Liquid biopsy. Simpler sampling
Liquid biopsies are easier on patients, yield faster results and cost less than tissue biopsies, but these blood draws don’t yet replace traditional tests.
During winter 2013, Larry Gershon had a bad cold that he couldn’t shake. The then 65-year-old print broker took cold medicine and steroids to fight off the symptoms, but the illness persisted. When the wheezing and coughing got worse, he went to urgent care.
“The doctor thought it was best to take a chest X-ray to check for pneumonia. I didn’t have pneumonia, but it showed a spot in my right lung,” says Gershon, a resident of Palo Alto, California. After CT and PET scans revealed a mass in his upper right mediastinum, the chest area between the lungs, doctors surgically removed part of his lymph node for a tissue biopsy.
In late January, he learned he had adenocarcinoma of the lung, the most common type of non-small cell lung cancer (NSCLC). Initially, doctors thought it might be stage 3b because it had spread to his lymph nodes, but a subsequent MRI uncovered metastasis to the brain.
“I was told I had stage 4 lung cancer, and I was shocked — a deer in headlights,” says Gershon, now 72. “Because I was self-employed, one of my biggest concerns, when I was diagnosed, was if I would be able to continue to work.”
Gershon was first treated with chemotherapy in February 2013. A genomic test from his surgical biopsy revealed a mutation in the EGFR gene. In 2015, he moved on to Tarceva (erlotinib), an oral targeted therapy approved by the Food and Drug Administration (FDA) to treat patients whose cancer has spread to other parts of the body and have an EGFR mutation. After 20 months, Gershon’s disease progressed, so he switched to Tagrisso (osimertinib), another oral EGFR inhibitor, which he remains on.
About a year ago, he enrolled in a clinical trial to test a new diagnostic technology, known as liquid biopsy, in patients with previously diagnosed cancer. Instead of analyzing a piece of tissue, as a traditional biopsy does, a liquid biopsy looks for traces of cancer in the bloodstream, also known as circulating tumor DNA (ctDNA).
For the past two-and-a-half years, Gershon’s imaging scans have shown no signs of cancer. Every three months, he has blood drawn in hopes that a liquid biopsy will detect any recurrence as soon as it arises.
“When I progressed in 2016, I had bone metastases, and so they’re currently giving me bone density injections along with a blood draw to check my counts,” he says. “The liquid biopsy is a couple more vials of blood than I already do, so it’s a very simple test.”
Liquid Biopsy for Simpler Sampling
In 2016, the FDA approved the first liquid biopsy to detect EGFR mutations, which are present in 10% to 20% of all NSCLCs and more commonly found in women, Asians and individuals who never smoked tobacco. Today, liquid biopsy has become the standard of care in clinics, either alone or alongside a tissue biopsy.
Although tissue biopsy remains the gold standard, doctors praise liquid biopsy for its many advantages, such as ease of administration, faster turnaround time and lower cost. In addition, even patients with hard-to-reach or small tumors can have blood drawn.
“In lung cancer, we are often dealing with very small tissue biopsies, and sometimes not even a biopsy but an aspirate, which is just a collection of cells, because it is too difficult to get a real tissue biopsy,” says Dr. Fred Hirsch, executive director of the Center for Thoracic Oncology in The Tisch Cancer Institute at Mount Sinai. “With such limited tissue available, it is reasonable to do a liquid biopsy to test for molecular abnormalities like EGFR for the possibility of targeted therapy.”
Liquid biopsy research has focused on NSCLC, which represents 84% of all cases, because of its many molecular biomarkers that have been identified. This disease can exhibit changes not just in EGFR but also in the ALK, BRAF, NTRK and ROS1 genes. A patient with an identified biomarker may be able to be treated with a medication that targets the mutation, potentially working better than chemotherapy and causing fewer side effects.
The noninvasive nature of a liquid biopsy translates to a much lower risk of injury compared with a tissue biopsy. For the latter, samples of the lung tissue are removed with a needle or during surgery, and possible complications include air leak (pneumothorax), bleeding and infection.
A liquid biopsy requires a blood sample drawn from a vein in the arm with a standard needle. Typically, 20 milliliters of blood are taken, but some tests need as little as five milliliters, or about a teaspoon. A centrifuge spins the blood, separating it into its component parts. The plasma, which makes up the largest portion and excludes the blood cells and platelets, is then analyzed for ctDNA with a genetic test.
Study findings have shown that the amount of ctDNA correlates to the amount of cancer in the body, tumor response and survival outcome. It also provides molecular information about the specific mutations of the original tumor.
