The Meaning of CTCs: More Research Needed
Circulating tumor cells (CTCs) are cells that ‘escape’ from primary tumor cells and migrate into the bloodstream, where they circulate through the body. CTCs were first identified in the 1980s as cells that had the morphology of tumor cells but were found in the blood. Over the past two decades technological innovations have eased the detection and analysis of CTCs. The resulting investigations of CTCs are an important area of clinical research because they are demonstrating that CTCs have potential clinical value as warning signals of cancer progression.
However we have only scratched the surface in deciphering the potential clinical significance and applications of CTCs. Much more clinical research is needed before we can achieve what I believe will be the biggest impact of CTCs for a cancer patient – the day when a newly diagnosed cancer patient can have a blood draw and within hours learn more about the characteristics of his/her cancer, such as the likelihood of metastasis, based on the CTC profile.
To get to the day of meaningful CTC-based cancer diagnostics, new technologies are needed for more comprehensive CTC analysis. We need to understand better the full array of CTCs associated with specific cancers and if/how particular CTC profiles are correlated with various clinical outcomes. Our present technology only gives us a limited view of CTCs. However, advances in the newest approach for detecting CTCs, microfluidic chips, holds promise for being the flexible platform needed for more complete CTC analysis to define and differentiate biomarkers.
Current CTC Technologies: Where are we?
The original technological innovation that enabled more efficient capturing and counting of CTCs is based on magnetic beads that are coated with an antibody that binds EpCAM (epithelial cell adhesion molecule), a common marker present on CTCs originating from epithelial cancers. Usually EpCAM positive (epithelial) cells are at a very low level in normal blood, so an increase in number of such cells in the blood of a patient with cancer is assumed to represent CTCs. Standardization and commercialization of this method for counting CTCs and correlating them to later stage cancer outcomes has been achieved by Veridex with its CTC magnetic bead-based system called CellSearch®.
However, the medical community has yet to fully incorporate the prognostic information currently provided by this CTC technology. For instance, although CTC technology is primarily limited to patients with metastatic cancer, the American Society of Clinical Oncology Tumor Marker Guidelines Committee has not recommended incorporation of CTC levels by any method into standard care of patients with metastatic breast cancer. This is because the technology is not yet sensitive enough to detect the lower numbers of CTCs that are likely present in early stages of cancer.
A limitation of the current CTC detection technology is its reliance on EpCAM positivity. EpCAM is not a perfect marker for CTCs since epithelial cancers cells within a single tumor mass will have heterogeneous expression of EpCAM. Although most CTCs studied are EpCAM positive, there is a whole subpopulation of EPCAM negative CTCs that are of growing interest in clinical research. Numerous studies have shown that not all cancer patients have detectable CTCs based on the CellSearch system.
This finding can be explained by the presence of EPCAM negative CTCs that have been shown to originate from the epithelial-mesenchymal transition (EMT). EMT cells are thought to be an early event in cancer invasion and metastasis and thus, the cells that we should be looking for if we want to detect cancer metastasis earlier. In fact, the acquisition of an EMT phenotype has been linked to tumor cell populations with an increased tumor initiating capacity and increased resistance to therapies. A deeper understanding of these EpCAM negative CTCs will help us get closer to the optimum use of CTCs for physicians and patients.
The challenge is how to capture and characterize these rarer EpCAM negative cells in a comprehensive and effective way. Once CTCs can reproducibly and accurately be identified and quantified in a more efficient way, clinical research correlating the number and type of CTCs in a patient’s blood will help physicians determine the real time status of the cancer and thus provide the necessary information for patient treatment.
Increasing the sensitivity and bringing CTC analysis to the clinical setting at an earlier stage is where much innovation is occurring both in ‘bead technology’ and in the microfluidic field.
Another CTC Technology: Microfluidic chips
Microfluidic chips are one technological innovation that may help catapult CTC detection and characterization forward. The main advantage to microfluidic devices is their flexibility and ability to detect larger numbers of CTCs. They have the potential to be applied to analysis at an earlier stage of cancer when CTCs are fewer. Microfluidic technology is on the verge of not only revolutionizing early cancer detection, but identifying subpopulations of CTCs that will directly impact the value of targeted therapies.
The microfluidic chip is a more flexible technology because it allows blood samples to be evaluated with a cocktail of antibodies simultaneously so both EpCAM positive and EpCAM negative cells can be captured, enumerated and characterized, which will hopefully lead to earlier detection of cancer. This technology will hopefully enable new correlations between CTCs and cancer outcomes, which could lead to more clinically meaningful decisions.
Another recent advance in microfluidic chip technology is the ability to integrate size capture with antibody-mediated affinity capture to ensure collecting CTCs with low expression levels of EpCAM. Microfluidic chips that do both –capture cells of a certain size and capture cells by antibody affinity can lead to a much greater understanding of circulating tumors cells and the subpopulations that compose them.
A third advantage of the microfluidic chip is that the captured cells can be released from the chip with a variety of methods allowing the analysis of DNA, RNA and proteins from intact CTCs. Clinical researchers are investigating how CTCs’ characteristics can change over time for patients and if a changing profile of the CTC populations could be used to assess cancer progression or response.
CTCs as Clinical biomarkers
A more robust CTC analysis based on microfluidic chip technology has immediate benefits. For example, a fuller picture of CTCs can lead to improved drug development because it will allow more accurate patient stratification and monitoring of therapy outcomes. CTCs can be used as biomarkers to stratify patient selection for targeted therapies, monitor the efficacy of therapies or be early surrogate biomarkers for predicting patient outcomes.
Another early application of deeper CTC knowledge will arise from the genomic analysis of CTCs that microfluidic chips will enable. A more complete genetic profile of specific CTCs and how they correlate with certain cancer outcomes will facilitate personalized medicine by providing real time monitoring of therapy effectiveness.
In the end, the driving force behind all of these technological improvements remains to enable cancer diagnostics in a way that makes the biggest difference to the cancer patient.
Walter P. Carney, Ph.D.
Chief Scientific Officer, Interim President, On-Q-ity
Walter Carney, PhD is Chief Scientific Officer and Interim President of On-Q-ity Inc., an innovative diagnostics company focused on informing and transforming cancer treatment cycle management and improving the quality of life for cancer patients. Dr. Carney oversees the scientific and business operations at On-Q-ity as it continues to optimize its CTC diagnostic platform. He comes to On-Q-ity from Oncogene Science, where he was Chief Executive Officer.
Dr. Carney received his Ph.D. in Medical Microbiology and Infectious Diseases from Thomas Jefferson Medical School in Philadelphia, PA in 1978. He became a Harvard and NIH Fellow in the Department of Infectious Diseases at the Mass General Hospital, Boston, MA where he trained extensively in virology and in particular the Herpes Virus family, and is considered one of the pioneers in HIV research. Following his fellowship, Dr. Carney joined the DuPont Medical Products Department where he managed a research group focused on HIV and oncogenes. These efforts resulted in a series of patents covering both tissue based tests and circulating tests for the ras and HER-2/neu Oncogenes. He was the first to discover and patent the circulating extracellular domain for HER-2/neu, which is currently used in the management of women with metastatic breast cancer. Dr. Carney is an author and frequent speaker at biomarker meetings on the topic of using diagnostic tests to guide the use of targeted therapies.