A scanning electron micrograph of human red blood cells. Courtesy of Wellcome Images.
When a tumor grows beyond a certain size, it begins to shed cells, not unlike particles flaking off dry skin. Exactly when or why this happens in humans isn’t known, but these cells, called “circulating tumor cells” or CTCs play a major role in the spread of cancer to other parts of the body, the process more formally known as metastasis.
Scientists believe CTCs could be a new and invaluable source of information in the diagnosis and prognosis of cancer, but a big part of the current challenge is finding enough them: For every million or so circulating blood cells, there may be only a few CTCs. It’s the proverbial search for a needle in a haystack, only the needle is infinitesimally smaller and moving inside the human body. CTCs are also not generally inclined to announce their presence – at least not until they’ve lodged somewhere else (a distant organ, for example) to colonize and grow a new tumor.
The existing gold standard for isolating and identifying CTCs is an assay in which blood samples are exposed to magnetic beads coated with an antibody that binds to specific proteins on the surface of cancer cells. The capturing efficacy of this method ranges between 60 and 90 percent, but it also takes time and is prone to contamination from leukocytes – white blood cells that may also stick to the beads.
Recently, researchers at the University of California, San Diego School of Medicine and Moores Cancer Center described a new, alternative filtering technique that employs microbubbles. Writing in the March issue of PLOS One, principal investigator Dmitri Simberg, PhD, assistant project scientist, and colleagues said each microbubble is about half the diameter of a blood cell, filled with perfluorocarbon gas (for buoyancy and stability) and coated with an antibody. Exposed to a blood sample, the bubbles quickly attach themselves to any CTC encountered and puls them into a greater concentration (think soda bubbles rising to the top of a glass).
In tests using blood samples from mice and humans, Simberg said the microbubble assay worked better and faster than existing approaches, reducing the risk of contamination or sample degradation.
Though more research is required, he noted that the microbubble method may represent “the emerging field of blood biopsies, in which highly pure CTCs could be used as a source of tissue for personalized molecular diagnostics.”