Over the past decade significant advances have been made in the development of non-invasive tools for diagnosing and monitoring treatment in a variety of solid cancers. While traditional imaging modalities and tissue biopsies are still the gold standards for establishing a cancer diagnosis, it has become apparent that these tools are ineffective at identifying cancers before they have metastasized. They are also ineffective at providing a complete picture of the complex genetic landscape present in most tumors, which is required to properly design and gauge the efficacy of treatments. In recent years, the detection of cancer cells in the circulation has emerged as a promising new technology in the fight against cancer. Analysis of these circulating tumor cells (or CTCs) allows physicians to obtain “liquid biopsies” of the tumor, which have the potential to provide real-time information on treatment responses, detect early relapse and serve as a biomarker for early detection.
Circulating tumor cells are released into the bloodstream from a tumor and allow us to scrutinize the biology of a tumor without performing invasive biopsies. The central challenge in isolating CTCs is their rarity in the circulation, with an estimated one to 10 cells per billion normal blood cells in 1 ml of blood. To find these ultra-rare cells, current technologies employ one of two approaches: (1) Positive selection, which entails capturing CTCs based on their physical properties or (2) Negative selection, whereby depletion of Red Blood Cells and White Blood Cells from blood samples results in a purified CTC population.
Methodologies aimed at detecting CTCs based on physical properties often rely on the presence of specific cell surface markers or differences in the cell size, electric charge or density of CTCs compared to surrounding leukocytes. Currently, the only FDA-approved CTC technology, CellSearch, utilizes magnetically tagged antibodies to the common epithelial cell surface marker EpCAM to capture tumor cells from the blood of cancer patients. This approach has been used to demonstrate the utility of CTCs as prognostic markers of survival and treatment response in metastatic breast, prostate and colon cancer.1 However, the broad applicability of the CellSearch system continues to be limited due to the low recovery rates for CTCs, as a result of the multiple processing steps, and the fact that the system often only detects CTCs in half of patients with metastatic disease.
In an effort to increase both the sensitivity and purity of CTC capture, bioengineering approaches using microfluidic-based technologies have been developed. Using methods originally developed for manufacturing microprocessors, we can now fabricate devices with microscopic channels coated with antibodies that can capture tumor cells directly from whole blood with minimal processing. Early studies using these “CTC-Chip” platforms show promise in detecting even small numbers of CTCs in patient samples, which can also be subjected to detailed molecular analysis.2 However, a significant limitation of all positive enrichment approaches is their reliance on tumor-cell-specific factors for isolation. It is increasingly recognized that CTCs are quite heterogeneous, coming in all shapes and sizes, and often expressing many different surface epitopes. This can make detection using a specific marker challenging.
Negative selection approaches attempt to resolve this issue using marker-agnostic methods to isolate CTCs. They are based on the premise that while CTCs may not be uniform in their expression of surface markers, leukocytes all invariably express certain well-characterized proteins. Using magnetically coated antibodies to standard leukocyte markers such as CD45, it is now possible to enrich for CTCs by depleting non-tumor cells from blood specimens. Initial results from limited patient cohorts are promising and demonstrate the ability to detect CTCs in nearly all patients with metastasis. Importantly this approach can also detect tumor microemboli (i.e. clusters of tumor cells) in the circulation, which are often associated with increased metastatic burden3 and lower patient survival.
As CTC technologies continue to evolve they will significantly improve our ability to diagnose and treat solid cancers.
As CTC technologies continue to evolve they will significantly improve our ability to diagnose and treat solid cancers. Currently, several clinical trials are underway to study CTCs as a “real-time” biomarker of patient response to chemotherapy. Through the monitoring of total number and genetic makeup of CTCs in a longitudinal fashion, these studies aim to target therapy based on the biology of each patient’s tumors and predict drug resistance before it happens. In addition to their role as therapeutic biomarkers, it is likely that CTCs will play an important role in the early detection of cancers in high-risk patient populations. As gastroenterologists, this will have the greatest impact on our ability to screen for pancreatic cancer in patients with cystic lesions of the pancreas or individuals with hereditary colon and pancreatic cancer syndromes.
While there are still many technical challenges to overcome, over the next few years CTCs will become a fixture in most cancer biomarker panels. As these technologies make their way into the clinic, they will undoubtedly change the way we risk stratify our patients and manage their cancer.
Dr. Maddipati has no conflicts to disclose.
1. Allard WJ, Matera J, Miller MC, et al. Tumor cells circulate in the peripheral blood of all major carcinomas but not in healthy subjects or patients with nonmalignant diseases. Clin Cancer Res 2004;10:6897-904.
2. Nagrath S, Sequist LV, Maheswaran S, et al. Isolation of rare circulating
tumour cells in cancer patients by microchip technology. Nature 2007;450:1235-9.
3. Maddipati R, Stanger BZ. Pancreatic Cancer Metastases Harbor Evidence of Polyclonality. Cancer Discov 2015.