Making the best of PD-L1 IHC testing

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Anne Paxton

July 2016—When Keith Kerr, MB ChB, describes the ideal biomarker, he isn’t hesitant about what pathologists and clinicians need. “Ideally, the biomarker would always be correct. It would be easy and practical to measure. It would either be present or absent, with no gray zone or doubt. The biomarker itself would be a stable and functionally unique factor related to the system being studied. And it would, of course, be 100 percent predictive.”

PD-L1 testing can’t reach that standard. But how far short of the ideal does PD-L1 testing fall? Dr. Kerr, consultant pathologist at Scotland’s Aberdeen Royal Infirmary and a professor of pulmonary pathology at the Aberdeen University Medical School, discussed PD-L1 testing’s key weaknesses and how pathologists can grapple with them, during a May webinar produced by CAP TODAY in collaboration with Dako. He sees immunotherapy as intensifying pressure on pathologists to produce reliable results—even as the nature of PD-L1 expression, the variability of scoring, and the lack of consensus standards make reliability challenging.

PD-L1 testing is imperfect, the factors surrounding evaluation of results are tricky issues, and pathologists are unavoidably treading in difficult territory, Dr. Kerr said. “But the oncology world is watching. It’s really incumbent upon the pathology community to get this right, or to get it as right as we can. If we cannot deliver good outcomes with this particular test, I think we will be doing immunohistochemistry a disservice.”

The stakes of the diagnostics that guide patient choices about therapy are high, said webinar co-presenter Annika Eklund, PhD, global product manager for companion diagnostics at Agilent Technologies (Dako is an Agilent company). Agilent developed the first two FDA-approved PD-L1 assays: a companion diagnostic (PD-L1 IHC 22C3 pharmDx) for Merck’s drug Keytruda (pembrolizumab) and a complementary diagnostic (PD-L1 IHC 28-8 pharmDx) for Bristol-Myers Squibb’s drug Opdivo (nivolumab).

The value of PD-L1 immunohistochemistry—for example, in non-small cell lung cancer—is that immunotherapy is more likely to help a PD-L1-positive patient’s T cells to stay active and attack the tumor, while a PD-L1-negative patient is less likely to be helped in that way, Dr. Eklund said. “PD-L1 testing is valuable, too, to identify which patients are most likely to respond to therapy.”

The CheckMate 057 study of nivolumab versus the chemotherapy docetaxel showed that nivolumab patients with a PD-L1 expression of more than one percent had 17.1 months median overall survival, versus nine months for docetaxel patients. Previous research has indicated that patients’ PD-L1 expression can be dynamic, Dr. Eklund added. “However, experience in NSCLC supports that either fresh or archival tissue can be assessed by PD-L1 IHC.”

To develop standardized and evidence-based PD-L1 IHC, the company starts by integrating the primary antibody into a standardized solution, optimizing reagents for antigen retrieval, and developing controls. It then optimizes the protocol with software and automation, Dr. Eklund said. “We develop interpretation and scoring guidance to identify responding patients for the unique drug, and this complete standardized solution is validated in clinical trials to provide reliable test performance.”

Users should not optimize these tests on their own, she warned, as any deviation from the validated practice could be harmful to patients.

So far, in lung cancer, Dr. Kerr said, the field of immunodiagnostics has tackled mainly the “low-hanging fruit” among potential biomarkers. “Our biomarker experiences are largely around EGFR mutation and ALK translocation, both addictive oncogenes that have very particular biological features and are the main, if not the sole, driver of the tumor. Yet the best response rates we’ve seen for selected drugs are 60 to 70 percent, not 100 percent. And we all know the testing is far from being foolproof.”

The new world of immunotherapy is complicated, he said, with many different cell types and many, many different molecular factors, soluble factors, and cell-to-cell interactions. “And the therapy we are thinking about and the biomarker we are thinking about are only one single factor in this very complex mix of things going on.”

As one of the most antigenic of solid tumors, lung cancers will have a high number of neoantigens and non-self antigens expressed on the tumor cells. “It’s very likely that a competent immune system will eliminate abnormal clones of neoplastic or near-neoplastic cells before they ever cause any clinically relevant cancer. So somehow, for a tumor to become clinically evident, it must escape the attention of the immune system,” Dr. Kerr said.

One likely escape route is through the actions of immune checkpoints, complicated systems of receptors that switch immune sector cells on or off when ligands are encountered. The interaction between PD-1 on the surface of T effector cells and its ligand PD-L1 is one of these possibly inhibitory interactions. In physiological terms, “We believe that these interactions are probably responsible for preventing autoimmune responses, for switching off our immune response to self antigens. It seems likely that some cancer cells manage to hijack this system in order to evade the immune system,” Dr. Kerr said.

It has been fairly consistently observed that PD-L1 expression, at least with non-small cell lung cancer, is a poor prognostic feature, indicating that the presence of PD-L1 somehow manages to switch off an immune system that otherwise might improve a patient’s prospects. Targeting and inhibiting the checkpoints might therefore be a sound approach to therapy, Dr. Kerr said. “The multiple interactions of PD-1 and PD-L1 with lymphocytes, T effector cells, and a whole range of other cell types, including tumor cells, might manage to convince an immune system that has been switched off to become reactivated and to attack the tumor.”

In response to an audience question, Dr. Kerr said that over the decades, “there is hardly a single consistent IHC biomarker that has been shown to be either a positive or negative prognostic in lung cancer.” He suspects this pattern has much to do with the enormous heterogeneity in the way immunohistochemistry is performed, using different clones and scoring standards, among other differences.

Several clinical studies in NSCLC have found higher response rates to anti-PD-1 or anti-PD-L1 drugs in patients with higher PD-L1 expression. “And we are now seeing that this effect and response rate in relation to PD-L1 expression are beginning to translate into progression-free survival and particularly into overall survival in those patients.”

Percentages and cutoffs in PD-L1 evaluation, however, vary in their significance.

A phase two trial for the 22C3 clone developed as a companion diagnostic for Merck’s pembrolizumab showed superior overall survival for previously treated NSCLC patients with a PD-L1 score greater than 50 percent, he noted. But a phase three trial made the additional observation that overall survival in the 1–49 percent group was better with pembrolizumab than the docetaxel group.