Co-opting the E3 ligase KLHDC2 for targeted protein degradation by small molecules.

Targeted protein degradation (TPD) by PROTAC (proteolysis-targeting chimera) and molecular glue small molecules is an emerging therapeutic strategy. To expand the roster of E3 ligases that can be utilized for TPD, we describe the discovery and biochemical characterization of small-molecule ligands targeting the E3 ligase KLHDC2. Furthermore, we functionalize these KLHDC2-targeting ligands into KLHDC2-based BET-family and AR PROTAC degraders and demonstrate KLHDC2-dependent target-protein degradation. Additionally, we offer insight into the assembly of the KLHDC2 E3 ligase complex. Using biochemical binding studies, X-ray crystallography and cryo-EM, we show that the KLHDC2 E3 ligase assembles into a dynamic tetramer held together via its own C terminus, and that this assembly can be modulated by substrate and ligand engagement.

High-resolution crystal structure and chemical screening reveal pantothenate kinase as a new target for antifungal development.

Fungal infections are the leading cause of mortality by eukaryotic pathogens, with an estimated 150 million severe life-threatening cases and 1.7 million deaths reported annually. The rapid emergence of multidrug-resistant fungal isolates highlights the urgent need for new drugs with new mechanisms of action. In fungi, pantothenate phosphorylation, catalyzed by PanK enzyme, is the first step in the utilization of pantothenic acid and coenzyme A biosynthesis. In all fungi sequenced so far, this enzyme is encoded by a single PanK gene. Here, we report the crystal structure of a fungal PanK alone as well as with high-affinity inhibitors from a single chemotype identified through a high-throughput chemical screen. Structural, biochemical, and functional analyses revealed mechanisms governing substrate and ligand binding, dimerization, and catalysis and helped identify new compounds that inhibit the growth of several Candida species. The data validate PanK as a promising target for antifungal drug development.

An allosteric site on MKP5 reveals a strategy for small-molecule inhibition.

The mitogen-activated protein kinase (MAPK) phosphatases (MKPs) have been considered "undruggable," but their position as regulators of the MAPKs makes them promising therapeutic targets. MKP5 has been suggested as a potential target for the treatment of dystrophic muscle disease. Here, we identified an inhibitor of MKP5 using a p38α MAPK-derived, phosphopeptide-based small-molecule screen. We solved the structure of MKP5 in complex with this inhibitor, which revealed a previously undescribed allosteric binding pocket. Binding of the inhibitor to this pocket collapsed the MKP5 active site and was predicted to limit MAPK binding. Treatment with the inhibitor recapitulated the phenotype of MKP5 deficiency, resulting in activation of p38 MAPK and JNK. We demonstrated that MKP5 was required for TGF-ß1 signaling in muscle and that the inhibitor blocked TGF-ß1-mediated Smad2 phosphorylation. TGF-ß1 pathway antagonism has been proposed for the treatment of dystrophic muscle disease. Thus, allosteric inhibition of MKP5 represents a therapeutic strategy against dystrophic muscle disease.

Discovery of Inhibitors of MicroRNA-21 Processing Using Small Molecule Microarrays.

The identification of small molecules that bind to and perturb the function of microRNAs is an attractive approach for the treatment for microRNA-associated pathologies. However, there are only a few small molecules known to interact directly with microRNAs. Here, we report the use of a small molecule microarray (SMM) screening approach to identify low molecular weight compounds that directly bind to a pre-miR-21 hairpin. Compounds identified using this approach exhibit good affinity for the RNA (ranging from 0.8-2.0 µM) and are not composed of a polycationic scaffold. Several of the highest affinity compounds inhibit Dicer-mediated processing, while in-line probing experiments indicate that the compounds bind to the apical loop of the hairpin, proximal to the Dicer site. This work provides evidence that small molecules can be developed to bind directly to and inhibit miR-21.

Screen-identified selective inhibitor of lysine demethylase 5A blocks cancer cell growth and drug resistance.

