Current achievements and future perspectives with liquid biopsy

Liquid biopsy with circulating tumor DNA (ctDNA) has become widely applied in clinical settings in step with progress in innovative technologies. Since circulating free DNA (cfDNA) was found in human plasma from patients with various cancers as well as auto-immune diseases, clinical application to oncology has spread, especially involving molecular analysis. cfDNA contains ctDNA derived from cancer cells, and it is not possible to isolate ctDNA from cfDNA derived from normal cells. Therefore, we need to recognize ctDNA by detection of somatic mutations corresponding to those observed in cancer tissue. Early on, these mutations were detected by polymerase chain reaction-based methods such as digital PCR. Recently, comprehensive genomic analysis with next generation sequencing (NGS) has been developed for liquid biopsy. Clinical applications include monitoring of tumor progression, analysis of mechanisms of resistance to molecular targeted therapy, and the possibility of detecting minimum residual disease after surgery. However, the sensitivity of ctDNA detection is unsatisfactory, and application is still limited to advanced cancers. To solve these problems, it is necessary to perform basic analysis of ctDNA and establish an efficient system for ctDNA isolation. Recent investigations showed that the cfDNA size distribution differs between ctDNA and cfDNA derived from normal cells, leading to efficient detection of ctDNA. Such new concepts could lead to greater development of liquid biopsy.