Date Published: July 16, 2015
Publisher: John Wiley and Sons Inc.
Author(s): Yong‐Qiang Li, Bevita K. Chandran, Chwee Teck Lim, Xiaodong Chen.
Originating from primary tumors and penetrating into blood circulation, circulating tumor cells (CTCs) play a vital role in understanding the biology of metastasis and have great potential for early cancer diagnosis, prognosis and personalized therapy. By exploiting the specific biophysical and biochemical properties of CTCs, various material interfaces have been developed for the capture and detection of CTCs from blood. However, due to the extremely low number of CTCs in peripheral blood, there exists a need to improve the efficiency and specificity of the CTC capture and detection. In this regard, a critical review of the numerous reports of advanced platforms for highly efficient and selective capture of CTCs, which have been spurred by recent advances in nanotechnology and microfabrication, is essential. This review gives an overview of unique biophysical and biochemical properties of CTCs, followed by a summary of the key material interfaces recently developed for improved CTC capture and detection, with focus on the use of microfluidics, nanostructured substrates, and miniaturized nuclear magnetic resonance‐based systems. Challenges and future perspectives in the design of material interfaces for capture and detection of CTCs in clinical applications are also discussed.
Metastasis, the spread of tumor cells from the primary tumor site to vital distant organs through the circulatory system, is directly responsible for most carcinoma‐related deaths in cancer patients.1, 2, 3 Understanding the metastasis process and investigating the cause of metastasis will benefit the diagnosis and therapy of cancers, and have long been a focal point in the fight against malignant tumors.4, 5, 6 More than a century ago, Ashworth found tumor cells in the blood of an individual with metastatic cancer and suggested that these circulating tumor cells (CTCs) could originate from several tumors present in the patient.7 CTCs up to now have been found in patients with malignant tumors including lung, prostate, breast, colon and pancreatic cancers, but not in healthy individuals or patients with non‐malignant tumors. Hence, the relationship between the presence of CTCs and the development of metastases has been an important subject in tumor studies, and the number of CTCs is believed to be an important indicator of carcinoma progression and metastasis.8, 9, 10 Therefore, CTC enumeration can be used as a novel approach for cancer prognosis in which the enumeration values have been demonstrated to correlate to overall survival of patients with metastatic cancer. For example, patients with metastatic breast and prostate cancer have a lower survival rate if their CTC count is more than 5 CTCs per 7.5 mL of whole blood when using the CellSearch System.11 Compared to routine clinical analysis by collecting disseminated tumor cells via surgical removal or tumor biopsy, CTC enumeration from peripheral blood as a “liquid biopsy” is more convenient and amenable in practical operation.12 Furthermore, for patients undergoing cancer treatment, the decline of CTCs number is reported with the decrease of tumor size.13, 14 Hence, in addition to serving as a prognostic marker of cancer metastasis, CTC enumeration can also be used as a novel non‐invasive method to assess the efficacy of cancer therapeutic treatment and realize personalized therapy.
By presenting the possibility of being exploited to discriminate CTCs from normal blood cells, the biophysical and biomechanical properties of CTCs have gained much attention.26, 27, 28 In this section, we first review historical and recent studies of the biophysical properties of CTCs including density, size and deformability. Next, we present studies of the unique biochemical properties of CTCs concentrating on the specific surface receptors that can be used for the selective capture and isolation of CTCs from peripheral blood samples.
Based on the differences in biophysical properties between CTCs and normal blood cells described above, label‐free strategy for direct capture and isolation of CTCs can be developed. As a powerful separation approach, microfluidic technique with small sample‐volume requirement, fast processing times, multiplexing capabilities and large surface area‐to‐volume ratios, offer a good option for label‐free CTCs capture and isolation.51, 52 Recently, with the progress in nanobiotechnology and microfabrication, various microfluidic devices with rationally designed material interfaces have been developed for efficient CTCs capture, isolation and enrichment.53, 54, 55 These exquisite microfluidic systems will be briefly introduced in this section, and their performances on CTCs capture and isolation are summarized in Table1.
