Liquid biopsies come of age: towards implementation of circulating tumour DNA

Key Points Cell-free DNA (cfDNA) is released predominantly by cell death into the bloodstream, although active secretion may have a role. Since the discovery of fetal cfDNA in the maternal circulation, cfDNA analysis has been rapidly implemented in clinical practice for noninvasive prenatal testing. Mutations were first detected in cfDNA more than two decades ago, and interest in circulating tumour DNA (ctDNA) as a noninvasive cancer diagnostic has increased considerably with the development of molecular methods that permit the sensitive detection and monitoring of multiple classes of mutation. ctDNA may have utility at almost every stage of the management of patients with cancer, including diagnosis, minimally invasive molecular profiling, treatment monitoring, the detection of residual disease and the identification of resistance mutations. ctDNA analysis may be broadly considered as a tool both for quantitative analysis of disease burden and for genomic analysis. The identification of ctDNA in individuals before a cancer diagnosis, and in presymptomatic individuals, suggests the possibility of ctDNA analysis as a tool for earlier detection and screening. Noninvasive cancer classification or subtyping has also emerged as a possibility, although for early detection, both technical and biological factors introduce challenges to the detection of mutant DNA in plasma and its interpretation. Monitoring multiple mutations in parallel can enhance the sensitivity of ctDNA detection, can be used to assess the clonal evolution of patients' disease and may identify resistance mutations before clinical progression is observed. ctDNA analysis is beginning to transition from the research setting into the clinic. The US Food and Drug Administration and the European Medicines Agency have approved ctDNA tests for specific indications in the absence of evaluable tumour tissue. Analysis of gene panels in plasma has now become available as a potential clinical tool. Large studies are under way to establish the overall performance and clinical utility of such assays when a tumour biopsy is not available for analysis. Potential applications of ctDNA have been demonstrated by a number of proof-of-principle studies. Prospective clinical trials are beginning to assess the clinical utility of ctDNA analysis for molecular profiling and disease monitoring. The increasing acceptance of ctDNA is enabling the field to move from exploratory ctDNA studies towards clinical trials in whic