Modulation of Ligand-Gated Glycine Receptors Via Functional Monoclonal Antibodies

Ion channels are targets of considerable therapeutic interest to address a wide variety of neurologic indications, including pain perception. Current pharmacological strategies have focused mostly on small molecule approaches that can be limited by selectivity requirements within members of a channel family or superfamily. Therapeutic antibodies have been proposed, designed, and characterized to alleviate this selectivity limitation; however, there are no Food and Drug Administration-approved therapeutic antibody-based drugs targeting ion channels on the market to date. Here, in an effort to identify novel classes of engineered ion channel modulators for potential neurologic therapeutic applications, we report the generation and characterization of six (EC₅₀ < 25nM) Cys-loop receptor family monoclonal antibodies with modulatory function against rat and human glycine receptor alpha 1 (GlyRα1) and/or GlyRα3. These antibodies have activating (i.e., positive modulator) or inhibiting (i.e., negative modulator) profiles. Moreover, GlyRα3 selectivity was successfully achieved for two of the three positive modulators identified. When dosed intravenously, the antibodies achieved sufficient brain exposure to cover their calculated in vitro EC₅₀ values. When compared head-to-head at identical exposures, the GlyRα3-selective antibody showed a more desirable safety profile over the nonselective antibody, thus demonstrating, for the first time, an advantage for GlyRα3-selectivity. Our data show that ligand-gated ion channels of the glycine receptor family within the central nervous system can be functionally modulated by engineered biologics in a dose-dependent manner and that, despite high protein homology between the alpha subunits, selectivity can be achieved within this receptor family, resulting in future therapeutic candidates with more desirable drug safety profiles. SIGNIFICANCE STATEMENT: This study presents immunization and multiplatform screening approaches to generate a diverse library of functional antibodies (agonist, potentiator, or inhibitory) raised against human glycine receptors (GlyRs). This study also demonstrates the feasibility of acquiring alpha subunit selectivity, a desirable therapeutic profile. When tested in vivo, these tool molecules demonstrated an increased safety profile in favor of GlyRα3-selectivity. These are the first reported functional GlyR antibodies that may open new avenues to treating central nervous system diseases with subunit selective biologics.

Abstract 2158: Preclinical evaluation of CFT1946 as a selective degrader of mutant BRAF for the treatment of BRAF driven cancers

The BRAF kinase is a critical node in the MAPK signaling pathway and is mutated in approximately 8% of human cancers including melanoma (~60%), thyroid (~60%), and lung adenocarcinoma (~10%). The most common mutation in BRAF is V600E (Class I), occurring in half of malignant melanomas. This mutation hyperactivates ERK and signals as a RAF inhibitor-sensitive monomer. BRAF inhibitors including vemurafenib, dabrafenib and encorafenib have produced impressive responses in V600X patients, however resistance usually emerges within a year, including RAS mutation, BRAFV600E amplification, and BRAFV600E intragenic deletion or splice variants. These inhibitors are also ineffective against non-V600 BRAF mutants (Class II & III). To address some of these limitations we have developed CFT1946, a bifunctional degradation activating compound (BiDAC™) degrader comprising a BRAF kinase domain targeting ligand linked to a cereblon ligand. CFT1946 is capable of degrading BRAF V600E (Class I), G469A (Class II), G466V (Class III) mutations, and the p61-BRAFV600E splice variant while maintaining exquisite selectivity against the proteome including WT BRAF and CRAF. In A375 cells, CFT1946 potently degraded BRAFV600E (Emax = 26%; DC50 = 14nM at 24hr) and, inhibited ERK phosphorylation (IC50 = 11nM at 24hr) and cell growth (GI50 = 94nM at 96hr) while having no effect in the mutant KRAS driven cell line HCT116. In A375 xenografts, oral delivery of CFT1946 resulted in deeper tumor regressions when dosed at 10 mg/kg PO BID and compared favorably to a clinically relevant dose of encorafenib. We further evaluated CFT1946 in an engineered A375-BRAFV600E/NRASQ61K double mutant model of BRAF inhibitor resistance. CFT1946 was able to degrade BRAFV600E in these cells and was much more effective than encorafenib at inhibiting viability in vitro. In this model, in vivo dosing of single agent CFT1946 caused robust tumor growth inhibition and combination with the MEK inhibitor, trametinib, resulted in tumor regressions. The combination of encorafenib and trametinib showed no activity in the same model. Next, we demonstrated that CFT1946 was able to degrade additional BRAF mutant proteins including G469A (Class II), G466V (Class III), and the p61-BRAFV600E splice variant using heterologous expression in HEK293T cells. Additionally, we also showed that CFT1946, but not encorafenib, inhibited proliferation of the BRAFG466V heterozygous lung tumor cell line H1666. Based on its activity in preclinical models, including models of BRAF inhibitor resistance, and its drug-like properties we are progressing CFT1946 as a candidate for clinical development in patients with solid tumors bearing BRAF V600X mutations. Further, given CFT1946’s activity on non-V600 BRAF mutations, we are continuing to explore CFT1946 and related BiDAC degraders as therapeutic options for patients bearing Class II or Class III BRAF mutations.

