BET Bromodomain Inhibitor Research Articles

Enhanced cellular death in liver and breast cancer cells by dual BET/BRPF1 inhibitors.

The acetylpyrrole scaffold is an acetylated lysine mimic that has been previously explored to develop bromodomain inhibitors. When tested on the hepatoma cell line Huh7 and the breast cancer cell line MDA-MB-231, a few compounds in our acetylpyrrole-thiazole library induced peculiar morphological changes, progressively causing cell death at increasing concentrations. Their evaluation on a panel of human bromodomains revealed concurrent inhibition of BRPF1 and BET bromodomains. To dissect the observed cellular effects, the acetylpyrrole derivatives were compared to JQ1 and GSK6853, chemical probes for the bromodomains of BET and BRPF1, respectively. The appearance of neurite-like extrusions, accompanied by βIII-tubulin overexpression, is caused by BET inhibition, with limited effect on cellular viability. Conversely, interference with BRPF1 induces cellular death but not phenotypic alterations. Combined treatment with JQ1 and GSK6853 showed additivity in reducing cellular viability, comparably to the acetylpyrrole-thiazole-based BET/BRPF1 inhibitors. In addition, we determined the crystallographic structures of the BRD4 and BRPF1 bromodomains in complex with the acetylpyrrole-thiazole compounds. The binding modes in the two bromodomains show similar interactions for the acetylpyrrole and different orientations of the moiety that point to the rim of the acetyl-lysine pocket.

Resistance of estrogen receptor function to BET bromodomain inhibition is mediated by transcriptional coactivator cooperativity.

The bromodomain and extraterminal domain (BET) family of proteins are critical chromatin readers that bind to acetylated histones through their bromodomains to activate transcription. Here, we reveal that bromodomain inhibition fails to repress oncogenic targets of estrogen receptor because of an intrinsic transcriptional mechanism. While bromodomains are necessary for the transcription of many genes, bromodomain-containing protein 4 (BRD4) binds to estrogen receptor binding sites and activates transcription of critical oncogenes such as MYC, independently of its bromodomains. BRD4 associates with the Mediator complex and disruption of Mediator reduces BRD4's enhancer occupancy. Profiling changes of the post-initiation RNA polymerase II (Pol II)-associated factors revealed that BET proteins regulate interactions between Pol II and elongation factors SPT5, SPT6 and the polymerase-associated factor 1 complex, which associate with BET proteins independently of their bromodomains and mediate their transcription elongation effect. Our findings highlight the importance of bromodomain-independent functions and interactions of BET proteins in the development of future therapeutic strategies.

BET degrader exhibits lower antiproliferative activity than its inhibitor via EGR1 recruiting septins to promote E2F1-3 transcription in triple-negative breast cancer

The bromodomain and extraterminal domain (BET) family proteins serve as primary readers of acetylated lysine residues and play crucial roles in cell proliferation and differentiation. Dysregulation of BET proteins has been implicated in tumorigenesis, making them important therapeutic targets. BET-bromodomain (BD) inhibitors and BET-targeting degraders have been developed to inhibit BET proteins. In this study, we found that the BET inhibitor MS645 exhibited superior antiproliferative activity than BET degraders including ARV771, AT1, MZ1 and dBET1 in triple-negative breast cancer (TNBC) cells. Treatment with MS645 led to the dissociation of BETs, MED1 and RNA polymerase II from the E2F1–3 promoter, resulting in the suppression of E2F1–3 transcription and subsequent inhibition of cell growth in TNBC. In contrast, while ARV771 displaced BET proteins from chromatin, it did not significantly alter E2F1–3 expression. Mechanistically, ARV771 induced BRD4 depletion at protein level, which markedly increased EGR1 expression. This elevation of EGR1 subsequently recruited septin 2 and septin 9 to E2F1–3 promoters, enhancing E2F1–3 transcription and promoting cell proliferation rate in vitro and in vivo. Our findings provide valuable insights into differential mechanisms of BET inhibition and highlight potential of developing BET-targeting molecules as therapeutic strategies for TNBC.

