Canadian Multicentre Project on Standardization of PD-L1 22C3 Immunohistochemistry Laboratory Developed Tests for Pembrolizumab Therapy in Non-Small Cell Lung Cancer.

Abstract Background PD-L1 immunohistochemistry (IHC) assay is used to select patients for first/second line pembrolizumab monotherapy in non-small cell lung carcinoma (NSCLC). The PD-L1 IHC 22C3 pharmDx assay requires Autostainer Link 48 instrument. Laboratories without this stainer may optionally develop highly accurate 22C3 IHC laboratory-developed test (LDT) on other instruments. The Canadian 22C3 IHC LDT validation project was initiated to harmonize the quality of PD-L1 22C3 IHC LDT protocols across 20 Canadian pathology laboratories. Methods Centrally optimized 22C3 LDT protocols were distributed to participating laboratories. The LDT results were assessed against result using reference PD-L1 IHC 22C3 pharmDx. Analytical sensitivity and specificity were assessed using cell lines with varying PD-L1 expression levels (Phase 1) and IHC Critical Assay Performance Controls (iCAPCs) (Phase 2B). Diagnostic sensitivity and specificity were assessed using whole sections of 50 NSCLC cases (Phase 2A), and tissue microarrays with additional 50 NSCLC cases (Phase 2C). Results In Phase 1, 80% participants reached acceptance criteria for analytical performance in the first attempt with disseminated protocols. However, in Phase 2A only 40% of participants reached desired diagnostic accuracy for both 1% and 50% TPS cut-off. In Phase 2B, further protocol modifications were conducted, which increased the number of successful laboratories to 75% in Phase 2C. Conclusions It is possible to harmonize highly accurate 22C3 LDTs for both 1% and 50% TPS in NSCLC across many laboratories with different platforms. However, despite centralized approach, diagnostic validation of predictive IHC LDTs can be challenging and not always successful.