Abstract 4517: Characterization of a standardized immunohistochemical assay for detecting PD-L1 expression in non-small cell lung cancer using anti-PD-L1 clone 73-10

Background: Recent data have demonstrated the clinical efficacy of anti-PD-1/PD-L1 checkpoint inhibitors in non-small cell lung cancer (NSCLC), and approvals by regulatory authorities in first-line and second-line settings have followed. In selected treatment scenarios, expression of PD-L1 by immunohistochemistry (IHC) is considered predictive of patient outcome. A recent comparative study using the rabbit anti-PD-L1 monoclonal antibody clone 73-10 and other commercialized IHC assays demonstrated strong intra-assay reliability in NSCLC tumor cell scoring and inter-assay differences in the number of tumor cells detected by each assay (Blueprint 2 data presented at IASLC 2017). Here we report precision around a ≥ 1% tumor cell cutoff and PD-L1 expression in 428 procured NSCLC tissues using an immunohistochemical assay developed with this antibody clone. Methods: PD-L1 expression was assessed using a standardized PD-L1 IHC assay. Scoring of tumor specimens was performed on a 0 - 3+ intensity scale, with membrane staining in ≥ 1% of tumor cells at ≥ 1+ intensity defining PD-L1 positivity. Assay precision around this cutoff was assessed in 28 specimens. PD-L1 expression was evaluated in 428 Stage IIIA, IIIB, and IV procured NSCLC tissues. Assay specificity was examined in 30 normal tissues. Results: The PD-L1 IHC 73-10 assay showed > 95% negative, positive, and overall agreement in inter-day, inter-operator, intra-run, inter-instrument, and inter-lot conditions around the ≥ 1% tumor cell cutoff. The lower bound of the 95% confidence interval was > 90% for all agreement types and precision conditions. PD-L1 protein localization in normal tissues was consistent with known patterns. The 428 procured NSCLC samples had primarily squamous cell (39.5%), adenocarcinoma (43.7%) and a mixture of other less common (16.8%) histologic subtypes; 397 (92.8%) of samples were from primary and 31 (7.2%) were from metastatic tumors. Crisp membrane staining was observed in tumor cells. Other labeled cell types included macrophages, lymphocytes, endothelial cells, and fibroblasts that exhibited membrane and/or cytoplasmic staining. Of the 428 procured specimens, 270 (63.0%) were PD-L1-positive based on the ≥ 1% cutoff. Tissues of each NSCLC stage expressed PD-L1 in the full dynamic range from 0% - 100% positive. Conclusions: These data provide insight into the prevalence of PD-L1 expression in procured NSCLC specimens when stained with PD-L1 IHC 73-10, and demonstrate that this assay is precise around a ≥ 1% tumor cell cutoff while exhibiting expected antigen specificity. PD-L1 positivity in the tumor stages examined using this assay is greater than in the literature, where prevalence using different antibodies has been reported at less than 50% in various stages. Citation Format: Darlene Krohn, Mai Nguyen, Marko Srdanov, Christina Samathanam, Peng Duan, Monika Vilardo, Alexander Prenta, Sharika Vasudevan, Landry Nicholson-Legg, Debra Hanks, Aaron R. Ellison. Characterization of a standardized immunohistochemical assay for detecting PD-L1 expression in non-small cell lung cancer using anti-PD-L1 clone 73-10 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4517.