The immunological landscape and silico analysis of key paraptosis regulator LPAR1 in gastric cancer patients

This study aims to identify key regulators of paraptosis in gastric cancer (GC) and explore their potential in guiding therapeutic strategies, especially in stomach adenocarcinoma (STAD). Genes associated with paraptosis were identified from the references and subjected to Cox regression analysis in the TCGA-STAD cohort. Using machine learning models, LPAR1 consistently ranked highest in feature importance. Multiple sequencing data showed that LPAR1 was significantly overexpressed in cancer-associated fibroblasts (CAFs). LPAR1 expression was significantly higher in normal tissues, and ROC analysis demonstrated its discriminative ability. Copy number alterations and microsatellite instability were significantly associated with LPAR1 expression. High LPAR1 expression correlated with advanced tumor grades and specific cancer immune subtypes, and multivariate analysis confirmed LPAR1 as an independent predictor of poor prognosis. LPAR1 expression was associated with different immune response metrics, including immune effector activation and upregulated chemokine secretion. High LPAR1 expression also correlated with increased sensitivity to compounds, such as BET bromodomain inhibitors I-BET151 and RITA, suggesting LPAR1 as a biomarker for predicting drug activity. FOXP2 showed a strong positive correlation with LPAR1 transcriptional regulation, while increased methylation of LPAR1 promoter regions was negatively correlated with gene expression. Knockdown of LPAR1 affected cell growth in most tumor cell lines, and in vitro experiments demonstrated that LPAR1 influenced extracellular matrix (ECM) contraction and cell viability in the paraptosis of CAFs. These findings suggest that LPAR1 is a critical regulator of paraptosis in GC and a potential biomarker for drug sensitivity and immunotherapy response. This underscores the role of CAFs in mediating tumorigenic effects and suggests that targeting LPAR1 could be a promising strategy for precision medicine in GC.

Resistance of estrogen receptor function to BET bromodomain inhibition is mediated by transcriptional coactivator cooperativity.

The Bromodomain and Extra-Terminal Domain (BET) family of proteins are critical chromatin readers that bind to acetylated histones through their bromodomains to activate transcription. Here, we reveal that bromodomain inhibition fails to repress oncogenic targets of estrogen receptor due to an intrinsic transcriptional mechanism. While bromodomains are necessary for the transcription of many genes, BRD4 binds to estrogen receptor binding sites and activates transcription of critical oncogenes independently of its bromodomains. BRD4 associates with the Mediator complex and disruption of Mediator complex reduces BRD4's enhancer occupancy. Profiling changes in the post-initiation RNA polymerase II (Pol II)-associated factors revealed that BET proteins regulate interactions between Pol II and elongation factors SPT5, SPT6, and PAF1 complex, which associate with BET proteins independently of their bromodomains and mediate their transcription elongation effect. Our findings highlight the importance of bromodomain-independent functions and interactions of BET proteins in the development of future therapeutic strategies.

BET Bromodomain Inhibition Potentiates Ocular Melanoma Therapy by Inducing Cell Cycle Arrest.

Ocular melanoma is a common primary malignant ocular tumor in adults with limited effective treatments. Epigenetic regulation plays an important role in tumor development. The switching/sucrose nonfermentation (SWI/SNF) chromatin remodeling complex and bromodomain and extraterminal domain family proteins are epigenetic regulators involved in several cancers. We aimed to screen a candidate small molecule inhibitor targeting these regulators and investigate its effect and mechanism in ocular melanoma. We observed phenotypes caused by knockdown of the corresponding gene and synergistic effects with BRD inhibitor treatment and SWI/SNF complex knockdown. The effect of JQ-1 on ocular melanoma cell cycle and apoptosis was analyzed with flow cytometry. Via RNA sequencing, we also explored the mechanism of BRD4. The best tumor inhibitory effect was observed for the BRD4 inhibitor (JQ-1), although there were no statistically obvious changes in the shBRD4 and shBRD9 groups. Interestingly, the inhibitory effect of JQ-1 was decrease in the shBRD4 group. JQ-1 inhibits the growth of melanoma in various cell lines and in tumor-bearing mice. We found 17 of these 28 common differentially expressed genes were downregulated after MEL270 and MEL290 cells treated with JQ-1. Four of these 17 genes, TP53I11, SH2D5, SEMA5A, and MDGA1, were positively correlated with BRD4. In TCGA database, low expression of TP53I11, SH2D5, SEMA5A, and MDGA1 improved the overall survival rate of patients. Furthermore, the disease-free survival rate was increased in the groups with low expression of TP53I11, SH2D5, and SEMA5A. JQ-1 may act downstream of BRD4 and suppress ocular melanoma growth by inducing G1 cell cycle arrest.

