Circulating tumor cell enrichment based on physical properties

Abstract

The metastatic dissemination and spread of malignant circulating tumor cells (CTCs) accounts for more than 90% of cancer-related deaths. CTCs detach from a primary tumor, travel through the circulatory system, and then invade and proliferate in distant organs. The detection of CTCs from blood has been established for prognostic monitoring and is predictive of patient outcome. Analysis of CTCs could enable the means for early detection and screening in cancer, as well as provide diagnostic access to tumor tissues in a minimally invasive way. The fundamental challenge with analyzing CTCs is the fact that they occur at extremely low concentrations in blood, on the order of one out of a billion cells. Various technologies have been proposed to isolate CTCs for enrichment. Here we focus on antigen-independent approaches that are not limited by specific capture antibodies. Intrinsic physical properties of CTCs, including cell size, deformability, and electrical properties, are reviewed, and technologies developed to exploit them for enrichment from blood are summarized. Physical enrichment technologies are of particular interest as they have the potential to increase yield and enable the analysis of rare CTC phenotypes that may not be otherwise obtained.

Keywords: antigen-independent; cancer; circulating tumor cells; enrichment; physical properties.

Fig. 1

Overview of the process of metastasis: Progression from a primary epithelial cancer cell to an invasive, metastatic cell involves several steps. First, cancer cells undergo EMT to (1) reduce adhesion to neighboring cells and (2) dissolve the basement membrane through the secretion of extracellular matrix metalloproteases (MMPs). (3) Intravasation, or the entry of a cancer cell into the bloodstream, is achieved by the release of molecules, such as vascular endothelial growth factor (VEGF), that stimulate angiogenesis. In the bloodstream, cancer cells can interact with platelets (4), which protect the cancer cell from the immune system. After reaching the secondary site, cancer cells can exit the bloodstream (5) by inducing endothelial cell retraction or death. Lastly, the cancer cells undergo MET (6) and continue to proliferate at the metastatic site.

Fig. 2

Technologies for CTC enrichment based on physical properties. A: Density gradient centrifugation with OncoQuick® (reproduced by courtesy of Grenier Bio-One GmbH, Germany). B: Track-etch filter for ISET. Arrows indicate 1: tumor cell; 2: filter pores; 3: leukocytes. (reproduced from ref. with permission from Elsevier). C: 3D–microfilter device for viable CTC enrichment (reproduced from ref. with permission from Springer). D: Crescent-shaped traps in a microfluidic device. Scale bar is 20 µm. (reproduced from ref. with permission from Elsevier). E: Array of microfluidic traps with varying geometrical restrictions. (reproduced from ref. with permission from Springer). F: Laminar vortices generated on a microfluidic device for size based CTC enrichment. (reproduced from ref. with permission from AIP). G: Spiral microchannel for CTC isolation using centrifugal forces. (reproduced from ref. with permission from NPG). H: DEP separation of CTCs using Apostream™ (reproduced from ref. with permission from AIP).