Liquid biopsy and circulating tumor cells
The current ‘gold standard’ procedure for obtaining information about a tumor is a solid biopsy. However, this is not without limitations, including:
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The primary tumor is not always easily accessible, for example a brain tumor. Even when it is, the invasive procedure is often painful and carries risk of infection and bleeding.
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Continual monitoring of tumor evolution is hampered once the primary tumor has been removed, with metastatic disease sites often difficult to access.
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Single-site biopsy may not provide a complete genomic landscape of the tumor due to intra-tumor heterogeneity, with anatomically distinct areas within a primary tumor, and the metastases, exhibiting clear differences in genomic architecture.4 For instance, in breast cancer, where HER2 status guides therapy, overt distant metastases and CTCs are found to have discordant HER2 statuses compared with the primary tumor in up to 30% of cases.5
CTCs are cells that have been shed from a solid tumor into the vasculature. They can be found even in patients with no overt evidence of metastasis and in whom the primary tumor has been completely removed. This population of cells includes viable tumor cells capable of initiating metastasis and the presence and quantity of CTCs has been shown to be indicative of patient prognosis in a number of cancers. CTCs also hold valuable information about the tumor, including the genetic mutations that drive the tumor’s growth and resistance mechanisms, the very information that makes each tumor unique. Thus, CTCs could serve an important role in putting precision medicine into practice, namely in diagnosis, treatment stratification on the basis of molecular characterization, real-time monitoring of treatment efficacy and remission surveillance.6
In addition to the information they can provide, the appeal of CTCs is that they can be harvested from a peripheral blood sample, or liquid biopsy, which is significantly less invasive and better suited for serial sampling. However, CTCs are extremely rare with estimates of just one CTC per 107 white blood cells per milliliter of blood.6 Therefore, their detection and capture is not straightforward.
CellSearch is the only FDA-approved CTC capturing device approved for CTC enumeration in breast, prostate and colon cancers. It isolates CTCs using magnetic particles coated with antibodies that bind to a cell surface marker called anti-epithelial cell adhesion molecule (EpCAM). Once extracted, CTCs are enumerated and their concentration has been proven to have prognostic value for progression-free survival and overall survival in primary and metastatic disease of patients. However, CTCs are heterogeneous and may express fewer cell surface markers, like EpCAM, in a process called epithelial-to-mesenchymal transition (EMT). The exact process of metastasis is still unclear, but emerging data in this field point to EMT transition being involved. EMT is the process in which malignant epithelial cells gain migratory and invasive properties.2 Due to EMT, circulating tumor cells may express fewer cell surface markers, like EpCAM. Also many cancers have weak epithelial expression, such as ovarian. Therefore, antibody-based systems, such as CellSearch, which are limited to detecting epithelial markers may fail to capture key subsets of CTCs, which are of particular clinical relevance.
Angle’s Parsortix system is a platform technology for harvesting rare cells from blood. The system uses a patented step-based microfluidic technology in the form of a disposable cassette, to capture and harvest the CTCs. The system processes the patient blood sample (volumes of <1ml up to 50ml) through the cassette, taking 60 to 90 minutes per standard 10ml sample. Based on their morphology, namely that they are less deformable and larger than other blood components, CTCs are caught in the cassette while the other blood components are able to pass through. The CTCs can then either be fixed and stained in the cassette for identification and enumeration, or can be harvested from the cassette to allow for external staining, genetic analysis and culture, Exhibit 1.
Exhibit 1: Parsortix system overview
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Source: Angle. Note: (A) The Parsortix system. (B) Diagram of the disposable isolation cassette. (C) Isolation principle inside the cassette. (D) Captured prostate cancer cells. The lines are the Parsortix cell separation steps.
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The Parsortix system, comprising a desktop machine and one-time use consumables, can be purchased for use on site alongside existing analysis platforms. In contrast, many of the competitor systems are so complex that the sample must be sent to a CLIA certified laboratory for processing. This has commercial downsides as it requires large in-house investment, is less scalable and deprives the hospital of the reimbursement, which instead goes to the external laboratory. The Parsortix system has potential to offer a number of advantages over other CTC enrichment technologies, particularly antibody-based approaches. These include the low cost and simplicity of the capture process, and the high purity of harvested cells, with minimal white blood cell contamination. In addition, the Parsortix system offers three important advantages:
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It is marker-independent and therefore is not limited by cell marker bias. It is able to capture multiple CTC types, including mesenchymal CTCs. Antibody-based approaches, including CellSearch (Janssen Diagnostics), the only FDA-approved system for CTC detection, rely on cell surface markers to select CTCs, often with an epithelial phenotype. As a result, these approaches are limited to certain types of CTCs and cancers. Furthermore, this approach may result in false negatives in cases where the cancer cells have undergone an EMT transition that results in reduced expression of the target markers. This is particularly important, as EMT cells have been implicated in the process of cancer metastasis.6
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Many of the other approaches have limited potential for the easy harvesting of CTCs, thus preventing genome analysis (CellSearch’s approval is limited to enumeration for prognostic purposes). A possible alternative is cell-free DNA (cfDNA), which is released as fragments from necrotic and apoptotic tumor cells, and can be detected from blood plasma for analysis using next generation sequencing. However, the analysis is limited to DNA only, whereas CTCs allow for analysis at the DNA, RNA and protein levels, providing more information.