DIPG-34. IDENTIFICATION OF CAR-T CELL ENHANCING COMPOUNDS USING HIGH THROUGHPUT SCREENING

Abstract BACKGROUND The infiltrative growth patterns and restrictiveness of the blood brain barrier greatly limit therapeutic options for children diagnosed with diffuse intrinsic pontine gliomas (DIPGs). However, clinical trials using chimeric antigen receptor (CAR)-T cells have shown promise in DIPG patients. Despite some success, the biological limitations plaguing CAR-T cells hinder their overall efficacy. We propose that leveraging chemical compounds in tandem with CAR-T cells may provide an increased therapeutic benefit against this lethal brain tumor. Our study is aimed at using high throughput screening of >2,000 FDA approved and investigational compounds to identify candidates that enhance CAR-T cell mediated cytotoxicity of DIPGs. METHODS In an initial limited screen (32 compounds) used for optimization, compounds were serially diluted using the Echo 650 acoustic liquid handler. Subsequent co-culture conditions of renilla luciferase transduced DIPG007 tumor cells and clinically relevant firefly luciferase transduced CAR-T cells were plated at a 2:1 effector: target (E: T) ratio. In comparison to DMSO control, drug induced changes on cell viability were measured using cell-specific luciferase activity at 24 and 72 hours. RESULTS The limited screen revealed the BET bromodomain inhibitor, GSK-046, as an initial target based on its ability to increase CAR-T cell viability after 72 hours. Preliminary validation of these findings supports this CAR-T enhancing role of GSK-046 via MTS assays. We also identified nonlethal concentrations of GSK-046 for DIPG007 cells that could be further tested in combination with CAR-T cells to determine therapeutic synergy. CONCLUSIONS Our initial findings warrant further investigation into the combinatorial benefit of GSK-046 and CAR-T cells against DIPGs. Currently, we are optimizing and expanding our drug screen to identify additional compounds that may complement CAR-T cell therapy. Furthermore, we intend to validate the cytotoxic benefit of our prioritized combinations in additional in vitro and in vivo settings.

HTS384 NCI60: The Next Phase of the NCI60 Screen.

The NCI60 human tumor cell line screen has been in operation as a service to the cancer research community for more than 30 years. The screen operated with 96-well plates, a 2-day exposure period to test agents, and following cell fixation, a visible absorbance endpoint by the protein-staining dye sulforhodamine B. In this study, we describe the next phase of this important cancer research tool, the HTS384 NCI60 screen. Although the cell lines remain the same, the updated screen is performed with 384-well plates, a 3-day exposure period to test agents, and a luminescent endpoint to measure cell viability based upon cellular ATP content. In this study, a library of 1,003 FDA-approved and investigational small-molecule anticancer agents was screened by the two NCI60 assays. The datasets were compared with a focus on targeted agents with at least six representatives in the library. For many agents, including inhibitors of EGFR, BRAF, MEK, ERK, and PI3K, the patterns of GI50 values were very similar between the screens with strong correlations between those patterns within the dataset from each screen. However, for some groups of targeted agents, including mTOR, BET bromodomain, and NAMPRTase inhibitors, there were limited or no correlations between the two datasets, although the patterns of GI50 values and correlations between those patterns within each dataset were apparent. Beginning in January 2024, the HTS384 NCI60 screen became the free screening service of the NCI to facilitate drug discovery by the cancer research community. Significance: The new NCI60 cell line screen HTS384 shows robust patterns of response to oncology agents and substantial overlap with the classic screen, providing an updated tool for studying therapeutic agents. See related commentary by Colombo and Corsello, p. 2397.

BET bromodomain inhibition potentiates radiosensitivity in models of H3K27-altered diffuse midline glioma.