Citation Format: Mathew E. Sowa, Bridget Kreger, Joelle Baddour, Yanke Liang, Jeffrey R. Simard, Laura Poling, Ping Li, Robert Yu, Ashley Hart, Roman V. Agafonov, Grace Sarkissian, Joe Sahil Patel, Richard Deibler, Kyle S. Cole, Scott Eron, David Cocozziello, Fazlur Rahman, Moses Moustakim, Christopher G. Nasveschuk, Katrina L. Jackson, Mark Fitzgerald, Victoria Garza, Morgan O’Shea, Gesine Veits, Jeremy L. Yap, Andrew J. Phillips, Elizabeth Norton, Adam S. Crystal, Stewart L. Fisher, Roy M. Pollock. Preclinical evaluation of CFT1946 as a selective degrader of mutant BRAF for the treatment of BRAF driven cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2158.

A Method for Determining the Kinetics of Small-Molecule-Induced Ubiquitination

Recent advances in targeted protein degradation have enabled chemical hijacking of the ubiquitin-proteasome system to treat disease. The catalytic rate of cereblon (CRBN)-dependent bifunctional degradation activating compounds (BiDAC), which recruit CRBN to a chosen target protein, resulting in its ubiquitination and proteasomal degradation, is an important parameter to consider during the drug discovery process. In this work, an in vitro system was developed to measure the kinetics of BRD4 bromodomain 1 (BD1) ubiquitination by fitting an essential activator kinetic model to these data. The affinities between BiDACs, BD1, and CRBN in the binary complex, ternary complex, and full ubiquitination complex were characterized. Together, this work provides a new tool for understanding and optimizing the catalytic and thermodynamic properties of BiDACs.

High-Throughput Quantitative Assay Technologies for Accelerating the Discovery and Optimization of Targeted Protein Degradation Therapeutics

The aberrant regulation of protein expression and function can drastically alter cellular physiology and lead to numerous pathophysiological conditions such as cancer, inflammatory diseases, and neurodegeneration. The steady-state expression levels of endogenous proteins are controlled by a balance of de novo synthesis rates and degradation rates. Moreover, the levels of activated proteins in signaling cascades can be further modulated by a variety of posttranslational modifications and protein-protein interactions. The field of targeted protein degradation is an emerging area for drug discovery in which small molecules are used to recruit E3 ubiquitin ligases to catalyze the ubiquitination and subsequent degradation of disease-causing target proteins by the proteasome in both a dose- and time-dependent manner. Traditional approaches for quantifying protein level changes in cells, such as Western blots, are typically low throughput with limited quantification, making it hard to drive the rapid development of therapeutics that induce selective, rapid, and sustained protein degradation. In the last decade, a number of techniques and technologies have emerged that have helped to accelerate targeted protein degradation drug discovery efforts, including the use of fluorescent protein fusions and reporter tags, flow cytometry, time-resolved fluorescence energy transfer (TR-FRET), and split luciferase systems. Here we discuss the advantages and disadvantages associated with these technologies and their application to the development and optimization of degraders as therapeutics.

SSO and other putative inhibitors of FA transport across membranes by CD36 disrupt intracellular metabolism, but do not affect FA translocation

