GSK789: A Selective Inhibitor of the First Bromodomains (BD1) of the Bromo and Extra Terminal Domain (BET) Proteins

Selective Synthesis of Tetrahydroisoquinoline and Piperidine Scaffolds by Oxidative Ring Opening/Ring Closing Protocols of Substituted Indenes and Cyclopentenes

Novel tetrahydroisoquinoline and piperidine derivatives were selectively synthesized from substituted indenes or cyclopentenes. The process starts with an oxidative cleavage of the ring olefin bond, which gives reactive diformyl intermediates. By a ring-closing step using chiral (R) or (S) α-methylbenzylamine under a reductive amination protocol facilitated ring formation with ring expansion of the corresponding nitrogen-containing heterocycles. The stereocontrolled methodology enabled accurate control of the stereochemistry of the final products. Additionally, the synthesized amino acid derivatives possessing an aryl moiety in their structure may be relevant building blocks for foldamer chemistry.

Discovery of the First BRD4 Second Bromodomain (BD2)-Selective Inhibitors

Pan-BD2 inhibitors have been shown to retain an antileukemia effect and display less dose-limiting toxicities than pan-BET inhibitors. However, it is necessary to consider the potential off-target toxicity associated with the inhibition of four BET BD2 proteins. To date, no BRD4 BD2 domain selective inhibitor has been reported. Based on our previous pan-BD2 inhibitor 12 (XY153), we successfully identified 16o (XY221) as the first BRD4 BD2-selective inhibitor. 16o demonstrated potent binding affinity for BRD4 BD2 (IC50 = 5.8 nM), along with high pan-BD2 selectivity (667-fold over BRD4 BD1) and BRD4 BD2 domain selectivity (9-32-fold over BRD2/3/T BD2). The BRD4 BD2 selectivity of 16o was further confirmed by the BLI assay, showing 66-144-fold selectivity over other BET BD2 domains. 16o exhibited good liver microsomal stability (T1/2 > 120 min) and pharmacokinetic properties (F = 13.1%). These data indicate that 16o may serve as a valuable candidate for BRD4 BD2 advancing epigenetic research.

The Development and Evaluation of a Novel Highly Selective PET Radiotracer for Targeting BET BD1

Background/Objectives: Small molecules that interfere with the interaction between acetylated protein tails and the tandem bromodomains of BET (bromodomain and extra-terminal) family proteins are pivotal in modulating immune/inflammatory and neoplastic diseases. This study aimed to develop a novel PET imaging tracer, [11C]GSK023, that targets the N-terminal bromodomain (BD1) of BET family proteins with high selectivity and potency, thereby enriching the chemical probe toolbox for epigenetic imaging. Methods: [11C]GSK023, a radio-chemical probe, was designed and synthesized to specifically target the BET BD1. In vivo PET imaging evaluations were conducted on rodents, focusing on the tracer's distribution and binding specificity in various tissues. Blocking studies were performed to confirm the probe's selectivity and specificity. Results: The evaluations revealed that [11C]GSK023 demonstrated good uptake in peripheral organs with limited brain penetration. Further blocking studies confirmed the probe's high binding specificity and selectivity for the BET BD1 protein, underscoring its potential utility in epigenetic imaging. Conclusions: The findings suggest that [11C]GSK023 is a promising PET probe for imaging the BET BD1 protein, offering the potential to deepen our understanding of the roles of BET bro-modomains in disease and their application in clinical settings to monitor disease progression and therapeutic responses.

Discovery of a potent, orally available furopyridine derivative as a novel selective bromodomain and extra-terminal domain (BET)-first bromodomain (BD1) inhibitor

We explored novel immunosuppressive agents with immune tolerance using a phenotypic drug discovery strategy, focusing on costimulatory molecules in T cells, and obtained triazolothienodiazepine derivatives. Their mechanism of action is to inhibit the bromodomain and extra-terminal domain (BET) family, as we have previously reported. Selective inhibition of the first bromodomain (BD1) of the BET family is expected to exert antitumor and immunosuppressive effects, similar to BET inhibitors. This study identified furopyridine derivatives 7 and 8 with high BD1 inhibitory activity and high selectivity over BD2. Compound 7 was found to be orally bioavailable and exhibited anti-inflammatory activity in a lipopolysaccharide-induced model.

Discovery of a potent orally available pyrazolopyridone derivative as a novel selective bromodomain and extra-terminal domain (BET)-first bromodomain (BD1) inhibitor

Clinical studies have shown that inhibitors of bromodomain and extra-terminal domain (BET) proteins, particularly BRD4, have antitumor activity and efficacy. The BET protein has two domains, BD1 and BD2, and we previously focused on BD1 and reported orally bioavailable BD1-selective inhibitors. In this study, we obtained a BD1 inhibitor, a more potent and highly selective pyrazolopyridone derivative 13a, and confirmed its in vivo efficacy.

Successes and challenges in the development of BD1-selective BET inhibitors: a patent review

INTRODUCTION

Bromodomain and ExtraTerminal (BET) domain proteins are transcriptional cofactors that, recognizing acetylated lysines of histone and non-histone proteins, can modulate gene expression. The BET family consists of four members, each of which contains two bromodomains (BD1 and BD2) able to recognize the acetylated mark. Pan-BET inhibitors (BETi) have shown a promising anticancer potential in many clinical trials; however, their further development has been in part hampered by the side effects due to their lack of selectivity. Mounting evidence suggests that BD1 is primarily involved in cancer and that its selective inhibition can phenocopy the anticancer effects of pan-BETi with increased tolerability. Therefore, the development of BD1 selective inhibitors is highly pursed in both academia and industry.

AREAS COVERED

This review aims at giving an overview of the patent literature of BD1-selective BETi between 2014 and 2023. WIPO, USPTO, EPO, and SciFinder® databases were used for the search of patents.

EXPERT OPINION

The development of BD1-selective BETi, despite challenging, is highly desirable as it could have a great impact on the development of new safer anticancer therapeutics. Several strategies could be applied to discover potent and selective compounds with limited side effects.

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.