Diffuse midline glioma (DMG) H3K27-altered is one of the most malignant childhood cancers. Radiation therapy remains the only effective treatment yet provides a 5-year survival rate of only 1%. Several clinical trials have attempted to enhance radiation antitumor activity using radiosensitizing agents, although none have been successful. Given this, there is a critical need for identifying effective therapeutics to enhance radiation sensitivity for the treatment of DMG. Using high-throughput radiosensitivity screening, we identified bromo- and extraterminal domain (BET) protein inhibitors as potent radiosensitizers in DMG cells. Genetic and pharmacologic inhibition of BET bromodomain activity reduced DMG cell proliferation and enhanced radiation-induced DNA damage by inhibiting DNA repair pathways. RNA-Seq and the CUT&RUN (cleavage under targets and release using nuclease) analysis showed that BET bromodomain inhibitors regulated the expression of DNA repair genes mediated by H3K27 acetylation at enhancers. BET bromodomain inhibitors enhanced DMG radiation response in patient-derived xenografts as well as genetically engineered mouse models. Together, our results highlight BET bromodomain inhibitors as potential radiosensitizer and provide a rationale for developing combination therapy with radiation for the treatment of DMG.

Transcriptional synergy in human aortic endothelial cells is vulnerable to combination p300/CBP and BET bromodomain inhibition

Combinatorial signaling by proinflammatory cytokines synergizes to exacerbate toxicity to cells and tissue injury during acute infections. To explore synergism at the gene-regulatory level, we investigated the dynamics of transcription and chromatin signaling in response to dual cytokines by integrating nascent RNA imaging mass spectrometry, RNA sequencing, amplification-independent mRNA quantification, assay for transposase-accessible chromatin using sequencing (ATAC-seq), and transcription factor profiling. Costimulation with interferon-gamma (IFNγ) and tumor necrosis factor alpha (TNFα) synergistically induced a small subset of genes, including the chemokines CXCL9, -10, and -11. Gene induction coincided with increased chromatin accessibility at non-coding regions enriched for p65 and STAT1 binding sites. To discover coactivator dependencies, we conducted a targeted chemogenomic screen of transcriptional inhibitors followed by modeling of inhibitor dose-response curves. These results identified high efficacy of either p300/CREB-binding protein (CBP) or bromodomain and extra-terminal (BET) bromodomain inhibitors to disrupt induction of synergy genes. Combination p300/CBP and BET bromodomain inhibition at half-maximal inhibitory concentrations (subIC50) synergistically abrogated IFNγ/TNFα-induced chemokine gene and protein levels.

Ribosome subunit attrition and activation of the p53–MDM4 axis dominate the response of MLL-rearranged cancer cells to WDR5 WIN site inhibition

The chromatin-associated protein WD Repeat Domain 5 (WDR5) is a promising target for cancer drug discovery, with most efforts blocking an arginine-binding cavity on the protein called the ‘WIN’ site that tethers WDR5 to chromatin. WIN site inhibitors (WINi) are active against multiple cancer cell types in vitro, the most notable of which are those derived from MLL-rearranged (MLLr) leukemias. Peptidomimetic WINi were originally proposed to inhibit MLLr cells via dysregulation of genes connected to hematopoietic stem cell expansion. Our discovery and interrogation of small-molecule WINi, however, revealed that they act in MLLr cell lines to suppress ribosome protein gene (RPG) transcription, induce nucleolar stress, and activate p53. Because there is no precedent for an anticancer strategy that specifically targets RPG expression, we took an integrated multi-omics approach to further interrogate the mechanism of action of WINi in human MLLr cancer cells. We show that WINi induce depletion of the stock of ribosomes, accompanied by a broad yet modest translational choke and changes in alternative mRNA splicing that inactivate the p53 antagonist MDM4. We also show that WINi are synergistic with agents including venetoclax and BET-bromodomain inhibitors. Together, these studies reinforce the concept that WINi are a novel type of ribosome-directed anticancer therapy and provide a resource to support their clinical implementation in MLLr leukemias and other malignancies.

Ribosome subunit attrition and activation of the p53-MDM4 axis dominate the response of MLL-rearranged cancer cells to WDR5 WIN site inhibition.

The chromatin-associated protein WD Repeat Domain 5 (WDR5) is a promising target for cancer drug discovery, with most efforts blocking an arginine-binding cavity on the protein called the 'WIN' site that tethers WDR5 to chromatin. WIN site inhibitors (WINi) are active against multiple cancer cell types in vitro, the most notable of which are those derived from MLL-rearranged (MLLr) leukemias. Peptidomimetic WINi were originally proposed to inhibit MLLr cells via dysregulation of genes connected to hematopoietic stem cell expansion. Our discovery and interrogation of small-molecule WINi, however, revealed that they act in MLLr cell lines to suppress ribosome protein gene (RPG) transcription, induce nucleolar stress, and activate p53. Because there is no precedent for an anticancer strategy that specifically targets RPG expression, we took an integrated multi-omics approach to further interrogate the mechanism of action of WINi in human MLLr cancer cells. We show that WINi induce depletion of the stock of ribosomes, accompanied by a broad yet modest translational choke and changes in alternative mRNA splicing that inactivate the p53 antagonist MDM4. We also show that WINi are synergistic with agents including venetoclax and BET-bromodomain inhibitors. Together, these studies reinforce the concept that WINi are a novel type of ribosome-directed anticancer therapy and provide a resource to support their clinical implementation in MLLr leukemias and other malignancies.

