New inhibitors for the BPTF bromodomain enabled by structural biology and biophysical assay development

Reevaluation of bromodomain ligands targeting BAZ2A

BAZ2A promotes migration and invasion in prostate cancer. Two chemical probes, the specific BAZ2-ICR, and the BAZ2/BRD9 cross-reactive GSK2801, interfere with the recognition of acetylated lysines in histones by the bromodomains of BAZ2A and of its BAZ2B paralog. The two chemical probes were tested in prostate cancer cell lines with opposite androgen susceptibility. BAZ2-ICR and GSK2801 showed different cellular efficacies in accordance with their unequal selectivity profiles. Concurrent inhibition of BAZ2 and BRD9 did not reproduce the effects observed with GSK2801, indicating possible off-targets for this chemical probe. On the other hand, the single BAZ2 inhibition by BAZ2-ICR did not phenocopy genetic ablation, demonstrating that bromodomain interference is not sufficient to strongly affect BAZ2A functionality and suggesting a PROTAC-based chemical ablation as an alternative optimization strategy and a possible therapeutic approach. In this context, we also present the crystallographic structures of BAZ2A in complex with the above chemical probes. Binding poses of TP-238 and GSK4027, chemical probes for the bromodomain subfamily I, and two ligands of the CBP/EP300 bromodomains identify additional headgroups for the development of BAZ2A ligands.

Design of Class I/IV Bromodomain-Targeting Degraders for Chromatin Remodeling Complexes

Targeted protein degradation is an emerging technology that can be used for modulating the activity of epigenetic protein targets. Among bromodomain-containing proteins, a number of degraders for the BET family have been developed, while non-BET bromodomains remain underexplored. Several of these proteins are subunits in chromatin remodeling complexes often associated with oncogenic roles. Here, we describe the design of class I (BPTF and CECR2) and IV (BRD9) bromodomain-targeting degraders based on two scaffolds derived from pyridazinone and pyrimidine-based heterocycles. We evaluate various exit vectors and linkers to identify analogues that demonstrate selectivity within these families. We further use an in-cell NanoBRET assay to demonstrate that these heterobifunctional molecules are cell-permeable, form ternary complexes, and can degrade nanoluciferase-bromodomain fusions. As a first example of a CECR2 degrader, we observe that our pyrimidine-based analogues degrade endogenous CECR2 while showing a smaller effect on BPTF levels. The pyridazinone-based compounds did not degrade BPTF when observed through Western blotting, further supporting a more challenging target for degradation and a goal for future optimization.

Discovery of new small molecule inhibitors of the BPTF bromodomain

Chromatin remodeling regulates many basic cellular processes, such as gene transcription, DNA repair, and programmed cell death. As the largest member of nucleosome remodeling factor (NURF), BPTF plays a vital role in the occurrence and development of cancer. Currently, BPTF bromodomain inhibitors are still in development. In this study, by conducting homogenous time-resolved fluorescence resonance energy transfer (HTRF) assay, we identified a potential, novel BPTF inhibitor scaffold Sanguinarine chloride with the IC₅₀ value of 344.2 ± 25.1 nM. Biochemical analysis revealed that compound Sanguinarine chloride exhibited high binding affinity to the BPTF bromodomain. Molecular docking predicted the binding mode of Sanguinarine chloride and elucidated the activities of its derivatives. Moreover, Sanguinarine chloride showed a potent anti-proliferative effect in MIAPaCa-2 cells and inhibited the expression of BPTF target gene c-Myc. Taken together, Sanguinarine chloride provides a qualified chemical tool for developing potent BPTF bromodomain inhibitors.

