Structure-Based Design of AG-946, a Pyruvate Kinase Activator.

Pyruvate kinase (PK) is the enzyme that catalyzes the conversion of phosphoenolpyruvate and adenosine diphosphate to pyruvate and adenosine triphosphate in glycolysis and plays a crucial role in regulating cell metabolism. We describe the structure-based design of AG-946, an activator of PK isoforms, including red blood cell-specific forms of PK (PKR). This was designed to have a pseudo-C2-symmetry matching its allosteric binding site on the PK enzyme, which increased its potency toward PKR while reducing activity against off-targets observed from the original scaffold. AG-946 (1) demonstrated activation of human wild-type PK (half-maximal activation concentration [AC50 ]=0.005 µM) and a panel of mutated PK proteins (K410E [AC50 =0.0043 µM] and R510Q [AC50 =0.0069 µM]), (2) displayed a significantly longer half-time of activation (>150-fold) compared with 6-(3-methoxybenzyl)-4-methyl-2-(methylsulfinyl)-4,6-dihydro-5H-thieno[2',3':4,5]pyrrolo[2,3-d]pyridazin-5-one, and (3) stabilized PKR R510Q, an unstable mutant PKR enzyme, and preserved its catalytic activity under increasingly denaturing conditions. As a potent, oral, small-molecule allosteric activator of wild-type and mutant PKR, AG-946 was advanced to human clinical trials.

Leveraging Structure-Based Drug Design to Identify Next-Generation MAT2A Inhibitors, Including Brain-Penetrant and Peripherally Efficacious Leads.

Inhibition of the S-adenosyl methionine (SAM)-producing metabolic enzyme, methionine adenosyltransferase 2A (MAT2A), has received significant interest in the field of medicinal chemistry due to its implication as a synthetic lethal target in cancers with the deletion of the methylthioadenosine phosphorylase (MTAP) gene. Here, we report the identification of novel MAT2A inhibitors with distinct in vivo properties that may enhance their utility in treating patients. Following a high-throughput screening, we successfully applied the structure-based design lessons from our first-in-class MAT2A inhibitor, AG-270, to rapidly redesign and optimize our initial hit into two new lead compounds: a brain-penetrant compound, AGI-41998, and a potent, but limited brain-penetrant compound, AGI-43192. We hope that the identification and first disclosure of brain-penetrant MAT2A inhibitors will create new opportunities to explore the potential therapeutic effects of SAM modulation in the central nervous system (CNS).

Discovery of AG-270, a First-in-Class Oral MAT2A Inhibitor for the Treatment of Tumors with Homozygous MTAP Deletion.

The metabolic enzyme methionine adenosyltransferase 2A (MAT2A) was recently implicated as a synthetic lethal target in cancers with deletion of the methylthioadenosine phosphorylase (MTAP) gene, which is adjacent to the CDKN2A tumor suppressor and codeleted with CDKN2A in approximately 15% of all cancers. Previous attempts to target MAT2A with small-molecule inhibitors identified cellular adaptations that blunted their efficacy. Here, we report the discovery of highly potent, selective, orally bioavailable MAT2A inhibitors that overcome these challenges. Fragment screening followed by iterative structure-guided design enabled >10 000-fold improvement in potency of a family of allosteric MAT2A inhibitors that are substrate noncompetitive and inhibit release of the product, S-adenosyl methionine (SAM), from the enzyme's active site. We demonstrate that potent MAT2A inhibitors substantially reduce SAM levels in cancer cells and selectively block proliferation of MTAP-null cells both in tissue culture and xenograft tumors. These data supported progressing AG-270 into current clinical studies (ClinicalTrials.gov NCT03435250).

Selective Vulnerability to Pyrimidine Starvation in Hematologic Malignancies Revealed by AG-636, a Novel Clinical-Stage Inhibitor of Dihydroorotate Dehydrogenase.