Membrane-bound proteins have been proposed to mediate the transport of long-chain FA (LCFA) transport through the plasma membrane (PM). These proposals are based largely on reports that PM transport of LCFAs can be blocked by a number of enzymes and purported inhibitors of LCFA transport. Here, using the ratiometric pH indicator (2',7'-bis-(2-carboxyethyl)-5-(and-6-)-carboxyfluorescein and acrylodated intestinal FA-binding protein-based dual fluorescence assays, we investigated the effects of nine inhibitors of the putative FA transporter protein CD36 on the binding and transmembrane movement of LCFAs. We particularly focused on sulfosuccinimidyl oleate (SSO), reported to be a competitive inhibitor of CD36-mediated LCFA transport. Using these assays in adipocytes and inhibitor-treated protein-free lipid vesicles, we demonstrate that rapid LCFA transport across model and biological membranes remains unchanged in the presence of these purported inhibitors. We have previously shown in live cells that CD36 does not accelerate the transport of unesterified LCFAs across the PM. Our present experiments indicated disruption of LCFA metabolism inside the cell within minutes upon treatment with many of the "inhibitors" previously assumed to inhibit LCFA transport across the PM. Furthermore, using confocal microscopy and a specific anti-SSO antibody, we found that numerous intracellular and PM-bound proteins are SSO-modified in addition to CD36. Our results support the hypothesis that LCFAs diffuse rapidly across biological membranes and do not require an active protein transporter for their transmembrane movement.

Direct monitoring of the conformational equilibria of the activation loop in the mitogen-activated protein kinase p38α

Conformational transitions in protein kinases are crucial for the biological function of these enzymes. Here, we characterize and assess conformational equilibria of the activation loop and the effect of small molecule inhibitors in the MAP kinase p38α. Our work experimentally revealed the existence of a two-state equilibrium for p38α while the addition of inhibitors shifts the equilibrium between these two states.

Applications of parallel synthetic lead hopping and pharmacophore-based virtual screening in the discovery of efficient glycine receptor potentiators

Glycine receptors (GlyRs) are pentameric glycine-gated chloride ion channels that are enriched in the brainstem and spinal cord where they have been demonstrated to play a role in central nervous system (CNS) inhibition. Herein we describe two novel classes of glycine receptor potentiators that have been developed using similarity- and property-guided scaffold hopping enabled by parallel synthesis and pharmacophore-based virtual screening strategies. This effort resulted in the identification of novel, efficient and modular leads having favorable in vitro ADME profiles and high CNS multi-parameter optimization (MPO) scores, exemplified by azetidine sulfonamide 19 and aminothiazole sulfone (ent2)-20.

The Discovery and Hit-to-Lead Optimization of Tricyclic Sulfonamides as Potent and Efficacious Potentiators of Glycine Receptors

Current pain therapeutics suffer from undesirable psychotropic and sedative side effects, as well as abuse potential. Glycine receptors (GlyRs) are inhibitory ligand-gated ion channels expressed in nerves of the spinal dorsal horn, where their activation is believed to reduce transmission of painful stimuli. Herein, we describe the identification and hit-to-lead optimization of a novel class of tricyclic sulfonamides as allosteric GlyR potentiators. Initial optimization of high-throughput screening (HTS) hit 1 led to the identification of 3, which demonstrated ex vivo potentiation of glycine-activated current in mouse dorsal horn neurons from spinal cord slices. Further improvement of potency and pharmacokinetics produced in vivo proof-of-concept tool molecule 20 (AM-1488), which reversed tactile allodynia in a mouse spared-nerve injury (SNI) model. Additional structural optimization provided highly potent potentiator 32 (AM-3607), which was cocrystallized with human GlyRα3_cryst to afford the first described potentiator-bound X-ray cocrystal structure within this class of ligand-gated ion channels (LGICs).

Discovery of inter-domain stabilizers-a novel assay system for allosteric akt inhibitors

In addition to the catalytically active kinase domain, most kinases feature regulatory domains that govern their activity. Modulating and interfering with these interdomain interactions presents a major opportunity for understanding biological systems and developing novel therapeutics. Therefore, small molecule inhibitors that target these interactions through an allosteric mode of action have high intrinsic selectivity, as these interactions are often unique to a single kinase or kinase family. Here we report the development of iFLiK (interface-Fluorescent Labels in Kinases), a fluorescence-based assay that can monitor such interdomain interactions. Using iFLiK, we have demonstrated selective detection of allosteric Akt inhibitors that induce an inactive closed conformation unique to Akt. This methodology easily distinguished small molecule allosteric inhibitors from classic ATP-competitive inhibitors. Screening an in-house compound library with iFLiK, we were able to identify novel compounds with a scaffold that has not been previously described for allosteric Akt inhibitors.