Epigenetic modulation through BET bromodomain inhibitors as a novel therapeutic strategy for progranulin-deficient frontotemporal dementia

Frontotemporal dementia (FTD) is a debilitating neurodegenerative disorder with currently no disease-modifying treatment options available. Mutations in GRN are one of the most common genetic causes of FTD, near ubiquitously resulting in progranulin (PGRN) haploinsufficiency. Small molecules that can restore PGRN protein to healthy levels in individuals bearing a heterozygous GRN mutation may thus have therapeutic value. Here, we show that epigenetic modulation through bromodomain and extra-terminal domain (BET) inhibitors (BETi) potently enhance PGRN protein levels, both intracellularly and secreted forms, in human central nervous system (CNS)-relevant cell types, including in microglia-like cells. In terms of potential for disease modification, we show BETi treatment effectively restores PGRN levels in neural cells with a GRN mutation known to cause PGRN haploinsufficiency and FTD. We demonstrate that BETi can rapidly and durably enhance PGRN in neural progenitor cells (NPCs) in a manner dependent upon BET protein expression, suggesting a gain-of-function mechanism. We further describe a CNS-optimized BETi chemotype that potently engages endogenous BRD4 and enhances PGRN expression in neuronal cells. Our results reveal a new epigenetic target for treating PGRN-deficient forms of FTD and provide mechanistic insight to aid in translating this discovery into therapeutics.

Zinc-chelating BET bromodomain inhibitors equally target islet endocrine cell types.

Inhibition of the bromodomain and extraterminal domain (BET) protein family is a potential strategy to prevent and treat diabetes; however, the clinical use of BET bromodomain inhibitors (BETis) is associated with adverse effects. Here, we explore a strategy for targeting BETis to β cells by exploiting the high-zinc (Zn2+) concentration in β cells relative to other cell types. We report the synthesis of a novel, Zn2+-chelating derivative of the pan-BETi (+)-JQ1, (+)-JQ1-DPA, in which (+)-JQ1 was conjugated to dipicolyl amine (DPA). As controls, we synthesized (+)-JQ1-DBA, a non-Zn2+-chelating derivative, and (-)-JQ1-DPA, an inactive enantiomer that chelates Zn2+. Molecular modeling and biophysical assays showed that (+)-JQ1-DPA and (+)-JQ1-DBA retain potent binding to BET bromodomains in vitro. Cellular assays demonstrated (+)-JQ1-DPA attenuated NF-ĸB target gene expression in β cells stimulated with the proinflammatory cytokine interleukin 1β. To assess β-cell selectivity, we isolated islets from a mouse model that expresses green fluorescent protein in insulin-positive β cells and mTomato in insulin-negative cells (non-β cells). Surprisingly, Zn2+ chelation did not confer β-cell selectivity as (+)-JQ1-DPA was equally effective in both β and α cells; however, (+)-JQ1-DPA was less effective in macrophages, a nonendocrine islet cell type. Intriguingly, the non-Zn2+-chelating derivative (+)-JQ1-DBA displayed the opposite selectivity, with greater effect in macrophages compared with (+)-JQ1-DPA, suggesting potential as a macrophage-targeting molecule. These findings suggest that Zn2+-chelating small molecules confer endocrine cell selectivity rather than β-cell selectivity in pancreatic islets and provide valuable insights and techniques to assess Zn2+ chelation as an approach to selectively target small molecules to pancreatic β cells.NEW & NOTEWORTHY Inhibition of BET bromodomains is a novel potential strategy to prevent and treat diabetes mellitus. However, BET inhibitors have negative side effects. We synthesized a BET inhibitor expected to exploit the high zinc concentration in β cells to accumulate in β cells. We show our inhibitor targeted pancreatic endocrine cells; however, it was less effective in immune cells. A control inhibitor showed the opposite effect. These findings help us understand how to target specific cells in diabetes treatment.

