Inertial Focusing for Tumor Antigen–Dependent and –Independent Sorting of Rare Circulating Tumor Cells

A multistage microfluidic chip is capable of sorting rare EpCAM+ and EpCAM− CTCs from cancer patients’ whole blood. Positive and Negative Outcomes Usually people want the good news first, to help cope with the bad news that inevitably follows. However, patients will soon desire both the positive and the negative outcomes together, according to the latest study by Ozkumur and colleagues. These authors have developed a multistage microfluidic device that is capable of sorting rare circulating tumor cells (CTCs) that are either positive or negative for the surface antigen epithelial cell adhesion molecule (EpCAM). EpCAM+ cells found in the bloodstream have long defined the typical CTC. Many sorting technologies have been developed to enumerate EpCAM+ CTCs in cancer patient’s blood; however, these cells are not always detectable in cancers with low EpCAM expression, like triple-negative breast cancer or melanoma. Ozkumur et al. engineered an automated platform, called the “CTC-iChip,” that captured both EpCAM+ and EpCAM− cancer cells in clinical samples using a series of debulking, inertial focusing, and magnetic separation steps. The sorted CTCs could then be interrogated using standard clinical protocols, such as immunocytochemistry. The authors tested the “positive mode” of their device using whole blood from patients with prostate, lung, breast, pancreatic, and colorectal cancers. After successfully separating out the EpCAM+ CTCs, they confirmed that the cells were viable and had high-quality RNA for molecular analysis, in one example, detecting the EML4-ALK gene fusion in lung cancer. Using the “negative mode” of their device, the authors were able to capture EpCAM− CTCs from patients with metastatic breast cancer, pancreatic cancer, and melanoma. The isolated CTCs showed similar morphology when compared with primary tumor tissue from these patients, suggesting that the microfluidic device can be used for clinical diagnoses—delivering both positive and negative news at once. Ozkumur et al. also demonstrated that CTCs isolated using the iChip could be analyzed on the single-cell level. One such demonstration with 15 CTCs from a prostate cancer patient reveals marked heterogeneity in the expression of mesenchymal and stem cell markers as well as typical prostate cancer–related antigens. The CTC-iChip can therefore process large volumes of patient blood to obtain not just EpCAM+ CTCs but also the EpCAM− ones, thus giving a broader picture of an individual’s cancer status and also allowing the device to be used for more cancer types. With the ability to further analyze the molecular characteristics of CTCs, this CTC-iChip could be a promising addition to current diagnostic tools used in the clinic. Circulating tumor cells (CTCs) are shed into the bloodstream from primary and metastatic tumor deposits. Their isolation and analysis hold great promise for the early detection of invasive cancer and the management of advanced disease, but technological hurdles have limited their broad clinical utility. We describe an inertial focusing–enhanced microfluidic CTC capture platform, termed “CTC-iChip,” that is capable of sorting rare CTCs from whole blood at 107 cells/s. Most importantly, the iChip is capable of isolating CTCs using strategies that are either dependent or independent of tumor membrane epitopes, and thus applicable to virtually all cancers. We specifically demonstrate the use of the iChip in an expanded set of both epithelial and nonepithelial cancers including lung, prostate, pancreas, breast, and melanoma. The sorting of CTCs as unfixed cells in solution allows for the application of high-quality clinically standardized morphological and immunohistochemical analyses, as well as RNA-based single-cell molecular characterization. The combination of an unbiased, broadly applicable, high-throughput, and automatable rare cell sorting technology with generally accepted molecular assays and cytology standards will enable the integration of CTC-based diagnostics into the clinical management of cancer.