J. Martin Herold

Discovery of a 2,4-diamino-7-aminoalkoxyquinazoline as a potent and selective inhibitor of histone lysine methyltransferase G9a.

SAR exploration of the 2,4-diamino-6,7-dimethoxyquinazoline template led to the discovery of 8 (UNC0224) as a potent and selective G9a inhibitor. A high resolution X-ray crystal structure of the G9a-8 complex, the first cocrystal structure of G9a with a small molecule inhibitor, was obtained. The cocrystal structure validated our binding hypothesis and will enable structure-based design of novel inhibitors. 8 is a useful tool for investigating the biology of G9a and its roles in chromatin remodeling.

Protein Lysine Methyltransferase G9a Inhibitors: Design, Synthesis, and Structure Activity Relationships of 2,4-Diamino-7-aminoalkoxy-quinazolines.

Protein lysine methyltransferase G9a, which catalyzes methylation of lysine 9 of histone H3 (H3K9) and lysine 373 (K373) of p53, is overexpressed in human cancers. Genetic knockdown of G9a inhibits cancer cell growth, and the dimethylation of p53 K373 results in the inactivation of p53. Initial SAR exploration of the 2,4-diamino-6,7-dimethoxyquinazoline template represented by 3a (BIX01294), a selective small molecule inhibitor of G9a and GLP, led to the discovery of 10 (UNC0224) as a potent G9a inhibitor with excellent selectivity. A high resolution X-ray crystal structure of the G9a-10 complex, the first cocrystal structure of G9a with a small molecule inhibitor, was obtained. On the basis of the structural insights revealed by this cocrystal structure, optimization of the 7-dimethylaminopropoxy side chain of 10 resulted in the discovery of 29 (UNC0321) (Morrison K(i) = 63 pM), which is the first G9a inhibitor with picomolar potency and the most potent G9a inhibitor to date.

Discovery of a chemical probe for the L3MBTL3 methyllysine reader domain

We describe the discovery of UNC1215, a potent and selective chemical probe for the methyl-lysine (Kme) reading function of L3MBTL3, a member of the malignant brain tumor (MBT) family of chromatin interacting transcriptional repressors. UNC1215 binds L3MBTL3 with a Kd of 120 nM, competitively displacing mono- or dimethyl-lysine containing peptides, and is greater than 50-fold selective versus other members of the MBT family while also demonstrating selectivity against more than 200 other reader domains examined. X-ray crystallography identified a novel 2:2 polyvalent mode of interaction. In cells, UNC1215 is non-toxic and binds directly to L3MBTL3 via the Kme-binding pocket of the MBT domains. UNC1215 increases the cellular mobility of GFP-L3MBTL3 fusion proteins and point mutants that disrupt the Kme binding function of GFP-L3MBTL3 phenocopy the effects of UNC1215. Finally, UNC1215 demonstrates a novel Kme-dependent interaction of L3MBTL3 with BCLAF1, a protein implicated in DNA damage repair and apoptosis.

Optimization of Cellular Activity of G9a Inhibitors 7-Aminoalkoxy-quinazolines

Protein lysine methyltransferase G9a plays key roles in the transcriptional repression of a variety of genes via dimethylation of lysine 9 on histone H3 (H3K9me2) of chromatin as well as dimethylation of nonhistone proteins including tumor suppressor p53. We previously reported the discovery of UNC0321 (3), the most potent G9a inhibitor to date, via structure-based design and structure-activity relationship (SAR) exploration of the quinazoline scaffold represented by BIX01294 (1). Despite its very high in vitro potency, compound 3 lacks sufficient cellular potency. The design and synthesis of several generations of new analogues aimed at improving cell membrane permeability while maintaining high in vitro potency resulted in the discovery of a number of novel G9a inhibitors such as UNC0646 (6) and UNC0631 (7) with excellent potency in a variety of cell lines and excellent separation of functional potency versus cell toxicity. The design, synthesis, and cellular SAR of these potent G9a inhibitors are described.

Screening for Inhibitors of Low-Affinity Epigenetic Peptide-Protein Interactions An AlphaScreen™-Based Assay for Antagonists of Methyl-Lysine Binding Proteins

The histone code comprises many posttranslational modifications that occur mainly in histone tail peptides. The identity and location of these marks are read by a variety of histone-binding proteins that are emerging as important regulators of cellular differentiation and development and are increasingly being implicated in numerous disease states. The authors describe the development of the first high-throughput screening assay for the discovery of inhibitors of methyl-lysine binding proteins that will be used to initiate a full-scale discovery effort for this broad target class. They focus on the development of an AlphaScreen™-based assay for malignant brain tumor (MBT) domain-containing proteins, which bind to the lower methylation states of lysine residues present in histone tail peptides. This assay takes advantage of the avidity of the AlphaScreen™ beads to clear the hurdle to assay development presented by the low micromolar binding constants of the histone binding proteins for their cognate peptides. The assay is applicable to other families of methyl-lysine binding proteins, and it has the potential to be used in screening efforts toward the discovery of novel small molecules with utility as research tools for cellular reprogramming and ultimately drug discovery.

