers", i.e., bromodomains (BRD) and post-translational modifications of nucleosomal histone proteins regulate gene expression in both cancerous and healthy cells. In this study, the new epigenetic agent BRD inhibitor PLX51107 and histone deacetylase (HDAC) inhibitor SAHA' s (Vorinostat) single/combined applications' reflections were analyzed in case of cell proliferation, cytotoxicity, apoptosis, cell cycle arrest, and finally target gene expression regulation upon both AML and healthy B-lymphocyte cells; HL60 and NCIBL2171, respectively; in vitro. Since mono treatments of either Vorinostat or Plx51107 regulated cellular responses such as growth, proliferation, apoptosis, and cell cycle arrest of tumor cells; their combination treatments exerted accelerated results. We detected that combined treatment of Plx51107 and Vorinostat strengthened effects detected upon leukemic cells for gaining more sensitization to the agents, decreasing cell proliferation, dramatically inducing apoptosis, and cell cycle arrest; thus regulating target gene expressions. We have shown for the first time that the newly analyzed BRD inhibitor Plx51107 could be a promising therapeutic approach for hematological malignancies and its mono or combined usage might support a rapid transition to clinical trials.

Apabetalone, a BET inhibitor, attenuates inflammation induced by viral RNA mimetic and reduces SARS-CoV-2 spike protein binding regardless of variants

Abstract Background/Introduction Hyperinflammatory responses to SARS-CoV-2 can cause myocarditis and cardiac dysfunction including congestive heart failure [1]. SARS-CoV-2 RNA induces type I interferon (IFN-I), activating IFN regulatory factors (IRFs) and downstream IFN stimulated genes (ISGs) to initiate inflammatory processes. SARS-CoV-2 variants may develop immune escape, undercutting benefits of vaccinations. These challenges highlight the need of variant-independent therapies to improve COVID-19 outcomes. Apabetalone is an epigenetic BD2-selective BET inhibitor in phase 3 trials for cardiovascular disease [2]. Apabetalone has the potential to treat COVID-19. It counters inflammatory signals caused by cytokine storm (CS), preventing cardiac dysfunction associated with severe COVID-19 symptoms in cardiac organoids [3]. It also downregulates angiotensin-converting enzyme 2 (ACE2) expression, the main host cell receptor for SARS-CoV-2 spike protein thus impeding propagation of wild-type SARS-CoV-2 [3,4]. Purpose 1) Evaluate apabetalone's effect on inflammatory processes induced by viral-RNA mimetic in human lung cells; 2) Assess apabetalone's ability to prevent binding of the highly contagious delta variant spike protein to human lung cells. Methods Inflammatory gene expression was examined by real-time PCR in apabetalone treated human bronchial epithelial cells (Calu-3) stimulated with poly I:C, a well-accepted viral RNA mimetic that elicits inflammatory signals similar to SARS-CoV-2 RNA [5]. Binding of SARS-CoV-2 delta or wild-type spike protein to apabetalone treated Calu-3 cells was determined by flow cytometry. Results In Calu-3 cells, apabetalone dose-dependently downregulated poly I:C induced transcription of key COVID-19 associated cytokines (IL6, CXCL10, CCL2) to a similar extent as baricitinib (up to 86%, p<0.0001), an anti-inflammatory agent in emergency use for COVID-19 treatment. Moreover, apabetalone but not baricitinib diminished IL1B mRNA levels (up to 66%, p<0.0001). Apabetalone and baricitinib opposed poly I:C induced expression of IFNB1 (an IFN-I), IRF1 and IRF9 (upstream regulators) as well as IFIT1 and IFIT2 (downstream ISGs that regulate CXCL10 expression; up to 90%, p<0.0001). Clinically relevant doses of apabetalone did not alter expression of anti-viral IFITM2, an ISG that blocks SARS-CoV-2, particularly omicron, endosomal entry [6]. Therefore, apabetalone counters the expression of inflammatory factors with roles in CS and IFN-I signaling in response to poly I:C. Additionally, apabetalone reduced delta and wild-type spike protein binding to unstimulated Calu-3 cells (up to 72%, p<0.0001). Conclusions Apabetalone's dual anti-viral and anti-inflammatory mechanism positions it as a variant-independent COVID-19 therapeutic. Together with an established safety profile from >2000 treatment-years with apabetalone, the data provide rationale for an ongoing clinical trial (NCT04894266) which includes analysis of cardiac damage. Funding Acknowledgement Type of funding sources: Private company. Main funding source(s): Resverlogix Corp

