Current Pharmaceutical Design - Volume 12, Issue 4, 2006

A significant difficulty the pharmaceutical industry has 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 macromolecular protein complexes embedded in the lipid bilayer and containing aqueous central pores allowing the passage of ions and sometimes of small molecules. Their functions are finely tuned by a variety of modulators, such as 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, arrhythmia or type II diabetes) are primarily treated by drugs that modulate ion channel activities. 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 second of three parts, for which I have the honour to be Executive Guest Editor, addresses topical issues to some of these channels. Despite their disease relevance, ion channels remain until now largely under exploited as drug targets. The ability to apply large-scale screening formats to measures of ion channel function offers immense opportunities for drug discovery and academic research. Several technologies now allow to screen large numbers of compounds and natural products on ion channel functions to find novel drugs. Application of these technologies has vastly improved the capabilities of ion channel drug discovery and provides an avenue to accelerate discoveries of ion channel biology. Mark Treherne describes [1] ion channel screening platforms now available, together with some of their inherent advantages and limitations. Neuronal acetylcholine ion channel receptors (nAChRs), that exist in several subtypes resulting from a different organisation of various subunits around the central ion channel, are involved in a variety of functions and disorders of the central nervous system. Cecilia Gotti et al. [2] discuss the molecular basis of brain nAChR structural and functional diversity mainly in pharmacological and biochemical terms, and summarise current knowledge concerning the newly discovered drugs used to classify the numerous receptor subtypes and to treat the brain diseases in which nAChRs are involved. Voltage-gated sodium channels mediate regenerative inward currents that are responsible for the initial depolarisation of action potentials in excitable cells. Advances in molecular biology have led to important new insights into the molecular structure of the sodium channel and have shed light on the relationship between channel structure and channel function. Kaoru Yamaoka et al. [3] present an overview of the various toxins and drug molecules affecting the gating behaviour of sodium channels, providing important clues on the nature of mobile structures involved in channel gating. Voltage-gated L-type Ca2+ channels control depolarisation-induced Ca2+ entry in different electrically excitable cells, including mammalian heart but play also crucial roles in other processes, as insulin secretory response, severe pain or ischemic stroke. David Triggle [4] discusses the mechanisms of action of L-type Ca2+ channels blockers as well as the limitations on their use (e.g. their little selectivity between subtypes of the L-type channels). Potassium channels are a diverse and ubiquitous family of membrane proteins present in both excitable and non-excitable cells. Members of this channel family play critical roles in cellular signalling processes regulating neurotransmitter release, heart rate, insulin secretion, neuronal excitability, epithelial electrolyte transport, smooth muscle contraction, cell volume regulation, auditory function, hormone secretion, immune function, cell proliferation, etc. Specific modulators have been identified for a limited number of K+ channel subtypes. Kim Lawson and Neil McKay [5] overview the current knowledge available concerning K+ channels as therapeutic targets. More than 1300 different mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) cause cystic fibrosis, a disease characterised by deficient epithelial Cl- secretion and enhanced Na+ absorption. Frédéric Becq [6] summarises the recent evolution of CFTR pharmacology and particularly how high throughput screening assays have been developed to identify novel molecules, some of them probably constituting a reservoir of future therapeutic agents for cystic fibrosis. Type-2 diabetes mellitus is considered to be due to the failure of glucose metabolism to stimulate pancreatic b-cell electrical activity, calcium influx, and insulin secretion via regulation of the open probability of the ATP-sensitive K (KATP) channels.....

Ion channels are increasingly being implicated in disease. Although existing drugs that modulate channel function currently represent a key class of pharmaceutical agents, future ion channel drugs could help to treat an even wider variety of diseases. Despite their disease relevance, ion channels remain largely under exploited as drug targets, chiefly resulting from the absence of screening technologies that provide the throughput and quality of data required to support medicinal chemistry. Although some technical challenges still lie ahead, this historic bottleneck in drug discovery is now being bypassed by newer technologies that can be fully integrated into the early stages of drug discovery and will allow the discovery of novel therapeutic agents. Sequencing the human genome has greatly added to the number of potential drug targets but selecting suitable ion channels for drug discovery research should be based on the potential therapeutic relevance of the channel and not just the availability of suitable screens. Currently, ion channel drug discovery is focused on the need to identify compounds that can provide tractable starting points for medicinal chemistry. Advances in laboratory automation have brought significant opportunities to increase screening throughput for ion channel assays but careful assay configuration to model drug-target interactions in a physiological manner remains an essential consideration. Ion channel screening platforms are described in this review to provide some insight into the variety of technologies available for screening, together with some of their inherent advantages and limitations.

