The Discovery of Novel Protein Kinase Inhibitors by Using Fragment-Based High-Throughput X-ray Crystallography

Kinematic analysis of kinases and their oncogenic mutations - Kinases and their mutation kinematic analysis

Protein kinases are crucial cellular enzymes that facilitate the transfer of phosphates from adenosine triphosphate (ATP) to their substrates, thereby regulating numerous cellular activities. Dysfunctional kinase activity often leads to oncogenic conditions. Chosen by using structural similarity to 5UG9, we selected 79 crystal structures from the PDB and based on the position of the phenylalanine side chain in the DFG motif, we classified these 79 crystal structures into 5 group clusters. Our approach applies our kinematic flexibility analysis (KFA) to explore the flexibility of kinases in various activity states and examine the impact of the activation loop on kinase structure. KFA enables the rapid decomposition of macromolecules into different flexibility regions, allowing comprehensive analysis of conformational structures. The results reveal that the activation loop of kinases acts as a "lock" that stabilizes the active conformation of kinases by rigidifying the adjacent α-helices. Furthermore, we investigate specific kinase mutations, such as the L858R mutation commonly associated with non-small cell lung cancer, which induces increased flexibility in active-state kinases. In addition, through analyzing the hydrogen bond pattern, we examine the substructure of kinases in different states. Notably, active-state kinases exhibit a higher occurrence of α-helices compared to inactive-state kinases. This study contributes to the understanding of biomolecular conformation at a level relevant to drug development.

The Application of High-Throughput Approaches in Identifying Novel Therapeutic Targets and Agents to Treat Diabetes

During the past decades, unprecedented progress in technologies has revolutionized traditional research methodologies. Among these, advances in high-throughput drug screening approaches have permitted the rapid identification of potential therapeutic agents from drug libraries that contain thousands or millions of molecules. Moreover, high-throughput-based therapeutic target discovery strategies can comprehensively interrogate relationships between biomolecules (e.g., gene, RNA, and protein) and diseases and significantly increase the authors' knowledge of disease mechanisms. Diabetes is a chronic disease primarily characterized by the incapacity of the body to maintain normoglycemia. The prevalence of diabetes in modern society has become a severe public health issue that threatens the well-being of millions of patients. Although a number of pharmacological treatments are available, there is no permanent cure for diabetes, and discovering novel therapeutic targets and agents continues to be an urgent need. The present review discusses the technical details of high-throughput screening approaches in drug discovery, followed by introducing the applications of such approaches to diabetes research. This review aims to provide an example of the applicability of high-throughput technologies in facilitating different aspects of disease research.

A computational fragment-based de novo design protocol guided by ligand efficiency indices (LEI)

We present a new protocol aimed at the structure-based design of drug-like molecules using a fragment approach. It starts from a suitably placed and well-defined "base fragment" and then uses an incremental construction algorithm and a scoring function to grow the molecule into prioritized candidates. The selection of the most promising solutions for synthesis and validation is guided by the optimization of the calculated ligand efficiency indices known as binding efficiency index (BEI) and surface efficiency index (SEI), which allow the user to navigate proficiently in chemico-biological space. A test case for the protocol is exemplified here using published data for inhibitors of protein kinase B, aka AKT, a key enzyme in several signal transduction pathways. Our procedure was able to identify the main features responsible for the binding of inhibitors and guided the selection process towards molecules that included or resembled those shown as the most active in the original studies.

A three-stage biophysical screening cascade for fragment-based drug discovery

This protocol describes the screening of a library of low-molecular-weight compounds (fragments) using a series of biophysical ligand-binding assays. Fragment-based drug discovery (FBDD) has emerged as a successful method to design high-affinity ligands for biomacromolecules of therapeutic interest. It involves detecting relatively weak interactions between the fragments and a target macromolecule using sensitive biophysical techniques. These weak binders provide a starting point for the development of inhibitors with submicromolar affinity. Here we describe an efficient fragment screening cascade that can identify binding fragments (hits) within weeks. It is divided into three stages: (i) preliminary screening using differential scanning fluorimetry (DSF), (ii) validation by NMR spectroscopy and (iii) characterization of binding fragments by isothermal titration calorimetry (ITC) and X-ray crystallography. Although this protocol is readily applicable in academic settings because of its emphasis on low cost and medium-throughput early-stage screening technologies, the core principle of orthogonal validation makes it robust enough to meet the quality standards of an industrial laboratory.

Targeting the unactivated conformations of protein kinases for small molecule drug discovery

BACKGROUND

The number of drugs in active clinical development or on the market that target the unactivated conformational states of protein kinases is growing and represents a significant portion of kinase research at biopharmaceutical companies. These non-classical kinase inhibitors have a mode of action which may overcome some of the liabilities of classical ATP-site inhibitors that substantially overlap the space that ATP occupies in the activated kinase.