In tissue engineering and regenerative medicine, nanostructured substrates have been widely employed to mimic the natural extracellular matrix (ECM) and basement membrane.85, 86, 87 These substrates can promote cell attachment due to enhanced local topographic interactions between nanostructures and nanoscale components of the cellular surface such as microvilli and filopodia, thereby assisting the capture and isolation of CTCs.88, 89 Furthermore, nanostructured substrates can provide more surface area for immobilization of CTC affinity molecules.90, 91 Hence, nanostructured substrates can be combined with the affinity interactions‐based CTC capture strategy, which can further improve CTC capture efficiency and emerge as a promising platform for isolation, and enrichment of CTCs. In this section, different types of nanostructured substrates‐based platforms for CTCs capture and isolation available will be briefly introduced including nanowires, nanopillars, nanodots, nanofibers, nanosheets, nanotubes and nanopores, and their performances on CTCs capture and isolation summarized in Table2.
As a novel sensing technology, micro‐nuclear magnetic resonance (μNMR) exploits magnetic resonance technology to detect target labelled with immunospecific magnetic nanoparticles (MNPs), showing great potential in rapid and highly sensitive biodetection.102 The typical MNPs used in μNMR are superparamagnetic and have small size (tens of nm), which is different from the conventional magnetic nanoparticles used in immunoseparation. The mechanism of μNMR‐based sensing technique is based on the phenomenon that MNP‐labeled targets exhibit faster relaxation of NMR signals due to local magnetic fields created by MNPs.103 By systematically optimization of nanoagents, MNP‐target conjugation method, and NMR detectors, several exquisite μNMR‐based platform have been developed for rapid and sensitive detection of biomolecules including nucleic acids, proteins, bacteria, and tumor cells.104, 105, 106, 107, 108, 109 Compared to conventional biosensing methods, μNMR‐based technique do not need sample purification procedures and can simultaneously achieve target capture and detection, gaining much attention in the fields of CTCs capture and detection. This section will briefly introduce recent developments of μNMR‐based biosening systems and their potential applications in CTCs capture and detection.
Once CTCs are captured and enriched, subsequent detection and identification are needed to investigate their origin and genetic profile from which more valuable insight into the biology of metastasis can be obtained.115, 116 In μNMR‐based platforms, CTCs detection can be easily achieved by analyzing the NMR signals of MNP‐labelled target tumor cells without prerequisite isolation and enrichment processes. Hence, this section focuses on the approaches of CTCs detection and identification used in microfluidic‐ and nanostructured substrates‐based platforms, which mainly involve immunological and molecular methods.
In the previous sections, we described various advanced materials interface mainly based on microfluidics, nanostructured substrates, and micro‐nuclear magnetic resonance systems for CTCs capture and detection. Although promising results have been achieved by these interfaces in terms of capture efficiency and detection sensitivity, most of them still remain in the laboratory level and little of them unequivocally shows clinical validity and utility.124 Heterogeneity among CTCs is a problem that cannot be ignored for CTCs isolation. CTCs always express variable biomarkers on their membrane, which affects their morphology and characteristics and makes cell‐to‐cell variations occur in same cancers, same patient or even within a single blood draw.125, 126, 127 Therefore, efficient CTCs capture and isolation are challenging due to this heterogeneity of CTCs, indicating that not a single cell surface biomarker can confidently be used for total CTCs isolation. That is why current widely used EpCAM antibodies‐based CTCs capture systems do have concerned limitations in clinical applications. Similarly, the immunological capture methods predominantly used in nanostructured substrates and nuclear magnetic resonance‐based platforms do have similar concerns. By exploiting the inherent unique biophysical properties of CTCs, label‐free microfluidic strategies seems to have greater potential in clinical capture and isolation of CTCs compared to the currently utilized biomarker‐based immunological methods by which only a subset of CTCs expressing the selected surface markers are isolated. However, the label‐free microfluidic approach might also introduce false positives results in clinical CTCs capture and isolation by capturing cells that may not directly originate from the primary tumors.128 In addition, captured CTCs by some label‐free microfluidic platforms are no longer intact after being subjected to shear forces, thus making subsequent CTCs identification and detection difficult.