Tissue biopsies—standard care for diagnosing cancer—can be invasive, risky, and painful. Some patients cannot have tissue biopsy because of tumors’ locations, multiple metastases, or other health conditions. Liquid biopsy poses an alternative. Compared to tissue biopsies, this type of testing is less invasive, easily repeated, quicker, and more useful when a tumor’s location makes tissue biopsy unfeasible.
Usually performed on blood, liquid biopsies analyze DNA from whole circulating tumor cells (CTCs) or cell-free DNA (cfDNA) from tumors. Many tests specifically target a type of cfDNA called circulating tumor DNA (ctDNA), most of which comes from cells in the bed of the tumors. Until recently, oncologists had at their disposal only two Food and Drug Administration (FDA)-approved liquid biopsy tests, and they targeted single genes. However, liquid biopsy entered a new era in August with FDA’s approval of two tests that use next-generation sequencing (NGS) to target many genes in advanced cancer patients, including mutations for which there are targeted drugs, and to profile any solid tumor. These new tests are Guardant Health’s Guardant 360 CDx assay, and FoundationOne Liquid CDx, marketed by Foundation Medicine.
“These tests allow oncologists to focus care on the molecule of origin rather than the cell type,” said William G. Cance, MD, chief medical and scientific officer of the American Cancer Society. “Approval of these tests is another step in the pathway to precision medicine and targeted therapeutics.”
While both tests represent a major advance in cancer diagnostics, their use involves several considerations, experts said. These issues include questions about the tests’ sensitivity, potential for false positive results, and how to use negative results and information about mutations in genes for which there are no targeted therapeutics.
Companion Diagnostics and More
Guardant 360Dx uses NGS to detect mutations in 55 tumor genes from circulating cfDNA in the blood. The test is approved both to provide information on multiple biomarkers of advanced solid tumors and as a companion diagnostic to identify specific EGFR mutations in patients who might benefit from treatment with osimertinib (Tagrisso) for a form of metastatic non-small cell lung cancer (NSCLC).
FoundationOne Liquid CDx targets 324 genes using circulating cfDNA in advanced cancer patients. This test also is indicated for use as a companion diagnostic, for assessing BRCA1 and BRCA2 mutation status in patients with metastatic, castration-resistant prostate cancer who might benefit from rucaparib (Rubraca), and for detecting EGFR exon 19 deletions or exon 21 L858R substitutions in patients with NSCLC for whom gefitinib (Iressa), osimertinib (Tagrisso), or erlotinib (Tarceva) might be effective.
FoundationOne Liquid CDx helps identify patients who might benefit from immunotherapy, including those whose tumors have cancer cells with a high degree of microsatellite instability (MSI), which affects proper repair of DNA inside cells and might respond to certain medications, said Nickolas Papadopoulos, PhD, professor of oncology and pathology at Johns Hopkins University in Baltimore. The test detects genomic signatures that include MSI.
Tumors with high mutational burden might also respond to immunotherapy. FoundationOne Liquid CDx provides information about the quantity of mutations within a tumor, added Christian Rolfo, MD, PhD, MBA, professor of medicine at University of Maryland School of Medicine in Baltimore.
Pros and Cons
Answers from liquid biopsy are quicker than those for tissue biopsy, which involves scheduling and a procedure, in addition to lab work. Generally, the entire process for both the Guardant and FoundationOne tests takes 1 to 2 weeks, versus at least 2 to 4 weeks for standard biopsy, said Maximilian Diehn, MD, PhD, associate professor of radiation oncology at Stanford University in Stanford, California.
Tests can also be used for real-time monitoring to see if therapy is working, added Papadopoulos. Additionally, liquid biopsies can be repeated easily to see whether tumor cells or cfDNA are disappearing from the blood. When results—back quicker than those of standard biopsy—show treatment failure, “it’s easy to pivot to another drug,” added Cance.
Oncologists also could use the new liquid biopsy tests to monitor patients for emerging new mutations that could be treated with a therapy included in the FDA approval or another drug, noted Richard L. Schilsky, MD, chief medical officer and executive vice president of the American Society of Clinical Oncology.
The new tests’ advantages must be balanced with their limitations. No guidelines directly address the newly approved tests, but 2019 National Comprehensive Cancer Network (NCCN) guidelines say for several reasons that older liquid biopsy tests should not replace tissue biopsy (nccn.org; ebulletin 1536). First, the guidelines point to low sensitivity and high false negative rates for tests that existed as of 2019. Meanwhile, no standards address analytical performance or recommend performance characteristics for cfDNA.
cfDNA testing can identify alterations that are unrelated to a lesion of interest, the NCCN guidelines add. For example, cfDNA testing could find clonal hematopoiesis (CH) of unknown potential. CH occurs when a hematopoietic stem cell develops into blood cells with the same genetic mutation, giving them a different genetic pattern than the rest of the blood cells. In rare cases, CH might lead to blood cancers.
Cance added that tumors are heterogenous, so it is impossible to know whether a liquid biopsy test analyzes DNA that represents a fraction of the tumor mutations or all mutations.