FLiK: a direct-binding assay for the identification and kinetic characterization of stabilizers of inactive kinase conformations

Despite the hundreds of kinase inhibitors currently in discovery and preclinical phases, the number of FDA-approved kinase inhibitors remains very low by comparison, a discrepancy which reflects the challenges which accompanies kinase inhibitor development. Targeting protein kinases with ATP-competitive inhibitors has been the classical approach to inhibit kinase activity, but the highly conserved nature of the ATP-binding site often contributes to the poor inhibitor selectivity. To address this problem, we developed a high-throughput screening technology that can discriminate for inhibitors, which stabilize inactive kinase conformations by binding within allosteric pockets in the kinase domain. Here, we describe how to use the Fluorescence Labels in Kinases approach to measure the K(d) of ligands as well as how to kinetically characterize the binding and dissociation of ligands to the kinase. We also describe how this technology can be used to rapidly screen small molecule libraries in high throughput.

Overcoming compound fluorescence in the FLiK screening assay with red-shifted fluorophores

In the attempt to discover novel chemical scaffolds that can modulate the activity of disease-associated enzymes, such as kinases, biochemical assays are usually deployed in high-throughput screenings. First-line assays, such as activity-based assays, often rely on fluorescent molecules by measuring a change in the total emission intensity, polarization state, or energy transfer to another fluorescent molecule. However, under certain conditions, intrinsic compound fluorescence can lead to difficult data analysis and to false-positive, as well as false-negative, hits. We have reported previously on a powerful direct binding assay called fluorescent labels in kinases ('FLiK'), which enables a sensitive measurement of conformational changes in kinases upon ligand binding. In this assay system, changes in the emission spectrum of the fluorophore acrylodan, induced by the binding of a ligand, are translated into a robust assay readout. However, under the excitation conditions of acrylodan, intrinsic compound fluorescence derived from highly conjugated compounds complicates data analysis. We therefore optimized this method by identifying novel fluorophores that excite in the far red, thereby avoiding compound fluorescence. With this advancement, even rigid compounds with multiple π-conjugated ring systems can now be measured reliably. This study was performed on three different kinase constructs with three different labeling sites, each undergoing distinct conformational changes upon ligand binding. It may therefore serve as a guideline for the establishment of novel fluorescence-based detection assays.

Development and implementation of a high-throughput AlphaLISA assay for identifying inhibitors of EZH2 methyltransferase

The methylation state of lysine residues within histone H3 is a major determinant of active and inactive regions of the genome. Enhancer of Zeste homolog 2 (EZH2) is a histone lysine methyltransferase that is part of the polycomb repressive complex 2 (PRC2). Elevated EZH2 expression levels have been linked to hypertrimethylation of histone H3 lysine 27 (H3K27), repression of tumor repressor genes, and the onset of several types of cancers. We used the AlphaLISA technology to develop a high-throughput assay for identifying small molecule inhibitors of EZH2. AlphaLISA Acceptor Beads coated with antibodies directed against methylated H3K27 provided a sensitive method of detecting EZH2 activity through measurement of K27 methylation of a biotinylated H3-based peptide substrate. Optimized assay conditions resulted in a robust assay (Z'>0.7) which was successfully implemented in a high-throughput screening campaign. Small molecule inhibitors identified by this method may serve as powerful tools to further elucidate the potential importance of EZH2 in the development and treatment of cancer.

Targeting GSK3 from Ustilago maydis: type-II kinase inhibitors as potential antifungals

Protein kinases are key enzymes in the complex regulation of cellular processes in almost all living organisms. For this reason, protein kinases represent attractive targets to stop the growth of eukaryotic pathogens such as protozoa and fungi. However, using kinase inhibitors to fight against these organisms bears several challenges since most of them are unselective and will also affect crucial host kinases. Here we present the X-ray structure of glycogen synthase kinase 3 from the fungal plant pathogen Ustilago maydis (UmGSK3) and its inhibition by type-II kinase inhibitors. Despite the high sequence homology between the human and the fungal variant of this vital kinase, we found substantial differences in the conformational plasticity of their active sites. Compounds that induced such conformational changes could be used to selectively inhibit the fungal kinase. This study serves as an example of how species-specific selectivity of inhibitors can be achieved by identifying and addressing the inactive state of a protein kinase. In addition to this, our study gives interesting insights into the molecular plasticity of UmGSK3 by revealing a previously unknown inactive conformation of this important kinase family.

