Circulating tumor cells: advances in isolation and analysis, and challenges for clinical applications

Abstract

Circulating tumor cells (CTCs) are rare cancer cells released from tumors into the bloodstream that are thought to have a key role in cancer metastasis. The presence of CTCs has been associated with worse prognosis in several major cancer types, including breast, prostate and colorectal cancer. There is considerable interest in CTC research and technologies for their potential use as cancer biomarkers that may enhance cancer diagnosis and prognosis, facilitate drug development, and improve the treatment of cancer patients. This review provides an update on recent progress in CTC isolation and molecular characterization technologies. Furthermore, the review covers significant advances and limitations in the clinical applications of CTC-based assays for cancer prognosis, response to anti-cancer therapies, and exploratory studies in biomarkers predictive of sensitivity and resistance to cancer therapies.

Keywords: AR; CK; CTC; Cancer biomarker; Circulating tumor cells; DEP; EGFR; EMT; ER; EpCAM; FDA; FISH; HER2; IF; ISET; PR; PSA; PSMA; Predictive biomarker; Prognostic biomarker; RT-PCR; US Food and Drug Administration; androgen receptor; circulating tumor cell; cytokeratin; dielectrophoresis; epidermal growth factor receptor; epithelial cell adhesion molecule; epithelial–mesenchymal transition; estrogen receptor; fluorescence in situ hybridization; human epidermal growth factor receptor 2; immunofluorescence; isolation by size of epithelial tumor cells technique; plasma tumor DNA; progesterone receptor; prostate specific antigen; prostate-specific membrane antigen; ptDNA; reverse transcription polymerase chain reaction.

Fig. 1

Approaches for CTC isolation from whole blood. 1: Immunoaffinity based techniques target specific markers to selectively enrich CTCs or deplete leukocytes. 2: Physical properties may be exploited to separate CTCs from blood cells based on differences in density, size, deformability and electrical properties. 3: Direct analysis is achieved by high throughput assaying of all cells in blood after erythrocyte lysis.

Fig. 2

Analytic tools for analysis of CTCs after isolation. 1: Immunophenotyping employs antibodies to label proteins for CTC detection and characterization. 2: Fluorescence in situ hybridization detects aberrant amplification and translocation of specific genes. 3: PCR and DNA sequencing techniques identify oncogenic mutations and alterations to the genome. 4: RT-PCR and RNA based profiling characterize CTC gene expression. Listed images are reproduced with permission and with all rights reserved from the following sources: Detection: IOP Publishing (Marrinucci et al., 2012) Characterization: BioMed Central (Alix-Panabieres et al., 2009) Amplification: Public Library of Science (Punnoose et al., 2010) Translocation: The American Association for the Advancement of Science (Stott et al., 2010b) DNA sequencing: John Wiley and Sons (Zhao et al., 2013) Expression profiling: Public Library of Science (Powell et al., 2012)