Abstract 3987: PD-L1 antigen stability using PD-L1 IHC pharmDx on human cancer tissues
Authors: Kalpakoff Megan , Olander Emily , Jones Kelci , Hund Stephanie , Evans Tiffany , Barreto Joseph , Moquin Deanna , Toland Grant , Tabuena-Frolli Siena , Kersch Kristopher , Kulangara Karina , Musser Jeanette 
Published: March 2024

Abstract
Diagnostic evaluation and selection of potential pembrolizumab-responsive patients is achieved by determining the programmed cell death ligand-1 (PD-L1) expression using the companion diagnostic PD-L1 IHC 22C3 pharmDx assay, for several indications. PD-L1 expression is evaluated by immunohistochemistry (IHC) and is currently the most widely used selective biomarker for response to pembrolizumab anti-PD-1 therapy. Antigen stability on cut tissue sections, also referred to as cut section stability (CSS), and its impact on IHC results is of interest when determining PD-L1 positive or negative status for clinical trials and diagnostic purposes. Since the stability of cut sections from formalin-fixed paraffin-embedded (FFPE) tissue relies on retention of PD-L1 antigenicity following tissue sectioning, the aim of these studies was to measure CSS across multiple tumor indications using combined positive score (CPS). The impact of aged cut sections was evaluated for PD-L1 expression based on pathologist CPS scores of blinded and randomized stained sections. To assess CSS, fresh unstained 4µm cut tissue sections were stored in the dark at 2-8 °C. Staining was performed according to the instructions for use at predefined timepoints. Studies included neoplastic specimens from biliary tract adenocarcinoma (BTAC), cervical cancer (CC), colorectal (CRC) and gastric (GC) carcinoma, esophageal cancer (EC), head and neck squamous cell carcinoma (HNSCC), nasopharyngeal (NPC), ovarian (OC), renal cell (RCC), and triple negative breast (TNBC) carcinoma. Data for each indication and timepoint were used to evaluate average drift in CPS scores over time, starting with scores from a set of respective slides stained at timepoint zero, prior to aging of unstained cut slides. CSS for each specimen within each indication was based on the intersection point of the two-sided 95% confidence interval for the regression line on CPS drift with allowable drift limits. Overall CSS for each indication was defined based on the specimen that exhibited the shortest shelf-life. Linear regression analysis was performed to evaluate average CPS drift over time and define the CSS per indication. This analysis identified the following CSS (in days) for each tumor type when stored at 2-8 °C: 105 (CC), 113 (OC), 115 (BTAC), 135 (EC), 163 (GC), 180 (RCC), 197 (HNSCC), 231 (TNBC), 240 (NPC), and 300 (CRC). The minimal shelf life for unstained cut sections stored at 2-8 °C across the 10 indications in these studies was 105 days, indicating a possible minimum threshold for indications tested when sectioned specimens are stored at 2-8 °C. These data provide high confidence in determining PD-L1 expression on properly stored aged cut sections. Commercial claims may be shorter based on region, as specified in the local instructions for use.
Citation Format: Megan Kalpakoff, Emily Olander, Kelci Jones, Stephanie Hund, Tiffany Evans, Joseph Barreto, Deanna Moquin, Grant Toland, Siena Tabuena-Frolli, Kristopher Kersch, Karina Kulangara, Jeanette Musser. PD-L1 antigen stability using PD-L1 IHC pharmDx on human cancer tissues [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 3987.