Abstract 1548: Olfactomedin 4 deficiency promote de novo progression of prostate cancer in TRAMP mouse model

Abstract Prostate cancer is the most diagnosed solid tumor and the second leading cause of cancer-related death in American men. Olfactomedin (OLFM4) plays tumor suppressor roles and is associated with the initiation, growth, and clinical outcome of human prostate cancer. Olfm4-null mouse sporadically developed prostate epithelial neoplasia through up-regulated hedgehog signaling pathways. In this study, we have generated Olfm4+/+/GFP/TRAMP and Olfm4−/−/TRAMP mouse models to investigate Olfm4 gene biological functions underlying prostate cancer de novo progression. We have observed that Olfm4−/−/TRAMP mice developed more advanced prostate cancer at 47.6% (20/42) compared to Olfm4+/+/TRAMP at 11.9 % (5/42) and had poor survival compared to Olfm4+/+/TRAMP mice (P<0.01). Histopathological and immunofluorescent staining revealed that tumor tissues containing more heterogeneous cells such as Ar+/Ck8+ epithelial or Syn+ neuroendocrinal cells or Vim+/Ck8+ epithelium mesenchymal transition cells in the primary tumors obtained from Olfm4−/−/TRAMP mouse. We found more tumor angiogenesis and metastases into lymph nodes, liver, lung, kidney, and soft tissues in Olfm4−/−/TRAMP mice. The accumulations of the dendritic cell population in primary tumor and lung metastasis tumors were detected. The primary tumor was sensitive to androgen deprivation therapy by using castration with both Olfm4+/+/GFP/TRAMP and Olfm4−/−/TRAMP mice. However, castration-resisted EMT tumor cells or expressing neuroendocrine marker cells regrew after castration 7-8 months in both Olfm4+/+/GFP/TRAMP and Olfm4−/−/TRAMP mice. Furthermore, we tested JQ1, a BET bromodomain inhibitor, as an epigenetic targeted therapy reagent and found that JQ1 significantly inhibited tumor cell growth of Olfm4−/−/TRAMP mouse tumor in the organoids culture. Taken together, we have provided novel evidence that Olfm4 plays a tumor-suppressing role during prostate cancer de novo progression. Citation Format: Hongzhen Li, Wenli Liu, Jianqiong Zhu, Kyung Chin, Griffin P. Rodgers. Olfactomedin 4 deficiency promote de novo progression of prostate cancer in TRAMP mouse model [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 1548.

Abstract 4737: The combination of the BET inhibitor JQ1 and gemcitabine induces regressions in gemcitabine resistant patient-derived xenograft models of pancreatic cancer

Abstract Efforts to develop targeted therapies for drug resistant tumors have been relatively unsuccessful. The 5-year survival for patients with advanced pancreatic cancer is ~3%. Gemcitabine is usually combined with nab-paclitaxel as a standard of care for treatment of locally advanced or metastatic pancreatic ductal adenocarcinoma (PDAC). However, no chemotherapeutic regimens thus far evaluated have been curative. We developed the first PDAC patient-derived xenograft (PDX) models in which resistance against gemcitabine was acquired in vivo. We used RNA-seq and ingenuity pathway analysis to compare expression profiles of paired parent vs gemcitabine resistant PDX models. This comparison suggested a previously unreported mechanism for gemcitabine resistance, involving aberrations of cholesterol biosynthesis and lipid metabolism pathways. We also made the unexpected observation that gemcitabine-resistant PDAC models derived from primary tumors have high levels of 3-hydroxy-3-methylglutaryl-CoA synthase 2 (HMGCS2), an enzyme essential to cholesterol biosynthesis and rate limiting in ketogenesis. Notably, the BET bromodomain inhibitor JQ1 decreases HMGCS2 levels; and BET inhibitors + gemcitabine exert synergistic cytotoxicity in gemcitabine-resistant PDAC cells. We determined that the BET bromodomain inhibitor JQ1 + gemcitabine induces regressions of two in vivo gemcitabine-resistant tumor models which we developed directly from primary human tumor tissue. These mechanism-based studies suggest that inhibiting HMGCS2 represents a novel approach to therapy for PDAC. We propose that combining gemcitabine with agents that decrease levels of HMGCS2 comprises effective treatment for patients with drug resistant PDAC. Citation Format: Aubrey L. Miller, Samuel C. Fehling, Eric Josh Brown, Eric O. Heard, Tracy L. Gamblin, Rebecca B. Vance, Karina J. Yoon. The combination of the BET inhibitor JQ1 and gemcitabine induces regressions in gemcitabine resistant patient-derived xenograft models of pancreatic cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 4737.