Alternative Mechanisms for DNA Engagement by BET Bromodomain-Containing Proteins

Epigenetic reader domains regulate chromatin structure and modulate gene expression through the recognition of post-translational modifications on histones. Recently, reader domains have also been found to harbor double-stranded (ds) DNA-binding activity, which is as functionally critical as histone association. Here, we explore the dsDNA recognition of the N-terminal bromodomain of the bromodomain and extra-terminal (BET) protein, BRD4. Using protein-observed ¹⁹F NMR, ¹H-¹⁵N HSQC NMR, electrophoretic mobility shift assays (EMSA), and competitive-inhibition assays, we establish the binding surface of dsDNA and find it to be largely overlapping with the acetylated histone (KAc)-binding site. Rather than engaging in electrostatic contacts, we find dsDNA to interact competitively within the KAc-binding pocket. These interactions are distinct from the highly homologous BET bromodomain, BRDT. Nine additional bromodomains have also been characterized for interacting with dsDNA, including tandem BET bromodomains. Together, these studies help establish a binding model for dsDNA interactions with BRD4 bromodomains and elucidate the chromatin-recognition mechanisms of the BRD4 protein for regulating gene expression.

New Design Rules for Developing Potent Cell-Active Inhibitors of the Nucleosome Remodeling Factor (NURF) via BPTF Bromodomain Inhibition

The nucleosome remodeling factor (NURF) alters chromatin accessibility through interactions with its largest subunit,the bromodomain PHD finger transcription factor BPTF. BPTF is overexpressed in several cancers and is an emerging anticancer target. Targeting the BPTF bromodomain presents a potential strategy for its inhibition and the evaluation of its functional significance; however, inhibitor development for BPTF has lagged behind those of other bromodomains. Here we describe the development of pyridazinone-based BPTF inhibitors. The lead compound, BZ1, possesses a high potency (K_d = 6.3 nM) and >350-fold selectivity over BET bromodomains. We identify an acidic triad in the binding pocket to guide future designs. We show that our inhibitors sensitize 4T1 breast cancer cells to doxorubicin but not BPTF knockdown cells, suggesting a specificity to BPTF. Given the high potency and good physicochemical properties of these inhibitors, we anticipate that they will be useful starting points for chemical tool development to explore the biological roles of BPTF.

Discovery of High-Affinity Inhibitors of the BPTF Bromodomain

The dysfunctional bromodomain PHD finger transcription factor (BPTF) exerts a pivotal influence in the occurrence and development of many human diseases, particularly cancers. Herein, through the structural decomposition of the reported BPTF inhibitor TP-238, the effective structural fragments were synthetically modified to obtain our lead compound DC-BPi-03. DC-BPi-03 was identified as a novel BPTF-BRD inhibitor with a moderate potency (IC₅₀ = 698.3 ± 21.0 nM). A structure-guided structure-activity relationship exploration gave rise to two BPTF inhibitors with much higher affinities, DC-BPi-07 and DC-BPi-11. Notably, DC-BPi-07 and DC-BPi-11 show selectivities 100-fold higher than those of other BRD targets. The cocrystal structures of BPTF in complex with DC-BPi-07 and DC-BPi-11 demonstrate the rationale of chemical efforts from the atomic level. Further study showed that DC-BPi-11 significantly inhibited leukemia cell proliferation.

Opportunity knocks for uncovering the new function of an understudied nucleosome remodeling complex member, the bromodomain PHD finger transcription factor, BPTF

Nucleosome remodeling provides access to genomic DNA for recruitment of the transcriptional machinery to mediate gene expression. The aberrant function of nucleosome remodeling complexes has been correlated to human cancer, making them emerging therapeutic targets. The bromodomain PHD finger transcription factor, BPTF, is the largest member of the human nucleosome remodeling factor NURF. Over the last five years, BPTF has become increasingly identified as a protumorigenic factor, prompting investigations into the molecular mechanisms associated with BPTF function. Despite a druggable bromodomain, small molecule discovery is at an early stage. Here we highlight recent investigations into the biology being discovered for BPTF, chemical biology approaches used to study its function, and small molecule inhibitors being designed as future chemical probes and therapeutics.

