Current Pharmaceutical Design - Volume 12, Issue 28, 2006

The production of new molecular entities endowed with salutary medicinal properties is a formidable challenge that involve several steps and requests rational target identification, recognition and avoidance of adverse properties of therapeutics before commitment to clinical trials, monitoring of clinical efficacy using surrogate markers and individualized approaches to disease treatment. The first to face up to is the initial identification and selection of macromolecular targets upon which de novo drug discovery programs can be initiated. A drug target needs to answer several criteria (as known biological function(s), robust assay systems for in vitro characterisation and high-throughput screening) and to be specifically modified by and accessible to small molecular weight compounds in vivo. Membrane channels have many of these attributes and can be viewed as suitable targets for small molecule drugs. Membrane channels are allosteric proteins that open and close a permeable pore (a process called gating), allowing ions and sometimes small molecules to flow across the cell membrane in a regulated manner. Their gating can be regulated by various stimuli including changes in membrane voltage, binding of extracellular or intracellular ligands, membrane stretch, enzymes and G-proteins. They play critical roles in a broad range of physiological processes, including electrical signal transduction, chemical signalling (involving different second messengers), transepithelial transport, regulation of cytoplasmic or vesicular ion concentration and pH, as well as regulation of cell volume. Channel dysfunction may lead to a number of diseases termed channelopathies, and a number of common diseases (e.g. epilepsy, hypertension, arrhythmia, chronic pain or type II diabetes) are primarily treated by drugs that modulate ion channel activities. The cell-based methods for evaluating membrane channel pharmacology are based on several distinct techniques such as electrophysiology, fluorescence, radioligand binding or displacement, and radiotracer flux assays. A better understanding of membrane channel structures and of channel functions has been achieved in recent years by three main scientific advances, the patch-clamp technique, the use of selective neurotoxins and the cloning and sequencing of genes. They allowed to investigate the pharmacological effects of traditional (antiarrhythmic, antiepileptic, ...) drugs and the development of new approaches. This issue of Current Pharmaceutical Design, the third of four parts, for which I have the honour to be Executive Guest Editor, addresses topical issues to some of these channels. A wide range of neurotransmitters, polypeptides and inflammatory mediators transduce their signals into the interior of cells by specific interactions with cell-surface receptors that are coupled to G-protein. Muscarinic acetylcholine receptors, one of the most familiar of them, mediate transmission of acetylcholine to neuronal or effector cells. There are five subtypes of closely homologous muscarinic receptors which are coupled by means of heterotrimeric G-proteins to a variety of signalling pathways resulting in a multitude of target cell effects. Masaru Ishii and Yoshihisa Kurachi [1] review the latest advances in the structural and functional characterization of these receptors and the pharmaceutical development of muscarinic receptor ligands. Glutamate is regarded as the most widespread excitatory neurotransmitter in the mammalian brain. Two classes of glutamate receptors have been cloned, the ionotropic (ligand-gated ion channels) and the metabotropic (G protein-coupled receptors). Ionotropic glutamate receptors mediate basic information processing in the brain and induce changes in synaptic efficacy, but their functional realm has been found to extend well outside the central nervous system to include roles in insulin secretion, bone resorption, cardiac pacemaking, as well as involvement in taste and tactile sensation. Rosa Planells-Cases, Juan Lerma and Antonio Ferrer-Montiel [2] overview the current knowledge available concerning the pharmacology of these channels. Cyclic nucleotide-gated ion channels are non-selective cation channels that are opened by the direct binding of intracellular cyclic nucleotides and behave as molecular amplifiers, with large changes in activity resulting from small changes in cyclic nucleotide concentration. Taking advantage of emerging structural information and the increasing knowledge of the biophysical properties of these channels, some promising compounds and strategies have begun to emerge. Lane Brown, Timothy Strassmaier, James Brady and Jeffrey Karpen [3] discuss progress on two fronts, cyclic nucleotide analogues as both activators and competitive inhibitors, and inhibitors that target the pore or gating machinery of the channel. Serotonin receptors are highly heterogeneous and they have been regrouped within seven different families (5-HT1 to 5-HT7)....