Agents targeting metabolic pathways form the backbone of standard oncology treatments, though a better understanding of differential metabolic dependencies could instruct more rationale-based therapeutic approaches. We performed a chemical biology screen that revealed a strong enrichment in sensitivity to a novel dihydroorotate dehydrogenase (DHODH) inhibitor, AG-636, in cancer cell lines of hematologic versus solid tumor origin. Differential AG-636 activity translated to the in vivo setting, with complete tumor regression observed in a lymphoma model. Dissection of the relationship between uridine availability and response to AG-636 revealed a divergent ability of lymphoma and solid tumor cell lines to survive and grow in the setting of depleted extracellular uridine and DHODH inhibition. Metabolic characterization paired with unbiased functional genomic and proteomic screens pointed to adaptive mechanisms to cope with nucleotide stress as contributing to response to AG-636. These findings support targeting of DHODH in lymphoma and other hematologic malignancies and suggest combination strategies aimed at interfering with DNA-damage response pathways.

High-throughput mutagenesis reveals unique structural features of human ADAR1.

Adenosine Deaminases that act on RNA (ADARs) are enzymes that catalyze adenosine to inosine conversion in dsRNA, a common form of RNA editing. Mutations in the human ADAR1 gene are known to cause disease and recent studies have identified ADAR1 as a potential therapeutic target for a subset of cancers. However, efforts to define the mechanistic effects for disease associated ADAR1 mutations and the rational design of ADAR1 inhibitors are limited by a lack of structural information. Here, we describe the combination of high throughput mutagenesis screening studies, biochemical characterization and Rosetta-based structure modeling to identify unique features of ADAR1. Importantly, these studies reveal a previously unknown zinc-binding site on the surface of the ADAR1 deaminase domain which is important for ADAR1 editing activity. Furthermore, we present structural models that explain known properties of this enzyme and make predictions about the role of specific residues in a surface loop unique to ADAR1.

Vorasidenib (AG-881): A First-in-Class, Brain-Penetrant Dual Inhibitor of Mutant IDH1 and 2 for Treatment of Glioma.

Inhibitors of mutant isocitrate dehydrogenase (mIDH) 1 and 2 cancer-associated enzymes prevent the accumulation of the oncometabolite d-2-hydroxyglutarate (2-HG) and are under clinical investigation for the treatment of several cancers harboring an IDH mutation. Herein, we describe the discovery of vorasidenib (AG-881), a potent, oral, brain-penetrant dual inhibitor of both mIDH1 and mIDH2. X-ray cocrystal structures allowed us to characterize the compound binding site, leading to an understanding of the dual mutant inhibition. Furthermore, vorasidenib penetrates the brain of several preclinical species and inhibits 2-HG production in glioma tissue by >97% in an orthotopic glioma mouse model. Vorasidenib represents a novel dual mIDH1/2 inhibitor and is currently in clinical development for the treatment of low-grade mIDH glioma.

Distinct Hepatic PKA and CDK Signaling Pathways Control Activity-Independent Pyruvate Kinase Phosphorylation and Hepatic Glucose Production.

Pyruvate kinase is an important enzyme in glycolysis and a key metabolic control point. We recently observed a pyruvate kinase liver isoform (PKL) phosphorylation site at S113 that correlates with insulin resistance in rats on a 3 day high-fat diet (HFD) and suggests additional control points for PKL activity. However, in contrast to the classical model of PKL regulation, neither authentically phosphorylated PKL at S12 nor S113 alone is sufficient to alter enzyme kinetics or structure. Instead, we show that cyclin-dependent kinases (CDKs) are activated by the HFD and responsible for PKL phosphorylation at position S113 in addition to other targets. These CDKs control PKL nuclear retention, alter cytosolic PKL activity, and ultimately influence glucose production. These results change our view of PKL regulation and highlight a previously unrecognized pathway of hepatic CDK activity and metabolic control points that may be important in insulin resistance and type 2 diabetes.

Structure and inhibition mechanism of the catalytic domain of human squalene epoxidase.