Fluorophore labeled kinase detects ligands that bind within the MAPK insert of p38α kinase

The vast majority of small molecules known to modulate kinase activity, target the highly conserved ATP-pocket. Consequently, such ligands are often less specific and in case of inhibitors, this leads to the inhibition of multiple kinases. Thus, selective modulation of kinase function remains a major hurdle. One of the next great challenges in kinase research is the identification of ligands which bind to less conserved sites and target the non-catalytic functions of protein kinases. However, approaches that allow for the unambiguous identification of molecules that bind to these less conserved sites are few in number. We have previously reported the use of fluorescent labels in kinases (FLiK) to develop direct kinase binding assays that exclusively detect ligands which stabilize inactive (DFG-out) kinase conformations. Here, we present the successful application of the FLiK approach to develop a high-throughput binding assay capable of directly monitoring ligand binding to a remote site within the MAPK insert of p38α mitogen-activated protein kinase (MAPK). Guided by the crystal structure of an initially identified hit molecule in complex with p38α, we developed a tight binding ligand which may serve as an ideal starting point for further investigations of the biological function of the MAPK insert in regulating the p38α signaling pathway.

Fluorescence labels in kinases: a high-throughput kinase binding assay for the identification of DFG-out binding ligands

Despite the hundreds of kinase inhibitors currently in discovery and pre-clinical phases, the number of kinase inhibitors which have been approved and are on the market remains low by comparison. This discrepancy reflects the challenges which accompany the development of kinase inhibitors which are relatively specific and less toxic. Targeting protein kinases with ATP-competitive inhibitors has been the classical approach to inhibiting kinase activity, but the highly conserved nature of the ATP-binding site contributes to poor inhibitor selectivity, issues which have particularly hampered the development of novel kinase inhibitors. We developed a high-throughput screening technology that can discriminate for inhibitors which stabilize the inactive "DFG-out" kinase conformation by binding within an allosteric pocket adjacent to the ATP-binding site. Here, we describe how to use this approach to measure the K (d) of ligands, as well as how to kinetically characterize the binding and dissociation of ligands to the kinase. We also describe how this technology can be used to rapidly screen small molecule libraries at high throughput.

Identification of Ustilago maydis Aurora kinase as a novel antifungal target

Infestation of crops by pathogenic fungi has continued to have a major impact by reducing yield and quality, emphasizing the need to identify new targets and develop new agents to improve methods of crop protection. Here we present Aurora kinase from the phytopathogenic fungus Ustilago maydis as a novel target for N-substituted diaminopyrimidines, a class of small-molecule kinase inhibitors. We show that Aurora kinase is essential in U. maydis and that diaminopyrimidines inhibit its activity in vitro. Furthermore, we observed an overall good correlation between in vitro inhibition of Aurora kinase and growth inhibition of diverse fungi in vivo. In vitro inhibition assays with Ustilago and human Aurora kinases indicate that some compounds of the N-substituted diaminopyrimidine class show specificity for the Ustilago enzyme, thus revealing their potential as selective fungicides.

Mutations in the DDR2 kinase gene identify a novel therapeutic target in squamous cell lung cancer

While genomically targeted therapies have improved outcomes for patients with lung adenocarcinoma, little is known about the genomic alterations which drive squamous cell lung cancer. Sanger sequencing of the tyrosine kinome identified mutations in the DDR2 kinase gene in 3.8% of squamous cell lung cancers and cell lines. Squamous lung cancer cell lines harboring DDR2 mutations were selectively killed by knock-down of DDR2 by RNAi or by treatment with the multi-targeted kinase inhibitor dasatinib. Tumors established from a DDR2 mutant cell line were sensitive to dasatinib in xenograft models. Expression of mutated DDR2 led to cellular transformation which was blocked by dasatinib. A squamous cell lung cancer patient with a response to dasatinib and erlotinib treatment harbored a DDR2 kinase domain mutation. These data suggest that gain-of-function mutations in DDR2 are important oncogenic events and are amenable to therapy with dasatinib. As dasatinib is already approved for use, these findings could be rapidly translated into clinical trials.

SIGNIFICANCE

DDR2 mutations are present in 4% of lung SCCs, and DDR2 mutations are associated with sensitivity to dasatinib. These findings provide a rationale for designing clinical trials with the FDA-approved drug dasatinib in patients with lung SCCs.

Synthesis and biological evaluation of 7-substituted-1-(3-bromophenylamino)isoquinoline-4-carbonitriles as inhibitors of myosin light chain kinase and epidermal growth factor receptor

Here we present the synthesis and biological activity of a series of 7-substituted-1-(3-bromophenylamino)isoquinoline-4-carbonitriles as inhibitors of myosin light chain kinase (MLCK) and the epidermal growth factor receptor kinase (EGFR). The inhibitory effect of these molecules was found to be dependent on the nature of the substituents at the 7-position of the isoquinoline scaffold.

