The use of conformationally thermostabilised GPCRs in drug discovery: application to fragment, structure and biophysical techniques

Recent developments in receptor stabilisation have facilitated major advances in G protein-coupled receptor (GPCR) research, notably structural biology, over the past eight years. Here we review the application of fragment, structure and biophysical techniques using stabilised GPCRs (StaR proteins), and their impact in the drug discovery process. These techniques have, most recently, been utilised in the discovery of the non-alkyne mGlu₅ negative allosteric modulator HTL14242, in addition to the dual orexin receptor antagonist HTL6641, with differentiated residence time kinetics.

Structurally Enabled Discovery of Adenosine A2A Receptor Antagonists

Over the past decade there has been a revolution in the field of G protein-coupled receptor (GPCR) structural biology. Many years of innovative research from different areas have come together to fuel this significant change in the fortunes of this field, which for many years was characterized by the paucity of high-resolution structures. The determination to succeed has been in part due to the recognized importance of these proteins as drug targets, and although the pharmaceutical industry has been focusing on these receptors, it can be justifiably argued and demonstrated that many of the approved and commercially successful GPCR drugs can be significantly improved to increase efficacy and/or reduce undesired side effects. In addition, many validated targets in this class remain to be drugged. It is widely recognized that application of structure-based drug design approaches can help medicinal chemists a long way toward discovering better drugs. The achievement of structural biologists in providing high-resolution insight is beginning to transform drug discovery efforts, and there are a number of GPCR drugs that have been discovered by use of structural information that are in clinical development. This review aims to highlight the key developments that have brought success to GPCR structure resolution efforts and exemplify the practical application of structural information for the discovery of adenosine A_2A receptor antagonists that have potential to treat multiple conditions.

Diamond: shedding light on structure-based drug discovery

Structure-based drug design has become a key tool for the development of novel drugs. The process involves elucidating the three-dimensional structure of the potential drug molecule bound to the target protein that has been identified as playing a key role in the disease state. Using this three-dimensional information facilitates the process of making improvements to the potential drug molecule by highlighting existing and possible new interactions within the binding site. This knowledge is used to inform increases in potency and selectivity of the molecules as well as to help improve other drug-like properties. The speed and numbers of samples that can be studied, combined with the improved resolution of the structures that can be obtained using synchrotron radiation, have had a significant impact on the utilization of crystallography in the drug discovery process.

Structure-based drug design for G protein-coupled receptors

Our understanding of the structural biology of G protein-coupled receptors has undergone a transformation over the past 5 years. New protein-ligand complexes are described almost monthly in high profile journals. Appreciation of how small molecules and natural ligands bind to their receptors has the potential to impact enormously how medicinal chemists approach this major class of receptor targets. An outline of the key topics in this field and some recent examples of structure- and fragment-based drug design are described. A table is presented with example views of each G protein-coupled receptor for which there is a published X-ray structure, including interactions with small molecule antagonists, partial and full agonists. The possible implications of these new data for drug design are discussed.

Structure-based drug design of chromone antagonists of the adenosine A2A receptor

Structure-based drug design identifies potent & selective GPCR antagonists.

Structure-based approaches to ligands for G-protein-coupled adenosine and P2Y receptors, from small molecules to nanoconjugates

Adenosine receptor (ARs) and P2Y receptors (P2YRs) that respond to extracellular nucleosides/nucleotides are associated with new directions for therapeutics. The X-ray structures of the A2AAR complexes with agonists and antagonists are examined in relationship to the G-protein-coupled receptor (GPCR) superfamily and applied to drug discovery. Much of the data on AR ligand structure from early SAR studies now are explainable from the A2AAR X-ray crystallography. The ligand-receptor interactions in related GPCR complexes can be identified by means of modeling approaches, e.g., molecular docking. Thus, molecular recognition in binding and activation processes has been studied effectively using homology modeling and applied to ligand design. Virtual screening has yielded new nonnucleoside AR antagonists, and existing ligands have been improved with knowledge of the receptor interactions. New agonists are being explored for central nervous system and peripheral therapeutics based on in vivo activity, such as chronic neuropathic pain. Ligands for receptors more distantly related to the X-ray template, i.e., P2YRs, have been introduced and are mainly used as pharmacological tools for elucidating the physiological role of extracellular nucleotides. Other ligand tools for drug discovery include fluorescent probes, radioactive probes, multivalent probes, and functionalized nanoparticles.

Fragment-based lead discovery on G-protein-coupled receptors

INTRODUCTION

G-protein-coupled receptors (GPCRs) form one of the largest groups of potential targets for novel medications. Low druggability of many GPCR targets and inefficient sampling of chemical space in high-throughput screening expertise however often hinder discovery of drug discovery leads for GPCRs. Fragment-based drug discovery is an alternative approach to the conventional strategy and has proven its efficiency on several enzyme targets. Based on developments in biophysical screening techniques, receptor stabilization and in vitro assays, virtual and experimental fragment screening and fragment-based lead discovery recently became applicable for GPCR targets.

AREAS COVERED

This article provides a review of the biophysical as well as biological detection techniques suitable to study GPCRs together with their applications to screen fragment libraries and identify fragment-size ligands of cell surface receptors. The article presents several recent examples including both virtual and experimental protocols for fragment hit discovery and early hit to lead progress.

EXPERT OPINION

With the recent progress in biophysical detection techniques, the advantages of fragment-based drug discovery could be exploited for GPCR targets. Structural information on GPCRs will be more abundantly available for early stages of drug discovery projects, providing information on the binding process and efficiently supporting the progression of fragment hit to lead. In silico approaches in combination with biological assays can be used to address structurally challenging GPCRs and confirm biological relevance of interaction early in the drug discovery project.

Biophysical fragment screening of the β1-adrenergic receptor: identification of high affinity arylpiperazine leads using structure-based drug design

Biophysical fragment screening of a thermostabilized β1-adrenergic receptor (β1AR) using surface plasmon resonance (SPR) enabled the identification of moderate affinity, high ligand efficiency (LE) arylpiperazine hits 7 and 8. Subsequent hit to lead follow-up confirmed the activity of the chemotype, and a structure-based design approach using protein-ligand crystal structures of the β1AR resulted in the identification of several fragments that bound with higher affinity, including indole 19 and quinoline 20. In the first example of GPCR crystallography with ligands derived from fragment screening, structures of the stabilized β1AR complexed with 19 and 20 were determined at resolutions of 2.8 and 2.7 Å, respectively.