One of the Cleveland Clinic’s Top Ten Medical Innovations for 2017, the liquid biopsy, can potentially be used to analyze the genetics of tumor cells from blood samples to provide an easier, faster, and comprehensive approach to understanding and treating solid tumors. But what exactly is a liquid biopsy? And can it replace tissue biopsy for detecting solid tumors?
The detection of cancer biomarkers in blood is not a new concept. Circulating protein tumor markers such as carcinoembryonic antigen or prostate-specific antigen have been used for decades to assess solid tumor burden. However, many malignancies do not have a reliable protein biomarker, and even for those tumors that do, levels of the protein may be elevated for reasons not related to tumor growth or progression.
A more promising alternative that is gaining traction is to evaluate tumor products such as circulating cell-free tumor DNA (ctDNA) and circulating tumor cells (CTCs) that are shed into the blood—or other bodily fluids, such as urine or saliva. Circulating tumor cells were first observed in 1869 in the peripheral circulation of a patient with metastatic cancer (Ashworth TR. Australian Med J. 1869;14:146-147), and ctDNA was first reported in the serum of people with cancer in 1977.
One reason it took the liquid biopsy so long to catch on is that CTCs and ctDNA are found in low concentrations in the bloodstream. However, the technology has now developed to the point of sufficient sensitivity and specificity to accurately detect these tumor products. A recent large-scale study reported that ctDNA analysis identified cancer mutations in 85% of all advanced tumors, and the genomic profiles obtained correlated highly with those from solid tumor biopsies.
A big advantage of the liquid biopsy is that it can be carried out in a simple, noninvasive way that can allow the tumor to be sampled in serial fashion, providing a more comprehensive view of a patient’s cancer than can be obtained through traditional methods, said Ryan Bruce Corcoran, MD, PhD, of Massachusetts General Hospital, Boston. Corcoran pointed out that while a tumor may lie deep within the body or be positioned in a critical organ, rendering tissue difficult or impossible to obtain, veins are easily accessible. And unlike tumor samples, liquid biopsies can capture how a tumor’s molecular profile evolves over time in response to various pressures, including cancer therapies.
Liquid biopsies are currently limited to only 2 validated uses: identifying targetable genetic mutations or fusions to match patients to a first-line therapy and detecting emerging resistance to treatment to justify switching to a different therapy, particularly when a tissue biopsy isn’t feasible.
To date, the only liquid biopsy with US Food and Drug Administration (FDA) approval is the cobas EGFR Mutation Test v2, a blood-based assay that identifies epidermal growth factor receptor (EGFR) gene mutations (exon 19 deletions and L858R substitutions) in ctDNA from patients with non–small cell lung cancer (NSCLC). Because these EGFR mutations predict sensitivity to front-line EGFR tyrosine kinase inhibitors (TKIs) such as erlotinib, the assay can be used at diagnosis of NSCLC to determine whether a patient would benefit from this therapy.
This test also identifies the acquired T790M EGFR mutation that can arise over time in the tumors of patients being treated with erlotinib that renders them resistant to this treatment. Patients who test positive for T790M can be prescribed a third-generation EGFR TKI, osimertinib, which overcomes this resistance.
“Our lung cancer patients don’t always have tissue biopsies that are sufficient to do the extensive molecular analyses that we need today to guide their therapy,” noted Pasi Jänne, MD, PhD, of the Dana Farber Cancer Institute, Boston. In these cases, a liquid biopsy can come to the rescue. Unfortunately, the test is not sensitive enough to say that a patient with a negative result would not benefit from these therapies. In these cases, when such mutations are not detected in the blood, a lung tissue biopsy is still recommended.
There are other commercially available assays for use in lung cancer and other solid tumors that have not received FDA approval but are making their way into the clinic. For example, last year Jänne and his colleagues showed in a prospective study involving patients with advanced NSCLC that plasma genotyping with droplet digital polymerase chain reaction can rapidly and accurately detect mutations in EGFR and KRAS, 2 key oncogenes involved in cell growth and survival and associated with NSCLC.
“This test is now available in our clinical lab,” he said.
Researchers are working to broaden the applications for liquid biopsies, and one of the aims for this tool is to improve monitoring of disease progression.
A number of studies involving patients with metastatic cancer—including breast, colon, and prostate—have shown that CTCs in the blood are a prognostic factor for survival, with higher CTC levels being associated with a poorer prognosis.
Liquid biopsies are also being investigated to assess a patient’s response to therapy. Two recent studies investigating metastatic cancer—1 in breast and 1 in prostate—demonstrated that changes in CTC counts measured with a standardized assay can contribute to the early assessment of therapeutic efficacy, said Klaus Pantel MD, PhD, of the University Medical Center Hamburg-Eppendorf, Germany.
The next goal, said Pantel, is to show that CTC count can be used to guide treatment in patients with metastatic cancer. French researchers have designed a large study (STIC CTC METABREAST trial) to determine if using the baseline CTC count to guide the initial treatment decision in hormone-sensitive metastatic breast cancer patients improves outcomes. In the CTC-count-driven arm, the patient receives endocrine therapy if the CTC count is low, but chemotherapy if the CTC count is high.