Structure-Guided Design and Synthesis of Pyridinone-Based Selective Bromodomain and Extra-Terminal Domain (BET)-First Bromodomain (BD1) Inhibitors

The bromodomain and extra-terminal domain (BET) proteins are epigenetic readers, regulating transcription via two highly homologous tandem bromodomains, BD1 and BD2. Clinical development of nonselective pan-BD BET inhibitors has been challenging, partly due to dose-limiting side effects such as thrombocytopenia. This has prompted the push for domain-selective BET inhibitors to achieve a more favorable therapeutic window. We report a structure-guided drug design campaign that led to the development of a potent BD1-selective BET inhibitor, 33 (XL-126), with a Kd of 8.9 nM and 185-fold BD1/BD2 selectivity. The high selectivity was first assayed by SPR, validated by a secondary time-resolved fluorescence energy transfer assay, and further corroborated by BROMOscan (∼57-373 fold selectivity). The cocrystal of 33 with BRD4 BD1 and BD2 demonstrates the source of selectivity: repulsion with His437 and lost binding with the leucine clamp. Notably, the BD1 selectivity of BET inhibitor 33 leads to both the preservation of platelets and potent anti-inflammatory efficacy.

DW71177: A novel [1,2,4]triazolo[4,3-a]quinoxaline-based potent and BD1-Selective BET inhibitor for the treatment of acute myeloid leukemia

The bromodomain and extraterminal domain (BET) family proteins recognize acetyl-lysine (Kac) at the histone tail through two tandem bromodomains, i.e., BD1 and BD2, to regulate gene expression. BET proteins are attractive therapeutic targets in cancer due to their involvement in oncogenic transcriptional activation, and bromodomains have defined Kac-binding pockets. Here, we present DW-71177, a potent BET inhibitor that selectively interacts with BD1 and exhibits strong antileukemic activity. X-ray crystallography, isothermal titration calorimetry, and molecular dynamic studies have revealed the robust and specific binding of DW-71177 to the Kac-binding pocket of BD1. DW-71177 effectively inhibits oncogenes comparable to the pan-BET inhibitor OTX-015, but with a milder impact on housekeeping genes. It efficiently blocks cancer-associated transcriptional changes by targeting genes that are highly enriched with BRD4 and histone acetylation marks, suggesting that BD1-selective targeting could be an effective and safe therapeutic strategy against leukemia.

Discovery of a brain-permeable bromodomain and extra terminal domain (BET) inhibitor with selectivity for BD1 for the treatment of multiple sclerosis

Multiple sclerosis (MS) is a neuroinflammatory autoimmune disease and lacks effective therapeutic agents. Dysregulation of transcription mediated by bromodomain and extra-terminal domain (BET) proteins containing two different bromodomains (BD1 and BD2) is an important factor in multiple diseases, including MS. Herein, we identified a series of BD1-biased inhibitors, in which compound 16 showed nanomolar potency for BD1 (Kd = 230 nM) and a 60-fold selectivity for BRD4 BD1 over BD2. The co-crystal structure of BRD4 BD1 with 16 indicated that the hydrogen bond interaction of 16 with BD1-specific Asp145 is important for BD1 selectivity. 16 showed favorable brain distribution in mice and PK properties in rats. 16 was able to inhibit microglia activation and had significant therapeutic effects on EAE mice including improvement of spinal cord inflammatory conditions and demyelination protection. Overall, these results suggest that brain-permeable BD1 inhibitors have the potential to be further investigated as therapeutic agents for MS.

Discovery of Novel Phenoxyaryl Pyridones as Bromodomain and Extra-Terminal Domain (BET) Inhibitors with High Selectivity for the Second Bromodomain (BD2) to Potentially Treat Acute Myeloid Leukemia

Bromodomain-selective BET inhibition has emerged as a promising strategy to improve the safety profiles of pan-BET inhibitors. Herein, we report the discovery of potent phenoxyaryl pyridones as highly BD2-selective BET inhibitors. Compound 23 (IC50 = 2.9 nM) exhibited a comparable BRD4 BD2 inhibitory activity relative to 10 (IC50 = 1.0 nM) and remarkably improved selectivity over BRD4 BD1 (23: 2583-fold; 10: 344-fold). This lead compound significantly inhibited the proliferation of acute myeloid leukemia (AML) cell lines through induction of G0/G1 arrest and apoptosis in vitro. Excellent in vivo antitumor efficacy with 23 was achieved in an MV;411 mouse xenograft model. Pleasingly, compound 23 (hERG IC50 > 30 μM) mitigated the inhibition of the human ether-à-go-go-related gene (hERG) ion channel compared with 10 (hERG IC50 = 2.8 μM). This work provides a promising BD2-selective lead for the development of more effective and safe BET inhibitors as anticancer agents.

Structure-Guided Design of a Domain-Selective Bromodomain and Extra Terminal N-Terminal Bromodomain Chemical Probe

Small-molecule-mediated disruption of the protein-protein interactions between acetylated histone tails and the tandem bromodomains of the bromodomain and extra-terminal (BET) family of proteins is an important mechanism of action for the potential modulation of immuno-inflammatory and oncology disease. High-quality chemical probes have proven invaluable in elucidating profound BET bromodomain biology, with seminal publications of both pan- and domain-selective BET family bromodomain inhibitors enabling academic and industrial research. To enrich the toolbox of structurally differentiated N-terminal bromodomain (BD1) BET family chemical probes, this work describes an analysis of the GSK BRD4 bromodomain data set through a lipophilic efficiency lens, which enabled identification of a BD1 domain-biased benzimidazole series. Structure-guided growth targeting a key Asp/His BD1/BD2 switch enabled delivery of GSK023, a high-quality chemical probe with 300-1000-fold BET BD1 domain selectivity and a phenotypic cellular fingerprint consistent with BET bromodomain inhibition.

Mutate and Conjugate: A Method to Enable Rapid In-Cell Target Validation

Target validation remains a challenge in drug discovery, which leads to a high attrition rate in the drug discovery process, particularly in Phase II clinical trials. Consequently, new approaches to enhance target validation are valuable tools to improve the drug discovery process. Here, we report the combination of site-directed mutagenesis and electrophilic fragments to enable the rapid identification of small molecules that selectively inhibit the mutant protein. Using the bromodomain-containing protein BRD4 as an example, we employed a structure-based approach to identify the L94C mutation in the first bromodomain of BRD4 [BRD4(1)] as having a minimal effect on BRD4(1) function. We then screened a focused, KAc mimic-containing fragment set and a diverse fragment library against the mutant and wild-type proteins and identified a series of fragments that showed high selectivity for the mutant protein. These compounds were elaborated to include an alkyne click tag to enable the attachment of a fluorescent dye. These clickable compounds were then assessed in HEK293T cells, transiently expressing BRD4(1)WT or BRD4(1)L94C, to determine their selectivity for BRD4(1)L94C over other possible cellular targets. One compound was identified that shows very high selectivity for BRD4(1)L94C over all other proteins. This work provides a proof-of-concept that the combination of site-directed mutagenesis and electrophilic fragments, in a mutate and conjugate approach, can enable rapid identification of small molecule inhibitors for an appropriately mutated protein of interest. This technology can be used to assess the cellular phenotype of inhibiting the protein of interest, and the electrophilic ligand provides a starting point for noncovalent ligand development.