OBJECTIVE

This review will discuss state-of-the-art methods of inhibiting protein kinases by targeting the unactivated conformations of the enzyme with small molecules directed to the ATP binding region.

METHODS

Biochemical and structural biology publications and public domain crystal structures were evaluated to identify key concepts in drug discovery for unactivated protein kinase inhibitors that target the ATP binding region.

CONCLUSION

The potential for enhanced selectivity, potency and duration of pharmacological action may allow non-classical kinase therapeutics to be used for chronic dosing in non-life-threatening indications. Moreover, by targeting additional conformational space on the kinase protein it is possible that new chemical matter will be discovered such that current intellectual property limitations on traditional ATP-site chemical scaffolds may be circumvented.

Potent, selective inhibitors of fibroblast growth factor receptor define fibroblast growth factor dependence in preclinical cancer models

We describe here the identification and characterization of 2 novel inhibitors of the fibroblast growth factor receptor (FGFR) family of receptor tyrosine kinases. The compounds exhibit selective inhibition of FGFR over the closely related VEGFR2 receptor in cell lines and in vivo. The pharmacologic profile of these inhibitors was defined using a panel of human tumor cell lines characterized for specific mutations, amplifications, or translocations known to activate one of the four FGFR receptor isoforms. This pharmacology defines a profile for inhibitors that are likely to be of use in clinical settings in disease types where FGFR is shown to play an important role.

Drugging challenging targets using fragment-based approaches

Fragment-based approaches have now become firmly established in the drug discovery armoury. After notable early successes against protein kinases, the versatility and power of fragment-based approaches are increasingly being demonstrated on more diverse and difficult protein targets. This review highlights seven examples including targeting protein-protein interactions, a RNA polymerase and a DNA-binding protein. It shows how fragment-based approaches using small libraries have been successful when large HTS screens have failed. It also highlights the range of biophysical approaches being used and the interplay between experimental and in silico screens. The examples all show the iterative way in which potency is built up by synthetic elaboration of the initial fragment hits.

Transforming fragments into candidates: small becomes big in medicinal chemistry

Fragment-based drug discovery (FBDD) represents a logical and efficient approach to lead discovery and optimisation. It can draw on structural, biophysical and biochemical data, incorporating a wide range of inputs, from precise mode-of-binding information on specific fragments to wider ranging pharmacophoric screening surveys using traditional HTS approaches. It is truly an enabling technology for the imaginative medicinal chemist. In this review, we analyse a representative set of 23 published FBDD studies that describe how low molecular weight fragments are being identified and efficiently transformed into higher molecular weight drug candidates. FBDD is now becoming warmly endorsed by industry as well as academia and the focus on small interacting molecules is making a big scientific impact.

Identification and selection of "privileged fragments" suitable for primary screening

The use of small molecule libraries for fragment-based primary screening (FBS) is a well-known approach to identify protein binders in the low affinity range. However, the search, analysis, and selection of suitable screening fragments can be a lengthy process, because of the large number of compounds that must be analyzed for different levels of ring/substituents identification and submitted to selection/exclusion criteria based on their physicochemical properties. The purpose of the present work is to propose a strategy to identify substructures from databases of known drugs, which can be used as templates for the generation of libraries of "privileged fragments" that are able to provide high-quality hits. The entire process has been developed integrating Pipeline Pilot (Accelrys Inc., San Diego, CA; http://www.accelrys.com ) native components and user-defined molecular files containing ISIS-like substructure query features (Symyx, San Ramon, CA; http://www.symyx.com ). The method is effortless, easy to put in place, and fast enough to be iteratively applied to different sources of druglike compounds.

Perspectives on NMR in drug discovery: a technique comes of age

In the past decade, the potential of harnessing the ability of nuclear magnetic resonance (NMR) spectroscopy to monitor intermolecular interactions as a tool for drug discovery has been increasingly appreciated in academia and industry. In this Perspective, we highlight some of the major applications of NMR in drug discovery, focusing on hit and lead generation, and provide a critical analysis of its current and potential utility.

Imidazolyl benzimidazoles and imidazo[4,5-b]pyridines as potent p38alpha MAP kinase inhibitors with excellent in vivo antiinflammatory properties

Herein we report investigations into the p38alpha MAP kinase activity of trisubstituted imidazoles that led to the identification of compounds possessing highly potent in vivo activity. The SAR of a novel series of imidazopyridines is demonstrated as well, resulting in compounds possessing cellular potency and enhanced in vivo activity in the rat collagen-induced arthritis model of chronic inflammation.