Squalene epoxidase (SQLE), also known as squalene monooxygenase, catalyzes the stereospecific conversion of squalene to 2,3(S)-oxidosqualene, a key step in cholesterol biosynthesis. SQLE inhibition is targeted for the treatment of hypercholesteremia, cancer, and fungal infections. However, lack of structure-function understanding has hindered further progression of its inhibitors. We have determined the first three-dimensional high-resolution crystal structures of human SQLE catalytic domain with small molecule inhibitors (2.3 Å and 2.5 Å). Comparison with its unliganded state (3.0 Å) reveals conformational rearrangements upon inhibitor binding, thus allowing deeper interpretation of known structure-activity relationships. We use the human SQLE structure to further understand the specificity of terbinafine, an approved agent targeting fungal SQLE, and to provide the structural insights into terbinafine-resistant mutants encountered in the clinic. Collectively, these findings elucidate the structural basis for the specificity of the epoxidation reaction catalyzed by SQLE and enable further rational development of next-generation inhibitors.

A chemical biology screen identifies a vulnerability of neuroendocrine cancer cells to SQLE inhibition.

Aberrant metabolism of cancer cells is well appreciated, but the identification of cancer subsets with specific metabolic vulnerabilities remains challenging. We conducted a chemical biology screen and identified a subset of neuroendocrine tumors displaying a striking pattern of sensitivity to inhibition of the cholesterol biosynthetic pathway enzyme squalene epoxidase (SQLE). Using a variety of orthogonal approaches, we demonstrate that sensitivity to SQLE inhibition results not from cholesterol biosynthesis pathway inhibition, but rather surprisingly from the specific and toxic accumulation of the SQLE substrate, squalene. These findings highlight SQLE as a potential therapeutic target in a subset of neuroendocrine tumors, particularly small cell lung cancers.

AG-221, a First-in-Class Therapy Targeting Acute Myeloid Leukemia Harboring Oncogenic IDH2 Mutations.

Somatic gain-of-function mutations in isocitrate dehydrogenases (IDH) 1 and 2 are found in multiple hematologic and solid tumors, leading to accumulation of the oncometabolite (R)-2-hydroxyglutarate (2HG). 2HG competitively inhibits α-ketoglutarate-dependent dioxygenases, including histone demethylases and methylcytosine dioxygenases of the TET family, causing epigenetic dysregulation and a block in cellular differentiation. In vitro studies have provided proof of concept for mutant IDH inhibition as a therapeutic approach. We report the discovery and characterization of AG-221, an orally available, selective, potent inhibitor of the mutant IDH2 enzyme. AG-221 suppressed 2HG production and induced cellular differentiation in primary human IDH2 mutation-positive acute myeloid leukemia (AML) cells ex vivo and in xenograft mouse models. AG-221 also provided a statistically significant survival benefit in an aggressive IDH2R140Q-mutant AML xenograft mouse model. These findings supported initiation of the ongoing clinical trials of AG-221 in patients with IDH2 mutation-positive advanced hematologic malignancies.Significance: Mutations in IDH1/2 are identified in approximately 20% of patients with AML and contribute to leukemia via a block in hematopoietic cell differentiation. We have shown that the targeted inhibitor AG-221 suppresses the mutant IDH2 enzyme in multiple preclinical models and induces differentiation of malignant blasts, supporting its clinical development. Cancer Discov; 7(5); 478-93. ©2017 AACR.See related commentary by Thomas and Majeti, p. 459See related article by Shih et al., p. 494This article is highlighted in the In This Issue feature, p. 443.

Outcome of the First wwPDB/CCDC/D3R Ligand Validation Workshop.

