Unraveling metastasis with circulating tumor cells

Outside of major academic research institutions, there has not yet been widespread uptake of CTCs in clinical practice. “In this era of personalized medicine, physicians are really looking beyond enumeration and prognosis toward using CTCs as a means of guiding drug selection and improving survival.” That is the focus now in the development of related technologies, Dr. Liu says.

Most research on CTCs has come from epithelial tumors such as those of the breast, colon, lung, and prostate, says Terence Friedlander, MD, assistant clinical professor of hematology/oncology at the University of California, San Francisco. His research in medical oncology has focused on development of novel therapeutics by looking at tumor cells in advanced cancer of the prostate.

“The CellSearch test allows us to count or enumerate CTCs in a very reliable way,” he says, noting the well-established link between the number of CTCs a patient has at baseline and prognosis for survival, and that patients whose CTC numbers drop in response to chemotherapy for prostate cancer have better overall survival.
But an estimate of patient prognosis has limited usefulness, he points out. He is much more interested in CTCs as a predictive marker that can be used to tailor the most clinically effective therapy. The liquid biopsy concept, though not yet validated clinically, would have special value in metastatic prostate cancer, Dr. Friedlander believes.

“We know no two cancers are alike between two people, or within the same person, because the cancer cells have different mutations. Prostate cancer is a great example of a disease that spreads mostly to the bone, and that is pretty inaccessible. It’s a costly and expensive procedure to biopsy bone just to get a piece of prostate tissue, and because there’s generally not enough tissue to get useful information, a lot of different labs are studying the genetics of CTCs to see how well they correlate with tumor biopsies and how they change over time in response to therapy.”

Metastatic prostate cancer patients can live up to four or five years, Dr. Friedlander points out. “Chemotherapy is a one-size-fits-all blanket approach that we take, hoping that it kills off a lot of the cancer cells. But it doesn’t address molecular changes that have happened over time in the cells. The hope is that by learning what’s driving the cancers molecularly, clinicians can select more appropriate therapy for individual patients. And CTCs are just one way of doing that.”

Over the last five or 10 years, he adds, the field has been moving from simply counting CTCs to characterizing them. With a treatment called enzalutamide that is used for metastatic castration-resistant prostate cancer, for example, “there is some evidence now that if you see a certain pattern of the androgen receptor staining in the cytoplasm of the prostate cancer cell, this may predict whether the patient is likely to respond to enzalutamide, and help us avoid wasting two months of therapy only to find that it didn’t work.”

Many mysteries about CTCs’ role in cancer progression remain, however. “We know patients have primary tumors and that metastases presumably develop from these cells getting into the circulation and establishing themselves at distant sites. But what is somewhat unclear when we capture CTCs is what they really represent. Are they cancer cells that have detached from the primary tumor or from the metastatic one, and more importantly, are they destined to become metastases in another spot? There’s not very good hard evidence about this.”

His bias is that they probably represent the advanced cancer, the most active part of the cancer that’s growing fast, and the cells are leaving tumors from different parts of the body and are being detected in the bloodstream. “But a lot of this is speculation. It’s what makes the field more challenging than the solid tumor field, where you can just biopsy the primary or metastatic tumor.”

Dr. Friedlander is excited by one recent study of hormonal therapy for advanced prostate cancer. “What the investigators showed was that change in CTC counts, along with change in lactate dehydrogenase—which is a simple biochemical marker—taken together, actually predicts survival for the trial. That means that if a patient had a decline in those two numbers, they were statistically likely to live longer.”

The striking thing was that the investigators found that CTCs met strict criteria for surrogacy, meaning that a biomarker completely captures and replicates an endpoint being studied. This is an important development, Dr. Friedlander says. “The reason is that generally when we do a clinical trial, we have to show that patients actually live longer in order to get the drug approved. You can imagine if we could say that a CTC was a complete surrogate for survival, then a clinical trial could just be run and the endpoint would just be the change in CTC count” (Goodman OB Jr., et al. Cancer Epidemiol Biomarkers Prev. 2009;18[6]: 1904–1913).