Bromodomain and extraterminal (BET) proteins: biological functions, diseases, and targeted therapy

BET proteins, which influence gene expression and contribute to the development of cancer, are epigenetic interpreters. Thus, BET inhibitors represent a novel form of epigenetic anticancer treatment. Although preliminary clinical trials have shown the anticancer potential of BET inhibitors, it appears that these drugs have limited effectiveness when used alone. Therefore, given the limited monotherapeutic activity of BET inhibitors, their use in combination with other drugs warrants attention, including the meaningful variations in pharmacodynamic activity among chosen drug combinations. In this paper, we review the function of BET proteins, the preclinical justification for BET protein targeting in cancer, recent advances in small-molecule BET inhibitors, and preliminary clinical trial findings. We elucidate BET inhibitor resistance mechanisms, shed light on the associated adverse events, investigate the potential of combining these inhibitors with diverse therapeutic agents, present a comprehensive compilation of synergistic treatments involving BET inhibitors, and provide an outlook on their future prospects as potent antitumor agents. We conclude by suggesting that combining BET inhibitors with other anticancer drugs and innovative next-generation agents holds great potential for advancing the effective targeting of BET proteins as a promising anticancer strategy.

Selectivity Mechanism of Pyrrolopyridone Analogues Targeting Bromodomain 2 of Bromodomain-Containing Protein 4 from Molecular Dynamics Simulations

Bromodomain and extra-terminal domain (BET) proteins play an important role in epigenetic regulation and are linked to several diseases; therefore, they are interesting targets. BET has two bromodomains: bromodomain 1 (BD1) and BD2. Selective targeting of BD1 or BD2 may produce different activities and greater effects than pan-BD inhibitors. However, the selective mechanism of the specific core must be studied at the atomic level. This study determined the effectiveness of pyrrolopyridone analogues to selectively inhibit BD2 using a pan-BD inhibitor (ABBV-075) and a selective-BD2 inhibitor (ABBV-744). Molecular dynamics simulations and calculations of binding free energies were used to systematically study the selectivity of BD2 inhibition by the pyrrolopyridone analogues. Overall, the pyrrolopyridone analogue inhibitors targeting BD2 interacted mainly with the following amino acid pairs between bromodomain-containing protein 4 (BRD4)-BD1 and BRD4-BD2 complexes: I146/V439, N140/N433, D144/H437, P82/P375, V87/V380, D88/D381, and Y139/Y432. The pyrrolopyridone analogues targeting BRD4-BD2 were divided into five regions based on selectivity mechanism. These results suggest that the R3 and R5 regions of pyrrolopyridone analogues can be modified to improve the selectivity between BRD4-BD1 and BRD4-BD2. The selectivity of BD2 inhibition by pyrrolopyridone analogues can be used to design novel BD2 inhibitors based on a pyrrolopyridone core.

Design and Characterization of 1,3-Dihydro-2H-benzo[d]azepin-2-ones as Rule-of-5 Compliant Bivalent BET Inhibitors

The 1,3-dihydro-2H-benzo[d]azepin-2-ones are potent and ligand-efficient pan-BET bromodomain inhibitors. Here we describe the extension of this template to exploit a bivalent mode of action, binding simultaneously to both bromodomains. Initially the linker length and attachment vectors compatible with bivalent binding were explored, leading to the discovery of exceptionally potent bivalent BET inhibitors within druglike rule-of-5 space.

Bromodomain inhibitors and therapeutic applications

The bromodomain acts to recognize acetylated lysine in histones and transcription proteins and plays a fundamental role in chromatin-based cellular processes including gene transcription and chromatin remodeling. Many bromodomain proteins, particularly the bromodomain and extra terminal domain (BET) protein BRD4 have been implicated in cancers and inflammatory disorders and recognized as attractive drug targets. Although clinical studies of many BET bromodomain inhibitors have made substantial progress toward harnessing the therapeutic potential of targeting the bromodomain proteins, the development of this new class of epigenetic drugs is met with challenges, especially on-target dose-limiting toxicity. In this review, we highlight the current development of new-generation small molecule inhibitors for the BET and non-BET bromodomain proteins and discuss the research strategies used to target different bromodomain proteins for a wide array of human diseases including cancers and inflammatory disorders.

Recent progress and structural analyses of domain-selective BET inhibitors

Epigenetic mechanisms for controlling gene expression through heritable modifications to DNA, RNA, and proteins, are essential processes in maintaining cellular homeostasis. As a result of their central role in human diseases, the proteins responsible for adding, removing, or recognizing epigenetic modifications have emerged as viable drug targets. In the case of lysine-ε-N-acetylation (Kac ), bromodomains serve as recognition modules ("readers") of this activating epigenetic mark and competition of the bromodomain-Kac interaction with small-molecule inhibitors is an attractive strategy to control aberrant bromodomain-mediated gene expression. The bromodomain and extra-terminal (BET) family proteins contain eight similar bromodomains. These BET bromodomains are among the more commonly studied bromodomain classes with numerous pan-BET inhibitors showing promising anticancer and anti-inflammatory efficacy. However, these results have yet to translate into Food and Drug Administration-approved drugs, in part due to a high degree of on-target toxicities associated with pan-BET inhibition. Improved selectivity within the BET-family has been proposed to alleviate these concerns. In this review, we analyze the reported BET-domain selective inhibitors from a structural perspective. We highlight three essential characteristics of the reported molecules in generating domain selectivity, binding affinity, and mimicking Kac molecular recognition. In several cases, we provide insight into the design of molecules with improved specificity for individual BET-bromodomains. This review provides a perspective on the current state of the field as this exciting class of inhibitors continue to be evaluated in the clinic.

