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Increased expression of programmed death-ligand 1 (PD-L1) is one mechanism neoplastic cells employ to protect themselves from immune attack. Binding of PD-L1 to its receptor, programmed cell death protein 1 (PD-1), results in inhibition of immune cells that would otherwise attack and destroy the tumour cell. Immunomodulatory drugs include immune checkpoint inhibitors that act on the PD-1/PD-L1 axis to interrupt this protective mechanism, thereby re-exposing the tumour to the immune system. These drugs have broadened and greatly improved treatment options for a variety of cancers, including non-small-cell lung carcinoma (NSCLC).1–3 As might be expected, immunomodulatory drugs are generally more effective against tumours in which there is high expression of PD-L1 on either the tumour cells themselves or on tumour-infiltrating immune cells, hence the rationale for PD-L1 testing.1,2,4,5
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Unfortunately, different immunochemical assays for detecting PD-L1 were developed for each immunomodulatory drug, resulting in a multitude of antibodies and platforms that were initially considered non-interchangeable. This, compounded by inherent heterogeneity of PD-L1 expression and the challenge of accurately quantifying its expression in tissue sections, has created a difficult environment for PD-L1 testing.1,5,6
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Four anti-PD-1/anti-PD-L1 immunomodulatory drugs are currently licensed for the treatment of cancer: nivolumab, pembrolizumab, atezolizumab and avelumab. Further agents, including durvalumab, are in clinical trials but are currently unlicensed. During their clinical trials, each agent was developed with a different PD-L1 test. As shown in Table 6.1, not only do these agents employ different antibody clones but they are run on different platforms and have different cut-offs for determination of positivity.1,4,6,7
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Inherent properties of PD-L1 expression
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PD-L1 expression within a primary cancer, and between a primary tumour and its metastases, may be markedly heterogeneous, altering not only with time as the tumour evolves, but also spatially within the tumour.6,7 Even across a small area of a tissue section, some tumour cells might demonstrate very strong expression and others very weak or no expression (Figure 6.1). Decisions to recommend treatment are based on a percentage score, usually in a small biopsy specimen, of ...