Such a surrogacy would mean that clinical trials could be years shorter, probably with fewer patients and lower cost, and would speed up delivery of new drugs into clinical use, Dr. Friedlander says. “Theoretically, a clinical trial that now takes four to five years to complete could be done in a year or two, and the implications would go far beyond the cancers being dealt with right now. There are a lot of challenges to confirming CTCs as a surrogate endpoint, but I think this looks very promising.”

For pathologists, he believes there is huge potential for CTCs to inform clinicians about what molecular changes are occurring in the patient in real time. “In the future, we may not be getting tissue biopsies; this may represent a whole different source of metastatic tissue for analysis, and you can imagine the amount of discoveries we could make using CTCs as a platform for learning about mechanisms of disease progression.”

UCSF has been collaborating with the State University of New York Stony Brook on a test platform that will yield live CTCs that can be grown in the lab to study the biology of the cells, and he notes that Janssen is developing a next-generation platform for CellSearch that will involve much more investigation of genomics. “Several other companies are really pushing to get CTC analysis into the hands of clinicians, so in the next 10 years I expect CTCs will become much more widely used by them.”

When Dr. Cote took up study of CTCs, publishing his first paper in 1988 and working closely with the Ludwig Cancer Research Institute, which pioneered much of the early key research, the field was called occult metastasis or micrometastasis. “We were interested in identifying these very rare populations of cancer cells in a massive population of normal cells; that is the CTC problem, and the CellSearch technology was a way to enrich the epithelial cells to target CTCs.”

A frustration in the early days was that researchers were targeting bone marrow aspirates for analysis. “The problem is that aspirate is a difficult and painful procedure, and that as rare as tumor cells were in bone marrow, they were about a log order more rare—about 10 times less common—in blood on a volume basis than they were in bone marrow. CellSearch answered the question of how to use blood to identify system tumor cells, but even with this technology, it was very difficult to identify cancer cells in patients with early-stage disease.”
That remains true to this day, Dr. Cote says. It was for that reason that the field shifted focus from patients with early-stage cancer to patients with established metastasis, using the CTCs as a tool of prognostication at first, and more recently, as a tool to predict whether a course of therapy is working.

“The evidence is very clear that CellSearch technology can predict patients who are going to have a worse outcome, but it can also indicate which patients with breast, prostate, and colorectal cancer are responding or not responding to the specific therapeutic intervention with which they are being treated,” Dr. Cote says. Patients are generally not going to be cured, but they could have prolonged survival—in the case of prostate cancer, substantially prolonged. “So these were meaningful observations and a step up from just saying ‘your prognosis is poor.’” Providing proof that changing therapy would benefit the patient will be the next step, he believes, despite the findings of the SWOG study.

Although he considers CellSearch to be the clinically validated gold standard for detecting CTCs, Dr. Cote and his laboratory have developed a novel filter-based microdevice as an alternative technology to CellSearch because they are seeking a broader array of targets than the epithelial cancers. “All solid tumors that we’ve studied are candidates for the filter technology, because cancer cells arising from solid tumors have one consistent characteristic: They tend to be larger than normal blood cells. Our filter works by enriching the tumor cell population away from the vast majority of the normal blood population.”

From a cost point of view, the filter technology is not significantly different from CellSearch, he believes. “But one advantage of filter technology is that it is very rapid, so processing for that initial enrichment procedure in the blood takes about five minutes. We can also handle much larger volumes of blood than can be handled by a single CellSearch test. This then allows you to use blood in earlier stages of the disease, in order to look at more blood and have a better chance of capturing a cancer cell.”

Dr. Cote does not expect that the filter will be useful as a cancer screening tool, but for early detection of metastases, he sees it as extremely promising. It’s a key reason why he thinks CTCs will become ever more important as a biomarker. “Testing for CTCs can potentially direct our therapeutic management, indicate prognosis, and if we can get it sensitive enough in patients with early-stage disease, it can really better direct systemic therapy in those patients. If the things we are seeing can be further validated, this would be the one general test that virtually every cancer patient would undergo.”

In the future, he hopes, the liquid biopsy will become standard practice for patients with metastatic disease. “In other words, you would simply do a blood test to assess the status of that tumor, then monitor the response and whether or not new targets are emerging, again with a simple blood test.”