Caveolins sequester FA on the cytoplasmic leaflet of the plasma membrane, augment triglyceride formation, and protect cells from lipotoxicity

Ectopic expression of caveolin-1 in HEK293 cells enhances FA sequestration in membranes as measured by a pH-sensitive fluorescent dye (1). We hypothesized that sequestration of FA is due to the enrichment of caveolin in the cytosolic leaflet and its ability to facilitate the formation of lipid rafts to buffer high FA levels. Here we show that ec-topic expression of caveolin-3 also results in enhanced FA sequestration. To further discriminate the effect that caveolins have on transmembrane FA movement and distribution, we labeled the outer membrane leaflet with fluorescein-phosphatidylethanolamine (FPE), whose emission is quenched by the presence of FA anions. Real-time measurements made with FPE and control experiments with positively charged fatty amines support our hypothesis that caveolins promote localization of FA anions through interactions with basic amino acid residues (lysines and arginines) present at the C termini of caveolins-1 and -3.

Fluorophore labeling of the glycine-rich loop as a method of identifying inhibitors that bind to active and inactive kinase conformations

Targeting protein kinases with small organic molecules is a promising strategy to regulate unwanted kinase activity in both chemical biology and medicinal chemistry research. Traditionally, kinase inhibitors are identified in activity-based screening assays using enzymatically active kinase preparations to measure the perturbation of substrate phosphorylation, often resulting in the enrichment of classical ATP competitive (Type I) inhibitors. However, addressing enzymatically incompetent kinase conformations offers new opportunities for targeted therapies and is moving to the forefront of kinase inhibitor research. Here we report the development of a new FLiK (Fluorescent Labels in Kinases) binding assay to detect small molecules that induce changes in the conformation of the glycine-rich loop. Due to cross-talk between the glycine-rich loop and the activation loop in kinases, this alternative labeling approach can also detect ligands that stabilize inactive kinase conformations, including slow-binding Type II and Type III kinase inhibitors. Protein X-ray crystallography validated the assay results and identified a novel DFG-out binding mode for a quinazoline-based inhibitor in p38alpha kinase. We also detected the high-affinity binding of a clinically relevant and specific VEGFR2 inhibitor, and we provide structural details of its binding mode in p38alpha, in which it stabilizes the DFG-out conformation. Last, we demonstrate the power of this new FLiK labeling strategy to detect the binding of Type I ligands that induce conformational changes in the glycine-rich loop as a means of gaining affinity for the target kinase. This approach may be a useful alternative to develop direct binding assays for kinases that do not adopt the DFG-out conformation while also avoiding the use of expensive kits, detection reagents, or radioactivity frequently employed with activity-based assays.

Proteus in the world of proteins: conformational changes in protein kinases

The 512 protein kinases encoded by the human genome are a prime example of nature's ability to create diversity by introducing variations to a highly conserved theme. The activity of each kinase domain is controlled by layers of regulatory mechanisms involving different combinations of post-translational modifications, intramolecular contacts, and intermolecular interactions. Ultimately, they all achieve their effect by favoring particular conformations that promote or prevent the kinase domain from catalyzing protein phosphorylation. The central role of kinases in various diseases has encouraged extensive investigations of their biological function and three-dimensional structures, yielding a more detailed understanding of the mechanisms that regulate protein kinase activity by conformational changes. In the present review, we discuss these regulatory mechanisms and show how conformational changes can be exploited for the design of specific inhibitors that lock protein kinases in inactive conformations. In addition, we highlight recent developments to monitor ligand-induced structural changes in protein kinases and for screening and identifying inhibitors that stabilize enzymatically incompetent kinase conformations.

High-throughput screening to identify inhibitors which stabilize inactive kinase conformations in p38alpha

Small molecule kinase inhibitors are an attractive means to modulate kinase activities in medicinal chemistry and chemical biology research. In the physiological setting of a cell, kinase function is orchestrated by a plethora of regulatory processes involving the structural transition of kinases between inactive and enzymatically competent conformations and vice versa. The development of novel kinase inhibitors is mainly fostered by high-throughput screening initiatives where the small molecule perturbation of the phosphorylation reaction is measured to identify inhibitors. Such setups require enzymatically active kinase preparations and present a risk of solely identifying classical ATP-competitive Type I inhibitors. Here we report the high-throughput screening of a library of approximately 35000 small organic molecules with an assay system that utilizes enzymatically inactive human p38alpha MAP kinase to detect stabilizers of the pharmacologically more desirable DFG-out conformation. We used protein X-ray crystallography to characterize the binding mode of hit compounds and reveal structural features which explain how these ligands stabilize and/or induce the DFG-out conformation. Lastly, we show that although some of the hit compounds were confirmed by protein X-ray crystallography, they were not detected in classic phosphorylation assays, thus validating the unique sensitivity of the assay system used in this study and highlighting the potential of screening with inactive kinase preparations.