Functional Roles of Bromodomain Proteins in Cancer

Histone acetylation is generally associated with an open chromatin configuration that facilitates many cellular processes including gene transcription, DNA repair, and DNA replication. Aberrant levels of histone lysine acetylation are associated with the development of cancer. Bromodomains represent a family of structurally well-characterized effector domains that recognize acetylated lysines in chromatin. As part of their fundamental reader activity, bromodomain-containing proteins play versatile roles in epigenetic regulation, and additional functional modules are often present in the same protein, or through the assembly of larger enzymatic complexes. Dysregulated gene expression, chromosomal translocations, and/or mutations in bromodomain-containing proteins have been correlated with poor patient outcomes in cancer. Thus, bromodomains have emerged as a highly tractable class of epigenetic targets due to their well-defined structural domains, and the increasing ease of designing or screening for molecules that modulate the reading process. Recent developments in pharmacological agents that target specific bromodomains has helped to understand the diverse mechanisms that bromodomains play with their interaction partners in a variety of chromatin processes, and provide the promise of applying bromodomain inhibitors into the clinical field of cancer treatment. In this review, we explore the expression and protein interactome profiles of bromodomain-containing proteins and discuss them in terms of functional groups. Furthermore, we highlight our current understanding of the roles of bromodomain-containing proteins in cancer, as well as emerging strategies to specifically target bromodomains, including combination therapies using bromodomain inhibitors alongside traditional therapeutic approaches designed to re-program tumorigenesis and metastasis.

Discovery of selective BPTF bromodomain inhibitors by screening and structure-based optimization

Bromodomain and PHD finger containing transcription factor (BPTF) is a multidomain protein that regulates the transcription of chromatin and is related to many cancers. Herein, we report the screening-based discovery of Cpd1, a compound with micromolar affinity to the BPTF bromodomain. Through structure-guided optimization, we synthesized a variety of new inhibitors. Among these compounds, Cpd8 and Cpd10 were highly potent and selective inhibitors, with K_D values of 428 nM and 655 nM in ITC assays, respectively. The high activity was explained by the cocrystal structure of Cpd8 in complex with the BPTF bromodomain protein. Cpd8 and Cpd10 were able to stabilize the BPTF bromodomain protein in cells in a cellular thermal shift assay (CETSA). Cpd8 downregulated c-MYC expression in A549 cells. All experiments prove that these two compounds are potential BPTF inhibitors.

Selective N-Terminal BET Bromodomain Inhibitors by Targeting Non-Conserved Residues and Structured Water Displacement*

Bromodomain and extra-terminal (BET) family proteins, BRD2-4 and T, are important drug targets; however, the biological functions of each bromodomain remain ill-defined. Chemical probes that selectively inhibit a single BET bromodomain are lacking, although pan inhibitors of the first (D1), and second (D2), bromodomain are known. Here, we develop selective BET D1 inhibitors with preferred binding to BRD4 D1. In competitive inhibition assays, we show that our lead compound is 9-33 fold selective for BRD4 D1 over the other BET bromodomains. X-ray crystallography supports a role for the selectivity based on reorganization of a non-conserved lysine and displacement of an additional structured water in the BRD4 D1 binding site relative to our prior lead. Whereas pan-D1 inhibitors displace BRD4 from MYC enhancers, BRD4 D1 inhibition in MM.1S cells is insufficient for stopping Myc expression and may lead to its upregulation. Future analysis of BRD4 D1 gene regulation may shed light on differential BET bromodomain functions.

Combined Protein- and Ligand-Observed NMR Workflow to Screen Fragment Cocktails against Multiple Proteins: A Case Study Using Bromodomains

As fragment-based drug discovery has become mainstream, there has been an increase in various screening methodologies. Protein-observed ¹⁹F (PrOF) NMR and ¹H CPMG NMR are two fragment screening assays that have complementary advantages. Here, we sought to combine these two NMR-based assays into a new screening workflow. This combination of protein- and ligand-observed experiments allows for a time- and resource-efficient multiplexed screen of mixtures of fragments and proteins. PrOF NMR is first used to screen mixtures against two proteins. Hit mixtures for each protein are identified then deconvoluted using ¹H CPMG NMR. We demonstrate the benefit of this fragment screening method by conducting the first reported fragment screens against the bromodomains of BPTF and Plasmodium falciparum (Pf) GCN5 using 467 3D-enriched fragments. The hit rates were 6%, 5% and 4% for fragments binding BPTF, PfGCN5, and fragments binding both proteins, respectively. Select hits were characterized, revealing a broad range of affinities from low µM to mM dissociation constants. Follow-up experiments supported a low-affinity second binding site on PfGCN5. This approach can be used to bias fragment screens towards more selective hits at the onset of inhibitor development in a resource- and time-efficient manner.