150 (PB030) - MEKi-based combination strategies for targeting KRAS-driven cancer

Background: KRAS is one of the most frequently mutated oncogenes in human cancer. Recent successes with KRAS G12C-specific inhibitors are changing the treatment paradigm in this subset of cancer patients. An alternative strategy is targeting downstream KRAS effectors, such as MEK. However, MEK inhibitors have historically proven unsuccessful as monotherapy in KRAS mutant cancers, highlighting the need for combinatory approaches. One possibility is the combination of MEK and SOS1 inhibition, which may overcome adaptive resistance to MEKi by blocking SOS1-mediated reactivation of MAPK signaling. Another promising approach is to combine MEKi with inhibitors of Bromodomain and extra-terminal (BET) proteins. While BET inhibitors exhibit broad anti-tumor activity in preclinical cancer models, BETi monotherapy demonstrated only moderate clinical activity. Resistance to BETi has been, at least partially, attributed to MAPK pathway upregulation, suggesting that a combination with MEKi may help to overcome resistance. Material and Methods: Here, we present BI 3011441, a potent and selective inhibitor of human MEK1/2. Using in vitro assays, we compare BI 3011441 to other clinical stage MEK inhibitors. We also combine BI 3011441 with BI 1701963 and BI 894999 - two potent, selective, and orally bioavailable inhibitors of SOS1 and BET, respectively. Both combinations are tested in vitro and/or in vivo across a panel of KRAS mutant pancreatic ductal adenocarcinoma (PDAC), colorectal cancer (CRC) and non-small cell lung cancer (NSCLC) models. Results: BI 3011441 efficiently inhibits ERK phosphorylation in cell-based assays and demonstrates the strongest in vitro growth inhibition in a panel of BRAF or KRAS mutant cell lines among the MEK inhibitors tested. In vivo, BI 3011441 leads to a dose-dependent modulation of MAPK pathway, which translates into anti-tumor activity in xenograft models of KRAS-driven cancer. The combination of BI 3011441 with SOS1i BI 1701963 synergistically decreases MAPK signaling resulting in enhanced efficacy in xenograft models, including NSCLC, CRC and PDAC. In NSCLC models this combination results in 10% of partial responses (PR) and 70% of stable disease (SD) as defined by modified Response Evaluation Criteria In Solid Tumors (mRECIST). In PDAC and CRC models the combination leads to 30% and 43% of SD, respectively. When combined with clinically relevant concentrations of BETi BI 894999, BI 3011441 strongly decreases in vitro proliferation and induces apoptosis in KRAS mutant cancer cells. Synergy is observed in 17 out of 44 cell lines across different indications and harboring different KRAS mutations. Conclusions: In summary, we show that combining BI 3011441 with SOS1 or BET inhibitors may lead to improved responses in KRAS mutant tumors, likely by circumventing MAPK pathway-related adaptive resistance. Conflict of interest: Other Substantive Relationships: The authors are either employees or have financial relationships with Boehringer Ingelheim.

335 (PB115) - Treatment strategies based on the molecular subtypes of transformed small cell lung cancer (t-SCLC)

