Medicinal chemistry strategies to 5-HT6 receptor ligands as potential cognitive enhancers and antiobesity agents

Although the 5-hydroxytryptamine 6 (5-HT 6 ) receptor was discovered only recently, its almost exclusive distribution in the brain makes it a promising, novel, target for central nervous system (CNS)-mediated diseases such as Alzheimer's disease (cognitive function), schizophrenia, anxiety and obesity. In the past few years a significant research interest has advanced the understanding of the functional roles and the pharmacophore requirements of this receptor. Two 5-HT 6 receptor antagonists have already entered Phase II clinical trials for the enhancement of cognitive function. Since the first discovery of selective ligands for the 5-HT 6 receptor by HTS in 1998, several medicinal-chemistry-driven approaches have delivered highly selective lead structures with well-defined functionalities, starting from either the endogenous ligand 5-HT or the chemical structures identified by HTS. The concept of ‘scaffold hopping’ has been employed to expand the variability of the available chemical scaffolds and to generate patentable ligands. Supported by pharmacophore models, which have been established recently, the binding and functionality (structure–activity relationships) of the lead structures have been optimized further.