Fluorescent GPCR ligands as new tools in pharmacology

The expansion of fluorescent techniques for studying the ligand-receptor interaction resulted in a burst of the novel fluorescent ligands development. The discovery of the ligand, that is of high affinity to the receptor and whose localization could be easily visualized, even on the single cell level, gave the researchers a strong impulse to investigate that field of GPCR ligands. Moreover, paying attention to the "non pharmacological" advantages of these ligands, as well as the techniques to be used, fluorescent ligands are becoming treated more seriously, as the ligands themselves, and as novel, useful tools for studying GPCRs. Herein, we review results described in the literature, starting from the year 2000, in the field of the fluorescent GPCR small, non-peptide ligands according to the affinity to the selected receptors (histamine, adenosine, adrenergic, cannabinoid, muscarinie, neuropeptide Y and serotonine) as well as the fluorophores that have been used to tag the molecules.     

A2A adenosine receptor and its modulators: overview on a druggable GPCR and on structure-activity relationship analysis and binding requirements of agonists and antagonists

Since the discovery of the biological effects of adenosine, the development of potent and selective agonists and antagonists of adenosine receptors has been the subject of medicinal chemistry research for several decades, even if their clinical evaluation has been discontinued. Main problems include side effects due to the ubiquity of the receptors and the possibility of side effects, or to low brain penetration (in particular for the targeting of CNS diseases), short half-life of compounds, lack of effects. Furthermore, species differences in the affinity of ligands make difficult preclinical testing in animal models. Nevertheless, adenosine receptors continue to represent promising drug targets. A(2A) receptor has proved to be a promising pharmacological target for small synthetic ligands, and while A(2A) agonists are undergoing clinical trials for myocardial perfusion imaging and as anti-inflammatory agents, A(2A) antagonists represent an attractive field of research to discover new drugs for the treatment of neurodegenerative disorders, such as Parkinson's disease. Furthermore, the information coming from bioinformatics and molecular modeling studies for the A(2A) receptor has made easier the understanding of ligand-target interaction and the rational design of agonists and antagonists for this subtype. The aim of this review is to show an overview of the most significant steps and progresses in developing A(2A) adenosine receptor agonists and antagonists.     

Medicinal chemistry of A2A adenosine receptor antagonists

Due to the clearly demonstrated receptor-receptor interaction between adenosine A(2A) and dopamine D(2) receptors in the basal ganglia, the discovery and development of potent and selective A(2A)adenosine receptor antagonists became, in the last ten years, an attractive field of research to discovery new drugs for the treatment of neurodegenerative disorders, such as Parkinsons disease. Different compounds have been deeply investigated as A(2A) adenosine receptor antagonists, which could be classified in two great families: xanthine derivatives and nitrogen poliheterocyclic systems. These studies led to the discovery of some highly potent and selective A(2A) adenosine receptor antagonists such as ZM241385, SCH58261 and some xanthine derivatives (KW6002), which have been used as pharmacological tools for studying this receptor subtype. However, those compounds showed some problems that do not permit their use in clinical studies, such as poor water solubility (SCH58261, and xanthine derivatives) or good affinity for A(2B) adenosine receptor subtype (ZM241385). In the last few years great efforts have been made to overcome these problems, trying to optimize not only the pharmacological profile but also the pharmacokinetic character of this class of compounds. The aim of this report is to briefly summarize the recent progress made in this attractive field of research.     

Non-peptide opioid receptor ligands - recent advances. Part I - agonists

Developments in the domain of non-peptide opioid receptor agonists, beginning from the first evidence of opiate binding to definite receptors, are briefly summarized. The recent achievements are in a more detailed way depicted and discussed. Novel agonists for each of three opioid receptor basic types (delta, kappa and micro) are presented with the special emphasis on one-type-selective ligands. Such selective or even specific agonists have been synthesized with a moderate success. Considerably more serious difficulties concern searching for selective ligands for opioid receptor subtypes (micro(1), micro(2), delta(1), delta(2, kappasub>1), kappasub>2, kappasub>3) which may be connected with the fact that dissimiliarities observed in vivo result from postbinding processes (signaling). For the large number of opioid receptor ligands, their structural diversity and relative easiness of generating them from combinatorial libraries (not comparable even with that of orphanine receptors) it is justified to consider the plasticity of opioid receptors (micro-receptor especially). This remark, in conjunction with the existence of opioid receptor types and subtypes, may enable to create new drugs with significantly reduced side-effects. The above facts and brand new reports about highly-active opioid agonists possessing no moieties thought to be essential for agonist activity make the need of reevaluation of classical opioid receptor pharmacophore model extremely important. In general, research results suggest that selective agonists of opioid receptors can be found both in morphine type of ligands and new structures like pyrido-acridine derivatives (COMP1) or diphenylmethylpiperazine derivatives (SNC 80). Better understanding of the structural prerequisites of the opioid receptors binding domains will certainly lead to even more potent and more selective ligands in a near future.     

