[Significance of imaging biomarkers in exploratory clinical trials].
Brain imaging tools such as positron emission tomography (PET) and functional magnetic resonance imaging have been successfully used to investigate in vivo human brain functions. In particular, PET has been widely used to visualize various molecules such as receptors, transporters, and amyloid proteins. Despite the fact that a considerable amount of time and money is invested in drug development, only a few drugs have been able to complete clinical trials; this can be partly attributed to the lack of appropriate information regarding the in vivo pharmacokinetics and pharmacodynamics of the drugs. Therefore, currently, PET is being employed to determine the optimal clinical dose and the pharmacokinetic profile of psychotropics by investigating the index of drug occupancy at its action sites. Furthermore, recently, the European Agency for the Evaluation of Medicinal Products and Food and Drug Administration have introduced microdose clinical trials and exploratory investigational new drug studies to screen candidate drugs using PET in the early phase of clinical trials. Subsequently, in 2008, the Japanese Ministry of Health, Labor and Welfare also released certain guidelines for microdose clinical trials. Thus, imaging biomarkers have facilitated the collection of useful information on drugs at every stage of drug development.