Muscarinic acetylcholine receptors mediate diverse physiological functions. At present, five receptor subtypes (M1 - M5) have been identified. The odd-numbered receptors (M1, M3, and M5) are preferentially coupled to Gq/11 and activate phospholipase C, which initiates the phosphatidylinositol trisphosphate cascade leading to intracellular Ca2+ mobilization and activation of protein kinase C. On the other hand, the even-numbered receptors (M2 and M4) are coupled to Gi/o, and inhibit adenylyl cyclase activity. They also activate G protein-gated potassium channels, which leads to hyperpolarization of the plasma membrane in different excitable cells. Individual members of the family are expressed in an overlapping fashion in various tissues and cell types. Recent gene targeting approaches have unraveled the specific function of these muscarinic receptor subtypes, which were not able to be fully elucidated with pharmacological approaches because of the non-selective effects of the available ligands. Based on these findings, muscarinic receptors have been emerging as an important therapeutic target for various diseases, including dry mouth, incontinence and chronic obstructive pulmonary disease. Here we review the latest advances in the structural and functional characterization of muscarinic acetylcholine receptors and the pharmaceutical development of muscarinic receptor ligands.

L-glutamate is considered the main excitatory neurotransmitter in the mammalian brain. Paradoxically, Lglutamate is also the most important excitotoxin pivotally involved in the aetiology of several neurodegenerative diseases such as stroke, Alzheimer, Parkinson, amyotropic lateral sclerosis, Huntington and neuropathic pain. L-glutamate signalling is transduced both presynaptically and postsynaptically by metabotropic and ionotropic receptors. Three types of glutamate-gated channels integrate the synaptic signal, namely AMPA, kainate and NMDA receptors. Sustained activation of these receptors, and especially of the NMDA receptor, is a casuistic phenomenon that leads to the neuronal death underlying neurodegeneration. Thus, pharmacological intervention at these neuronal receptors and their synaptic protein complexes is a valuable therapeutic strategy. The approval of memantine, a safe, well-tolerated uncompetitive NMDA antagonist for the treatment of moderate to severe Alzheimer dementia validates ionotropic glutamate receptors as key therapeutic targets of neurodegenerative diseases in humans. As a consequence, an enormous effort is being carried out to identify and develop safe and potent antagonists for the clinics. In this review, we will describe progress in this important arena of human health.

Cyclic nucleotide-gated (CNG) ion channels play a central role in vision and olfaction, generating the electrical responses to light in photoreceptors and to odorants in olfactory receptors. These channels have been detected in many other tissues where their functions are largely unclear. The use of gene knockouts and other methods have yielded some information, but there is a pressing need for potent and specific pharmacological agents directed at CNG channels. To date there has been very little systematic effort in this direction - most of what can be termed CNG channel pharmacology arose from testing reagents known to target protein kinases or other ion channels, or by accident when researchers were investigating other intracellular pathways that may regulate the activity of CNG channels. Predictably, these studies have not produced selective agents. However, taking advantage of emerging structural information and the increasing knowledge of the biophysical properties of these channels, some promising compounds and strategies have begun to emerge. In this review we discuss progress on two fronts, cyclic nucleotide analogs as both activators and competitive inhibitors, and inhibitors that target the pore or gating machinery of the channel. We also discuss the potential of these compounds for treating certain forms of retinal degeneration.

The 5-HT3 receptor is a member of the Cys-loop family of ligand-gated ion channels. These receptors are located in both the peripheral and central nervous systems, where functional receptors are constructed from five subunits. These subunits may be the same (homopentameric 5-HT3A receptors) or different (heteropentameric receptors, usually comprising of 5-HT3A and 5-HT3B receptor subunits), with the latter having a number of distinct properties. The 5-HT3 receptor binding site is comprised of six loops from two adjacent subunits, and critical ligand binding amino acids in these loops have been largely identified. There are a range of selective agonists and antagonists for these receptors and the pharmacophore is reasonably well understood. There are also a wide range of compounds that can modulate receptor activity. Studies have suggested many diverse potential disease targets that might be amenable to alleviation by 5-HT3 receptor selective compounds but to date only two applications have been fully realised in the clinic: the treatment of emesis and irritable-bowel syndrome.

