Cell adhesion molecules are glycoproteins expressed on the cell surface and play an important role in inflammatory as well as neoplastic diseases. There are four main groups: the integrin family, the immunoglobulin superfamily, selectins, and cadherins. The integrin family has eight subfamilies, designated as beta 1 through beta 8. The most widely studied subfamilies are beta 1 (CD29, very late activation [VLA] members), beta 2 (leukocyte integrins such as CD11a/CD18, CD11b/CD18, CD11c/CD18, and alpha d beta 2), beta 3 (CD61, cytoadhesions), and beta 7 (alpha 4 beta 7 and alpha E beta 7). The immunoglobulin superfamily includes leukocyte function antigen-2 (LFA-2 or CD2), leukocyte function antigen-3 (LFA-3 or CD58), intercellular adhesion molecules (ICAMs), vascular adhesion molecule-1 (VCAM-1), platelet-endothelial cell adhesion molecule-1 (PE-CAM-1), and mucosal addressin cell adhesion molecule-1 (MAdCAM-1). The selectin family includes E-selectin (CD62E), P-selectin (CD62P), and L-selectin (CD62L). Cadherins are major cell-cell adhesion molecules and include epithelial (E), placental (P), and neural (N) subclasses. The binding sites (ligands/receptors) are different for each of these cell adhesion molecules (e.g., ICAM binds to CD11/CD18; VCAM-1 binds to VLA-4). The specific cell adhesion molecules and their ligands that may be involved in pathologic conditions and potential therapeutic strategies by modulating the expression of these molecules will be discussed.

Drug-induced valvulopathy is a serious liability for certain compound classes in development and for some marketed drugs intended for human use. Reports of valvulopathy led to the withdrawal of fenfluramines (anorexigens) and pergolide (antiparkinson drug) from the United States market in 1997 and 2007, respectively. The mechanism responsible for the pathogenesis of valvulopathy by these drugs is likely a result of an "off-target" effect via activation of 5-hydroxytryptamine (5-HT) 2B receptor (5-HT2BR) expressed on heart valve leaflets. Microscopically, the affected valve leaflets showed plaques of proliferative myofibroblasts in an abundant extracellular matrix, composed primarily of glycosaminoglycans. However, the valvular effects caused by fenfluramines and pergolide were not initially predicted from routine preclinical toxicity studies, and to date there are no specific validated animal models or preclinical/toxicologic screens to accurately predict drug-induced valvulopathy. This review covers the structure and function of heart valves and highlights major advances toward understanding the 5-HT2BR-mediated pathogenesis of the lesion and subsequently, development of appropriate animal models using novel techniques/experiments, use of functional screens against 5-HT2BR, and more consistent sampling and pathologic evaluation of valves in preclinical studies that will aid in avoidance of future drug-induced valvulopathy in humans.

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Animal models of human disease are commonly utilized to gain insight into the potential efficacy and mode of action of novel pharmaceuticals. However, conventional (healthy) rodent and nonrodent models are generally utilized in nonclinical safety testing. Animal models of human disease may be helpful in understanding safety risks of compounds in nonclinical or clinical development, with their greatest value being in targeted or hypothesis-driven studies to help understand the mechanism of a particular toxicity. Limitations of animal models of disease in nonclinical safety testing include a lack of historical control, heterogeneity in disease expression, a limited life span, and confounding effects of the disease. In most instances, animal models of human disease should not be utilized to supplant testing in conventional animal models. While of potential benefit, testing in an animal model of human disease should only be taken after adequate consideration of relevance along with benefits and limitations of the proposed model.

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Obesity is a well-established risk factor for hypertension, hyperlipidemia, type II diabetes, coronary heart disease, stroke, obstructive sleep apnea, asthma, orthopedic disorders, and certain cancers. Despite this risk, the prevalence of obesity continues to increase worldwide, and there is a growing demand for safe and effective antiobesity drugs. Previous antiobesity drugs or anorexigens, particularly centrally acting agents, have poor safety records. Life-threatening safety issues led to the withdrawal of aminorex in 1968, fenfluramine and dexfenfluramine in 1997, and phenylpropanolamine in 2000. Many of the safety issues, such as valvulopathy with fenfluramine and pulmonary arterial hypertension with aminorex, were initially not predicted by routine preclinical toxicology studies. To date, there are no validated animal models or preclinical and/or toxicologic screens to accurately predict anorexigen-induced valvulopathy and pulmonary arterial hypertension in humans. This review covers the current state of antiobesity drugs and their safety concerns, and highlights new therapeutic targets and scientific advances toward the development of appropriate animal models by using novel techniques that will aid in understanding pathogenesis and pathophysiology of anorexigen-related safety issues.

S U M M A R Y 5-Hydroxytryptamine-2B receptor (5HT2BR) stimulation is known to cause fibroblast mitogenesis, and the mitogenic effect has been proposed to trigger valvular heart disease in humans. In this study, we used real-time polymerase chain reaction (TaqMan) to quantify transcript levels of 5HT2B, 5HT2C, and 5HT1B receptors and immunohistochemistry (IHC) to detect the tissue localization of these receptors in the normal heart valves of cynomolgus (CM) monkeys and Sprague-Dawley (S-D) rats. In both species, positive immunostaining was noted for 5HT1B and 5HT2B receptors in mitral, tricuspid, aortic, and pulmonary valves, and the cell types showing positive staining were interstitial cells and endothelial cells lining the valve leaflet. In CM monkeys, 5HT2CR was expressed only in the endothelial cells lining the leaflet, whereas S-D valves were negative for this receptor. IHC results were correlated with 5HT2B and 5HT1B receptor transcripts for all four valves. However, 5HT2C receptor transcripts were lower than 5HT2B or 5HT1B in all CM monkey valves, whereas 5HT2C transcripts were below the level of detection in any of the S-D rat valves. Our data showed the expression of 5HT2B, 5HT1B, and 5HT2C receptors in the normal heart valves of CM monkeys and S-D rats, and IHC and TaqMan techniques may be used to study the potential mechanism of compounds with 5HT2BR agonist activity.

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