The FDA approvals call for use of Guardant 360Dx and FoundationOne Liquid CDx in conjunction with tissue biopsy, just as both the NCCN guidelines and a 2018 International Association for the Study of Lung Cancer (IASLC) statement did for older liquid biopsy tests that target single genes (Crit Rev Oncol Hematol 2020;150:102978).
Experts agree. “The tests may miss mutations because the tumor isn’t shedding enough DNA into the blood,” explained Diehn. “Just because these tests don’t find a mutation doesn’t mean it’s not there. Therefore, doctors should consider a tissue test if the blood test is negative.” Conversely, he added, sometimes liquid biopsy picks up mutations biopsy misses. “It may be advantageous to do both. Sometimes there’s low tumor content in a biopsy specimen and tumor heterogeneity can lead to missing mutations. So using both tests could identify more candidates to target with particular therapies.”
The NCCN guidelines do suggest that liquid biopsy alone can be considered in patients who are not healthy enough for biopsy or who cannot provide sufficient tissue samples, Diehn pointed out.
New liquid biopsy tests are “not as accurate as looking at tissue. . . However, the technology is getting more sensitive,” added Papadopoulos.
Rolfo, first author of the 2018 IASLC statement, is working on an updated version that will comment on the use of Guardant 360Dx and FoundationOne Liquid CDx and other recent innovations in the field.
In the meantime, oncologists will need education about the new liquid biopsy tests. Clinical laboratory professionals can help, Schilsky said. In particular, laboratorians would do well to explain liquid biopsies’ sensitivity and specificity and what negative results might mean, as well as advise on how to navigate next steps. Oncologists will ponder their next steps after a negative result: accept that this finding means a patient truly has no cancer, repeat the test, or proceed to a tissue biopsy, if one hasn’t already been performed.
Due to the tests’ complexity, a molecular tumor board should interpret and assist treatment decisions, maintained Rolfo. Liquid biopsies detect both driver mutations that promote cancer development and passenger mutations that do not directly contribute to the cancer phenotype but could impact a treatment’s success. “For example, responses to anti-EGFR drugs can be different, according to the presence of other mutations in the tumor,” Rolfo said.
“Interdisciplinary molecular tumor boards should comprehensively analyze results in the context of the tumor and passenger, and their relationship to one another,” added Umberto Malapelle, PhD, assistant professor in the department of public health at University of Naples (Italy) Federico II and a co-author with Rolfo of the 2018 IASLC statement.
The Road Ahead
Experts pointed to potential future uses for the newest breed of liquid biopsies. For example, even if new tests do not identify a mutation with a targeted therapy right now, they could qualify more patients for clinical trials, Schilsky noted. “For most trials, patients need measurable disease on imaging studies. If imaging doesn’t find disease, patients are excluded from trials. If ctDNA level can be used as a validated end point for responding to treatment, you can have a baseline measurement and follow to see if ctDNA levels decline. It’s like when you scan and see a tumor has shrunk as a response to treatment.”
The tests might also monitor how patients do on treatment. “If the ctDNA level fails to decrease on treatment, the oncologist might have reason to switch treatment if there’s a good backup drug. Test results that show a spike in ctDNA or whole cells may be a sign of recurrence,” Schilsky added.
While these uses by themselves would be welcome, all eyes are on the day when liquid biopsy can be employed to spot cancer early. “The blood-first approach is almost here,” Rolfo said. He predicted that liquid biopsy will be a tool not only for screening, but also for finding minimal residual disease and targeting processes that lead to cancer formation.
Having FDA approval for tests built on NGS means that FDA understands how to evaluate and regulate similar tests, added Papadopoulos. “The trials on which FDA based approvals say the assays are safe. Ongoing trials will show if they are useful.”
For example, another Guardant test called Lunar, now available for research use only, is being evaluated for screening and detecting disease recurrence. Research is also determining whether Signatera, a Natera test that uses ctDNA, can monitor treatment and assess molecular residual disease. Life sciences company Grail is developing an early detection test for multiple cancer types. PapGene is engineering another early detection test that uses ctDNA and protein biomarkers to find cancer in average-risk, asymptomatic patients over age 65.
“Liquid biopsy will be part of routine care one day,” Schilsky predicted. “Research needs to determine what type of cancer someone has and where in the body it’s located. It needs more tissue specificity than we have today.”
Deborah Levenson is a freelance writer in College Park, Maryland.+Email: email@example.com
Diehn is a consultant to Roche Diagnostics, which has liquid biopsy and tissue products.
Malapelle reports personal fees (for service in the speaker bureau and as an advisor) from Boehringer Ingelheim, AstraZeneca, Roche, MSD, Amgen, Merck, Eli Lilly, and Diaceutics outside the submitted work.
Papadopoulos is cofounder of Thrive Earlier Detection, which is developing a liquid biopsy screening test, and Personal Genome Diagnostics, which has tissue and liquid biopsy products. He is on the Thrive Earlier Detection board.
Rolfo is on the MSD, Astra Zeneca, and Roche speaker boards and the MD Serono, ArcherDX, and Invitae advisory boards.
Schilsky’s disclosures represent funding to ASCO for the Targeted Agent Profiling and Utilization Registry trial from AstraZeneca, Bayer, Bristol-Myers Squibb, Genetech/Roche, Lilly, Merck, Pfizer, and Boehringer Ingelheim.