Targeting bromodomain-containing proteins: research advances of drug discovery

Bromodomain (BD) is an evolutionarily conserved protein module found in 46 different BD-containing proteins (BCPs). BD acts as a specific reader for acetylated lysine residues (KAc) and serves an essential role in transcriptional regulation, chromatin remodeling, DNA damage repair, and cell proliferation. On the other hand, BCPs have been shown to be involved in the pathogenesis of a variety of diseases, including cancers, inflammation, cardiovascular diseases, and viral infections. Over the past decade, researchers have brought new therapeutic strategies to relevant diseases by inhibiting the activity or downregulating the expression of BCPs to interfere with the transcription of pathogenic genes. An increasing number of potent inhibitors and degraders of BCPs have been developed, some of which are already in clinical trials. In this paper, we provide a comprehensive review of recent advances in the study of drugs that inhibit or down-regulate BCPs, focusing on the development history, molecular structure, biological activity, interaction with BCPs and therapeutic potentials of these drugs. In addition, we discuss current challenges, issues to be addressed and future research directions for the development of BCPs inhibitors. Lessons learned from the successful or unsuccessful development experiences of these inhibitors or degraders will facilitate the further development of efficient, selective and less toxic inhibitors of BCPs and eventually achieve drug application in the clinic.

Bromodomain and Extra-Terminal Proteins in Brain Physiology and Pathology: BET-ing on Epigenetic Regulation

BET proteins function as histone code readers of acetylated lysins that determine the positive regulation in transcription of genes involved in cell cycle progression, differentiation, inflammation, and many other pathways. In recent years, thanks to the development of BET inhibitors, interest in this protein family has risen for its relevance in brain development and function. For example, experimental evidence has shown that BET modulation affects neuronal activity and the expression of genes involved in learning and memory. In addition, BET inhibition strongly suppresses molecular pathways related to neuroinflammation. These observations suggest that BET modulation may play a critical role in the onset and during the development of diverse neurodegenerative and neuropsychiatric disorders, such as Alzheimer’s disease, fragile X syndrome, and Rett syndrome. In this review article, we summarize the most recent evidence regarding the involvement of BET proteins in brain physiology and pathology, as well as their pharmacological potential as targets for therapeutic purposes.

Rational Design, Synthesis and Biological Evaluation of Benzo[d]isoxazole Derivatives as Potent BET Bivalent Inhibitors for Potential Treatment of Prostate Cancer

Multivalency is an attractive strategy for effective binding to target protein. Bromodomain and extra-terminal (BET) family features two tandem bromodomains (BD1, BD2), which are considered to be potential new targets for prostate cancer. Herein, we report the rational design, optimization, and evaluation of a class of novel BET bivalent inhibitors based on our monovalent BET inhibitor 7 (Y06037). The representative bivalent inhibitor 17b effectively inhibited the cell growth of LNCaP, exhibiting 32 folds more potency than monovalent inhibitor 7. Besides, 17b induced 95.1 % PSA regression in LNCaP cell at 2 μM. Docking study was further carried out to reveal the potential binding mode of 17b with two BET bromodomains. Our study demonstrates that 17b (Y13021) is a promising BET bivalent inhibitor for the treatment of prostate cancer.

Cytosporone E analogues as BRD4 inhibitors for cancer treatment: molecular docking and molecular dynamic investigations

Cancer is considered one of the worldwide life-threatening and leading causes of human mortality. In 2020, 19,292,789 cancer cases and 9,958,133 cancer deaths have been estimated worldwide. Therefore, efforts have been devoted to discover novel anticancer agents. Bromodomains have a vital role in the regulation of transcription. Many reports have shown that bromodomain-containing protein 4 (BRD4) is an important target for cancer therapeutics. In this study, several in silico approaches were utilized to discover new inhibitors against the BRD4 protein using the Schrodinger suite. A library of 27 cytosporone E derivatives was docked into the active site of the BRD4 protein. Docked ligands showed docking scores ranging between -11.289 to -3.992 Kcal/mol. Ligands 1-4 showed better binding affinities with docking scores ranging from -11.289 to -8.917 Kcal/mol compared to the reference ligand BI-2536 (-8.426 Kcal/mol). These ligands displayed favorable MM-GBSA free binding energy. Also, ligands 1-4 were subjected to molecular dynamics simulations for 100 ns to get insight into the ligand-binding stability. These compounds exhibited an average RMSD below 2.8 Å, indicating the stability of the compounds with BRD4 protein. Further, Moreover, ligands 1-3 displayed favorable AMDET properties (absorption, distribution, metabolism, excretion, and toxicity). These new compounds might be potential leads to combat cancer.Communicated by Ramaswamy H. Sarma.

Discovery of Benzo[d]imidazole-6-sulfonamides as Bromodomain and Extra-Terminal Domain (BET) Inhibitors with Selectivity for the First Bromodomain

The bromodomain and extra-terminal (BET) family of proteins includes BRD2, BRD3, BRD4, and the testis-specific protein, BRDT, each containing two N-terminal tandem bromodomain (BRD) modules. Potent and selective inhibitors targeting the two bromodomains are required to elucidate their biological role(s), with potential clinical applications. In this study, we designed and synthesized a series of benzimidazole-6-sulfonamides starting from the azobenzene compounds MS436 (7 a) and MS611 (7 b) that exhibited preference for the first (BD1) over the second (BD2) BRD of BET family members. The most-promising compound (9 a) showed good binding potency and improved metabolic stability and selectivity towards BD1 with respect to the parent compounds.