Lysine demethylase 5A (KDM5A/RBP2/JARID1A) is a histone lysine demethylase that is overexpressed in several human cancers including lung, gastric, breast and liver cancers. It plays key roles in important cancer processes including tumorigenesis, metastasis, and drug tolerance, making it a potential cancer therapeutic target. Chemical tools to analyze KDM5A demethylase activity are extremely limited as available inhibitors are not specific for KDM5A. Here, we characterized KDM5A using a homogeneous luminescence-based assay and conducted a screen of about 9,000 small molecules for inhibitors. From this screen, we identified several 3-thio-1,2,4-triazole compounds that inhibited KDM5A with low µM in vitro IC50 values. Importantly, these compounds showed great specificity and did not inhibit its close homologue KDM5B (PLU1/JARID1B) or the related H3K27 demethylases KDM6A (UTX) and KDM6B (JMJD3). One compound, named YUKA1, was able to increase H3K4me3 levels in human cells and selectively inhibit the proliferation of cancer cells whose growth depends on KDM5A. As KDM5A was shown to mediate drug tolerance, we investigated the ability of YUKA1 to prevent drug tolerance in EGFR-mutant lung cancer cells treated with gefitinib and HER2+ breast cancer cells treated with trastuzumab. Remarkably, this compound hindered the emergence of drug-tolerant cells, highlighting the critical role of KDM5A demethylase activity in drug resistance. The small molecules presented here are excellent tool compounds for further study of KDM5A's demethylase activity and its contributions to cancer.

Small Molecule Microarrays Enable the Identification of a Selective, Quadruplex-Binding Inhibitor of MYC Expression.

The transcription factor MYC plays a pivotal role in cancer initiation, progression, and maintenance. However, it has proven difficult to develop small molecule inhibitors of MYC. One attractive route to pharmacological inhibition of MYC has been the prevention of its expression through small molecule-mediated stabilization of the G-quadruplex (G4) present in its promoter. Although molecules that bind globally to quadruplex DNA and influence gene expression are well-known, the identification of new chemical scaffolds that selectively modulate G4-driven genes remains a challenge. Here, we report an approach for the identification of G4-binding small molecules using small molecule microarrays (SMMs). We use the SMM screening platform to identify a novel G4-binding small molecule that inhibits MYC expression in cell models, with minimal impact on the expression of other G4-associated genes. Surface plasmon resonance (SPR) and thermal melt assays demonstrated that this molecule binds reversibly to the MYC G4 with single digit micromolar affinity, and with weaker or no measurable binding to other G4s. Biochemical and cell-based assays demonstrated that the compound effectively silenced MYC transcription and translation via a G4-dependent mechanism of action. The compound induced G1 arrest and was selectively toxic to MYC-driven cancer cell lines containing the G4 in the promoter but had minimal effects in peripheral blood mononucleocytes or a cell line lacking the G4 in its MYC promoter. As a measure of selectivity, gene expression analysis and qPCR experiments demonstrated that MYC and several MYC target genes were downregulated upon treatment with this compound, while the expression of several other G4-driven genes was not affected. In addition to providing a novel chemical scaffold that modulates MYC expression through G4 binding, this work suggests that the SMM screening approach may be broadly useful as an approach for the identification of new G4-binding small molecules.

An analysis of FDA-approved drugs: natural products and their derivatives.

Natural products contribute greatly to the history and landscape of new molecular entities (NMEs). An assessment of all FDA-approved NMEs reveals that natural products and their derivatives represent over one-third of all NMEs. Nearly one-half of these are derived from mammals, one-quarter from microbes and one-quarter from plants. Since the 1930s, the total fraction of natural products has diminished, whereas semisynthetic and synthetic natural product derivatives have increased. Over time, this fraction has also become enriched with microbial natural products, which represent a significant portion of approved antibiotics, including more than two-thirds of all antibacterial NMEs. In recent years, the declining focus on natural products has impacted the pipeline of NMEs from specific classes, and this trend is likely to continue without specific investment in the pursuit of natural products.

An analysis of original research contributions toward FDA-approved drugs.

Academic researchers shaped the landscape of drug discovery for nearly two centuries, and their efforts initiated programs for more than half of the US Food and Drug Administration (FDA)-approved new molecular entities (NMEs). During the first 50 years of the 20th century, contributions from industry-based discovery programs steadily increased, stabilizing near half of all first publications for NMEs. Although academia and industry have made similar contributions to the discovery of FDA-approved NMEs, there remains a substantial difference in the gap-to-approval; on average, industry NMEs are 12 years closer to market at the time of the first publication. As more drug discovery efforts shift from industry to academia, including high-throughput screening resources, academia could have an increasingly crucial role in drug discovery.

Identification of biologically active, HIV TAR RNA-binding small molecules using small molecule microarrays.