BET bromodomain inhibitors PFI-1 and CPI-203 suppress the development of follicular lymphoma via regulating Wnt/β-catenin signaling

ObjectiveFollicular lymphoma (FL) is an indolent B-cell lymphoproliferative disorder, characterized by a lymphoid follicular pattern of growth. PFI-1 or CPI-203 has been known to effectively promote the inhibition of primary effusion lymphoma progression. This study aimed at investigating the anti-tumor properties of PFI-1 and CPI-203 on FL cells and uncover the underlying mechanism of action. MethodsFL cells were treated with PFI-1 and CPI-203, and the treated cells were evaluated for their cell viability, cell cycle and apoptosis using CCK8, flow cytometry, and Western blot assays. A xenograft mouse model was used for assessing the in vivo effects of CPI-203 on tumorigenesis. ResultsPFI-1 or CPI-203 showed potential inhibitory effects on the cell viability of DOHH2 and RL cells in a dose-response-dependent manner. Furthermore, PFI-1 and CPI-203 inhibited cell growth, induced apoptosis of FL cells in vitro, and facilitated the translocation of β-catenin into cytoplasm both in vitro and in vivo. After engrafted with FL cells, CPI-203-treated mice got a longer duration of survival and a smaller tumor size than control mice. Mechanistically, PFI-1 and CPI-203 impede the activity of β-catenin and its downstream molecules by regulating the DVL2/GSK3β axis. ConclusionIn conclusion, PFI-1 and CPI-203 may serve as potential anti-tumor inhibitors for the therapy of FL.

ARV-825 Showed Antitumor Activity against BRD4-NUT Fusion Protein by Targeting the BRD4.

The bromodomain-containing 4 (BRD4) is a member of the bromodomain and extra terminal domain (BET) family, which is an important epigenetic reader. It is currently a promising oncology target. In some tumors, BET bromodomain inhibitors have demonstrated promising results. Proteolysis-targeting methods (PROTAC), which rapidly and effectively degrade BRD4, have displayed considerable potential in the treatment of tumors in recent years. The purpose of this study is to examine the potential impact of BRD4 PROTAC compounds ARV-825 on oncogene BRD4-NUT fused protein in NUT carcinoma. The effectiveness of ARV-825 was evaluated at the cellular level using the cell counting kit 8 test, wound healing, cell transfection, western blotting analysis, and RNA sequencing. The effectiveness of ARV-825 was also examined in vivo using a xenograft model. The BRD4-NUT fusion gene was overexpressed in 3T3 cells, and the pathogenic fusion gene was simulated. The results showed that the overexpression of BRD4-NUT could promote the proliferation and migration of 3T3 cells, but the expression of BRD4 protein was degraded after the addition of the novel cereblon-based PROTAC compound ARV-825 against BRD4, resulting in inhibition of BRD4-NUT 3T3 cell proliferation and migration. Further RNA-seq analysis showed that overexpression of BRD4-NUT was accompanied by increased expression of gene (e.g., Myc, E2F, TRAFs, Wnt, Gadd45g, and Sox6) with significantly enriched pathway (e.g., small cell lung cancer, NF-kappa B signaling pathway, and breast cancer), promoted cell cycle from G 1 phase to S phase, and increased cell proliferation and migration, activated the antiapoptosisi signal, led to abnormal cell growth, and ultimately led to tumorigenesis. The addition of ARV-825 effectively rescued this process and effectively inhibited cell vitality, proliferation, and migration. In vivo studies demonstrated that treatment with ARV-825 greatly suppressed tumor growth without causing harmful side effects and downregulated the BRD4-NUT expression level. Through the induction of BRD4 protein degradation, ARV-825 can successfully limit BRD4-NUT 3T3 cell proliferation in vitro and in vivo. These findings suggested that the BRD4 inhibitor ARV-825 would be an effective therapeutic strategy for treating NUT carcinoma that with the genetic feature of BRD4-NUT fusion event.

