Abstract PS2-03: Comparison of pathologist reads of sp142 and sp263 with quantitative measurement of protein and mRNA in triple negative breast cancer

Background: PD-L1 SP142 immunohistochemistry (IHC) assay has been approved as a companion test by the US Food and Drug Administration (FDA) to identify eligibility for atezolizumab therapy in patients with advanced triple negative breast cancer (TNBC) but a number of studies suggest the assay suffers from poor reproducibility. Using readings of 70 TNBC chromogenic FDA approved assays from 19 pathologists in a previous study as a baseline, we compared pathologist reads to quantitatively measured mRNA and protein expression Methods: Formalin-fixed paraffin-embedded (FFPE) slides representing primary invasive triple negative breast cancer (stage I-III) from 100 patients between 2012-16 were selected from the Yale Pathology archives. Slides were macrodissected to tumor enrichment for quantitative assessment of CD274 (PD-L1 mRNA) measured using a closed-system, real-time quantitative reverse transcription polymerase chain reaction (RT-qPCR) research use only (RUO)* prototype assay on the GeneXpert® instrument. We also measured protein expression levels using the AQUA method of quantitative immunofluorescence (QIF) in both the tumor and non-tumor compartments on full sections by QIF stained using SP142 in a lab derived test (LDT). The IHC stained slides were prepared using SP142 and SP263 assays prepared exactly according the FDA approved label followed by reading by 19 pathologists. This study was approved by Yale Human Investigation IRB protocol ID 9505008219.Results: Previous work from our group showed overall percent agreement for both the SP142 and SP263 IHC assays read by pathologists was in the 40-50% range. We used the median CD274 score to compare positive (IC≥1) vs negative (IC *For Research Use Only - Not for use in diagnostic procedures. Not approved or reviewed by any regulatory body Citation Format: Swati Gupta, Vesal Yaghoobi, Aileen Fernandez, Leena McCann, Jodi Weidler, Michael Bates, Emily Reisenbichler, David Rimm. Comparison of pathologist reads of sp142 and sp263 with quantitative measurement of protein and mRNA in triple negative breast cancer [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS2-03.