Potassium channels form highly K+ ion-selective pores in the plasma membrane of excitable cells. Voltagegated potassium (Kv) channels open in response to membrane depolarization to allow rapid diffusion of K+ ions out of the cell, thus repolarizing the cell to restore a negative resting membrane potential. Inherited mutations in Kv channel genes produce abnormal cellular repolarization and cause diseases of excitable tissues. Small molecule interactions with Kv channels can cause similar pathologies. During the last decade of research into Kv channels and associated diseases - termed ‘channelopathies’ - we have begun to understand Kv channel function and dysfunction at the molecular level. In this review, the molecular mechanisms of Kv channelopathies are discussed, with particular emphasis on the overlap between inherited and acquired disease, and the drive towards novel channel-targeted therapies.

Mechanosensitivity is a property common to many cell types. Because channels gated by mechanical stimuli (mechanosensitive channels, MSCs) are implicated in many normal and pathological cellular responses, they present a valid target for therapeutic agents. However the process of mechanotranduction, the structure, function and pharmacology of eukaryotic MSCs are not well understood and matching experimental and in vivo stimuli is difficult. With respect to the pharmacology of these channels, a further complication arises because agents that modulate the activity of MSCs may not even bind to the channel itself, but may cause their effects by changing the properties of the tension-sensing lipid bilayer and/or cytoskeleton. MSCs in the myocardium are discussed in the context of their probable role in the generation of stretch-activated arrhythmias. The actions of the three most prominent agents used to study MSCs in the heart, the lanthanide gadolinium, the aminoglycosidic antibiotic streptomycin, and a peptide toxin isolated from tarantula venom, GsMTx- 4, are compared. While all three can prevent mechanically-induced cardiac arrhythmias in experimental situations, only GsMTx-4 seems to have the potential as a novel therapeutic agent for the targeting of arrhythmias provoked by MSCs.

The use of general anaesthetics has facilitated great advantages in surgery within the last 150 years. General anaesthesia is composed of several components including analgesia, amnesia, hypnosis and immobility. To achieve these components, general anaesthetics have to act via multiple molecular targets at different anatomical sites in the central nervous system. Much of our current understanding of how anaesthetics work has been obtained within the last few years on the basis of genetic approaches, in particular knock-out or knock-in mice. Anaesthetic drugs can be grouped into volatile and intravenous anaesthetics according to their route of administration. Common volatile anaesthetics induce immobility via molecular targets in the spinal cord, including glycine receptors, GABAA receptors, glutamate receptors, and TREK-1 potassium channels. In contrast, intravenous anaesthetics cause immobility almost exclusively via GABAA receptors harbouring β3 subunits. Hypnosis is predominantly mediated by β3-subunit containing GABAA receptors in the brain, whereas β2 subunit containing receptors, which make up more than 50% of all GABAA receptors in the central nervous system, mediate sedation. At clinically relevant concentrations, ketamine and nitrous oxide block NMDA receptors. Unlike all other anaesthetics in clinical use they produce analgesia. Not only desired actions of anaesthetics, but also undesired side effects are linked to certain receptors. Respiratory depression involves β3 containing GABAA receptors whereas hypothermia is largely mediated by GABAA receptors containing β2 subunits. These recent insights into the clinically desired and undesired actions of anaesthetic agents provide new avenues for the design of drugs with an improved side-effect profile. Such agents would be especially beneficial for the treatment of newborn children, elderly patients and patients undergoing ambulatory surgery.

Early detection and treatment of cancers have increased survival and improved clinical outcome. The development of metastases is often associated with a poor prognostic of survival. Finding early markers of metastasis and developing new therapies against their development is a great challenge. Since a few years, there is more evidence that ionic channels are involved in the oncogenic process. Among these, voltage-gated sodium channels expressed in non-nervous or non-muscular organs are often associated with the metastatic behaviour of different cancers. The aim of this review is to describe the current knowledge on the functional expression of voltage-gated sodium channels and their biological roles in different cancers such as prostate, breast, lung (small cells and non-small cells) and leukaemia. In the conclusion, we develop conceptual approaches to understand how such channels can be involved in the metastatic process and conclude that blockers targeted toward these channels are promising new therapeutic solutions against metastatic cancers.