In Silico Positional Analogue Scanning with Amber GPU-TI

Positional analogue scanning (PAS) is an accepted strategy for multiparameter lead optimization (MPO) in drug discovery. Small structural changes as introduced by PAS can lead to 10-fold changes in binding potency in ∼10-20% of cases, a significant parameter shift irrespective of other MPO objectives. Sometimes performing a complete PAS is challenging due to resource and time constraints, building block availability, or difficulty in synthesis. Calculating relative binding free energies (RBFEs) for all positions can contribute to prioritizing the most promising analogues for synthesis. We tested a well-established RBFE calculation method, Amber GPU-TI, for 20 positional analogue scans in 14 test systems (cyclin-dependent kinase 8 (CDK8), hepatitis C virus nonstructural protein 5B (HCV NS5B), tankyrase, RAC-α serine/threonine-protein kinase (Akt), phosphodiesterase 1B (PDE1B), orexin/hypocretin receptor type 1 (OX1R), orexin/hypocretin receptor type 2 (OX2R), histone acetyltransferase K (lysine) acetyltransferase 6A (KAT6A), peroxisome proliferator-activated receptor γ (PPARγ), extracellular signal-regulated kinases (ERK1/2), coactivator-associated arginine methyltransferase 1 (PRMT4), α_vβ₆, bromodomain 1 (BD1), human immunodeficiency virus-1 (HIV-1) entry) involving nitrogen, methyl, halogen, methoxy, and hydroxyl scans with at least four analogues per set. Among the 66 analogue positions explored, we found that in 18 cases Amber GPU-TI calculations predicted a more than 10-fold change in potency. In all of these cases, the experimentally observed direction of potency changes agreed with the predictions. In 16 cases, more than 10-fold changes in experimental potency were observed. Again, in all of these cases, Amber GPU-TI predicted the direction of the potency changes correctly. In none of these cases would a decision made for or against synthesis based on a 10-fold change in potency have resulted in missing an important analogue. Therefore, in silico RBFE calculations using Amber GPU-TI can meaningfully contribute to the prioritization of positional analogues before synthesis.

Recent Progress in the Selective Fluorinations of Some Functionalized Cycloalkenes

Organofluorine compounds have had an increasing impact in synthetic organic chemistry and pharmaceutical research over the past two decades. Their syntheses and the development of novel synthetic approaches towards versatile fluorinated small molecules have received great interest. Our research team has designed various selective and stereocontrolled methods for the construction of fluorine-containing small molecular entities, involving the transformation of various functionalized cycloalkenes across their ring olefin bond. The synthetic methodologies developed to access various pharmacologically interesting fluorinated derivatives with multiple chiral centers might be valuable protocols for the preparation of other classes of organic compounds as well.

Discovery of potent BET bromodomain 1 stereoselective inhibitors using DNA-encoded chemical library selections

BET bromodomain inhibition is therapeutic in multiple diseases; however, pan-BET inhibitors have induced significant myelosuppression and gastrointestinal toxicity, perhaps due to inhibition of both tandem bromodomains (BD) of all BET family members. However, selective inhibition of just the first BD (BD1) phenocopies pan-BET inhibitor activity in preclinical models of cancer, other diseases, and, for BRDT, in the testes for a contraceptive effect. Here, we leveraged our multibillion-molecule collection of DNA-encoded chemical libraries (DECLs) to identify BET BD1-selective inhibitors of specific chirality with high potency, stability, and good cellular activity. Our findings highlight the robustness and efficiency of the DECL platform to identify specific, potent protein binders that have promise as potential anticancer and anti-inflammatory agents and as male contraceptives.

BRDT, BRD2, BRD3, and BRD4 comprise the bromodomain and extraterminal (BET) subfamily which contain two similar tandem bromodomains (BD1 and BD2). Selective BD1 inhibition phenocopies effects of tandem BET BD inhibition both in cancer models and, as we and others have reported of BRDT, in the testes. To find novel BET BD1 binders, we screened >4.5 billion molecules from our DNA-encoded chemical libraries with BRDT-BD1 or BRDT-BD2 proteins in parallel. A compound series enriched only by BRDT-BD1 was resynthesized off-DNA, uncovering a potent chiral compound, CDD-724, with >2,000-fold selectivity for inhibiting BRDT-BD1 over BRDT-BD2. CDD-724 stereoisomers exhibited remarkable differences in inhibiting BRDT-BD1, with the R-enantiomer (CDD-787) being 50-fold more potent than the S-enantiomer (CDD-786). From structure–activity relationship studies, we produced CDD-956, which maintained picomolar BET BD1 binding potency and high selectivity over BET BD2 proteins and had improved stability in human liver microsomes over CDD-787. BROMOscan profiling confirmed the excellent pan-BET BD1 affinity and selectivity of CDD-787 and CDD-956 on BD1 versus BD2 and all other BD-containing proteins. A cocrystal structure of BRDT-BD1 bound with CDD-956 was determined at 1.82 Å and revealed BRDT-BD1–specific contacts with the αZ and αC helices that explain the high affinity and selectivity for BET BD1 versus BD2. CDD-787 and CDD-956 maintain cellular BD1-selectivity in NanoBRET assays and show potent antileukemic activity in acute myeloid leukemia cell lines. These BET BD1-specific and highly potent compounds are structurally unique and provide insight into the importance of chirality to achieve BET specificity.

Discovery of 4-Hydroxyquinazoline Derivatives as Small Molecular BET/PARP1 Inhibitors That Induce Defective Homologous Recombination and Lead to Synthetic Lethality for Triple-Negative Breast Cancer Therapy

The effective potency and resistance of poly(ADP-ribose) polymerase (PARP) inhibitors limit their application. Here, we exploit a new paradigm that mimics the effects of breast cancer susceptibility genes (BRCA) mutations to trigger the possibility of synthetic lethality, based on the previous discovery of a potential synthetic lethality effect between bromodomain-containing protein 4 (BRD4) and PARP1. Consequently, the present study describes compound BP44 with high selectivity for BRD4 and PARP1. Fortunately, BP44 inhibits the homologous recombination in triple-negative breast cancer (TNBC) and triggers synthetic lethality, thus leading to cell cycle arrest and DNA damage. In conclusion, we optimized the BRD4-PARP1 inhibitor based on previous studies, and we expect it to become a candidate drug for the treatment of TNBC in the future. This strategy aims to expand the use of PARPi in BRCA-competent TNBC, making an innovative approach to address unmet oncology needs.

Combined noncanonical NF-κB agonism and targeted BET bromodomain inhibition reverse HIV latency ex vivo

Latency reversal strategies for HIV cure using inhibitor of apoptosis protein (IAP) antagonists (IAPi) induce unprecedented levels of latent reservoir expression without immunotoxicity during suppressive antiretroviral therapy (ART). However, full targeting of the reservoir may require combinatorial approaches. A Jurkat latency model screen for IAPi combination partners demonstrated synergistic latency reversal with bromodomain (BD) and extraterminal domain protein inhibitors (BETi). Mechanistic investigations using CRISPR-CAS9 and single-cell RNA-Seq informed comprehensive ex vivo evaluations of IAPi plus pan-BET, bD-selective BET, or selective BET isoform targeting in CD4+ T cells from ART-suppressed donors. IAPi+BETi treatment resulted in striking induction of cell-associated HIV gag RNA, but lesser induction of fully elongated and tat-rev RNA compared with T cell activation-positive controls. IAPi+BETi resulted in HIV protein induction in bulk cultures of CD4+ T cells using an ultrasensitive p24 assay, but did not result in enhanced viral outgrowth frequency using a standard quantitative viral outgrowth assay. This study defines HIV transcriptional elongation and splicing as important barriers to latent HIV protein expression following latency reversal, delineates the roles of BET proteins and their BDs in HIV latency, and provides a rationale for exploration of IAPi+BETi in animal models of HIV latency.