Crystallographic studies of ligands bound to biological macromolecules (proteins and nucleic acids) represent an important source of information concerning drug-target interactions, providing atomic level insights into the physical chemistry of complex formation between macromolecules and ligands. Of the more than 115,000 entries extant in the Protein Data Bank (PDB) archive, ∼75% include at least one non-polymeric ligand. Ligand geometrical and stereochemical quality, the suitability of ligand models for in silico drug discovery and design, and the goodness-of-fit of ligand models to electron-density maps vary widely across the archive. We describe the proceedings and conclusions from the first Worldwide PDB/Cambridge Crystallographic Data Center/Drug Design Data Resource (wwPDB/CCDC/D3R) Ligand Validation Workshop held at the Research Collaboratory for Structural Bioinformatics at Rutgers University on July 30-31, 2015. Experts in protein crystallography from academe and industry came together with non-profit and for-profit software providers for crystallography and with experts in computational chemistry and data archiving to discuss and make recommendations on best practices, as framed by a series of questions central to structural studies of macromolecule-ligand complexes. What data concerning bound ligands should be archived in the PDB? How should the ligands be best represented? How should structural models of macromolecule-ligand complexes be validated? What supplementary information should accompany publications of structural studies of biological macromolecules? Consensus recommendations on best practices developed in response to each of these questions are provided, together with some details regarding implementation. Important issues addressed but not resolved at the workshop are also enumerated.

Crystal structure of human GDF11.

Members of the TGF-ß family of proteins are believed to play critical roles in cellular signaling processes such as those involved in muscle differentiation. The extent to which individual family members have been characterized and linked to biological function varies greatly. The role of myostatin, also known as growth differentiation factor 8 (GDF8), as an inhibitor of muscle differentiation is well understood through genetic linkages. In contrast, the role of growth differentiation factor 11 (GDF11) is much less well understood. In humans, the mature forms of GDF11 and myostatin are over 94% identical. In order to understand the role that the small differences in sequence may play in the differential signaling of these molecules, the crystal structure of GDF11 was determined to a resolution of 1.50 Å. A comparison of the GDF11 structure with those of other family members reveals that the canonical TGF-ß domain fold is conserved. A detailed structural comparison of GDF11 and myostatin shows that several of the differences between these proteins are likely to be localized at interfaces that are critical for the interaction with downstream receptors and inhibitors.

Synthesis, SAR, and series evolution of novel oxadiazole-containing 5-lipoxygenase activating protein inhibitors: discovery of 2-[4-(3-{(r)-1-[4-(2-amino-pyrimidin-5-yl)-phenyl]-1-cyclopropyl-ethyl}-[1,2,4]oxadiazol-5-yl)-pyrazol-1-yl]-N,N-dimethyl-acetamide (BI 665915).

The synthesis, structure-activity relationship (SAR), and evolution of a novel series of oxadiazole-containing 5-lipoxygenase-activating protein (FLAP) inhibitors are described. The use of structure-guided drug design techniques provided compounds that demonstrated excellent FLAP binding potency (IC50 < 10 nM) and potent inhibition of LTB4 synthesis in human whole blood (IC50 < 100 nM). Optimization of binding and functional potencies, as well as physicochemical properties resulted in the identification of compound 69 (BI 665915) that demonstrated an excellent cross-species drug metabolism and pharmacokinetics (DMPK) profile and was predicted to have low human clearance. In addition, 69 was predicted to have a low risk for potential drug-drug interactions due to its cytochrome P450 3A4 profile. In a murine ex vivo whole blood study, 69 demonstrated a linear dose-exposure relationship and a dose-dependent inhibition of LTB4 production.

Integrated strategies for identifying leads that target the NS3 helicase of the hepatitis C virus.

Future treatments for individuals infected by the hepatitis C virus (HCV) will likely involve combinations of compounds that inhibit multiple viral targets. The helicase of HCV is an attractive target with no known drug candidates in clinical trials. Herein we describe an integrated strategy for identifying fragment inhibitors using structural and biophysical techniques. Based on an X-ray structure of apo HCV helicase and in silico and bioinformatic analyses of HCV variants, we identified that one site in particular (labeled 3 + 4) was the most conserved and attractive pocket to target for a drug discovery campaign. Compounds from multiple sources were screened to identify inhibitors or binders to this site, and enzymatic and biophysical assays (NMR and SPR) were used to triage the most promising ligands for 3D structure determination by X-ray crystallography. Medicinal chemistry and biophysical evaluations focused on exploring the most promising lead series. The strategies employed here can have general utility in drug discovery.