Background: Small-cell transformation is one of the resistance mechanisms (4–14%) to Epidermal Growth Factor Receptor (EGFR) Tyrosine Kinase Inhibitors (TKIs) in EGFR-mutant lung adenocarcinomas (LADCs). Although bi-allelic inactivation of TP53 and RB1 is identified as the major genetic footprint of t-SCLC, the molecular driving force to the small cell transformation is still unknown. Recent studies to classify the genetic subtypes of de novo SCLC have guided subtype-specific treatment strategies. Here, we characterized the subtypes of nine t-SCLC cases and two t-SCLC patient-.derived cell lines. Material and methods: We compared RNA profiles of two t-SCLC cell lines (SNU-2962A and SNU-4505) with 57 LADC and 48 SCLC cell lines from the Cancer Cell Line Encyclopedia (CCLE). We investigated the molecular subtypes of t-SCLC and their therapeutic strategies. SNU-2962A and SNU-4505 cells were seeded in 384-well plates containing the siRNA of 709 kinases for 72hr. Furthermore, cell viability assays were performed on drugs targeting genes suggested as subtype-specific vulnerabilities in de novo SCLC. We also examined four subtype markers (ASCL1, NEUROD1, POU2F3, and YAP1) by immunohistochemistry (IHC) in t-SCLC tumor samples of 5 EGFR-mutant and 3 EGFR wild-type patients. Results: RNA-seq results showed that the SNU-2962A cell line was classified into the NEUROD1 subtype with high expression of neuroendocrine genes and typical SCLC markers (SYP, CHGA, and NCAM1). In contrast, the SNU-4505 cell line, classified as a YAP1 subtype, has EMT characteristics with low neuroendocrine and SCLC marker expressions. A dimension reduction plot of RNA expression showed that the SNU-2962A cell line was clustered close to the NEUROD1 subtype. The SNU-4505 cell line was located close to LADC with other SCLC-YAP1 cell lines. We tested the drug sensitivities of the targets based on kinome screening and the previously proposed subtype-specific vulnerabilities. In both cell lines, PLK1 knockdown significantly reduced cell viability. SNU-2962A cell line was sensitive to bromodomain and extra-terminal (BET) inhibitors, and the apoptotic pathway was activated through Caspase signaling. SNU-4505 cell lines did not show sensitivity to the drugs tested except for paclitaxel. Subtype classification by IHC in patient samples was positive for at least one subtype marker. In most cases, ASCL1 or NEUROD1 that exhibits neuroendocrine characteristics was identified as a dominant subtype. In addition, there was no difference of subtypes, according to the EGFR status. Conclusions: Our in vitro studies suggest that EGFR-mutant t-SCLC can be classified into subtypes like de novo SCLC. NEUROD1-positive SNU-2962A cell line was sensitive to BET inhibition, whereas YAP1-positive SNU-4505 cell line was only sensitive to paclitaxel, suggesting a subtype-specific therapeutic strategy in t-SCLC. Conflict of interest: Advisory Board: T.M. Kim reports advisory role or honorarium from AstraZeneca/MedImmune, Bayer, Boryung, Hanmi, Janssen, Novartis, Roche/Genentech, Sanofi, and Takeda. D.W. Kim reports Uncompensated consultation or advisory role from Amgen, AstraZeneca, BMS/ONO Pharmaceuticals, Daiichi-Sankyo, GSK, Janssen, Meck, MSD, Oncobix, Pfizer, SK Biopharm, and Takeda Corporate-sponsored Research: T.M. Kim reports research funding from Amgen, AstraZeneca/Medimmune, Boehringer-Ingelheim, BMS, Hanmi, Janssen, Merck, MSD, Novartis, Regeneron, Roche/Genentech, Sanofi, and Takeda. D.W. Kim reports research funding from Alpha Biopharma, Amgen, Astrazeneca/Medimmune, Boehringer-Ingelheim, BMS, Bridge BioTherapeutics, Chong Keun Dang, Daiichi-Sankyo, GSK, Hanmi, Janssen, Merck, Merus, Mirati Therapeutics, MSD, Novartis, ONO Pharmaceutical, Pfizer, Roche/Genentech, Takeda, TP Therapeutics, Xcovery, and Yuhan and laboratory research funding from InnoN. Other Substantive Relationships: D.W. Kim reports travel and accomodation support for advisory board meeting attendance from Amgen, Daiichi-Sankyo and medical writing assistance from Amgen, AstraZeneca, Boehringer-Ingelheim, BMS, Chong Keun Dang, Daiichi-Sankyo, GSK, Pfizer, MSD, Meck, Novartis, Roche, Takeda, and Yuhan.

