4543 Background: The combination of immune checkpoint inhibitor (ICI) and vascular endothelial growth factor tyrosine kinase inhibitor (VEGF-TKI) is recommended as the first-line treatment in patients with metastatic clear cell renal cell carcinoma (ccRCC). However, no effective serum biomarker has been used to predict the therapy response and potential benefits. Abnormal content and changes of immunoglobulin (IG) in serum or cancer tissue may indicate cancer progression and impact the prognosis. We examined the content of serum IG and analyzed the alteration after treatment to assess the efficacy of ICI and VEGF-TKI. Methods: Eighty-four patients with metastatic ccRCC were enrolled in this study. The total serum IG level was determined as the sum of IgA, IgG, and IgM. The data of baseline level and alteration of IG after 3-mo treatment was complete in 72 ccRCC patients. These patients were then divided into two groups based on alteration in their IG levels: the IG-increased group and the IG-decreased group. This study aimed to investigate the correlation between the baseline level of immunoglobulin (IG), changes in its content after 3-mo of treatment, and clinical outcomes. Results: Eighty-four patients were retrospectively enrolled with a median follow-up of 17.3 months (2.5-45.1). The objective response rate (ORR) was 41.67%, and the median overall survival (OS) was 31 months in our cohort. The baseline level of IG was not associated with ORR or OS. However, dynamic alteration in IG level after 3-mo treatment had a significant impact on ORR and OS in ccRCC patients. The ORR was 2.7% in the IG-increased group and 80.0% in the IG-decreased group. The OS was 21.0 months (95% CI 14.0–NA) in the IG-increased group and 48.0 months (28·0–NA) in the IG decreased-group (Hazard ratio [HR] 4.59, p=0·003). Conclusions: The change of serum IG level after 3-mo treatment served as an effective predictor of drug response and the prognosis for metastatic ccRCC patients. Value of serum IG needs to be validated in multi-centers and the mechanism of it should be further investigated.