Banking on Blood Tests

from The Scientist by Jyoti Madhusoodanan

How close are liquid biopsies to replacing current diagnostics?

No matter where a tumor lurks in the body, its secrets circulate in the blood. Stray tumor cells begin metastatic migrations by slipping into the vasculature. Vesicles secreted by cancer cells and free-floating DNA are also released into the bloodstream. Because these bits of cellular debris are a grab-bag of biomarkers that could both signal a cancer’s presence and predict its progression and response to treatment, the use of blood-based tests, or liquid biopsies, to detect and evaluate them is now drawing significant commercial interest.

In January, sequencing giant Illumina announced that it was launching GRAIL, a new company that aims to develop a single diagnostic blood test for multiple cancer types. Illumina joins other big hitters such as Johnson & Johnson, Qiagen, and Roche, along with at least 30 other new companies, both small and large, in committing to develop such tests. In February, the business consulting firm RNCOS estimated that the global liquid biopsy industry could cross the $1 billion mark by 2020.

One goal of liquid biopsies is to screen for cancer before symptoms appear, as most tumors are much more easily treated in the early stages of disease. So far, only one test is approved by the US Food and Drug Administration (FDA) as a cancer screen based on free-floating tumor DNA, and it relies on fecal samples, not blood. In 2014, Exact Sciences of Madison, Wisconsin, became the first company to receive approval for a cancer screening test, called ColoGuard, which spots colon cancer–linked mutations in cell-free DNA collected from patient stool. The test is approved to screen people with an average risk of colon cancer; if positive, the results must be further verified by colonoscopy and other methods.

A third target of liquid biopsies in development bases cancer detection on secreted vesicles known as exosomes. Once thought to be cellular junk, these vesicles are more abundant in plasma than CTCs or cell-free DNA and carry several kinds of molecules that diagnostic tests can intercept. “They’re a relative newcomer compared to circulating tumor cells or cell-free DNA,” says Vincent O’Neill of Exosome Diagnostics. “But they contain DNA, proteins, lipids, many species of RNA. . . . From a diagnostic point of view, exosomes are really the jackpot.” (See “Cancer’s Vanguard” and “Written in Blood” fromt this issue).

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