Identifying small molecules that selectively bind to structured RNA motifs remains an important challenge in developing potent and specific therapeutics. Most strategies to find RNA-binding molecules have identified highly charged compounds or aminoglycosides that commonly have modest selectivity. Here we demonstrate a strategy to screen a large unbiased library of druglike small molecules in a microarray format against an RNA target. This approach has enabled the identification of a novel chemotype that selectively targets the HIV transactivation response (TAR) RNA hairpin in a manner not dependent on cationic charge. Thienopyridine 4 binds to and stabilizes the TAR hairpin with a Kd of 2.4 µM. Structure-activity relationships demonstrate that this compound achieves activity through hydrophobic and aromatic substituents on a heterocyclic core, rather than cationic groups typically required. Selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE) analysis was performed on a 365-nucleotide sequence derived from the 5' untranslated region (UTR) of the HIV-1 genome to determine global structural changes in the presence of the molecule. Importantly, the interaction of compound 4 can be mapped to the TAR hairpin without broadly disrupting any other structured elements of the 5' UTR. Cell-based anti-HIV assays indicated that 4 inhibits HIV-induced cytopathicity in T lymphocytes with an EC50 of 28 µM, while cytotoxicity was not observed at concentrations approaching 1 mM.

A HaloTag-based small molecule microarray screening methodology with increased sensitivity and multiplex capabilities.

Small Molecule Microarrays (SMMs) represent a general platform for screening small molecule-protein interactions independent of functional inhibition of target proteins. In an effort to increase the scope and utility of SMMs, we have modified the SMM screening methodology to increase assay sensitivity and facilitate multiplex screening. Fusing target proteins to the HaloTag protein allows us to covalently prelabel fusion proteins with fluorophores, leading to increased assay sensitivity and an ability to conduct multiplex screens. We use the interaction between FKBP12 and two ligands, rapamycin and ARIAD's "bump" ligand, to show that the HaloTag-based SMM screening methodology significantly increases assay sensitivity. Additionally, using wild type FKBP12 and the FKBP12 F36V mutant, we show that prelabeling various protein isoforms with different fluorophores allows us to conduct multiplex screens and identify ligands to a specific isoform. Finally, we show this multiplex screening technique is capable of identifying ligands selective for a specific PTP1B isoform using a 20,000 compound screening deck.

Targeting the von Hippel-Lindau E3 ubiquitin ligase using small molecules to disrupt the VHL/HIF-1α interaction.

E3 ubiquitin ligases, which bind protein targets, leading to their ubiquitination and subsequent degradation, are attractive drug targets due to their exquisite substrate specificity. However, the development of small-molecule inhibitors has proven extraordinarily challenging as modulation of E3 ligase activities requires the targeting of protein-protein interactions. Using rational design, we have generated the first small molecule targeting the von Hippel-Lindau protein (VHL), the substrate recognition subunit of an E3 ligase, and an important target in cancer, chronic anemia, and ischemia. We have also obtained the crystal structure of VHL bound to our most potent inhibitor, confirming that the compound mimics the binding mode of the transcription factor HIF-1α, a substrate of VHL. These results have the potential to guide future development of improved lead compounds as therapeutics for the treatment of chronic anemia and ischemia.

Strategies for recognition of stem-loop RNA structures by synthetic ligands: application to the HIV-1 frameshift stimulatory sequence.

Production of the Gag-Pol polyprotein in human immunodeficiency virus (HIV) requires a -1 ribosomal frameshift, which is directed by a highly conserved RNA stem-loop. Building on our discovery of a set of disulfide-containing peptides that bind this RNA, we describe medicinal chemistry efforts designed to begin to understand the structure-activity relationships and RNA sequence-selectivity relationships associated with these compounds. Additionally, we have prepared analogues incorporating an olefin or saturated hydrocarbon bioisostere of the disulfide moiety, as a first step toward enhancing biostability. The olefin-containing compounds exhibit affinity comparable to the lead disulfide and, importantly, have no discernible toxicity when incubated with human fibroblasts at concentrations up to 1 mM.

Identification and characterization of a peptidic ligand for Ras.

The development of new ligands for the oncoprotein Ras can provide tools for the study of this important signaling component or potentially serve as therapeutic agents for the treatment of Ras-associated diseases. Herein, we report a peptidic Ras ligand identified through naïve phage display. Panning a phage library with a diversity of 10(9) transormants successfully identified a peptide dodecamer that contains two internal consensus motifs and binds Ras in both the active GTP- and inactive GDP-bound conformations with low micromolar dissociation constants. The dodecamer does not alter the intrinsic GTPase activity of Ras, does not compete for Ras binding to the Ras binding domain of Raf, and does not alter cell viability. This novel Ras ligand has the potential to serve in the development of higher-affinity ligands and chemical tools targeting Ras.