“For every patient who has advanced adenocarcinoma of the lung, you have to test for EGFR mutations, ALK mutations, BRAF mutations, ROS1 mutations— and that’s just the bare minimum,” says Dr. Nathan Pennell, a medical oncologist and director of the lung cancer medical oncology program at Cleveland Clinic’s Taussig Cancer Institute. “By the end of 2020, there will be approved drugs for RET and MET mutations, most likely, and there are lots of drugs in trials for KRAS and HER2 mutations.”
Genetic tests that analyze blood instead of tissue samples can detect the same kinds of mutations but with a much faster turnaround time. In a 2019 study, Dr. Bob T. Li, a medical oncologist at Memorial Sloan Kettering Cancer Center, and colleagues compared liquid biopsy with tissue biopsy in 210 patients with advanced lung cancer and found median turnarounds of nine days versus 20, respectively.
“It takes much longer to get tissue biopsy results, and sometimes patients can’t wait that long,” Li says. “Time matters, because there’s a risk of rapid deterioration, and you don’t want to miss the boat. Treating the patient expeditiously can mean the difference between life or death.”
Sensitivity of Liquid Biopsy
Meanwhile, tissue biopsy remains the most accurate test. Medical test performance often is based on sensitivity, or the proportion of correctly identified true positives, and specificity, or the proportion of correctly identified true negatives. A blood-based biopsy’s specificity for ctDNA is very high, ranging from 95% to 100%, which means a positive result for a given mutation can be trusted and acted on immediately.
The sensitivity, on the other hand, varies from 60% to 85%. Therefore, a negative result cannot be fully trusted, and doctors will order a tissue biopsy to double-check.
“It’s very unlikely that you’re going to have a false- positive result with liquid biopsy, but you can have a false negative result,” Pennell says. “For EGFR mutations, the sensitivity is only about 80% compared with tissue. That means if 100 people have EGFR mutations, you’ll miss about 20 of them if you just do the blood-based test.”
In 2018, the International Association for the Study of Lung Cancer released a statement paper with recommendations on liquid biopsies for NSCLC. The association stated that liquid biopsy can be considered at the time of initial diagnosis in all patients who need tumor mutations identified but is particularly recommended when tumor tissue is scarce or unavailable or won’t be obtained for more than two weeks.
Blood Based Biopsy
Ongoing research on liquid biopsies continues to improve on the test’s accuracy for diagnostics, but clinical trials like the one Gershon participated in aim to uncover new applications. Many experts foresee liquid biopsy as a tool for monitoring treatment or detecting cancer early, even before it shows up on imaging scans.
“In the near future, we would be able to use a blood test to see a molecular response based on the treatment. This would tell us whether the treatment is working or not before the CT scan,” Li says. “If the treatment is not working, we may be able to learn why and overcome it with another drug.”
These applications remain in the research phase and aren’t ready for prime time, but Li believes the field will get there within the next decade. “In the long term, perhaps we can have advanced technology that goes beyond ctDNA and looks at other genetic material to increase the sensitivity of detection,” Li says. “One day, we may be able to do a blood test, find cancer in its earliest form and eliminate it.”
For now, investigators in several studies are attempting to tackle a less ambitious goal: treatment monitoring. Because doctors can take many blood samples throughout the course of a patient’s treatment, they can use liquid biopsy to track tumor response and drug resistance. For instance, the T790M mutation in EGFR is found in about half of patients who develop resistance to EGFR-targeted drugs. Liquid biopsies conducted at certain intervals may reveal these mutations earlier.
“Even if a patient has a certain mutation at the start of treatment, this mutation can disappear and come back. Or it can disappear, and other mutations can come,” Hirsch says. “That is why repeated molecular characterization is important. But it is very difficult to get a new tissue biopsy, so there is a role here for liquid biopsy.”
Clinical trials like the one Gershon participated in are determining if liquid biopsy can detect minimum residual disease (MRD), a small number of cancer cells left in the body after treatment that may be too scarce to cause physical signs or symptoms.
Natera, the company that made Gershon’s liquid biopsy test for MRD, uses the unique mutational signature of a patient’s tumor from a tissue biopsy to determine the kinds of ctDNA most likely to show up in the bloodstream during progression.
The company hopes this personalized approach can find MRD far earlier than an imaging scan. For example, Natera used Gershon’s brain tumor biopsy from 2016 for the clinical trial to create a custom-built ctDNA monitoring assay to test his liquid biopsies.
“They take a blood draw and match it against the assay they did from my tumor biopsy based on what they think will indicate progression,” Gershon says. “If they see any circulating tumor DNA in my blood, it’ll be likely that I’ll progress at some point. I’ve had four of these blood draws so far, and thankfully, they have shown no circulating tumor DNA.”