BackgroundThe dynamic change in C-reactive protein (CRP) levels, CRP kinetics, is a prognostic factor for metastatic renal cell carcinoma (mRCC) in the tyrosine kinase inhibitor era. We investigated the impact of early CRP kinetics on the efficacy of nivolumab in patients with mRCC.MethodsWe performed a retrospective analysis of 42 mRCC patients who were treated with nivolumab as a second-line or later therapy between 2016 and 2019. All patients had received previous TKI therapy. Patients were divided into three groups based on their early CRP kinetics: CRP levels increased to more than double compared with baseline within 1 month after initiation of nivolumab (flare) and then decreased to a lower value than baseline within 3 months (CRP flare-responders); CRP levels decreased by ≥30% within 3 months without “flare” (CRP responders); and the remaining patients (non-CRP responders). The maximum tumor shrinkage, objective response rate (ORR), progression-free survival (PFS), and overall survival (OS) were evaluated. The association of the early CRP kinetics and oncological outcomes was assessed.ResultsThe median follow-up period was 8 months. The median baseline CRP level was 23 mg/L. CRP flare-responders, CRP responders, and non-CRP responders included 11 (26%), 15 (36%), and 16 (38%) patients, respectively. Thirteen patients (31%) died of mRCC. The maximum changes in target lesions from baseline of CRP flare-responder, CRP-responder, and non-CRP responder groups were −38%, −13%, and 16%, on average, respectively (p<0.001). ORRs of these three groups were 73%, 27%, and 6%, respectively (p<0.001). The median PFS values of each group were not reached, 12 months, and 2.4 months (p=0.005), and the median OS values were not reached, not reached, and 12 months (p=0.048). In a multivariate analysis, early CRP kinetics was a significant independent factor for objective response, PFS, and OS (p<0.001, p=0.004, and p=0.006, respectively).ConclusionsCRP flare-response was associated with significant tumor shrinkage and improved survival outcomes in patients with mRCC who were treated with nivolumab. Early CRP kinetics could be useful for evaluating nivolumab treatment efficacy.