Abstract A059: Small molecule microarray lysate screen identifies bromodomain ligands that target the MYC transcription regulatory network

Abstract MYC is the most frequently amplified gene in human cancers and one of the most sought-after drug targets for cancer therapy. Its function as a transcription factor is essential for its oncogenic potential. However, development of small molecules that target oncogenic MYC function in cells has been intractable due to its lack of known ligand binding sites. To overcome these structural challenges, we leveraged Kronos Bio’s small molecule microarray (SMM) screening platform to identify small molecules that can bind to MYC transcriptional complexes in cell lysates from cancer cells with deregulated MYC function. Unbiased transcriptional signature-based profiling identified multiple SMM hits that modulated the MYC transcription regulatory network (TRN) in MYC-dependent cancer cell lines. One of the SMM hits mimicked signature changes due to MYC loss of function, which were also similar to signature changes effected by BET bromodomain inhibitors. Biochemical, biophysical, and structural analyses found that the SMM hit could directly engage with BET bromodomains in purified systems. Cell-based proximity labeling demonstrated binding to BET proteins in live cells, and comparative pharmacology approaches suggested that the BET protein engagement was likely driving the observed gene expression changes. These results demonstrate the utility of the SMM platform in identifying ligands that modulate transcription factor TRNs through binding to critical cofactors. Citation Format: Emily B Cohen, Zhihua Ma, Wayne L Glore, Oleg A Volkov, Tong Liang, Nikolaus D Obholzer, Andrew C Clay, Mulini Reddy Pingili, Yupeng Zheng, Jun Li, Shi Yun Wang, Tom Chen, David B Freeman, Hua Gao, Minyun Zhou, Priscilla Cheung, Tamara D Hopkins, Marius S Pop, Christopher J Dinsmore, B. Wesley Trotter, Charles Y Lin, Peter B Rahl. Small molecule microarray lysate screen identifies bromodomain ligands that target the MYC transcription regulatory network [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2023 Oct 11-15; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2023;22(12 Suppl):Abstract nr A059.

Abstract LB_A21: NCI60 HTS384: The next phase of the NCI60 screen

Abstract The NCI60 human tumor cell line screen, which has operated as a free service to the cancer research community for over 20 years, has proven to be an important drug discovery tool. The screen has operated using cutting-edge technology of the 1990s including a 96-well microplate format, a 2-day drug exposure period and following cell fixation, and a visible absorbance endpoint by the protein-staining dye sulforhodamine B (SRB). The SRB endpoint determines cell density based on protein content with a lower resolution limit of 1,000 – 2,000 cells. This report describes the next phase of this important cancer research tool, the NCI60 HTS384 screen format. While the cell lines remain the same, the screen is performed using 384-well microplates with a 3-day compound exposure period and a CellTiter-Glo luminescence endpoint. The CellTiter-Glo endpoint determines cell number based upon ATP content with a lower resolution limit of <10 cells. To develop a baseline comparison, data were generated from a library of 1,000 FDA-approved and investigational small molecule anticancer agents by the two assay formats. Although the mean-graph patterns and COMPARE analyses are very similar for many compounds, there is a divergence between the two screens for some molecular targets. Within subsets of approved and investigational agents directed toward the same target (e.g., EGFR, BRAF/MEK/ERK, and PI3K inhibitors), the clustering of compound data from the NCI60 HTS384 screen is qualitatively similar to the clustering of compound data from the classic NCI60 SRB screen and there is a strong COMPARE correlation between the two screens. However, for some drug targets there is very little or no clustering between data from the two screens, although the clustering within each screen is moderate-to-good. Examples include mTOR rapalogs, BET bromodomain and NAMPRTase inhibitors. Differences in data between the two screens might be partially attributed to increases in the assay endpoint sensitivity and compound exposure time of the NCI60 HTS384 assay. Compounds submitted for NCI60 screening after September 30, 2023, will be tested in the modernized NCI60 HTS384 format. This project was funded in part with federal funds from the NCI, NIH, under contract no. HHSN261201500003I and by the Intramural Research Program of the NIH, National Cancer Institute. Citation Format: Beverly A Teicher, Mark W Kunkel, Joel Morris, Ronald C Taylor, Thomas S Dexheimer, Nathan P Coussens, James H Doroshow. NCI60 HTS384: The next phase of the NCI60 screen [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2023 Oct 11-15; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2023;22(12 Suppl):Abstract nr LB_A21.