Berbamine Suppresses the Growth of Gastric Cancer Cells by Inactivating the BRD4/c-MYC Signaling Pathway

PURPOSE

Berbamine (Ber), a bioactive constituent extracted from a traditional Chinese medicinal herb, has been shown to exhibit broad inhibitory activity on a panel of cancer cell types. However, its effects and the underlying molecular mechanisms on gastric cancer (GC) remain poorly understood.

METHODS

The anti-growth activity of Ber on two GC cell lines and normal gastric epithelial cell line were evaluated using MTS and clone formation assay. Flow cytometry analysis was employed to evaluate the cell cycle distribution and apoptosis of GC cells. Western blot and quantitative PCR (qPCR) analysis were employed to investigate the anti-GC mechanism of Ber. The inhibitory activity and binding affinity of Ber against BRD4 were evaluated by homogeneous time-resolved fluorescence (HTRF) and surface plasmon resonance (SPR) assay, respectively. Molecular docking and molecular simulations were conducted to predict the interaction mode between BRD4 and Ber.

RESULTS

The results demonstrated that Ber reduced the proliferation of GC cell lines SGC-7901 and BGC-823 and induced cell cycle arrest and apoptosis. Mechanistically, Ber was identified as a novel natural-derived BRD4 inhibitor through multiple experimental assay, and its anti-GC activity was probably mediated by BRD4 inhibition. Molecular modeling studies suggested that Ber might bind to BRD4 primarily through hydrophobic interactions.

CONCLUSION

Our study uncovered the underlying anti-GC activity of Ber in vitro and suggested that Ber holds promise as a potential lead compound in the discovery of novel BRD4 inhibitors.

Structure-Based Discovery and Optimization of Furo[3,2-c]pyridin-4(5H)-one Derivatives as Potent and Second Bromodomain (BD2)-Selective Bromo and Extra Terminal Domain (BET) Inhibitors

Pan-bromodomain and extra terminal (Pan-BET) inhibitors show profound efficacy but exhibit pharmacology-driven toxicities in clinical trials. The development of domain-selective BET inhibitors to separate efficacy and toxicity is urgently needed. Herein, we report a series of furo[3,2-c]pyridin-4(5H)-one derivatives as novel BD2-selective BET inhibitors. The representative compound 8l (XY153) potently bound to BRD4 BD2 with an half-maximum inhibitory concentration (IC₅₀) value of 0.79 nM and displayed 354-fold selectivity over BRD4 BD1. Besides, 8l exhibited 6-fold BRD4 BD2 domain selectivity over other BET BD2 domains. Compound 8l displayed potent antiproliferative activity against multiple tumor cell lines, especially MV4-11 (IC₅₀ = 0.55 nM), while showing weak cytotoxicity against the normal lung fibroblast cell line. It highlights the safety profile of this series of BD2 inhibitors. 8l also demonstrated good metabolic stability in vitro. These data indicate that 8l may serve as a new and valuable lead compound for the development of potential therapeutics against acute myeloid leukemia (AML).

Targeting Bromodomain-Selective Inhibitors of BET Proteins in Drug Discovery and Development

Blocking the interactions between bromodomain and extraterminal (BET) proteins and acetylated lysines of histones by small molecules has important implications for the treatment of cancers and other diseases. Many pan-BET inhibitors have shown satisfactory results in clinical trials, but their potential for poor tolerability and toxicity persist. However, recently reported studies illustrate that some BET bromodomain (BET-BD1 or BET-BD2)-selective inhibitors have advantage over pan-inhibitors, including reduced toxicity concerns. Furthermore, some selective BET inhibitors have similar or even better therapeutic efficacy in inflammatory diseases or cancers. Therefore, the development of selective BET inhibitors has become a hot spot for medicinal chemists. Here, we summarize the known selective BET-BD1 and BET-BD2 inhibitors and review the methods for enhancing the selectivity and potency of these inhibitors based on their different modes of interactions with BET-BD1 or BET-BD2. Finally, we discuss prospective strategies that selectively target the bromodomains of BET proteins.

Recent Developments in Targeting Bromodomain and Extra Terminal Domain Proteins for Cancer Therapeutics

Abstract:

Bromodomain and extra-terminal domain (BET) proteins are a well-studied family of proteins associated with a variety of diseases including malignancy and chronic inflammation. Currently, numerous pan BET inhibitors have exhibited potent efficacy in several in vivo preclinical models and entered clinical trials, but have largely stalled due to their adverse events. Therefore, the development of new selective inhibitors and PROTACs (Proteolysis Targeting Chimeras) targeting BET is urgently needed. In the present review, we summarize the BET protein structure, the recent development of BET inhibitors, focusing mainly on BRD4-selective inhibitors and PROTAC degraders.

Design, Synthesis, and Characterization of I-BET567, a Pan-Bromodomain and Extra Terminal (BET) Bromodomain Oral Candidate

Through regulation of the epigenome, the bromodomain and extra terminal (BET) family of proteins represent important therapeutic targets for the treatment of human disease. Through mimicking the endogenous N-acetyl-lysine group and disrupting the protein-protein interaction between histone tails and the bromodomain, several small molecule pan-BET inhibitors have progressed to oncology clinical trials. This work describes the medicinal chemistry strategy and execution to deliver an orally bioavailable tetrahydroquinoline (THQ) pan-BET candidate. Critical to the success of this endeavor was a potency agnostic analysis of a data set of 1999 THQ BET inhibitors within the GSK collection which enabled identification of appropriate lipophilicity space to deliver compounds with a higher probability of desired oral candidate quality properties. SAR knowledge was leveraged via Free-Wilson analysis within this design space to identify a small group of targets which ultimately delivered I-BET567 (27), a pan-BET candidate inhibitor that demonstrated efficacy in mouse models of oncology and inflammation.

