Overall, iPSC-based drug screening has been used to evaluate more than 1,000 compounds for several diseases (Table 5)121,125,128,129, and several clinical candidates have been identified (Table 6)126,127,130. However, these studies require considerable time (several weeks or more) to differentiate iPSCs into disease-relevant cell types. Although this may not seem long for phenotypic screening, a shorter differentiation period is preferable to avoid variation in cell quality. Therefore, faster and more stable differentiation methods that result in higher maturity and purity are being sought. An alternative approach is to perform drug screen using cells derived from direct conversion131,132. Direct conversion forces the target somatic cells (e.g. fibroblasts) to express cell-specific transcription factors and reprogram one somatic cell state to another somatic cell state without passing through the iPSC state49,132. Direct conversion has been used to reprogram myocardial cells, liver cells, neural cells, or other type of somatic cells from a different type of somatic cells, such as fibroblasts. As an advantage of direct conversion, authentic human neurons that reflect important aspects of cellular aging can be generated50. However, the non-renewable source of
