RUSSCO-RSP study of immunohistochemistry (IHC) diagnostic assays for PD-L1 expression in urothelial cancer (UC).

489Background: This collaborative Russian Society of Clinical Oncology (RUSSCO) and Russian Society of Pathology (RSP) study assess the extent of concordance between three validated, commercially available PD-L1 IHC assays for UC patients. Methods: Tumors from 100 UC patients were stained with SP142 (Ventana), 22C3 (Dako) and SP263 (Ventana) clones as used in the clinical trials of second-line therapy of atezolizumab, pembrolizumab and durvalumab. Four trained pathologists independently evaluated the percentages of tumor (TC) and tumor infiltrating immune cells (IC) staining positive at any intensity. One test-specific cutoff rule for each assay in this analysis was pre-specified as: IC ≥5% for SP142, TC+IC ≥10% or TC ≥10% for 22C3, and TC ≥25% or IC ≥25% for SP263. Results: 300 IHC slides were scored. Patients were predominantly male (89%) with T1 (45%) and T2 (45%) stages, respectively. The percent of IC staining across 3 assays is shown to be higher than for TC staining (45% vs. 8% for SP142; 55% vs. 2...