Discovery of potent, selective chymase inhibitors via fragment linking strategies.

Chymase plays an important and diverse role in the homeostasis of a number of cardiovascular processes. Herein, we describe the identification of potent, selective chymase inhibitors, developed using fragment-based, structure-guided linking and optimization techniques. High-concentration biophysical screening methods followed by high-throughput crystallography identified an oxindole fragment bound to the S1 pocket of the protein exhibiting a novel interaction pattern hitherto not observed in chymase inhibitors. X-ray crystallographic structures were used to guide the elaboration/linking of the fragment, ultimately leading to a potent inhibitor that was >100-fold selective over cathepsin G and that mitigated a number of liabilities associated with poor physicochemical properties of the series it was derived from.

Indole RSK inhibitors. Part 1: discovery and initial SAR.

A series of inhibitors for the 90 kDa ribosomal S6 kinase (RSK) based on an 1-oxo-2,3,4,5-tetrahydro-1H-[1,4]diazepino[1,2-a]indole-8-carboxamide scaffold were identified through high throughput screening. An RSK crystal structure and exploratory SAR were used to define the series pharmacophore. Compounds with good cell potency, such as compounds 43, 44, and 55 were identified, and form the basis for subsequent kinase selectivity optimization.

Fragment-based discovery of indole inhibitors of matrix metalloproteinase-13.

Matrix metalloproteases (MMPs) play an important role in cartilage homeostasis under both normal and inflamed disease states and, thus, have become attractive targets for the treatment of arthritic diseases. Herein, we describe the identification of a potent, selective MMP-13 inhibitor, developed using fragment-based structure-guided lead identification and optimization techniques. Virtual screening methods identified a novel, indole-based MMP-13 inhibitor that bound into the S1' pocket of the protein exhibiting a novel interaction pattern hitherto not observed in MMP-13 inhibitors. X-ray crystallographic structures were used to guide the elaboration of the fragment, ultimately leading to a potent inhibitor that was >100-fold selective over nine other MMP isoforms tested.

Lateral energy transfer model for adjacent light-harvesting antennae rods of C-phycocyanins.

Modeling of excitation transfer pathways have been carried out for the structure of Spirulina platensis C-phycocyanin. Calculations by Förster mechanism using the crystal structure coordinates determined in our laboratory indicate ultra-fast lateral energy transfer rates between pairs of chromophores attached to two adjacent hexamer disks. The pairwise transfer times of the order of a few pico-seconds correspond to resonance transitions between peripheral beta155 chromophores. A quantitative lateral energy transfer model for C-phycocyanin light-harvesting antenna rods that is suggestive to its native structural organization emerges from this study.

Structural basis for autoinhibition and mutational activation of eukaryotic initiation factor 2alpha protein kinase GCN2.

The GCN2 protein kinase coordinates protein synthesis with levels of amino acid stores by phosphorylating eukaryotic translation initiation factor 2. The autoinhibited form of GCN2 is activated in cells starved of amino acids by binding of uncharged tRNA to a histidyl-tRNA synthetase-like domain. Replacement of Arg-794 with Gly in the PK domain (R794G) activates GCN2 independently of tRNA binding. Crystal structures of the GCN2 protein kinase domain have been determined for wild-type and R794G mutant forms in the apo state and bound to ATP/AMPPNP. These structures reveal that GCN2 autoinhibition results from stabilization of a closed conformation that restricts ATP binding. The R794G mutant shows increased flexibility in the hinge region connecting the N- and C-lobes, resulting from loss of multiple interactions involving Arg794. This conformational change is associated with intradomain movement that enhances ATP binding and hydrolysis. We propose that intramolecular interactions following tRNA binding remodel the hinge region in a manner similar to the mechanism of enzyme activation elicited by the R794G mutation.