LAUSR

454Background: Identification of biomarkers predictive of response to ICI could help guide treatment (tx) decisions. We assessed the correlation between PD1/PDL1 expression in key immunomodulatory subsets (myeloid-derived suppressor cells [MDSC]; CD8+ T cells) and tx response in mUC pts treated with ICI. Methods: Serial peripheral blood samples were collected from mUC pts treated with ICI. Flow cytometry was used to quantify PD1/PDL1 expression in MDSC (CD33+HLADR−) and CD8+ T cells (CD8+CD4−) from live peripheral blood mononuclear cells. MDSC were subdivided into monocytic (M)-MDSC (CD14+CD15−), polymorphonuclear (PMN)-MDSC (CD14− CD15+), and immature (I)-MDSC (CD14− CD15−). Mixed-model regression and Wilcoxon rank-sum tests were performed to assess post-ICI changes in immune marker expression and identify correlations between PD1/PDL1 expression and best overall response (BOR) to ICI. Results: Of 36 ICI-treated pts with ≥2 blood samples, 24 received anti-PDL1 (22 atezolizumab/2 avelumab; [A]) and 12 rec...