N-terminal BET bromodomain inhibitors disrupt a BRD4-p65 interaction and reduce inducible nitric oxide synthase transcription in pancreatic β-cells

Chronic inflammation of pancreatic islets is a key driver of β-cell damage that can lead to autoreactivity and the eventual onset of autoimmune diabetes (T1D). In the islet, elevated levels of proinflammatory cytokines induce the transcription of the inducible nitric oxide synthase (iNOS) gene, NOS2, ultimately resulting in increased nitric oxide (NO). Excessive or prolonged exposure to NO causes β-cell dysfunction and failure associated with defects in mitochondrial respiration. Recent studies showed that inhibition of the bromodomain and extraterminal domain (BET) family of proteins, a druggable class of epigenetic reader proteins, prevents the onset and progression of T1D in the non-obese diabetic mouse model. We hypothesized that BET proteins co-activate transcription of cytokine-induced inflammatory gene targets in β-cells and that selective, chemotherapeutic inhibition of BET bromodomains could reduce such transcription. Here, we investigated the ability of BET bromodomain small molecule inhibitors to reduce the β-cell response to the proinflammatory cytokine interleukin 1 beta (IL-1β). BET bromodomain inhibition attenuated IL-1β-induced transcription of the inflammatory mediator NOS2 and consequent iNOS protein and NO production. Reduced NOS2 transcription is consistent with inhibition of NF-κB facilitated by disrupting the interaction of a single BET family member, BRD4, with the NF-κB subunit, p65. Using recently reported selective inhibitors of the first and second BET bromodomains, inhibition of only the first bromodomain was necessary to reduce the interaction of BRD4 with p65 in β-cells. Moreover, inhibition of the first bromodomain was sufficient to mitigate IL-1β-driven decreases in mitochondrial oxygen consumption rates and β-cell viability. By identifying a role for the interaction between BRD4 and p65 in controlling the response of β-cells to proinflammatory cytokines, we provide mechanistic information on how BET bromodomain inhibition can decrease inflammation. These studies also support the potential therapeutic application of more selective BET bromodomain inhibitors in attenuating β-cell inflammation.

The binding mechanism of NHWD-870 to bromodomain-containing protein 4 based on molecular dynamics simulations and free energy calculation

Bromodomain and extra-terminal (BET) proteins (BRD2, BRD3, BRD4, and BRDT) are epigenetic readers with tandem bromodomains.

Progress in the development of domain selective inhibitors of the bromo and extra terminal domain family (BET) proteins

Dysfunction of the bromo and extra terminal domain (BET) family proteins is associated with many human diseases, therefore the BET family proteins have been considered as promising targets for drug development. Numerous small molecular compounds targeting the N-terminal two tandem bromodomains BD1 and BD2 of the BET family proteins have been reported, and a number of them have been advanced into clinical trials. Most of the BET inhibitors entered clinical trials are pan-BET inhibitors which show poor selectivity among BET members and bind to the BD1 and BD2 of the BET family proteins with comparable binding affinities. In order to elucidate the distinct functions of BD1s and BD2s, many BD1 and BD2 selective BET inhibitors have also been developed. In this review, we summarized the recent progress in the development of BD1 and BD2 selective BET inhibitors, and provided the perspectives for future studies of BET inhibitors.

Design, Synthesis, and Evaluation of Thienodiazepine Derivatives as Positron Emission Tomography Imaging Probes for Bromodomain and Extra-Terminal Domain Family Proteins

To better understand the role of bromodomain and extra-terminal domain (BET) proteins in epigenetic mechanisms, we developed a series of thienodiazepine-based derivatives and identified two compounds, 3a and 6a, as potent BET inhibitors. Further in vivo pharmacokinetic studies and analysis of in vitro metabolic stability of 6a revealed excellent brain penetration and reasonable metabolic stability. Compounds 3a and 6a were radiolabeled with fluorine-18 in two steps and utilized in positron emission tomography (PET) imaging studies in mice. Preliminary PET imaging results demonstrated that [¹⁸F]3a and [¹⁸F]6a have good brain uptake (with maximum SUV = 1.7 and 2, respectively) and binding specificity in mice brains. These results show that [¹⁸F]6a is a potential PET radiotracer that could be applied to imaging BET proteins in the brain. Further optimization and improvement of the metabolic stability of [¹⁸F]6a are still needed in order to create optimal PET imaging probes of BET family members.

Selective Transformation of Norbornadiene into Functionalized Azaheterocycles and β-Amino Esters with Stereo- and Regiocontrol

Novel functionalized azaheterocycles with multiple chiral centers have been accessed from readily available norbornene β-amino acids or β-lactams across a stereocontrolled synthetic route, based on ring-opening metathesis (ROM) of the staring unsaturated bicyclic amino esters, followed by selective cyclization through ring-closing metathesis (RCM). The RCM transformations have been studied under various experimental conditions to assess the scope of conversion, catalyst, yield, and substrate influence. The structure of the starting norbornene β-amino acids predetermined the structure of the new azaheterocycles, and the developed synthetic route took place with the conservation of the configuration of the chiral centers.

Selectively targeting individual bromodomain: Drug discovery and molecular mechanisms

Bromodomain-containing proteins include bromodomain and extra-terminal (BET) and non-BET families. Due to the conserved bromodomain (BD) module between BD-containing proteins, and especially BETs with each member having two BDs (BD1 and BD2), the high degree of structural similarity makes BD-selective inhibitors much difficult to be designed. However, increasing evidences emphasized that individual BDs had distinct functions and different cellular phenotypes after pharmacological inhibition, and selectively targeting one of the BDs could result in a different efficacy and tolerability profile. This review is to summarize the pioneering progress of BD-selective inhibitors targeting BET and non-BET proteins, focusing on their structural features, biological activity, therapeutic application and experimental/theoretical mechanisms. The present proteolysis targeting chimeras (PROTAC) degraders targeting BDs, and clinical status of BD-selective inhibitors were also analyzed, providing a new insight into future direction of bromodomain-selective drug discovery.

