Cytopathology in focus: Non-small cell lung carcinoma

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Cytology samples and immunotherapy predictive testing

Kaitlin E. Sundling, MD, PhD

January 2020—Requests for predictive biomarkers in oncology patients are becoming increasingly common in the cytology laboratory. At the time of rapid on-site evaluation, cytologists are now keenly aware of the need to collect adequate material not just for a diagnosis of malignancy but also for diagnostic and predictive molecular and immunohistochemical testing. This article provides an overview of current practices and some of the recent literature regarding predictive testing for immunotherapy in cytologic preparations in non-small cell lung carcinoma.

Biological rationale of immunotherapy. Tumor cells may evade the immune system through inhibition of the immune synapse between T cells and antigen presenting cells.¹ The inhibitory molecules programmed cell death-1 (PD-1) and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) are referred to as immune checkpoint molecules. Immunotherapy using PD-1, PD-L1/2 (programmed cell death ligand 1/2), and CTLA-4 targeted antibodies are immune checkpoint inhibitors. These therapies may restore the ability of the cytotoxic T cells to recognize and attack tumor cells.¹ In the appropriate context, high levels of PD-L1 expression on tumor cells suggest that the patient may be more likely to respond to inhibition of PD-1.²

Notably, immune checkpoint inhibitors have a unique set of side effects including immune-related adverse events as well as others seemingly unrelated to the immune system.³ Rarely, these adverse events may prove fatal. Thus, efforts continue to better predict which patients will be most likely to benefit from immunotherapy. While the most evidence has accumulated for non-small cell lung carcinoma, other cancers with the potential to respond to immune checkpoint inhibitors include gastroesophageal adenocarcinoma, cervical carcinoma, urothelial carcinoma, esophageal squamous cell carcinoma, head and neck squamous cell carcinoma, and triple-negative breast carcinoma.⁴

PD-L1 immunohistochemistry. Although specific immune checkpoint inhibitors may be used clinically without testing of the tumor itself, many situations require quantification of the level of PD-L1 expression on tumor cells to allow the patient to receive specific therapies or to participate in clinical trials.² It is important to know which antibody clone or specific assay is requested, as some assays have been FDA approved/cleared as companion diagnostic tests for a specific drug,⁴ while others may not be.

PD-L1 is quantified by the percentage of tumor cells with membranous staining. For PD-L1 clone 22C3 in advanced non-small cell lung carcinoma, greater than or equal to 50 percent staining may result in the use of pembrolizumab as first-line therapy, while a greater than or equal to one percent staining is considered positive and pembrolizumab may be considered as second-line therapy.^5,6 Interpretive criteria are likely to vary by antibody clone/testing platform,⁷ and some investigational applications may explore PD-L1 expression in immune cells and/or stroma in addition to tumor cell expression.