Allosteric site variants of Haemophilus influenzae beta-carbonic anhydrase.

Haemophilus influenzae beta-carbonic anhydrase (HICA) is hypothesized to be an allosteric protein that is regulated by the binding of bicarbonate ion to a non-catalytic (inhibitory) site that controls the ligation of Asp44 to the catalytically essential zinc ion. We report here the X-ray crystallographic structures of two variants (W39F and Y181F) involved in the binding of bicarbonate ion in the non-catalytic site and an active-site variant (D44N) that is incapable of forming a strong zinc ligand. The alteration of Trp39 to Phe increases the apparent K(i) for bicarbonate inhibition by 4.8-fold. While the structures of W39F and Y181F are very similar to the wild-type enzyme, the X-ray crystal structure of the D44N variant reveals that it has adopted an active-site conformation nearly identical to that of non-allosteric beta-carbonic anhydrases. We propose that the structure of the D44N variant is likely to be representative of the active conformation of the enzyme. These results lend additional support to the hypothesis that HICA is an allosteric enzyme that can adopt active and inactive conformations, the latter of which is stabilized by bicarbonate ion binding to a non-catalytic site.

Ribosomal frameshifting: an emerging drug target for HIV.

Combating HIV currently remains one of the key public health challenges. While immense effort has been invested in the development of HIV-targeted therapeutic agents, interfering with protein synthesis is a relatively unexplored area. HIV has an absolute requirement for a -1 ribosomal frameshift event during translation that constitutes a potentially attractive target for interfering with the viral life cycle. Research suggests that while a considerable amount of investigation is still required, targeting frameshifting in HIV with small molecules, peptides and oligonucleotides is feasible.

Dynamic combinatorial selection of molecules capable of inhibiting the (CUG) repeat RNA-MBNL1 interaction in vitro: discovery of lead compounds targeting myotonic dystrophy (DM1).

Myotonic dystrophy type 1 (DM1), the most common form of muscular dystrophy in adults, is an RNA-mediated disease. Dramatically expanded (CUG) repeats accumulate in nuclei and sequester RNA-binding proteins such as the splicing regulator MBNL1. We have employed resin-bound dynamic combinatorial chemistry (RBDCC) to identify the first examples of compounds able to inhibit MBNL1 binding to (CUG) repeat RNA. Screening an RBDCL with a theoretical diversity of 11 325 members yielded several molecules with significant selectivity for binding to (CUG) repeat RNA over other sequences. These compounds were also able to inhibit the interaction of GGG-(CUG)(109)-GGG RNA with MBNL1 in vitro, with K(i) values in the low micromolar range.

Selective recognition of alkyl pyranosides in protic and aprotic solvents.

The design and synthesis of receptors capable of selective, noncovalent recognition of carbohydrates continues to be a signature challenge in bioorganic chemistry. We report a new generation of tripodal receptors incorporating three pyridine (compound 2) or quinoline (compound 3) rings around a central cyclohexane core for use in molecular recognition of monosaccharides in apolar and polar protic solvents. These tripodal receptors were investigated using (1)H NMR, UV, and fluorescence titrations in order to determine their binding abilities toward a set of octyl glycosides. Receptor 2 displayed the highest binding affinity reported to date for noncovalent 1:1 binding of an alpha-glucopyranoside in chloroform (Ka = 212,000 +/- 27,000 M(-1)) and an approximately 8-fold selectivity for the alpha anomer over the beta anomer of the glucopyranoside. Most importantly, 2 retained its micromolar range of affinities toward monosaccharides in a polar and highly competitive solvent (methanol). The quinoline variant 3 also displayed micromolar binding affinities for selected monosaccharides in methanol (as measured by fluorescence) that were generally smaller than those of 2. Compound 3 was found to follow a selectivity pattern similar to that of 2, displaying higher affinities for glucopyranosides than for other monosaccharides. The binding stoichiometry was estimated to be 1:1 for the complexes formed by both 2 and 3 with glucopyranosides, as determined by Job plots. Nuclear Overhauser effect spectroscopy allowed for the derivation of a binding model consistent with the observed selectivities.