Device Captures CTC Clusters in Blood

Individual blood-borne tumor cells have been linked to metastasis in patients with cancer, and clusters of these circulating tumor cells (CTC) spell an even worse prognosis. To facilitate the study of these aggregates, researchers have created Cluster-Chip, a microfluidic device that captures CTC clusters directly from the blood without using antibodies or chemical labels.

Isolating CTCs is tricky because they are uncommon—about one per billion cells in blood—and CTC clusters are rarer still. Many methods use antibodies, which bind CTCs in order to capture them directly or bind other blood cells in order to separate them from CTCs. However, because antibody strength and specificity often vary and CTCs can be heterogeneous in their expression of surface markers, antibody-based techniques are not ideal.

Cluster-Chip, on the other hand, is “antigen-agnostic,” says A. Fatih Sarioglu, PhD, a biomedical engineer at Georgia Institute of Technology in Atlanta. Unlike other methods that target single CTCs with the hope of capturing clusters, the new device, Sarioglu says, is specifically designed to catch clumps of CTCs.

Scanning electron micrograph of Cluster-Chip shows rows of triangular pillars that trap CTC clusters.

Sarioglu and molecular biologist Nicola Aceto, PhD, of Massachusetts General Hospital (MGH) in Boston, are co–first authors of a paper describing Cluster-Chip (Nat Methods 2015;12:685–91). They initiated the research while working with Mehmet Toner, PhD, and Daniel Haber, MD, PhD, at MGH.

A three-cell cluster of fluorescently labeled human prostate adenocarcinoma cells captured on Cluster-Chip.

Measuring about 7.50 cm × 3.75 cm, Cluster-Chip is a glass slide that traps CTC clusters as blood passes through a matrix of triangular pillars with 12-μm gaps between vertices. Although CTCs are slightly larger (about 15 μm wide), single cells are squishy and easily squeeze through the matrix. However, CTC clusters catch on the points of the triangular pillars and remain trapped on the slide.

To measure the device's capture efficiency, the researchers spiked healthy human blood samples with clusters of fluorescently labeled MDA-MB-231 human breast cancer cells. Cluster-Chip captured 99% of clusters containing at least four tumor cells, 70% of three-cell clusters and 41% of two-cell clusters. For optimal performance, the researchers used a flow rate about 10 times slower than normal blood circulation and processed samples at 4°C instead of room temperature.

Compared with other methods, Sarioglu says, Cluster-Chip may give a truer estimate of how common CTC clusters are. When used on blood samples from 60 patients with metastatic breast or prostate cancer or with melanoma, CTC clusters were found in 30% to 40% of patients. Earlier experiments with antibody-based microfluidic tools developed by the same researchers detected CTC clusters in only 5% to 10% of patient samples.

Chwee Teck Lim, PhD, of the National University of Singapore, says devices like Cluster-Chip provide a “means of capturing circulating tumor microemboli.” Isolating CTC clusters can give insight into their role and function, including how they might differ from single CTCs. For example, “Do CTC clusters contain specific subpopulations of CTCs that make them more likely to metastasize?” Lim asks. “We do not know yet.”