Development of a fluorescent-tagged kinase assay system for the detection and characterization of allosteric kinase inhibitors

Kinase disregulation disrupts the intricate network of intracellular signaling pathways and contributes to the onset of diseases such as cancer. Although several kinase inhibitors are on the market, inhibitor selectivity and drug resistance mutations persist as fundamental challenges in the development of effective long-term treatments. Chemical entities binding to less conserved allosteric sites would be expected to offer new opportunities for scaffold development. Because no high-throughput method was previously available, we developed a fluorescence-based kinase binding assay for identifying and characterizing ligands which stabilize the inactive kinase conformation. Here, we present a description of the development and validation of this assay using the serine/threonine kinase p38alpha. By covalently attaching fluorophores to the activation loop of the kinase, we were able to detect conformational changes and measure the K(d), k(on), and k(off) associated with the binding and dissociation of ligands to the allosteric pocket. We report the SAR of a synthesized focused library of pyrazolourea derivatives, a scaffold known to bind with high affinity to the allosteric pocket of p38alpha. Additionally, we used protein X-ray crystallography together with our assay to examine the binding and dissociation kinetics to characterize potent quinazoline- and quinoline-based type II inhibitors, which also utilize this binding pocket in p38alpha. Last, we identified the b-Raf inhibitor sorafenib as a potent low nanomolar inhibitor of p38alpha and used protein X-ray crystallography to confirm a unique binding mode to the inactive kinase conformation.

Fast diffusion of very long chain saturated fatty acids across a bilayer membrane and their rapid extraction by cyclodextrins: implications for adrenoleukodystrophy

Abnormalities in the transport of saturated very long chain fatty acids (VLCFA; >C18:0) contribute to their toxic levels in peroxisomal disorders of fatty acid metabolism, such as adrenoleukodystrophy and adrenomyeloneuropathy. We previously showed that VLCFA desorb much slower than normal dietary fatty acids from both albumin and protein-free lipid bilayers. The important step of transbilayer movement (flip-flop) was not measured directly as a consequence of this very slow desorption from donors, and the extremely low aqueous solubility of VLCFA precludes addition of unbound VLCFA to lipid membranes. We have overcome these limitations using methyl-beta-cyclodextrin to solubilize VLCFA for rapid delivery to "acceptor" phosphatidylcholine vesicles (small and large unilamellar) and to cells. VLCFA binding was monitored in real time with the fluorescent probe fluorescein-labeled phosphatidylethanolamine in the outer membrane leaflet, and entrapped pyranine was used to detect flip-flop across the membrane. The upper limit of the rate of flip-flop across the membrane was independent of temperature and media viscosity and was similar for model raft and non-raft membranes as well as living cells. We further showed that cyclodextrins can extract VLCFA rapidly (within seconds) from vesicles and cells, which have implications for the mechanism and potential alternative approaches to treat adrenoleukodystrophy. Because VLCFA diffuse through the lipid bilayer, proteins may not be required for their transport across the peroxisomal membrane.

Hybrid compound design to overcome the gatekeeper T338M mutation in cSrc

The emergence of drug resistance remains a fundamental challenge in the development of kinase inhibitors that are effective over long-term treatments. Allosteric inhibitors that bind to sites lying outside the highly conserved ATP pocket are thought to be more selective than ATP-competitive inhibitors and may circumvent some mechanisms of drug resistance. Crystal structures of type I and allosteric type III inhibitors in complex with the tyrosine kinase cSrc allowed us to employ principles of structure-based design to develop these scaffolds into potent type II kinase inhibitors. One of these compounds, 3c (RL46), disrupts FAK-mediated focal adhesions in cancer cells via direct inhibition of cSrc. Details gleaned from crystal structures revealed a key feature of a subset of these compounds, a surprising flexibility in the vicinity of the gatekeeper residue that allows these compounds to overcome a dasatinib-resistant gatekeeper mutation emerging in cSrc.