Post-translational lysine ac(et)ylation in health, ageing and disease

The acetylation/acylation (ac(et)ylation) of lysine side chains is a dynamic post-translational modification (PTM) regulating fundamental cellular processes with implications on the organisms' ageing process: metabolism, transcription, translation, cell proliferation, regulation of the cytoskeleton and DNA damage repair. First identified to occur on histones, later studies revealed the presence of lysine ac(et)ylation in organisms of all kingdoms of life, in proteins covering all essential cellular processes. A remarkable finding showed that the NAD⁺-dependent sirtuin deacetylase Sir2 has an impact on replicative lifespan in Saccharomyces cerevisiae suggesting that lysine acetylation has a direct role in the ageing process. Later studies identified sirtuins as mediators for beneficial effects of caloric/dietary restriction on the organisms' health- or lifespan. However, the molecular mechanisms underlying these effects are only incompletely understood. Progress in mass-spectrometry, structural biology, synthetic and semi-synthetic biology deepened our understanding of this PTM. This review summarizes recent developments in the research field. It shows how lysine ac(et)ylation regulates protein function, how it is regulated enzymatically and non-enzymatically, how a dysfunction in this post-translational machinery contributes to disease development. A focus is set on sirtuins and lysine acyltransferases as these are direct sensors and mediators of the cellular metabolic state. Finally, this review highlights technological advances to study lysine ac(et)ylation.

Optimization of Naphthyridones into Selective TATA-Binding Protein Associated Factor 1 (TAF1) Bromodomain Inhibitors

Bromodomain containing proteins and the acetyl-lysine binding bromodomains contained therein are increasingly attractive targets for the development of novel epigenetic therapeutics. To help validate this target class and unravel the complex associated biology, there has been a concerted effort to develop selective small molecule bromodomain inhibitors. Herein we describe the structure-based efforts and multiple challenges encountered in optimizing a naphthyridone template into selective TAF1(2) bromodomain inhibitors which, while unsuitable as chemical probes themselves, show promise for the future development of small molecules to interrogate TAF1(2) biology. Key to this work was the introduction and modulation of the basicity of a pendant amine which had a substantial impact on not only bromodomain selectivity but also cellular target engagement.

Discovery of a Highly Selective BET BD2 Inhibitor from a DNA-Encoded Library Technology Screening Hit

Second-generation bromodomain and extra terminal (BET) inhibitors, which selectively target one of the two bromodomains in the BET proteins, have begun to emerge in the literature. These inhibitors aim to help determine the roles and functions of each domain and assess whether they can demonstrate an improved safety profile in clinical settings compared to pan-BET inhibitors. Herein, we describe the discovery of a novel BET BD2-selective chemotype using a structure-based drug design from a hit identified by DNA-encoded library technologies, showing a structural differentiation from key previously reported greater than 100-fold BD2-selective chemotypes GSK620, GSK046, and ABBV-744. Following a structure-based hypothesis for the selectivity and optimization of the physicochemical properties of the series, we identified 60 (GSK040), an in vitro ready and in vivo capable BET BD2-inhibitor of unprecedented selectivity (5000-fold) against BET BD1, excellent selectivity against other bromodomains, and good physicochemical properties. This novel chemical probe can be added to the toolbox used in the advancement of epigenetics research.

Identification of a Series of N-Methylpyridine-2-carboxamides as Potent and Selective Inhibitors of the Second Bromodomain (BD2) of the Bromo and Extra Terminal Domain (BET) Proteins

Domain-specific BET bromodomain ligands represent an attractive target for drug discovery with the potential to unlock the therapeutic benefits of antagonizing these proteins without eliciting the toxicological aspects seen with pan-BET inhibitors. While we have reported several distinct classes of BD2 selective compounds, namely, GSK620, GSK549, and GSK046, only GSK046 shows high aqueous solubility. Herein, we describe the lead optimization of a further class of highly soluble compounds based upon a picolinamide chemotype. Focusing on achieving >1000-fold selectivity for BD2 over BD1 ,while retaining favorable physical chemical properties, compound 36 was identified as being 2000-fold selective for BD2 over BD1 (Brd4 data) with >1 mg/mL solubility in FaSSIF media. 36 represents a valuable new in vivo ready molecule for the exploration of the BD2 phenotype.

Structural variation of protein-ligand complexes of the first bromodomain of BRD4

The bromodomain-containing protein 4 (BRD4), a member of the bromodomain and extra-terminal domain (BET) family, plays a key role in several diseases, especially cancers. With increased interest in BRD4 as a therapeutic target, many X-ray crystal structures of the protein in complex with small molecule inhibitors are publicly available over the recent decade. In this study, we use this structural information to investigate the conformations of the first bromodomain (BD1) of BRD4. Structural alignment of 297 BRD4-BD1 complexes shows a high level of similarity between the structures of BRD4-BD1, regardless of the bound ligand. We employ WONKA, a tool for detailed analyses of protein binding sites, to compare the active site of over 100 of these crystal structures. The positions of key binding site residues show a high level of conformational similarity, with the exception of Trp81. A focused analysis on the highly conserved water network in the binding site of BRD4-BD1 is performed to identify the positions of these water molecules across the crystal structures. The importance of the water network is illustrated using molecular docking and absolute free energy perturbation simulations. 82% of the ligand poses were better predicted when including water molecules as part of the receptor. Our analysis provides guidance for the design of new BRD4-BD1 inhibitors and the selection of the best structure of BRD4-BD1 to use in structure-based drug design, an important approach for faster and more cost-efficient lead discovery.

Synthesis and Characterization of a Positron Emission Tomography Imaging Probe Selectively Targeting the Second Bromodomain of Bromodomain Protein BRD4

Two tandem bromodomains (BD1 and BD2) of bromodomain and extraterminal domain (BET) family proteins have shown distinct roles in mediating gene transcription and expression. Inhibitors that interact with a specific bromodomain may contribute to a specific therapeutic potential with fewer side effects. However, little is known about this disease-related target. Positron emission tomography (PET) imaging could allow us to achieve in-depth knowledge of the BD2 bromodomain. Herein we describe the radiosynthesis and evaluation of [¹¹C]1 as a BRD4 BD2 bromodomain PET imaging radioligand. Our preliminary PET imaging results in rodents demonstrated that [¹¹C]1 had suitable biodistribution in peripheral organs and tissues. Further blocking studies indicated that [¹¹C]1 had good binding specificity toward the BD2 bromodomain. This study may pave the way for the development of a PET radioligand specifically targeting BD1/2 bromodomains as well as for the biological mechanism investigation of BD1/2 bromodomains.