Recruitment is finished and the first analysis will be done at the end of this year, he said.
Research is also focused on developing liquid biopsies that could be used to determine which patients are at increased risk for developing metastatic cancers. In one recent study, researchers identified a genotypically distinct subset of CTCs associated with breast cancer brain metastases. The future goal is to use this information to predict brain metastases and monitor treatment efficacy in patients with metastatic brain tumors.
Another potential clinical application for liquid biopsy is early detection of minimal residual disease (MRD), the state in which a patient has no gross disease present but does have microscopic disease that is undetectable with standard clinical imaging methods but that will ultimately lead to recurrence, said Maximillian Diehn, MD, PhD, of Stanford Cancer Institute, Palo Alto, California.
Feasibility studies have shown that ctDNA can be a good marker for MRD, said Diehn, but these studies have focused on using personalized assays developed for each patient, which can be expensive and time intensive. In a study presented this year at the 2017 annual American Society of Clinical Oncology (ASCO) meeting, his team used an “off the shelf” approach that measures all major mutations and could be applied to any patient with lung cancer. With an ultrasensitive ctDNA quantitation method, the group analyzed before and after treatment blood samples in patients with localized lung cancer and showed that ctDNA analysis can track multiple mutations and identify those at high risk of recurrence.
Another trial looking at detection of ctDNA in early-stage breast cancer is just getting under way. In this prospective trial, Ben Ho Park, MD, of Johns Hopkins University, and his colleagues will be collecting blood in patients with early-stage breast cancer—those with high-risk disease, either triple negative or ERBB2 (formerly HER2)–positive disease—to see if the presence or absence of ctDNA in blood plasma (ptDNA) can predict response to preoperative chemotherapy.
Park explained that about 30% of patients with these 2 types of breast cancer who receive chemotherapy before surgery don’t have any detectable cancer in their breasts or lymph nodes when surgery is performed. “This is a pathological complete response, a good prognostic sign in these subgroups,” he said. “Our hypothesis is those patients don’t really need surgery—but the catch is we don’t know if they’ve had a pathological complete response until you’ve done the surgery.”
The researchers will be using droplet digital PCR to determine whether the absence of ptDNA in the blood correlates with pathologic complete response after preoperative chemotherapy. The patients in this study will still undergo surgery, but the idea is that in the future, if the findings are positive, they could offer patients the option of foregoing surgery based on their ptDNA levels, explained Park.
Despite its promise, there is still much work to be done to demonstrate the clinical validity and utility of liquid biopsies, said Daniel Hayes, MD, of the University of Michigan Comprehensive Care Center, Ann Arbor.
Hayes, who served as president of ASCO from 2016 to 2017, emphasized that researchers need to verify that the tumor products found in circulation accurately represent what is found in tissue. At this point, he said, CTC and ctDNA analysis can potentially provide complementary information to tissue biopsies and imaging, but they won’t replace them.
Besides validity, one technical challenge of liquid biopsies is that both healthy and malignant cells shed fragments of DNA into circulation, but only a fraction of this cell-free DNA (cfDNA) is tumor-derived ctDNA, noted Diehn. “The fraction of ctDNA within the pool of cfDNA is usually much lower in patients with early-stage disease, making the use of these assays at this stage more difficult,” he said.
There is also a glaring need to standardize methods, said Park. “Everyone is kind of doing their own thing. No one has come up with standardizations,” he said. “You can’t really compare studies because they are like apples and oranges.”
Various efforts are being made to address these issues. Workshops on liquid biopsies cosponsored the last 2 years by the American Association of Cancer Research and FDA aim to bring experts together to discuss how to take the technology forward. The ASCO and the College of American Pathologists are also working to develop a formal position statement on ctDNA liquid biopsies, with the goal of standardizing the methods used so that they can be scientifically validated, said Diehn.
Howard Scher, MD, of Memorial Sloan Kettering Cancer Center, New York, emphasized that the development of liquid biopsy assays is similar to developing a drug. Even the way a blood sample is collected and stored needs to be standardized because different ways of handling the sample can affect the results, he said. Scher is one of many experts involved in the Blood Profiling Atlas in Cancer (BloodPAC) Consortium, which aims to develop, validate, and assess the clinical utility of liquid biopsies for cancer care.
“The bottom line is clinical utility—does the result of the assay lead to a clinical decision that’s been shown with a high level of evidence to improve outcome?” said Hayes.
“That’s what those of us in white coats need to have before we take care of a patient using these assays,” he said. “Because a bad tumor marker is every bit as bad as a bad drug.”
Note: The print version excludes source references. Please go online to jama.com.
Friedrich M. Going With the Flow: The Promise and Challenge of Liquid Biopsies. JAMA. Published online September 08, 2017. doi:10.1001/jama.2017.10203