New and emerging treatments for irritable bowel syndrome and functional dyspepsia

Treatment approaches of gastrointestinal dysfunction in Parkinson's disease, therapeutical options and future perspectives

Gastrointestinal (GI) dysfunction is a common but underestimated feature in Parkinson's disease (PD). Out of the multimodal spectrum of treatment options, there currently are only a few pharmacological treatments available to improve gastrointestinal motility and symptoms. Because enteric nervous function is mainly regulated by transmitters different from those involved in the brain, dopamine replacement is not a treatment option in PD patients. This article focuses on the known regulative mechanism of GI function and presents known and upcoming treatment options for GI dysfunction in PD.

Existing and emerging therapies for irritable bowel syndrome

INTRODUCTION

Irritable bowel syndrome is a common disorder that is associated with a significant impact on both affected individuals and society. While the pathophysiology of irritable bowel syndrome remains unknown, knowledge regarding the normal and abnormal functions of the gut and its complex interaction with the body's nervous systems continues to shed light on the multifactorial origins of irritable bowel syndrome symptoms. This article provides an overview of the current knowledge of the therapeutic approaches to irritable bowel syndrome.

AREAS COVERED

A search of the online bibliographic databases MEDLINE and EMBASE was performed in order to identify all relevant articles published between 1980 and 2010. The search was enhanced with the use of a medical librarian. Bibliographies from potentially relevant articles were manually searched.

EXPERT OPINION

The therapeutic options for irritable bowel syndrome are rapidly evolving beyond traditional symptom-based therapies, such as fiber, antispasmodics, antidiarrheals and laxatives, and are moving toward agents with organ-specific receptor selectivity directed, in many cases, at specific gastrointestinal functions.

In vitro and in vivo pharmacological characterization of PF-01354082, a novel partial agonist selective for the 5-HT(4) receptor

The pharmacological profile of PF-01354082, a selective 5-HT(4) receptor partial agonist, was investigated. PF-01354082 displayed high affinity for human 5-HT(4d) and dog 5-HT(4h) receptors in binding studies, having Ki values of 2.0 nM and 4.2 nM, respectively. By contrast, PF-01354082 did not show significant affinity for several other 5-HT receptors (5-HT(1A), 5-HT(1B), 5-HT(1D), 5-HT(2A), 5-HT(2B), 5-HT(2C), 5-HT(3A), and 5-HT(7)) or the dopamine D(2long) receptor. Functional assays using either cells expressing human recombinant 5-HT(4d) receptors or rat tunica muscularis mucosae demonstrated that PF-01354082 exhibited partial agonist activity at the 5-HT(4) receptor. The effects of PF-01354082 on in vitro receptor binding, ion channel activity, and sites of uptake were further investigated. PF-01354082 did not show biologically relevant binding activity at concentrations up to 10 microM except for binding to the 5-HT(4e) receptor. Furthermore, PF-01354082 decreased I(HERG) current by only 11% at a concentration of 300 microM, indicating that the compound had greater than 150,000-fold selectivity for the human 5-HT(4d) receptor over hERG channels. An in vivo study using a gastric motility model in conscious dogs demonstrated that oral administration of PF-01354082 resulted in marked and sustained stimulation of gastric motility in a dose-dependent manner. These results indicate that PF-01354082 is an orally active, highly selective, partial agonist of the human 5-HT(4) receptor that is expected to exert a favorable effect on gastrointestinal motor disorders with reduced adverse effects mediated by other related receptors.