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CTCs harvested using Parsortix are not subject to antibody binding or other chemical reactions. In some of the pilot trials captured CTCs have been found to be viable after harvesting; in contrast, many of the antibody-based systems lead to damage to, or death of, the cell, which can limit detailed analysis. A number of Angle’s collaborators are now investigating the potential to culture the CTCs, which could allow for the testing of proposed treatments prior to patient administration.
There a number of CTC detection systems in development. Exhibit 2 lists a selection of these.
Exhibit 2: Selected CTC detection technologies
Product (company) |
Status |
Notes |
Antibody-based systems |
CellSearch (Veridex/Janssen Diagnostics) |
FDA approved, CE marked for clinical use |
FDA approved for enumeration of CTCs for prognostic purposes in metastatic breast, colorectal and prostate cancer. Isolates CTCs using magnetic particles coated with anti-EpCAM antibodies. Limited to epithelial CTCs. Captured CTCs typically have low yield and purity, and are not viable. |
IsoFlux (Fluxion Biosciences) |
Lab-run test |
Antibody-coated magnetic beads combined with microfluidic processing, not limited to epithelial markers. High-sensitivity (>80%), tumor DNA purity >10%. CTCs can be analyzed using a number of analysis platforms. |
GILUPI CellCollector (GILUPI) |
CE marked for clinical use |
Anti-EpCAM antibody-coated functionalized medical wire which is placed directly into the antecubital vein for 30 minutes to sample a large blood volume. High CTC sensitivity of c 70%. Captured CTCs can be used for enumeration and analysis. Limited to epithelial CTCs. |
AdnaTest (ADnaGen AG) |
CE marked for clinical use |
Immunomagnetic beads with MUC1-coupled and EpCAM-coupled antibodies. Specificity of >90% and a sensitivity of two CTCs per 5ml of blood at a recovery rate of >90%. Cell lysis means that enumeration is not possible. Obtained mRNA can be analyzed by PCR. |
MagSweeper (Illumina) |
Validation |
Enriches CTCs using a magnetic rod stirred through a blood sample pre-labelled with anti-EpCAM antibody-coated magnetic beads. In one study mean capture of spiked cells was 81%, also able to isolate viable CTCs with high purity. Limited to epithelial CTCs. |
LiquidBiopsy (Cynvenio) |
Lab-run test |
Immunomagnetic capture of CTCs and cfDNA within a microfluidic chip. Reports capture sensitivity of one CTC per ml of blood with high purity. Automated platform means cell populations can be directly analyzed by NGS and other platforms. |
CTC-iChip (Veridex-partnered) |
Prototype |
Magnetic bead capture of WBCs combined with microfluidic inertial focusing to isolate CTCs. Not limited to epithelial CTCs and cells are viable after capture, however purity is reported to be low as a result of WBC contamination |
Lung and breast cancer offering (Biocept) |
Lab-run test |
Dual platform of proprietary antibody-based enrichment technique and cfDNA capture. The latter is also innovative with an extra sample preparation step that ensures almost all the DNA it sequences comes from the mutant cells shed by tumors. Enriched CTCs can be checked for certain surface biomarkers. |
Membrane-based systems |
ScreenCell (ScreenCell) |
CE marked for clinical use |
Filtration based on cell size through a microporous membrane filter. Capture sensitivity reported to be two CTCs per ml of blood. CTCs can then be analyzed in situ, or harvested for analysis and/or culture. Not limited to epithelial CTCs. |
ISET (RareCell Diagnostics) |
CE marked for clinical use |
Filtration based on cell size. Capture sensitivity reported to be one CTC per ml of blood. Captured CTCs can then be analyzed by FISH and PCR. WBC contamination due to membrane becoming clogged. Not limited to epithelial CTCs. |
Centrifugation and Vortex flows |
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DeanFlow Fractionation |
Prototype |
Size-based selection using centrifugal force. Reported recovery of CTCs >85% with high purity. High throughput, no issues with clogging. Captured CTCs are easily harvested for further analysis and are viable. |
ClearCell FX System (Clearbridge BioMedics) |
Marketed for research use only |
Automated machine using the CTChip FR1 microfluidic chip to isolate CTCs on the basis of their size and inertia. Recovery >40% with spiked samples. Reports ultra-high purity and high throughput. Harvested CTCs are intact and viable. The system can be integrated with a number of downstream analysis technologies and culture of CTCs. |
Vortex Biosciences (NetScientific) |
Validation |
Microfluidic chip to isolate CTCs on the basis of their size and other physical properties. Preliminary testing suggests >80% purity and high throughput. CTCs are viable and can be harvested for downstream analysis and culture. |
Microfluidic |
Parsortix (Angle) |
CE marked for clinical use |
Microfluidic disposable cassette captures CTCs on the basis of their size and morphology. CTCs can be fixed and stained in situ or harvested for analysis or culture. See text for further details. |
Source: Edison Investment Research., company websites. Note: Published data is limited on many systems. EpCAM: epithelial cell adhesion molecule; WBC: white blood cell; FISH: fluorescence in situ hybridization; NGS: next generation sequencing; PCR: polymerase chain reaction.