MPN-238 Single-Cell RNA Profiling of Myelofibrosis Patients Reveals Pelabresib-Induced Decrease of Megakaryocytic Progenitors and Normalization of CD4+ T Cells in Peripheral Blood.

Abnormal differentiation of the megakaryocytic lineage and overproduction of proinflammatory cytokines, resulting in bone marrow fibrosis, anemia, symptoms, extramedullary hematopoiesis, and often hepatosplenomegaly, are key characteristics of myelofibrosis. Bromodomain and extraterminal domain (BET) proteins play a significant role in the regulation of neoplastic myeloproliferation and proinflammatory cytokine production. Pelabresib (CPI-0610) is an investigational, oral, small-molecule BET inhibitor. Characterize cellular composition and transcriptional alterations in peripheral blood (PB) obtained from myelofibrosis patients enrolled in three arms (Arm 1: pelabresib monotherapy; Arm 2: pelabresib 'add-on' to ruxolitinib in patients with suboptimal ruxolitinib response; Arm 3: pelabresib with ruxolitinib in JAKi-naïve patients) of the ongoing MANIFEST Phase 2 study (NCT02158858). We performed single-cell RNA sequencing on 234,904 CD34+ HSPCs and 135,970 CD34- mature PB cells. Baseline and on-treatment samples were obtained from a random pool of 20 patients (Arm 1: n=5; Arm 2: n=8; Arm 3: n=7). Mobilized PB cells from healthy donors (HD) were used as controls (n=11). Analysis of CD34+ HSPCs at baseline demonstrated increased numbers of megakaryocytic, neutrophilic, and erythroid progenitors in myelofibrosis patients compared with HDs and a significant reduction of myeloid and B-cell lineage progenitors. Pelabresib as monotherapy and combined with ruxolitinib led to significant reduction of megakaryocytic, neutrophilic, and erythroid progenitors compared with baseline. Analysis of CD34- cells from myelofibrosis patients identified a significantly lower proportion of CD4+ T cells and increased numbers of erythroid cells at baseline versus HD. Individual patients exhibited reduction in natural killer cells and CD16+ monocytes as well as elevated megakaryocytic lineage cells. Pelabresib as monotherapy and combined with ruxolitinib increased the proportion of CD4+ T cells, and, importantly, reduced megakaryocytic lineage cells in both treatment-naïve and ruxolitinib-relapsed/refractory patients. In myelofibrosis patients at baseline, larger spleen volume was observed in patients with lower numbers of CD4+ T cells and increased numbers of megakaryocytic and myeloid CSF3R+ cells. Single-cell profiling of a subset of myelofibrosis patients in the MANIFEST study suggests that pelabresib alone and in combination with ruxolitinib induces improvement of the myeloid-lymphoid imbalance. This potential disease-modifying effect warrants further investigation.

MPN-375 BET Inhibitor Pelabresib (CPI-0610) Combined With Ruxolitinib in Patients With Myelofibrosis - JAK Inhibitor-Naïe or With Suboptimal Response to Ruxolitinib - Preliminary Data From the MANIFEST Study.