Small-Molecule Ligands of Methyl-Lysine Binding Proteins: Optimization of Selectivity for L3MBTL3

Lysine methylation is a key epigenetic mark, the dysregulation of which is linked to many diseases. Small-molecule antagonism of methyl-lysine (Kme) binding proteins that recognize such epigenetic marks can improve our understanding of these regulatory mechanisms and potentially validate Kme binding proteins as drug-discovery targets. We previously reported the discovery of 1 (UNC1215), the first potent and selective small-molecule chemical probe of a methyl-lysine reader protein, L3MBTL3, which antagonizes the mono- and dimethyl-lysine reading function of L3MBTL3. The design, synthesis, and structure-activity relationship studies that led to the discovery of 1 are described herein. These efforts established the requirements for potent L3MBTL3 binding and enabled the design of novel antagonists, such as compound 2 (UNC1679), that maintain in vitro and cellular potency with improved selectivity against other MBT-containing proteins. The antagonists described were also found to effectively interact with unlabeled endogenous L3MBTL3 in cells.

Discovery of a Selective, Substrate-Competitive Inhibitor of the Lysine Methyltransferase SETD8

The lysine methyltransferase SETD8 is the only known methyltransferase that catalyzes monomethylation of histone H4 lysine 20 (H4K20). Monomethylation of H4K20 has been implicated in regulating diverse biological processes including the DNA damage response. In addition to H4K20, SETD8 monomethylates non-histone substrates including proliferating cell nuclear antigen (PCNA) and promotes carcinogenesis by deregulating PCNA expression. However, selective inhibitors of SETD8 are scarce. The only known selective inhibitor of SETD8 to date is nahuoic acid A, a marine natural product, which is competitive with the cofactor. Here, we report the discovery of the first substrate-competitive inhibitor of SETD8, UNC0379 (1). This small-molecule inhibitor is active in multiple biochemical assays. Its affinity to SETD8 was confirmed by ITC (isothermal titration calorimetry) and SPR (surface plasmon resonance) studies. Importantly, compound 1 is selective for SETD8 over 15 other methyltransferases. We also describe structure–activity relationships (SAR) of this series.

Structure–activity relationships of methyl-lysine reader antagonists

The interaction between methyl-lysine binding proteins and methylated histones plays a crucial role in the regulation of gene expression. Herein we describe the development of structure–activity relationships (SAR) surrounding UNC669, the first reported small molecule ligand for a methyl-lysine binding domain, using multiple assay formats. These studies revealed the key features required for successful inhibition of the L3MBTL1-methylated histone protein-protein interaction, while the selectivity of designed compounds against a panel of related methyl-lysine readers was also evaluated. Additionally, an optimized compound was demonstrated to successfully inhibit the recognition of H4K20me1 by L3MBTL1 in the context of an affinity pull down assay.

Drug Discovery Toward Antagonists of Methyl-Lysine Binding Proteins

The recognition of methyl-lysine and -arginine residues on both histone and other proteins by specific “reader” elements is important for chromatin regulation, gene expression, and control of cell-cycle progression. Recently the crucial role of these reader proteins in cancer development and dedifferentiation has emerged, owing to the increased interest among the scientific community. The methyl-lysine and -arginine readers are a large and very diverse set of effector proteins and targeting them with small molecule probes in drug discovery will inevitably require a detailed understanding of their structural biology and mechanism of binding. In the following review, the critical elements of methyl-lysine and -arginine recognition will be summarized with respect to each protein family and initial results in assay development, probe design, and drug discovery will be highlighted.

Assessment of Free Energy Predictors for Ligand Binding to a Methyllysine Histone Code Reader

Methyllysine histone code readers constitute a new promising group of potential drug targets. For instance, L3MBTL1, a malignant brain tumor (MBT) protein, selectively binds mono‐ and di‐methyllysine epigenetic marks (KMe, KMe2) that eventually results in the negative regulation of multiple genes through the E2F/Rb oncogenic pathway. There is a pressing need in potent and selective small‐molecule probes that would enable further target validation and might become therapeutic leads. Such an endeavor would require efficient tools to assess the free energy of protein–ligand binding. However, due to an unparalleled function of the MBT binding pocket (i.e., selective binding to KMe/KMe2) and because of its distinctive structure representing a small aromatic “cage,” an accurate assessment of its binding affinity to a ligand appears to be a challenging task. Here, we report a comparative analysis of computationally affordable affinity predictors applied to a set of seven small‐molecule ligands interacting with L3MBTL1. The analysis deals with novel ligands and targets, but applies widespread computational approaches and intuitive comparison metrics that makes this study compatible with and incremental to earlier large scale accounts on the efficiency of affinity predictors. Ultimately, this study has revealed three top performers, far ahead of the other techniques, including two scoring functions, PMF04 and PLP, along with a simulation‐based method MM‐PB/SA. We discuss why some methods may perform better than others on this target class, the limits of their application, as well as how the efficiency of the most CPU‐demanding techniques could be optimized.