CTCs and Circulating Tumor DNA for the Monitoring of Breast Cancer Patients

Massimo Cristofanilli, MD

As part of our coverage of the American Association for Cancer Research (AACR) Annual Meeting, held April 1–5 in Washington, DC, we are speaking with Massimo Cristofanilli, MD, associate director of precision medicine and translational research at the Robert H. Lurie Comprehensive Cancer Center at Northwestern University in Chicago. At the meeting, Dr. Cristofanilli discussed how circulating tumor cells (CTCs) and cell-free circulating tumor DNA may be used as prognostic markers and for disease monitoring in breast cancer.

 -Interviewed by Anna Azvolinsky 

Cancer Network: First, what do we know about CTCs and circulating tumor DNA in patients with breast cancer? Are there any general trends or patterns emerging?

Dr. Cristofanilli: CTCs and circulating tumor DNA have been studied for quite a long time in breast cancer and solid tumors in general. Particularly, CTCs were found to be prognostic and predictive from the first study published in the New England Journal of Medicine back in 2004. At that time, there was only one technology that allowed us to isolate and enrich the sample for cancer cells with epithelial characteristics in the blood, and that was the CellSearch system, which was approved by the US Food and Drug Administration (FDA). Since then a number of studies have shown that, in fact, more than five CTCs in the peripheral blood in patients with metastatic breast cancer is associated with a strong prediction of progression or survival.

Over the years several companies have tried to improve the technology, allowing us not only to enumerate the cells but also to look at the molecular characteristics of the cells, first looking at expression of estrogen receptors and HER2 because these are our common markers that we use for treatment planning.

Most recently, there has been a trend in trying to isolate cell-free DNA, which represents, I think, a complement to what we have already discovered about CTC research. In this case we are not just looking at the tumor burden in the amount of DNA present, but also looking for actionable mutations. The PI3 kinase mutations and estrogen receptor mutations may be important to monitor in patients receiving endocrine therapy or any other targeted therapy in breast cancer.

Cancer Network: You mentioned that for the circulating tumor DNA assays that you can look at both the number and also some of the mutations present in these cells. Is that also true for the currently available CTC assays?

Dr. Cristofanilli: The CTC technology allows us to not only look at the number, to visualize the cells, but also to extract the DNA or RNA from the cell and look at expression and specific mutations. So, it is possible to identify the same mutations that I mentioned for the cell-free DNA within the DNA extracted from the CTCs. In some respects, this may be considered more accurate. This critique has been raised with regard to the functional value of cell-free DNA and the source-this is most likely DNA from cells undergoing apoptosis in the tumor, but we don’t know if this is the most accurate representation of the tumor. With the CTCs, you have the possibility to make sure that this DNA actually comes from tumor cells. The problem with CTCs is that the enrichment technologies don’t capture all of the cells, the epithelial and mesenchymal, and some may have lost some characteristics of their cell of origin over time. There will always be some limitations of both of these technologies, and that is why I think combining the two in some respects, and monitoring the tumor burden during therapy, will give a more global view of what the disease biology looks like.

Cancer Network: Are there assays for either CTC or circulating tumor DNA that are now used in clinical practice to either help diagnose or monitor patients with breast cancer?

Dr. Cristofanilli: I mentioned that the CellSearch assay is FDA-approved, and there have been several studies looking at the clinical utility, to prospectively test whether changing the therapy, particularly going from one chemotherapy to another, will change outcomes. This has been mentioned in the American Society of Clinical Oncology biomarker guidelines-that CTCs may be used in the metastatic setting to monitor these patients-but there is no strong recommendation because it has not been shown, with regard to the clinical utility, that changing the chemotherapy just based on CTC could be useful. But the guidelines confirmed the prognostic value of CTCs. The CellSearch test is available and a physician is free to use it if prognosis is an important consideration for deciding treatment for patients with metastatic disease.

There are other technologies that are not approved by the FDA that may allow for some of these patients to be monitored with CTCs. I think an emerging trend is to monitor patients with a high risk of recurrence who have completed primary therapy; some companies have focused on detecting CTCs or cell-free DNA in this high-risk population. With regard to circulating tumor DNA, there are commercial tests available for detecting single mutations or panels of mutations. The most popular has been Guardant360, which was launched several years ago and provides a 73-gene panel that can be used at baseline to detect actionable mutations. However, some patients cannot have a next-generation sequencing analysis from tissue because of the location of the metastatic lesion or because of sampling issues. The test can also be used for longitudinal monitoring, which is probably the most important application for liquid biopsies.

Cancer Network: Are there any recent notable studies that suggest that either of these two types of assays could be useful for prognosis in breast cancer? You mentioned CellSearch, but are there any new ones?

Dr. Cristofanilli: Yes, I think with regard to the cell-free DNA assays, it’s clear that testing for some of the mutations associated with endocrine therapy resistance appears to be important, because this is associated with overall survival benefit and sensitivities to some therapies. Clearly, the mutations that are most important for estrogen receptor–positive breast cancer are estrogen receptor mutations and PI3 kinase mutations. In retrospective studies, estrogen receptor mutations have been shown to be prognostic and can be associated with poor outcomes when patients are on an aromatase inhibitor, so they are predictive of a lack of benefit from an aromatase inhibitor. They are associated with a prediction of benefit for treatment with fulvestrant compared with an aromatase inhibitor. The PI3 kinase mutations appear to predict for resistance to some endocrine therapy, but also for response to some of the PI3 kinase inhibitors currently in clinical development. I think there will be much more use of these tests, particularly for these actionable mutations, in clinical practice and certainly for drug development, for monitoring of emerging resistance during therapy.

Cancer Network: You already mentioned this for a few specific mutations but, as far as disease monitoring, what do we know about how these assays may help clinicians understand whether a therapy is working or whether the cancer has mutated and may be resistant to a drug?

Dr. Cristofanilli: I think that this is something that is evolving. What appears to be important is that over time in treatment there is a much more complex evolution of disease with regard to some accompanying mutations together with the driver mutations, which may represent an even more complex disease than the way we think about it now or have dealt with. The genes for the cell-free DNA panel change over time for each patient and there are mechanisms of resistance that we have not known about before. At the same time, we need to understand how we use this information. Primarily, what do we do when we see a change in the patterns of mutations, besides the ones that we know have actionable therapeutic targets?

Understanding the disease is important in the context of precision medicine, so at our institution and at other academic institutions, there is an important role for incorporating cell-free DNA in the management of patients. Discussing the context of this at molecular tumor boards is also important because many times the type of mutations and the pattern is not so obvious. We need to work together to modify the therapy based on the changes in the mutations and the quantitation of the DNA to possibly improve patient outcomes. This is something that we need to generate, the clinical utility of longitudinal monitoring of cell-free DNA and the testing of actionable mutations, as well as quantifying the DNA to monitor the tumor burden. When you put all of this together, this will probably be the most important information that we can provide for patient care and for payers because they will see the utility of using this type of test compared, for example, with imaging studies.

Cancer Network: Thank you so much for joining us today, Dr. Cristofanilli.

Dr. Cristofanilli: You’re welcome. Thank you!