Myelofibrosis is characterized by splenomegaly, symptoms, cytopenias, and bone marrow (BM) fibrosis. Pelabresib is an investigational, oral, small-molecule BET inhibitor designed to selectively inhibit the BD1 and BD2 bromodomains of BET proteins. Evaluation of pelabresib combined with ruxolitinib in patients with myelofibrosis. In Arm 3 of the Phase 2 MANIFEST study (NCT02158858), JAKi-naïve myelofibrosis patients are treated with pelabresib combined with ruxolitinib. In Arm 2, myelofibrosis patients with suboptimal response to ruxolitinib are treated with pelabresib as 'add-on' to ruxolitinib (Arm 2A: transfusion-dependent [TD]; Arm 2B: non-TD). The primary endpoints are ≥35% spleen volume reduction (SVR35) at Week 24 for Arms 3 and 2B and TD to transfusion independence (TI) in Arm 2A. The key secondary endpoint is ≥50% total symptom score reduction (TSS50) at Week 24; in Arm 2A, SVR35 is an additional key secondary endpoint. BM biopsies to assess BM fibrosis and safety data are also evaluated. As of September 2021, at Week 24 in Arm 3 (N=84), 68% (57/84) of patients achieved SVR35 (median change: -50%), and 56% (46/82) of patients achieved TSS50 (median change: -59%). At Week 24 in Arm 2 (N=86), 20% (16/81; 17% in Arm 2A and 26% in Arm 2B) of patients achieved SVR35 (median change: -18%), and 37% (30/81) of patients achieved TSS50 (median change: -47%). In Arm 2A, the TD to TI rate was 16% (6/38). At Week 24, BM fibrosis improvement ≥ 1 grade was achieved in 28% (16/57) and 26% (12/47) of patients in Arms 3 and 2, respectively. Hematologic treatment-emergent adverse events (TEAEs) included thrombocytopenia, in 52% (≥Grade 3: 12%) and 52% (≥Grade 3: 33%) of patients, and anemia, in 42% (≥Grade 3: 35%) and 27% (≥Grade 3: 19%) of patients in Arms 3 and 2, respectively. Low-grade gastrointestinal TEAEs and respiratory infections were observed but rarely resulted in discontinuation. In ruxolitinib treatment-naïve and previously treated patients with myelofibrosis, pelabresib combined with ruxolitinib resulted in splenic and symptom responses and BM fibrosis improvement and was generally well tolerated. The Phase 3 MANIFEST-2 study is evaluating pelabresib combined with ruxolitinib in JAKi treatment-naïve patients with myelofibrosis (NCT04603495).

Evaluation of the Synergistic Potential of Simultaneous Pan- or Isoform-Specific BET and SYK Inhibition in B-Cell Lymphoma: An In Vitro Approach.

Simple SummaryB-cell lymphomas represent the majority of non-Hodgkin lymphomas and are the most common lymphoid malignancies in the Western world. Genetic alterations or epigenetic modulations can lead to tumor initiation and tumor progression. Aside from standard care, targeted, individualized therapies can be highly effective. Here, we evaluated the impact of simultaneous specific inhibition of two key regulators involved in B lymphoid tumor progression. Spleen tyrosine kinase (SYK) is a B-cell receptor-associated kinase acting as a proto-oncogene in B-cell malignancies, while bromodomain and extra-terminal domain (BET) proteins are epigenetic reader proteins involved in histone recognition and transcription regulation. The simultaneous inhibition of SYK and BET showed enhanced anti-proliferative effects, as well as inducing a distinct combination-specific gene expression profile, suggesting SYK and BET inhibition as a promising combination in the treatment of B-cell lymphoma.Background: Both bromodomain and extra-terminal domain (BET) proteins and spleen tyrosine kinase (SYK) represent promising targets in diffuse large B-cell (DLBCL) and Burkitt’s lymphoma (BL). We evaluated the anti-lymphoma activity of the isoform-specific bivalent BET inhibitor AZD5153 (AZD) and the pan-BET inhibitor I-BET151 (I-BET) as single agents and in combination with SYK inhibitor Entospletinib (Ento) in vitro. Methods: The effect of the single agents on cell proliferation and metabolic activity was evaluated in two DLBCL and two BL cell lines. Proliferation, metabolic activity, apoptosis, cell cycle and morphology were further investigated after a combined treatment of AZD or I-BET and Ento. RNAseq profiling of combined AZD+Ento treatment was performed in SU-DHL-4 cells. Results: Both BET inhibitors reduced cell proliferation and metabolic activity in a dose- and time-dependent manner. Combined BET and SYK inhibition enhanced the anti-proliferative effect and induced a G0/G1 cell cycle arrest. SU-DHL-4 demonstrated a pronounced modulation of gene expression by AZD, which was markedly increased by additional SYK inhibition. Functional enrichment analyses identified combination-specific GO terms related to DNA replication and cell division. Genes such as ADGRA2, MYB, TNFRSF11A, S100A10, PLEKHH3, DHRS2 and FOXP1-AS1 were identified as possible key regulators. Conclusion: Simultaneous inhibition of BET and SYK enhanced the anti-proliferative effects, and induced a combination-specific gene expression signature.