Ligand design by a combinatorial approach based on modeling and experiment: application to HLA-DR4

Combinatorial synthesis and large scale screening methods are being used increasingly in drug discovery, particularly for finding novel lead compounds. Although these "random" methods sample larger areas of chemical space than traditional synthetic approaches, only a relatively small percentage of all possible compounds are practically accessible. It is therefore helpful to select regions of chemical space that have greater likelihood of yielding useful leads. When three-dimensional structural data are available for the target molecule this can be achieved by applying structure-based computational design methods to focus the combinatorial library. This is advantageous over the standard usage of computational methods to design a small number of specific novel ligands, because here computation is employed as part of the combinatorial design process and so is required only to determine a propensity for binding of certain chemical moieties in regions of the target molecule. This paper describes the application of the Multiple Copy Simultaneous Search (MCSS) method, an active site mapping and de novo structure-based design tool, to design a focused combinatorial library for the class II MHC protein HLA-DR4. Methods for the synthesizing and screening the computationally designed library are presented; evidence is provided to show that binding was achieved. Although the structure of the protein-ligand complex could not be determined, experimental results including cross-exclusion of a known HLA-DR4 peptide ligand (HA) by a compound from the library. Computational model building suggest that at least one of the ligands designed and identified by the methods described binds in a mode similar to that of native peptides.

Fragment-based lead discovery: a chemical update

Fragment-based lead discovery constructs drug leads from small molecular fragments. In theory, this is a highly efficient method for drug discovery, and the technique has become enormously popular in the past few years. In this review, I describe how a variety of approaches in fragment-based lead discovery--including NMR, X-ray crystallography, mass spectrometry, functional screening, and in silico screening--have produced drug leads. Although the examples show that the technique can reliably generate potent molecules, there is still much work to be done to maintain the efficiency of molecules' binding affinities as fragments are linked, expanded, and otherwise improved.

Structure-based development of target-specific compound libraries

The success or failure of a small-molecule drug discovery project ultimately lies in the choice of the scaffolds to be screened -- chosen from among the many millions of available compounds. Therefore, the methods used to design compound screening libraries are key for the development of new drugs that target a wide range of diseases. Currently, there is a trend towards the construction of receptor-structure-based focused libraries. Recent advances in high-throughput computational docking, NMR and crystallography have facilitated the development of these libraries. A structure-based target-specific library can save time and money by reducing the number of compounds to be experimentally tested, also improving the drug discovery success rate by identifying more-potent and specific binders.

Antibacterial drug discovery and structure-based design

Bacterial resistance continues to develop and pose a significant threat, both in hospitals and, more recently, in the community. A focus on other therapeutic areas by the larger pharmaceutical companies has left a shortfall in the pipeline of novel antibacterials. Recently, many new structures have been studied by structure-genomics initiatives, delivering a wealth of targets to consider. Using the tools of structure-based design, antibacterial discovery must exploit these targets to accelerate the process of drug discovery.

Second-generation kinase inhibitors

An increasing number of kinase inhibitor candidates are entering clinical development, representing an important change in the pharmaceutical industry; notably, the development of small-molecule kinase inhibitors for signal transduction therapies. Today, kinase inhibitors garner substantial attention in cancer research. Over the last few years, three distinct small-molecule kinase inhibitors reached the market for treatment of chronic myeloid leukaemia, gastrointestinal stromal tumours, and non-small cell lung cancers. These three drugs, imatinib, gefitinib and erlotinib, act on a distinct subset of dysregulated, and often cancer-relevant kinases. Imatinib, gefitinib and erlotinib are considered the front-runners of targeted kinase inhibitor drugs. The entire research field gains tremendous insights through the ongoing research and clinical trials with these three drugs and with fast following first-generation kinase inhibitors, many of which are in different phases of clinical development. In addition, novel chemogenomic and chemoproteomic technologies are emanating from the current kinase research area, focussing efforts on the generation of spectrum-selective inhibitors for anticancer therapies as opposed to the monospecific inhibitors for the remaining therapeutic areas.

Solution Nuclear Magnetic Resonance Spectroscopy Techniques for Probing Intermolecular Interactions

Nuclear magnetic resonance (NMR) spectroscopy in solution has evolved into a powerful technique for structure determination of proteins and nucleic acids. More recently, a number of NMR-based approaches have been developed to monitor and characterize intermolecular interactions. These approaches offer unique advantages over other techniques and find their utility in both structural biology and drug discovery. We will report on basic principles and recent examples of the application of such NMR methodologies to characterize protein-protein interactions and for ligand binding studies and drug discovery.