Immunotherapy has ushered in a new age of cancer care. The Food and Drug Administration (FDA) has approved six checkpoint inhibitors and two CAR T cell therapies in recent years, and the American Society of Clinical Oncology has named immunotherapy “the advance of the year” for the past 3 years running.

Checkpoint inhibitors act on cancer-mediated cell surface proteins that otherwise keep protective T cells from mounting a response against an invading cancer. CAR T cell therapies focus on T cells themselves, engineering patient or donor T cells so that they target antigens on the surface of cancer cells.

Despite their promise, however, these new therapies are far from perfect. More than half of patients treated with checkpoint inhibitor monotherapy do not respond to treatment, and immunotherapy causes devastating side effects in some patients. Clinicians and patients are eager for tests that not only identify who will benefit from treatment but also who is at risk for adverse events—and researchers are busy trying to develop such tests.

In the meantime, clinical labs are on the forefront of deploying currently available tests that help determine which immunotherapies patients should receive, and help monitor their treatment response.

Treatment Gatekeepers

PD-L1 testing to help identify which patients are most likely to respond to anti-PD-1/PD-L1 checkpoint inhibitor therapy is widespread, according to experts. David Rimm, MD, PhD, director of pathology tissue services at Yale University in New Haven, Connecticut, said that his lab tests PD-L1 levels in all lung cancer patients, and that oncologists sometimes also order off-label PD-L1 testing to aid therapeutic decisions.

To date, FDA has approved four immunohistochemistry (IHC)-based PD-L1 assays: one as a companion diagnostic and three as complementary diagnostics for their associated checkpoint inhibitors (See Table).

Unlike companion diagnostics, which are necessary before a patient is prescribed a drug, “complementary diagnostics are a new category that FDA is still defining for us,” explained Christina Lockwood, PhD, DABMGG, DABCC, FAACC, director of the Genetics and Solid Tumor Diagnostics Laboratory at the University of Washington in Seattle. Rather than being the sole input, “overexpression of PD-L1 is one piece of information that is used in conjunction with clinical information to determine whether or not to treat with a drug.”

Given the number of PD-L1 assays available, clinical labs face a choice. “Many have elected to go the route of offering all of the tests, because clinicians are interested in the test that goes with the drug that they might be prescribing for the patient,” Lockwood said.

However, others only offer one test, due to cost and logistical constraints. Dartmouth Hitchcock Medical Center in Hanover, New Hampshire is one such example. The Laboratory for Clinical Genomics and Advanced Technology offers one PD-L1 test, the Dako PD-L1 IHC 28-8, due to the expense and operational challenges of offering more than one test, according to the lab’s director, Gregory Tsongalis, PhD, FAACC. He added that the lab went with this test because it was the first PD-L1 assay approved by FDA.

Rimm indicated that his lab recently switched to a lab-developed E1L3N version of the test, as it is less expensive to perform and equally reliable as the FDA-approved tests.

Which Test Is Best?

Given FDA’s unprecedented 2017 approval of the checkpoint inhibitor pembrolizumab to treat any microsatellite instability (MSI)-high or mismatch repair (MMR)-deficient tumor, interest in MSI and MMR testing has grown dramatically, said experts. “What FDA didn’t tell us is how you measure MSI—there are no FDA-approved assays to evaluate this,” explained Lockwood.

Her lab measures MSI via next-generation sequencing to analyze different areas of the genome for different types of tumor, as she and her colleagues have found that the most informative regions differ across cancers. However, most clinical labs currently use a polymerase chain reaction-based commercial kit that analyzes five genomic markers selected for colorectal and endometrial cancer, according to Lockwood. This poses a problem when clinicians need to analyze MSI in a different type of tumor. 

Given these issues with MSI testing, and that the standard IHC-based test for MMR is faster and less expensive, most labs test for MMR first, Rimm added. “If any of the markers is clearly missing, then you have an MMR defect and don’t need to do the MSI test.”

However, he noted that because the MSI test has historically been popular for colon and endometrial cancers, his lab sometimes performs both tests for these tumors. In these cases, “it’s not clear to me that one test is better than the other, and there aren’t really guidelines.”

Tumor mutational burden (TMB) is also being investigated as a predictor of response to checkpoint inhibitor therapy. However, Lockwood indicated that only a few labs nationwide offer such testing at this time, due to its expense and the length of time it takes to perform. “You have to do a large sequencing-based test to get a good estimate of what the TMB is,” she observed.

Tsongalis said that his lab is one of many looking at how TMB should be used as a biomarker, if at all. His lab is using a new next-generation sequencing panel to measure TMB.

Rimm said his lab is holding off until TMB testing is shown to offer an advantage over much less expensive MMR testing, as many MMR-deficient tumors also have a high mutational burden. He would also like to see more evidence that TMB testing improves patient outcomes. “If patients have mutationally unstable tumors that generate a lot of mutations, they’re also likely to generate mutations that ultimately lead to resistance to immunotherapy,” he explained.

Thus, the increases in progression-free survival reported among patients treated with checkpoint inhibitor therapies who have high TMB levels may not ultimately translate into the increases in survival that patients and clinicians care about.

Monitoring Adverse Events

Qing Meng, MD, PhD, DABCC, director of clinical chemistry and special chemistry laboratories at the MD Anderson Cancer Center in Houston, noted that studies suggest cytokine testing should be performed for patients receiving CAR T cell therapy. The concept in doing so is to promptly detect cytokine release syndrome—the most common and severe adverse effect after CAR T cell infusion—so that it can be managed.

Labs measure interleukin 6, tumor necrosis factor alpha, and interferon gamma levels at baseline and daily for the first 1 to 2 weeks after the infusion of CAR T cells. C-reactive protein and ferritin levels are often measured as well, to better identify systemic inflammation.

Testing of these molecules is also typically performed prior to initiating other immunotherapies, such as checkpoint inhibitors, to serve as a baseline for patients who experience toxicities, added Meng.

Research, Reports, Recommendations

Clinical lab directors emphasize the role of lab reports in communicating with oncologists about immunotherapy testing. Rimm made a point of working with oncologists to design his lab’s reports to be easy to use. A typical report might include a statement such as “this tumor tests positive for first-line treatment with pembrolizumab,” for example.

At the University of Washington, Lockwood and her colleagues often will also call or email an oncologist the same day an important result is delivered, to make sure that the findings are clear and that treatment can begin as soon as possible. Finally, many labs work with oncologists via one-on-one sessions and tumor boards as well. “While some cases have straightforward interpretations, many are complex—and the more data we have, the more decisions need to be made,” said Tsongalis.

Given the pace of immunotherapy research, a major part of lab directors’ jobs is deciding which new tests to offer. Tsongalis explained that when considering a new test, clinical labs like his typically evaluate how important the test is to patients (based on clinical trial data), the technology needed to perform it, staffing and lab logistics, and the cost of and reimbursement for the test.

“It’s an exciting time to be a laboratory director,” said Lockwood. “The challenge is to separate out which tests and areas of research are going to be most significant for improving clinical care.”

Kristin Harper is a freelance writer in Seattle, Washington. Email: kristin.nicole.harper@gmail.com