Previously unsuspected widespread cellular and tissue distribution of β-adrenoceptors and its relevance to drug action

The discovery of β-adrenoceptors in previously unsuspected cell types is contributing to the rethinking of new drug targets. Recent developments in β-adrenoceptor pharmacology might have excited and surprised James Black, given his interest in developing drugs based on the selective manipulation of receptors to alter physiological responses. β-adrenoceptors continue to generate surprises at molecular and pharmacological levels that often require knowledge of receptor location to interpret. In this review, we emphasize the use of fluorescent ligands as the most selective means of demonstrating receptor localization. Fluorescent ligand binding in live tissues can provide quantitative pharmacological data, under carefully controlled conditions, relevant to other signalling parameters. Consideration of the role of β-adrenoceptors in many cell types (previously ignored) is needed to understand the actions of drugs at β-adrenoceptors throughout the body, particularly in the lung epithelium, vascular endothelium, immune cells and other 'structural' and 'restorative' cell types.     

Physiological relevance of GPCR oligomerization and its impact on drug discovery

The potentially large functional and physiological diversity of G-protein coupled receptor (GPCR) dimers has generated a great deal of excitement about the opportunity that dimerization provides for enabling novel drug discovery. The discovery of physiologically relevant GPCR dimers suggests that new drug targets for diseases such as schizophrenia and pre-eclampsia can be developed by targeting dimers. Most of the previous work on GPCR dimers made use of the overexpression of differentially tagged GPCRs in heterologous cell systems. Current emphasis on the development of physiologically relevant cell systems that endogenously express the appropriate combination of GPCR dimers and accessory proteins is leading to dramatic increases in our understanding of GPCR dimers. These and other new tools such as GPCR-specific antibodies will be required to develop GPCR dimer specific drugs. Given that ligands are available for only a small percentage of the large number of potentially druggable GPCRs, the use of GPCR dimers might provide the necessary targets to increase the breadth and depth of receptors available for therapeutic interventions.     

Structural basis for computational screening of non-steroidal androgen receptor ligands

Importance of the field: Deep structural and chemical understanding of the protein target and computational methods for detection of receptor-selective ligands are important for the early drug discovery in the steroid receptor field. Areas covered in this review: This review focuses on the use of currently available structural information of the androgen receptor (AR) and known AR ligands to make computational strategies for the discovery of AR ligands in order to offer new chemical platforms for drug development. What the reader will gain: AR is a challenging target for drug discovery and modeling even if there is a wealth of experimental data available. First, only the active structure of AR is currently known, which hampers the design of AR antagonists. Second, the structural similarity between the ligand-binding sites of AR and its mutated forms and closely related steroid receptors (SRs) such as progesterone receptors presents challenges for the development of drugs with receptor-selective function. Take home message: Research indicates that a very small chemical change in the structure of a non-steroidal ligand can cause a complete change in its activity. One source of this effect arises from binding to similar binding sites in related SRs and other proteins in the signaling pathway. Currently, computational methods are not able to predict the subtle differences between AR ligand activities but modeling does offer the possibility of generating new lead structures that might have the desired properties.     

Physical and virtual screening methods for marine toxins and drug discovery targeting nicotinic acetylcholine receptors

Introduction: Nicotinic acetylcholine receptors (nAChRs) have been extensively studied because of their importance in physiological processes and their involvement in a number of muscle and neuronal human pathologies. However, the role of specific subtypes remains poorly understood due to the lack of selective nAChR probes. During the last decade, drug design strategies have been powered by a wide variety of natural compounds with diverse chemical structures, and by the structural characterization of several nAChRs structural homologs.

Areas covered: In this review, the authors present a short overview of nAChRs, and some natural sources of bioactive molecules targeting these receptors. The authors provide an emphasis on α-conotoxins from Conus venoms, which provide the most diverse selective antagonists of nAChRs known to date, as well briefly discussing macrocyclic imine toxins. The authors, furthermore, review valuable radioactive and non-radioactive methods used for discovering novel ligands targeting nAChRs and highlight high-throughput developments in receptor-binding and electrophysiological assays. Finally, the authors review the molecular modeling approaches used in the last few years with an aim to provide an overview of their potential to identify and optimize selective nAChR ligands.