The potential ability of CTCs to home in on appropriate targets for therapy is likewise the feature of most interest to Daniel J. Boffa, MD, associate professor of surgery at Yale School of Medicine. A thoracic surgeon, he operates mostly on lung cancer and esophageal cancer patients, but for select patients, he also removes deposits of metastatic cancer that originate from other parts of the body that end up in the lungs in hopes of curing them.
“Frustratingly few patients with metastatic cancer are eligible for a curative-intent approach. Even in that highly selected population with only a few metastatic lesions, two-thirds will succumb to their disease despite complete removal of all radiographically identifiable areas of cancer. We have been studying CTCs not only to prognosticate among patients with limited spread of cancer but, perhaps more interestingly, to understand how some patients appear to contain the traditionally lethal process of metastatic progression.”

The prognostic ability of CTCs, as measured by the CellSearch test, has not shaped up as clearly as hoped in the oligometastatic cancer population, Dr. Boffa says. “What has been painfully clear is the first generation of CTC assays turned up as many questions as they answered, with clear inconsistencies between measured cell numbers and clinical outcome. All of us who evaluate CTCs in cancer patients have identified some early-stage patients who were apparently cured yet had persistent CTC populations, and other patients experience rapid progression, despite the absence of CTCs.”

It’s become more and more evident, he says, that the fluid base of cancer progression is not as simple as previously thought. The circulating population of cells originally thought to be tumor cells includes a mixture of epithelial-derived cells that are not all cancer cells but likely play a role in the process and may have independent prognostic potential. CTC enumeration has offered many new perspectives on the process of metastatic progression. The simplistic notion that a single cell leaves a tumor, enters the bloodstream, and then exits at the next available opportunity and forms a metastasis does not appear to be the case at all, Dr. Boffa says. “There’s much more to the circulation than just a simple downstream flow pattern, and CTC study has exposed flaws in this model and has real potential to clarify the true pathway.”

When he meets a patient, he tries to consider what is the “driver of their demise.” “Once you get a sense for whether it is the established sites of disease that are going to get the patient into trouble or yet-to-be manifested progression, you can plan treatment. For patients whose outcomes are dominated by established areas of tumor, we have a wide range of progressively more effective and less invasive surgical and ablative techniques to eradicate these areas of cancer. Some patients have tumors whose natural history is to grow where they start with little potential for hematogenous spread. These ‘local growers’ would benefit from surgery, even if it was high risk. On the other hand, ‘early spreaders’ are destined for systemic progression from a seemingly unimpressive primary tumor. Even low-risk surgery is less likely to help these patients. In this way, we need a ‘metastameter’ to estimate potential for dissemination to optimize treatment. CTC analysis has real potential for this type of information.”

One of the most exciting aspects of CTC research is the potential to target the circulating phase of hematogenous tumor progression. “The important tumor cell attributes for successful dissemination likely vary at different phases of progression. Just as a triathlete uses different skills as she swims, runs, and bikes to complete the race, there are likely tumor cell attributes that are uniquely important for the successful circulating phase of progression. It may be possible to target these attributes and contain disseminated cancers,” Dr. Boffa says. “Converting patients from disseminated cancers back to local growers is one of the ultimate frontiers for CTCs research.”

He hopes that CTCs could bring about a change in the surgeon’s role in cancer treatment. “As we gain an understanding of how a disease is going to behave, it could mean we don’t operate on some traditional patients and we do operate on some higher-risk patients we traditionally haven’t operated on. For the stage four cancer patient who has five areas deposited in a single lung, you would go as far as removing the entire lung, if necessary, to completely eradicate those lesions if you knew those five areas were the only areas that would ever give that patient problems. So that is my hope—that using CTCs as a biomarker to identify those patients, we might be able to reset our understanding of a patient’s tumor cell biology and adapt our approach to them.”

The field has evolved from counting cells that were atypical in the circulation of healthy patients, Dr. Boffa says, to sophisticated profiling and characterization of the cells as a reflection of the patient’s global disease burden. “We now have a better understanding of what role cells play in cancer progression and a better ability to estimate prognosis. I hope the evolution will continue as CTCs provide a window to the patient’s cancer, exposing treatable, targetable aspects of the tumor, and that ultimately CTCs will serve not only as a window of what is happening with the patient’s established tumor burden, but also as the Achilles’ heel for metastatic progression of cancer.”
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Anne Paxton is a writer in Seattle.