Location of high and low affinity fatty acid binding sites on human serum albumin revealed by NMR drug-competition analysis

Human serum albumin (HSA) is an abundant plasma protein that transports a wide variety of drugs and endogenous compounds. The complex binding capacity of HSA has made it a challenging system to study in detail but in order to develop our understanding of the interactions between ligands for HSA, the locations and relative affinities of different ligand binding sites must be determined. Albumin possesses multiple binding sites for its primary physiological ligand, non-esterified fatty acids (FA). Previously, titration of BSA with (13)C-labeled FA revealed multiple chemical shifts and allowed identification of a subset of three chemical shifts that were associated with high affinity FA binding. Recent crystallographic studies of HSA have mapped at least seven FA binding sites for long-chain FA and delineated the overlap with binding sites for drugs and other endogenous compounds. We aim to correlate NMR and structural data for FA to provide a more complete description of the binding capacity of HSA. Our recent mutagenesis studies allowed us to identify two high affinity binding sites in domain III of HSA. Here, we use NMR to study the binding of (13)C-carboxyl labeled palmitate to HSA in the presence and absence of competitor ligands to complete the correlation of NMR chemical shifts with specific structural binding sites. We carefully selected ligands with specific binding sites identified by crystallography and used them, either singly or in combination, to compete with [(13)C]palmitate for binding to HSA. We show that FA sites 2, 4 and 5 bind FA with high affinity, while sites 1, 3, 6 and 7 exhibit low affinity for FA, thus providing the first complete determination of relative affinities of all seven long-chain FA sites on HSA. Our results also yield direct insights into the interactions between FA and other ligands.

Role of caveolin-1 and cholesterol in transmembrane fatty acid movement

We have created by transfection a series of HEK 293 cell lines that express varying amounts of caveolin-1 to test the possible effect of this protein on the transport and metabolism of long chain fatty acids (FA) in cells with this gain of function. We used an extracellular fluorescent probe (ADIFAB) to monitor binding of exogenous FA to the plasma membrane and an intracellular pH probe to monitor FA equilibration across the plasma membrane. Real-time fluorescence measurements showed rapid binding of oleic acid to the extracellular side of the plasma membrane and a rapid translocation across the lipid bilayer by the flip-flop mechanism (<5 s). Two cell lines expressing levels of caveolin-1 roughly comparable to that of adipocytes, which have a very high level of endogenous expression of caveolin-1, showed a relatively slow change in intracellular pH (t(1/2) < 100 s) in addition to the fast changes in fluorescence. We interpret this additional second phase to represent translocation of additional FA from the outer to inner leaflet of the plasma membrane. The slower kinetics could represent either slower flip-flop of FA across highly organized, rigid regions of the plasma membrane or binding of FA to caveolin-1 in the intracellular leaflet of the plasma membrane. The kinetics of palmitate and elaidate (a trans FA) transmembrane movement were identical to that for oleate. These results were observed in the absence of the putative FA transport protein, CD36, and in the absence of any changes in expression of fatty acid transport proteins (FATP) 2 and 4, and are in direct correlation with increased cellular free cholesterol content. FA metabolism was slow in all cell lines and was not enhanced by caveolin-1 expression. We conclude that transport of FA across the plasma membrane is modulated by caveolin-1 and cholesterol and is not dependent on the putative FA transport proteins CD36 and FATP.

Locating high-affinity fatty acid-binding sites on albumin by x-ray crystallography and NMR spectroscopy

Human serum albumin (HSA) is a versatile transport protein for endogenous compounds and drugs. To evaluate physiologically relevant interactions between ligands for the protein, it is necessary to determine the locations and relative affinities of different ligands for their binding site(s). We present a site-specific investigation of the relative affinities of binding sites on HSA for fatty acids (FA), the primary physiological ligand for the protein. Titration of HSA with [(13)C]carboxyl-labeled FA was used initially to identify three NMR chemical shifts that are associated with high-affinity binding pockets on the protein. To correlate these peaks with FA-binding sites identified from the crystal structures of FA-HSA complexes, HSA mutants were engineered with substitutions of amino acids involved in coordination of the bound FA carboxyl. Titration of [(13)C]palmitate into solutions of HSA mutants for either FA site four (R410A/Y411A) or site five (K525A) within domain III of HSA each revealed loss of a specific NMR peak that was present in spectra of wild-type protein. Because these peaks are among the first three to be observed on titration of HSA with palmitate, sites four and five represent two of the three high-affinity long-chain FA-binding sites on HSA. These assignments were confirmed by titration of [(13)C]palmitate into recombinant domain III of HSA, which contains only sites four and five. These results establish a protocol for direct probing of the relative affinities of FA-binding sites, one that may be extended to examine competition between FA and other ligands for specific binding sites.