Expert opinion: Recent years have provided new valuable techniques for the detection and identification of new nAChRs ligands, along with an increasing use of different molecular modeling tools. This furthering of knowledge has had an impact on the design and discovery of more potent and selective nAChRs ligands. There is still however a lack of high-resolution structural information that will require new developments.     

Discovery of an ectopic activation site on the M(1) muscarinic receptor

Receptors have well-conserved regions that are recognized and activated by hormones and neurotransmitters. Most drugs bind to these sites and mimic or block the action of the native ligands. Using a high-throughput functional screen, we identified a potent and selective M(1) muscarinic receptor agonist from a novel structural class. Using a series of chimeric receptors, we demonstrated that this ligand activates the receptor through a region that is not conserved among receptor subtypes, explaining its unprecedented selectivity. This region of the receptor is distinct from the conserved region that is recognized by traditional ligands. The finding that receptors for small-molecule transmitters can have multiple, structurally distinct activation sites has broad implications for the study of receptor structure/function and the potential for the discovery of novel ligands with high selectivity.     

Fluorescent probes for G-protein-coupled receptor drug discovery

ABSTRACT

Introduction: G-protein-coupled receptors (GPCRs) mediate the effects of approximately 33% of all marketed drugs. The development of tools to study GPCR pharmacology is urgently needed as it can lead to the discovery of safer and more effective medications. Fluorescent GPCR ligands represent highly sensitive and safe small-molecule tools for real-time exploration of the life of the receptor, cellular signaling, and ligand–/receptor–receptor interactions in cellulo and/or in vivo.

Areas covered: This review summarizes relevant information from published literature and provides critical insights into the design of successful small-molecule fluorescent probes for Class A GPCRs as potential major targets for drug development.

Expert opinion: Considering the rapid progress of fluorescence technologies, effective small-molecule fluorescent probes represent valuable pharmacological tools for studying GPCRs. However, the design and development of such probes are challenging, largely due to the low affinity/specificity of the probe for its target, inadequate photophysical properties, extensive non-specific binding, and/or low signal-to-noise ratio. Generally speaking, fluorescent and luminescent small-molecule probes, receptors, and G proteins in combination with FRET and BRET technologies hold great promise for studying kinetic profiles of GPCR signaling.     

The future of G protein-coupled receptors as targets in drug discovery

G protein-coupled receptors (GPCRs) represent the most abundant drug targets today. A large number of GPCR-based drugs have already been developed for a variety of indications in human disease. However, orphan receptors with unidentified ligands serve as potential targets still to be explored. Moreover, research on the interaction of GPCRs with different molecules in the signal transduction pathways, and further studies on receptor dimerization may also lead to the discovery of new drugs. Structure-based drug design will eventually play a key role in generating better and more selective drugs more rapidly when high-resolution structures of GPCRs can be provided by expression, purification and crystallography technologies.     

Non-peptide δ opioid agonists and antagonists

Following a flurry of medicinal chemistry activity in the late 1980s, a number of non-peptide pharmacological tools, selective for the δ opioid receptors, became available to challenge the pre-eminent position occupied by the existing peptide δ ligands. The first non-peptide δ antagonist NTI (1) represented a breakthrough in this field. Several analogues have been subsequently synthesised and are currently being used to clarify the pharmacology associated with the δ opioid receptors. The discovery of the selective δ agonists TAN-67 (50), BW373U86 (56) and SNC 80 (62) represented another step towards the understanding of the involvement of the δ opioid receptor in a number of possible pathophysiological conditions. This review addresses the recent highlights and developments that have been made by several research groups in the design of potent and selective non-peptide δ ligands. Focus has been given to the different pharmacological actions of δ agonists and antagonists. Analgesia can be considered the historical target for drugs acting through the opioidergic system. However, contrary to existing μ opioid narcotic drugs, there is substantial evidence to suggest that selective δ opioid agonists may be safe and effective analgesics. Beside this very important therapeutic target, recent studies have revealed that such drugs may elicit a variety of other beneficial pharmacological effects. They may also positively modulate some activities of μ agonists such as morphine. Animal models have demonstrated that δ antagonists may also play an important pharmacological role per se having possible clinical applications in preventing drug abuse, in organ transplantation and as antitussive agents.     

