The Blueprint PD-L1 IHC Assay Comparison Project revealed that three of the four assays were closely aligned on tumor cell staining whereas the fourth showed consistently fewer tumor cells stained. All of the assays demonstrated immune cell staining, but with greater variability than with tumor cell staining. By comparing assays and cutoffs, the study indicated that despite similar analytical performance of PD-L1 expression for three assays, interchanging assays and cutoffs would lead to "misclassification" of PD-L1 status for some patients. More data are required to inform on the use of alternative staining assays upon which to read different specific therapy-related PD-L1 cutoffs.

- Combined positive score is a robust, reproducible PD-L1 scoring method that predicts response to pembrolizumab in patients with G/GEJ cancer. This novel scoring method supported US Food and Drug Administration approval of pembrolizumab as third-line therapy for G/GEJ cancer and has facilitated investigation in other indications.

e14589 Background: Developing clinically relevant and highly reproducible scoring methods for PD-L1 to identify patients who will respond effectively to anti-PD-1 therapy is key in the development of companion or complementary diagnostic assays. Methods: Scoring method for PD-L1 IHC 22C3 pharmDx in NSCLC have only captured the percentage on stained tumor cells using the tumor proportion score (TPS) (Roach et al. 2016, Appl Immunohistochem Mol Morphol.) (Garon et al. 2015, N Engl J Med.). In the KN012 study 2 out of 11 responders to pembrolizumab were detected with the TPS for the gastric indication. More and more additional data indicate that in some tumor indications PD-L1 staining on both tumor and tumor-associated immune cells is associated with clinical outcome. Therefore a method evaluating both tumor and immune cells in one sitting using the combined positive score was evaluated. Results: Scoring the same patient specimens with the new combined positive score method resulted in the detection of 9 out of 11 responders. Our internal inter- and intra- observer data shows that the CPS can be scored reproducibly with an overall agreement point estimates of 93% for intra-observer and an overall agreement of 87.6% for inter-observer reproducibility in gastric carcinoma. Additionally, CPS builds on the scoring method for NSCLC as it shares the same denominator, namely total number of tumor cells. CPS is evaluated based on the number of PD-L1 positive cells (tumor, lymphocytes and macrophages) in relation to total tumor cells, and hence allows the capture of tumor and immune cells in a single read. Conclusions: Our data demonstrates that the CPS scoring method is reproducible when scored by pathologists and clinically relevant for a number of indications.

The objectives were to develop a standardized Ki-67 immunohistochemistry (IHC) method for precise, robust, and reproducible assessment of patients with early breast cancer, and utilize this assay to evaluate patients participating in the mon-archE study (NCT03155997). The Ki-67 assay was developed and validated for sensitivity, specificity, repeatability, precision, and robustness using a predefined ≥ 20% cutoff. Reproducibility studies (intersite and intrasite, interobserver and intraobserver) were conducted at 3 external laboratories using detailed scoring instructions designed for monarchE. Using the assay, patient tumors were classified as displaying high ( ≥ 20%) or low (< 20%) Ki-67 expression; Kaplan-Meier methods evaluated 2-year invasive disease-free survival rates for these 2 groups among patients treated with endocrine therapy (ET) alone. All analytical validation and reproducibility studies achieved point estimates of > 90% for negative, positive, and overall percent agreement. Intersite reproducibility produced point estimate values of 94.7%, 100.0%, and 97.3%. External interobserver reproducibility produced point estimate values of 98.9%, 97.8%, and 98.3%. Among 1954 patients receiving ET alone, 986 (50.5%) had high and 968 (49.5%) had low Ki-67 expression. Patients with high Ki-67 had a clinically meaningful increased risk of developing invasive disease within 2 years compared with those with low Ki-67 [2-y invasive disease-free survival rate: 86.1% (95% confidence interval: 83.1%-88.7%) vs. 92.0% (95% confidence interval: 89.7%-93.9%), respectively]. This standardized Ki-67 methodology resulted in high concordance across multiple