Neuronal nicotinic receptors (nAChRs) are a heterogeneous family of ion channels differently expressed in the nervous system where, by responding to the endogenous neurotransmitter acetylcholine, they contribute to a wide range of brain activities and influence a number of physiological functions. Over recent years, the application of newly developed molecular and cellular biological techniques has made it possible to correlate the subunit composition of nAChRs with specific nicotine-elicited behaviours, and refine some of the in vivo physiological functions of nAChR subtypes. The major new findings are the widespread expression of nAChRs, outside the nervous system, their specific and complex organisation, and their relevance to normal brain function. Moreover, the combination of clinical and basic research has better defined the involvement of nAChRs in a growing number of nervous pathologies other than degenerative diseases. However, there are still only a limited number of nicotinic-specific drugs and, although some nicotinic agonists have an interesting pharmacology, their clinical use is limited by undesirable side effects. Some selective nicotinic ligands have recently been developed and used to explore the complexity of nAChR subtype structure and function in the expectation that they will become rational therapeutic alternatives in a number of neurodegenerative, neuropsychiatric and neurological disorders. In this review, we will discuss the molecular basis of brain nAChR structural and functional diversity mainly in pharmacological and biochemical terms, and summarise current knowledge concerning the newly discovered drugs used to classify the numerous receptor subtypes and treat the brain diseases in which nAChRs are involved.

Electrogenesis of efficiently propagated action potentials requires synchronized opening of transmembrane Na+ channels possessing a sodium selectivity-filter, a high-throughput ion-conductance pathway, and voltage-dependent gating functions. These properties of the Na+ channel have long been the target of molecular analysis. Several toxins and drugs, known to selectively bind to Na + channels, have been used as pharmacological tools to investigate Na+ channel properties either electrophysiologically or chemically. Recent analyses of the protein crystal structure of bacterial voltage-dependent K+ channels have provided important clues to the identity of mobile structures involved in channel gating. The new information may be applicable to Na+ channels, and may well require a total revision of our understanding of gating mechanisms of sodium channels. Several experiments challenge the emerging view that channel gating by S6 transmembrane segments is triggered by signals from voltage sensors floating in membrane lipid. Herein, we review the various toxin and drug molecules that affect the gating behavior of Na+ channels in this new structural framework, by characterizing the binding sites of these toxins, and assessing the pharmacological effects resulting from changes in the structure of the toxin or sodium channel.

The Ca2+ channel blockers represent a successful group of therapeutic agents directed against cardiovascular targets, including hypertension and angina. These drugs, including the first-generation verapamil, nifedipine and diltiazem are directed against a subclass of voltage-gated Ca2+ channel - the L-type channel. Other subclasses of Ca2+ channel exist and are targets for new indications. The mechanisms of actions of the L-type blockers are discussed and the origins of their cardiovascular selectivity discussed. Although new drugs of this class directed against hypertension could be developed, there are both clinical and economic reasons that argue against such development. However, there are other possible targets to investigate where antagonists and activators of the L-type channel may be useful: such targets include fertility, neuronal growth, bone formation and epilepsy. Limitations to these approaches are discussed.

Regulation of potassium (K+) channels evokes hyperpolarization or repolarization of the cell membrane to prevent or reverse cell excitability and is fundamental in the control of cellular activity throughout the range of tissue types within the human body. Genome projects predict that in excess of 80 K+ channel-related genes exist, resulting in a high degree of K+ channel diversity. In addition, dysfunction of K+ channels, as a result of mutations of the genes for the channel proteins or alterations in channel regulation, has been associated with the pathophysiology of diseases. These observations support K+ channels as therapeutic targets to regulate cellular homeostasis in pathophysiological conditions. Molecular cloning and expression of K+ channels offer important information in the identification of selective compounds to provide unique tissue management. Specific modulators have been identified for a limited number of K+ channel subtypes. Unfortunately the conversion of data obtained in the laboratory to success in the clinical setting has been limited. Tissue delivery of genes, in combination with drugs, may be an avenue enabling specific modulation of ion channel function and improved drug selectivity. Using specific examples (HERG, IKs, KCNQs, KCa, Kv1.3), issues regarding distribution, function and diversity related to advances made in the identification of modulators having therapeutic potential are discussed. The scope of this field is just emerging and the number of likely therapeutic indications for K+ channel modulators will increase as insight into the dynamics of expression of these channels in various diseases grows and the issue of the required selectivity is resolved.