Novel benzodiazepine photoaffinity probe stereoselectively labels a site deep within the membrane-spanning domain of the cholecystokinin receptor

An understanding of the molecular basis of drug action provides opportunities for refinement of drug properties and for development of more potent and selective molecules that act at the same biological target. In this work, we have identified the active enantiomers in racemic mixtures of structurally related benzophenone derivatives of 1,5-benzodiazepines, representing both antagonist and agonist ligands of the type A cholecystokinin receptor. The parent compounds of the 1,5-benzodiazepine CCK receptor photoaffinity ligands were originally prepared in an effort to develop orally active drugs. The enantiomeric compounds reported in this study selectively photoaffinity-labeled the CCK receptor, resulting in the identification of a site of attachment for the photolabile moiety of the antagonist probe deep within the receptor's membrane-spanning region at Leu(88), a residue within transmembrane segment two. In contrast, the agonist probe labeled a region including extracellular loop one and a portion of transmembrane segment three. The antagonist covalent attachment site to the receptor served as a guide in the construction of theoretical three-dimensional molecular models for the antagonist-receptor complex. These models provided a means for visualization of physically plausible ligand-receptor interactions in the context of all currently available biological data that address small molecule interactions with the CCK receptor. Our approach, featuring the use of novel photolabile compounds targeting the membrane-spanning receptor domain to probe the binding site region, introduces powerful tools and a strategy for direct and selective investigation of nonpeptidyl ligand binding to peptide receptors.     

Discovery of Potent Dual PPARα Agonists/CB1 Ligands

相似文献   

Therapeutic and chemical developments of cholecystokinin receptor ligands

Cholecystokinin (CCK) is an important 'brain-gut' hormone located both in the gastrointestinal (GI) system and in the CNS. At least two different G-coupled high affinity receptors have been identified: the CCK-A and the CCK-B receptors. Although the complex biological role of CCK is, as yet, not fully understood, its connection with many different physiological processes both at the GI level and at the CNS level is now well established. There is much potential for therapeutic use of CCK receptor ligands, however, clear investigations have yet to be completed. Several chemical families have been investigated over the last 20 years to find potent, subtype selective and stable CCK receptor agonists and antagonists. The main goal was to discover new therapeutic drugs acting on GI and/or on CNS diseases and also, to obtain powerful pharmacological tools that could permit a better understanding of the biological role of CCK. Despite promising results from investigations into medicinal chemistry of CCK receptor ligands, the therapeutical applications of these ligands still remains to be defined. This article reviews the main biological role of CCK, the therapeutic potential of CCK-A and CCK-B receptor agonists and antagonists and the common compounds from the different families of ligands.     

Discovery of potent and selective small-molecule PAR-2 agonists

Proteinase activated receptor-2 plays a crucial role in a wide variety of conditions with a strong inflammatory component. We present the discovery and characterization of two structurally different, potent, selective, and metabolically stable small-molecule PAR-2 agonists. These ligands may be useful as pharmacological tools for elucidating the complex physiological role of the PAR-2 receptors as well as for the development of PAR-2 antagonists.     

Steroid hormone receptors and drug discovery: therapeutic opportunities and assay designs

Steroid receptors belong to the nuclear hormone receptor superfamily, which represents one of the important families of drug targets in the pharmaceutical industry. These receptors are generally expressed in a broad range of tissues and play roles in multiple physiological pathways. They represent one of the most complex target classes for drug research as the typical pharmacologically selective, potent, and pure agonist or antagonist may not be sufficient for the development of an optimal compound for therapeutic use. In many cases, the professed compound that is a selective modulator of a steroid hormone receptor that functions as an agonist in one tissue and as an antagonist in another can offer a better therapeutic advantage. In this review, we will selectively summarize members of the steroid hormone receptor subfamily, their therapeutic opportunities, and a glimpse of the methods that can be utilized in the development of drugs that target these receptors.     

Kappa-opioid receptor ligands

For years the design of κ-opioid receptor (KOR) agonists focussed on generating yet more potent and selective ligands that could access the CNS. As it became apparent that this approach was not going to provide analgesics without significant side effects, effort was directed towards the design of peripherally-acting ligands. This line of research has continued in recent years, together with work towards ligands with a mixed KOR/μ-opioid receptor (MOR) profile. In the area of KOR antagonists, significant advances have been made since the discovery of norbinaltorphimine (norBNI). These studies have provided significant insight into the structural determinants for selective interaction with the KOR. This has led to a number of new classes of KOR antagonist being discovered using rational drug design techniques. This review of the patent literature covering the years 2000 – 2003 will focus on these activities and describe the therapeutic potential of these compounds.