Current Drug Targets-CNS & Neurological Disorders - Volume 3, Issue 5, 2004

The prevalence in obesity has increased dramatically over the past 30 years, more than double in the United States alone. Obesity is associated with an increased risk for type 2 diabetes mellitus, dyslipidemia, hypertension, biliary disease, obstructive sleep apnea, and certain types of cancer. The pathophysiology of obesity is complex, involving behavioral, environmental, and genetic factors. Current treatment options include behavior modification and lifestyle changes which incorporate weight-reducing diets and physical activity, FDA approved long-term anti-obesity pharmacological agents sibutramine and orlistat, non-FDA approved over-the-counter (OTC) supplements and nutriceuticals, and, when appropriate, bariatric surgery. Without adequate prevention and treatment of obesity, government agencies have suggested that the direct and indirect costs associated with obesity may overwhelm the healthcare system. This brief review explores the current data available on treatments for the obese patient including the relative merits of different types of macronutrient composition (e.g., low carbohydrate vs. high carbohydrate diets) of weight-reducing diets, the value of resistance / strength training in physical activity programs designed for the obese patient, the safety and efficacy associated with OTC supplements and nutriceuticals for weight reduction (e.g., Ephedra, conjugated linoleic acid (CLA), Garcinia cambogia / hydroxycitric acid (HCA), chromium, pyruvate), the safety and efficacy of FDA-approved long-term obesity treatments sibutramine and orlistat, and bariatric surgery.

In the last decade, the G-Protein-Coupled Receptor (GPCR) superfamily has emerged as a very promising and enriched source of therapeutic targets for the treatment of obesity. GPCRs represent the largest family of mammalian proteins, with approximately 1000 members. It is estimated that the GPCR family may comprise greater than 1% of the human genome and is the molecular target for approximately 30% of currently marketed drugs. Human GPCRs are modulated by a large variety of ligands, including peptides, lipids, neurotransmitters, nucleotides, ions and external sensory signals such as pheromones, tastes or odors. Many of the above ligands have been implicated in the physiological control of energy balance. This article will examine the biological rationale, assets, identified liabilities and current drug development status of these receptors as anti-obesity drug targets.

Many peptides are synthesised and released from the gastrointestinal tract. Whilst their roles in regulation of gastrointestinal function have been known for some time, it is now evident that they also influence eating behaviour and thus potential anti obesity targets. Peptide YY (PYY) is released post prandially from the gastrointestinal L-cells with glucagon-like peptide 1 (GLP-1) and oxyntomodulin. Following peripheral administration of PYY 3-36, the circulating form of PYY, to mouse, rat or human there is marked inhibition of food intake. PYY 3-36 is thought to mediate its actions through the NPY Y2 GPCR. Obese subjects have lower basal fasting PYY levels and have a smaller post prandial rise. However, obesity does not appear to be associated with resistance to PYY (as it is with leptin) and exogenous infusion of PYY 3-36 results in a reduction in food intake by 30% in an obese group and 31% in a lean group. GLP-1 or oxyntomodulin, products of the prepreglucagon gene, decrease food intake when administered either peripherally or directly into the CNS. In addition, both have been shown to decrease food intake in humans. These effects are thought to be mediated by the GLP-1 receptor. Ghrelin, a huger hormone produced by the stomach, increases in the circulation following a period of fasting. Administration of ghrelin either peripherally or directly into the CNS increases food intake and chronic administration leads to obesity. Further infusion into normal healthy volunteers increases both food intake and appetite. Ghrelin is thought to act through the growth hormone secretagogue receptor (GHS-R). Obesity is the current major cause of premature death in the UK, killing almost 1000 people a week. Worldwide its prevalence is accelerating. The administration of the naturally occurring gut hormone may offer a long-term therapeutic approach to weight control. Here we consider the therapeutic potential of some gut hormones, and the GPCR's through which they act, in the treatment of obesity.

The possibility of developing a pill to increase energy expenditure is explored by examining the metabolic processes involved. Such a pill should be targeted at organ systems involved in facultative thermogenesis. In rodents, these are brown adipose tissue (BAT) and skeletal muscle. Since BAT-mediated thermogenesis is not available in adult humans, emphasis here is on skeletal muscle. A hypothesis is presented based on three known facts: (1) plasticity of skeletal muscle, with interconversion of fiber types that differ in their fuel efficiency; (2) presence of thyroxine 5'-deiodinase type 2 (TD2) in human skeletal muscle; (3) gradual increase in thermogenesis that occurs during rehabilitation after starvation, probably in muscle. A low capacity thermogenic system, muscle efficiency thermogenesis (MET), is proposed to occur as adipose stores refill during the transition from famine to feasting to obesity. This system involves increased activity of TD2 and a T3-induced increase in proportion of type II fibers, less efficient at rest and during activity. The protective effect of this system is probably overwhelmed by long-term eating in excess of energy needs. Better understanding of the complex remodeling of differentiated muscle fibers in the conversions proposed and of the regulation of TD2 activity in human skeletal muscle may reveal targets for increasing energy expenditure in humans. In addition, the possibility of exploiting the plasticity of the adipose organ, with conversion of white adipocytes in white adipose tissue to atypical brown adipocytes and increasing thermogenesis in them is considered as another potential target for increasing energy expenditure in humans.

The increasing prevalence of overweight and obesity worldwide is daunting and requires prompt attention by the affected, health care profession, government and the pharmaceutical industry. Because overweight / obesity are defined as an excess of adipose tissue mass, all approaches in prevention and treatment must consider redirecting lipid storage in adipose tissue to oxidative metabolism. Lipid partitioning is a complex process that involves interaction between fat and other macronutrients, particularly carbohydrate. In an isocaloric environment, when fat is stored carbohydrate is oxidized and vice versa. Processes that influence fat partitioning in a manner in which weight is maintained must be modified by changes in organ-specific fat transport and metabolism. When therapy is considered, however, changes in lipid partitioning alone will be ineffective unless a negative energy balance is also achieved, i.e. energy expenditure exceeds energy intake. The intent of this review is to focus on molecules including hormones, enzymes, cytokines, membrane transport proteins, and transcription factors directly involved in fat trafficking and partitioning that could be potential drug targets. Some examples of favorably altering body composition by systemic and / or tissue specific modification of these molecules have already been provided with gene knockout and / or transgenic approaches in mice. The translation of this science to humans remains a challenging task.

The metabolic syndrome is a cluster of easy-to-measure clinical phenotypes that serve as markers for increased risk for CVD and diabetes. There is no universal agreement as to the underlying pathophysiology of the metabolic syndrome. At its core, the metabolic syndrome is the result of energy excess; therefore treating obesity is a good strategy to reverse the clinical features of the metabolic syndrome. Hypertension is a special case, may not be part of the core pathophysiology of the metabolic syndrome, and will not be discussed. After a brief review of recent developments in the pathophysiology of the metabolic syndrome, this review will concentrate on peripheral targets in the following categories: ectopic fat and fat oxidation, intrinsic defects in substrate switching and mitochondrial biogenesis, lipolysis and lipid turnover, adipose tissue as an endocrine organ, nutrient / energy sensing systems, and inflammation. The advantages and pitfalls of these targets will be discussed with an eye towards the relevant literature.