Tumor metastases cause ninety percent of cancer deaths, and occur when cells detach from a primary tumor and form new malignancies elsewhere. This process involves circulating tumor cells (CTC) which intravasate and extravasate (enter and exit the blood vessels) allowing migration to other parts of the body.
Recent technological developments enable detection of the minute amounts of circulating tumor cells in patient blood samples. These non-invasive ‘liquid biopsy’ techniques allow patient monitoring and early detection of metastasis, and provide the possibility for cell-specific information to direct personalized, targeted therapy at an early stage of the disease.
In Towards the Biological Understanding of CTC: Capture Technologies, Definitions and Potential to Create Metastasis, Barradas and Terstappen review the current state of CTC research and technology in a highly informative work recently published in the journal Cancers Special Issue Circulating Tumor Cells in Cancer. Below, Leon Terstappen, discusses challenges and directions in CTC research and technology.
The definition of CTC is currently not clear, yet is critical for the diagnostic and therapeutic value of CTC capture technologies, can you explain the challenge?
Leon Terstappen: Each technology is using a different definition. The most common technologies are chasing the EpCAM (Epithelial Cell Adhesion Molecule) antigen, at least for carcinomas, so that represents a large camp of technologies, but even within that camp there is enormous discrepancy because how do they then identify the cells? Many are doing it with microscopy technologies but even there, what is the numerical aperture of the objective, which reagents and colors do you use to identify the cells?
The standards are basically from the CellSearch system using a nucleic acid dye (DAPI), using antibodies against some of the cytokeratins, labeled with PE, and CD45, a molecule expressed on leukocytes labeled with APC. So then the original definition is that it has to express cytokeratins, lack CD45, be at least 4 microns and look like a cell, which is of course, subjective .
When you change any of the dyes or the reagents, the cells which you will be counting will not be the same cells anymore, but how do you know what is the correct definition? There are reports of people detecting a thousand times more circulating tumor cells, but then you are in the range of the white blood cells, so if that were so, everyone should have seen them. Sometimes detection might include tumor microparticles, not really cells, since a lot of technologies do not allow you to actually see that. As the technologies develop, the number of detected cells goes down because you add criteria, which makes it more strict.
So the real challenge here is to have the various investigators use a common platform at least to share definitions or let other people look at cells and say this is one, that’s not one, otherwise you never get consensus, so in the end, you will have to do a clinical study to demonstrate that the presence of these cells indeed strongly correlates with the clinical outcome. The CTCs detected by the CellSearch system have been proven in clinical trials in that the more CTC you have the quicker you die. When you suddenly find ten times more, also in patients where you didn’t see them with the CellSearch system, that is confusing because these patients live much longer. So, perhaps then the cells, if they are in fact cells, which you are detecting, are not really relevant.
The CTCtrap method you are developing allows detection from the entire patient blood volume along with cell analysis. Can you explain your perspective on current CTC research and how CTCtrap will help advance progress in this area?
Leon Terstappen: The technologies to reliably capture CTC and investigate them are fairly recent and allow us to gather information on what you see in these cells at the protein and gene level. But the ability to investigate heterogeneity means also you have to do so at the single cell level, so we are currently developing novel technologies to actually enable that because the most critical aspect in cancers is the heterogeneity and the constant changing. You are diagnosed today with metastatic disease and you get treated. But the phenotype and genotype of the tumor cells change and they will become resistant to the therapy. These cell specific investigations are proceeding currently, but will progress faster if most people would at least have similar definitions about what they call a CTC, so that then in reports about gene expression, protein expression, you are talking about the same thing.
The real promise of this technology is to personalize therapy, and the only way to do that is to have tumor cells available at all times during the disease process, so that you can actually change therapy when needed according to the new information. The information to direct therapy options is contained within these cells, so we hope to get as much information out as possible and hopefully in the future we can treat the patients based on the phenotype and genotype of the tumor cell that is actually circulating in the patient’s blood.