Technologies for circulating tumor cell separation from whole blood

Abstract

The importance of early cancer diagnosis and improved cancer therapy has been clear for years and has initiated worldwide research towards new possibilities in the care strategy of patients with cancer using technological innovations. One of the key research fields involves the separation and detection of circulating tumor cells (CTC) because of their suggested important role in early cancer diagnosis and prognosis, namely, providing easy access by a liquid biopsy from blood to identify metastatic cells before clinically detectable metastasis occurs and to study the molecular and genetic profile of these metastatic cells. Provided the opportunity to further progress the development of technology for treating cancer, several CTC technologies have been proposed in recent years by various research groups and companies. Despite their potential role in cancer healthcare, CTC methods are currently mainly used for research purposes, and only a few methods have been accepted for clinical application because of the difficulties caused by CTC heterogeneity, CTC separation from the blood, and a lack of thorough clinical validation. Therefore, the standardization and clinical application of various developed CTC technologies remain important subsequent necessary steps. Because of their suggested future clinical benefits, we focus on describing technologies using whole blood samples without any pretreatment and discuss their advantages, use, and significance. Technologies using whole blood samples utilize size-based, immunoaffinity-based, and density-based methods or combinations of these methods as well as positive and negative enrichment during separation. Although current CTC technologies have not been truly implemented yet, they possess high potential as future clinical diagnostic techniques for the individualized therapy of patients with cancer. Thus, a detailed discussion of the clinical suitability of these new advanced technologies could help prepare clinicians for the future and can be a foundation for technologies that would be used to eliminate CTCs in vivo.

Keywords: CTC; Cancer; Circulating; Separation; Tumor cell; Whole blood.

Fig. 1

Different types of whole blood methods where immunocapture and physical selection (size and density) methods are separated into subgroups depending on the main properties of the technique. (“Immunoaffinity,” “Size based,” “Density based,” “Combined” sections; i.c immunocapture)

Fig. 2

Immunoaffinity-based techniques where the EpCAM antigen is typically targeted on the surface of CTCs for positive enrichment, while the CD45 antigen is targeted on the surface of noncancerous cells for negative enrichment. For positive enrichment, only a subpopulation with a distinct antigen is captured. For negative enrichment, the obtained cancer cells are label-free, and a heterogeneous CTC population is obtained

Fig. 3

Different CTC membrane microfilters where a an FMSA contains highly porous and flexible micro spring structures in a single layer [32] and b shows a separable bilayer structure where capture is achieved by a gap between the top and bottom porous membranes with 8-μm-diameter holes arranged hexagonally on the bottom layer and larger holes with a diameter of 40 μm on the top layer [33]

Fig. 4

Microfluidic CTC sorting devices where a Parsortix contains a microscale stepped separation structure (image on the left side) with a cross-sectional gap that gradually decreases the dimension of the fluid path (schematic on the right side) [70, 107] and another microfluidic device and b an MCA contains microcavity arrays with circular- (image on the left side) or rectangular-shaped cavities (image on the right side) [31]