Chloride channels play important roles in vital cellular signalling processes contributing to homeostasis in both excitable and non-excitable cells. Since 1987, more than ten ion channel genes have been identified as causing human hereditary diseases among them the genes for the voltage-dependent chloride channel ClC-1 (myotonia) and the cystic fibrosis transmembrane conductance regulator (CFTR) protein (cystic fibrosis). The CFTR gene was cloned in 1989 and its protein product identified as an ATP-gated and phosphorylation-regulated chloride channel during the following two years. Since then, searching for potent and specific small molecules able to modulate normal and mutated CFTR has become a crucial endpoint in the field for both our understanding of the physiological role that CFTR plays in epithelial cells and more importantly for the development of therapeutic agents to cure cystic fibrosis (CF). It is predicted that a pharmacological approach would help not only to restore the defective transport activity of mutant CFTR but also to correct the regulatory function of CFTR. This review describes the evolution of CFTR pharmacology and how during the last five years, high throughput screening assays have been developed to identify novel molecules, some of them probably constituting a reservoir of future therapeutic agents for CF.

Type-2, or non-insulin-dependent diabetes mellitus is a serious disease that is now widespread throughout Western society. Glucose intolerance, or failure of glucose to stimulate insulin secretion, is a primary factor in the manifestation of this disease and is likely to be due to the failure of glucose metabolism to stimulate pancreatic βcell electrical activity, calcium influx, and insulin secretion. In this review we describe how ion channels regulate the electrical behaviour of the βcell and how the membrane potential depolarises in response to a rise in glucose metabolism. Central to these electrical events is the inhibition of ATP-sensitive potassium channel by ATP, and we summarise recent advances in our understanding of the properties of this ion channel in coupling βcell metabolism to electrical activity. We discuss the mechanism, specificity, and clinical implications of the pharmacological inhibition of KATP channels by sulphonyureas and other antidiabetic drugs. The roles of other ion channels in regulating electrical activity are considered, and also their potential use as targets for drug action in treating βcell disorders.

In spite of recent progress in the pharmacotherapy of depression major issues are still unresolved. These include the non-response rate of approximately 30% to conventional antidepressant pharmacotherapy, side effects of available antidepressants and the latency of several weeks until clinical improvement. The only non-pharmacological biological treatment options available so far which exert more rapid antidepressant efficacy are electroconvulsive therapy and, as an augmentation strategy, sleep deprivation. Current pharmacological treatments aim to enhance serotonergic and/or noradrenergic neurotransmission. In spite of emerging knowledge, the crucial mechanisms underlying both non-pharmacological treatments, which are responsible for antidepressant efficacy, are not yet clear so far. In the meantime several new pharmacological principles are under investigation with regard to their putative antidepressant potency. These include 5-HT1A receptor agonists, tachykinin receptor antagonists and various interventions within the hypothalamic-pituitary-adrenal system. While there is evidence for antidepressant properties of these new treatments in animal studies, in case series, in open studies and to some degree also in placebo controlled studies, no definite proof for the antidepressant efficacy of these new pharmacological strategies according to the requirements for evaluation of antidepressant drugs has been furnished so far. In contrast, for the established non-pharmacological treatment strategies including bright light therapy the clinical efficacy has been proven at least in subgroups of depression, but more knowledge of the main mechanisms underlying their antidepressant efficacy is still necessary. In addition new non-pharmacological treatments like repetitive transcranial magnetic stimulation, magnetic seizure therapy and Vagus nerve stimulation are currently under development. Nevertheless, a follow-up of both the new pharmacological strategies and non-pharmacological treatment options is of major importance to provide even better strategies for the clinical management of depression, which also is of great socio-economic impact.

Animal studies have shown angiotensin converting enzyme (ACE) inhibitors to be effective agents for myocardial protection. They protect against lethal arrhythmias, preserve ventricular function, improve coronary reserve (especially after ischemia/reperfusion), and reverse myocardial hypertrophy. Human studies, on the other hand, have shown inconsistent results. The beneficial effects of ACE inhibitors demonstrated in animal studies provide major advantages for cardiac surgery. First, most cardiac surgery is performed under ischemic arrest induced by a cardioplegic solution, and the protective effects of ACE inhibition against reperfusion injury can reduce peri-operative mortality and morbidity. Second, most patients who undergo such surgery have myocardial hypertrophy due to hypertension, pressure or volume overload mediated by valve disease, or myocardial infarction. Ventricular hypertrophy is a strong risk factor for sudden death, probably from arrhythmia. Regression of the hypertrophy may prevent post-operative sudden death, thereby allowing for long-term benefits of surgery. In this paper, I review ACE inhibitor studies in animals and humans and the protective mechanisms involved. I also discuss why human studies show inconsistent results in spite of the fact that ACE inhibition is consistently protective in animal studies. Finally, I explore the potential clinical applications of ACE inhibitors in cardiac surgery.