| Summary: | Abstract Background Despite improvements in the management of renal cell carcinomas (RCC) with the advent of immunotherapy, only a few patients respond to these treatments. Predictors of response to nivolumab are currently being investigated but are still lacking. Aim of the study To evaluate eosinophil levels and their variations during treatment as an accurate biomarker for outcome in metastatic RCC treated with nivolumab. Methods A retrospective analysis was carried out for patients with metastatic RCC treated with nivolumab. Absolute eosinophil counts, their variation, and relative change were evaluated at six weeks. Relative eosinophil change was categorized in three groups (≥10%‐decrease, no change, ≥10%‐increase). Univariable and multivariable analyses were performed to determine whether eosinophils and their variations were prognostic markers for response at the first scan evaluation, progression‐free survival, and overall survival. Results Sixty‐five patients aged on average 66 years, 68% men, and 77% with good or intermediate International Metastatic Renal Cell Carcinoma Database Consortium (IMDC) risk group were included. The median follow‐up was 16.6 months. Median overall survival (OS) was not reached for good prognosis and was 22.5 and 6.5 months for intermediate and poor prognosis, respectively. An increase in eosinophils and relative eosinophil change at six weeks of nivolumab was associated with a good response to immunotherapy (p = 0.012 and p = 0.024 respectively). In the group of patients with a 10%‐decrease in relative change, PFS reduced significantly compared to the other groups (p = 0.0044 with the 10%‐increase group and p = 0.03 with the no‐change group). This relative increase was independent of IMDC risks factors for better OS (HR = 3.3 [1.45–7.4]; p = 0.004). The eosinophil baseline level was not associated with response to treatment. Conclusion Eosinophil levels and relative eosinophil change at 6 weeks might be good prognostic markers for response to nivolumab for metastatic RCC, and were associated with better PFS and OS.
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