5-HT4 receptor agonists: similar but not the same

5-Hydroxytryptamine(4) (5-HT(4)) receptors are an interesting target for the management of patients in need of gastrointestinal (GI) promotility treatment. They have proven therapeutic potential to treat patients with GI motility disorders. Lack of selectivity for the 5-HT(4) receptor has limited the clinical success of the agonists used until now. For instance, next to their affinity for 5-HT(4) receptors, both cisapride and tegaserod have appreciable affinity for other receptors, channels or transporters [e.g. cisapride: human ether-a-go-go-related gene (hERG) is K(+) channel and tegaserod: 5-HT(1) and 5-HT(2) receptors]. Adverse cardiovascular events observed with these compounds are not 5-HT(4) receptor-related. Recent efforts have led to the discovery of a series of selective 5-HT(4) receptor ligands, with prucalopride being the most advanced in clinical development. The selectivity of these new compounds clearly differentiates them from the older generation compounds by minimizing the potential of target-unrelated side effects. The availability of selective agonists enables the focus to shift to the exploration of 5-HT(4) receptor-related differences between agonists. Based on drug- and tissue-related properties (e.g. differences in receptor binding, receptor density, effectors, coupling efficiency), 5-HT(4) receptor agonists are able to express tissue selectivity, i.e. behave as a partial agonist in some and as a full agonist in other tissues. Furthermore, the concept of ligand-directed signalling offers great opportunities for future drug development by enlarging the scientific basis for the generation of agonist-specific effects in different cell types, tissues or organs. Selective 5-HT(4) receptor agonists might thus prove to be innovative drugs with an attractive safety profile for better treatment of patients suffering from hypomotility disorders.

Relationship of functional gastrointestinal disorders and psychiatric disorders: Implications for treatment

This article revisits the links between psychopathology and functional gastrointestinal disorders such as irritable bowel syndrome (IBS), discusses the rational use of antidepressants as well as non-pharmacological approaches to the management of IBS, and suggests guidelines for the treatment of IBS based on an interdisciplinary perspective from the present state of knowledge. Relevant published literature on psychiatric disorders, especially somatization disorder, in the context of IBS, and literature providing direction for management is reviewed, and new directions are provided from findings in the literature. IBS is a heterogeneous syndrome with various potential mechanisms responsible for its clinical presentations. IBS is typically complicated with psychiatric issues, unexplained symptoms, and functional syndromes in other organ systems. Most IBS patients have multiple complaints without demonstrated cause, and that these symptoms can involve systems other than the intestine, e.g. bones and joints (fibromyalgia, temporomandibular joint syndrome), heart (non-cardiac chest pain), vascular (post-menopausal syndrome), and brain (anxiety, depression). Most IBS patients do not have psychiatric illness per se, but a range of psychoform (psychological complaints in the absence of psychiatric disorder) symptoms that accompany their somatoform (physical symptoms in the absence of medical disorder) complaints. It is not correct to label IBS patients as psychiatric patients (except those more difficult patients with true somatization disorder). One mode of treatment is unlikely to be universally effective or to resolve most symptoms. The techniques of psychotherapy or cognitive-behavioral therapy can allow IBS patients to cope more readily with their illness. Specific episodes of depressive or anxiety disorders can be managed as appropriate for those conditions. Medications designed to improve anxiety or depression are not uniformly useful for psychiatric complaints in IBS, because the psychoform symptoms that sound similar to those seen in psychiatric disorders may not have the same significance in patients with IBS.

The 5-HT3 receptor as a therapeutic target

The 5-HT3 receptor is a neurotransmitter-gated ion channel. It is a member of the Cys-loop family of receptors, which also includes nicotinic acetylcholine, glycine and GABAA receptors. Each member of the family consists of an arrangement of five subunits surrounding a central ion-conducting pore. The 5-HT3 receptor binding site is composed of six loops from two adjacent subunits, and the critical ligand binding residues within these loops are well documented. There are a range of 5-HT3 receptor agonists and competitive antagonists, but it is the antagonists that dominate their clinical use. Studies have proposed a range of disease symptoms that might be amenable to 5-HT3 receptor selective compounds; however, so far only the treatment of emesis and irritable bowel syndrome have been fully realised. In this review, the authors look at the structure, function and distribution of 5-HT3 receptors and how this may influence their role in disease. The authors also describe the existing clinical applications of 5-HT3 antagonists and the future potential of these drugs.

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