Pretreatment eosinophil counts (PEC) in metastatic urothelial carcinoma (mUC) treated with anti-PD1/PD-L1 checkpoint inhibitors (CPI)

e16035 Background: Eosinophils may influence anti-tumor immunity, having been associated with outcomes in CPI-treated melanoma. We sought to examine the association between PEC and outcomes of CPI-treated mUC. Methods: Independent CPI-treated cohorts were identified: discovery (n = 60, Teo et al JCO 2018) and validation (n = 111, non-overlapping CPI-treated mUC patients [pts] from 2014-19). Uni- and multivariate analyses were performed using Cox proportional hazard models to evaluate prognostic value of PEC on (i) overall survival (OS) and (ii) time on treatment (ToT). A platinum-treated metastatic cohort (n = 81, Teo et al, CCR 2017) was used as non-CPI comparator. Results: The cohorts showed comparable OS and pts characteristics, except higher proportion of females and lower neutrophil at baseline (Neut) in the validation cohort. In the discovery cohort, PEC as a continuous variable showed a significant positive association with OS (HR 0.01, 95% CI 0.00-0.29, P = .009). PEC of 100 cells/µL was set as the optimal cut-off point (Eos-L: < 100 cells/µL, n = 9; Eos-H: ≥100 cells/µL, n = 51). Eos-L showed inferior outcomes (OS: HR 3.98, 95%CI 1.85-8.56, P < .001; ToT: HR 2.45, 95%CI 1.17-5.10, P = .017). Similar association was seen in the validation cohort (Eos-L, n = 17; Eos-H, n = 94; OS: HR 2.51, 95%CI 1.31-4.80, P = .006; ToT: HR 2.22, 95%CI 1.29 - 3.80, P = .004). A multivariate model adjusting for other prognostic variables was created (table). In the chemotherapy cohort, continuous Eos or Eos-L was not associated with OS/ToT, with or without adjusting for known prognostic factors (P > 0.5). Conclusions: The association between Eos-L and shorter OS in CPI- but not chemotherapy-treated cohorts might represent a potential negative predictive biomarker for CPI in mUC. [Table: see text]