Sunitinib is an oral multi-targeted tyrosine kinase inhibitor approved for the first-line treatment of metastatic renal cell carcinoma (mRCC). Sunitinib has wide inter- and intra- individual variability in dose-concentration relationship due to its pharmacokinetic characteristics and genetic polymorphism. Moreover, it has drug-drug interactions that can profoundly alter/affect its plasma concentration. It has excellent exposure-efficacy relationship. A 10-fold difference in plasma exposure was found in mRCC patients who received the standard dose of sunitinib. Furthermore, it has a narrow therapeutic index and its treatment is often associated with severe toxicity, necessitating dose reductions or discontinuation. We found that therapeutic drug monitoring is routinely feasible and may both contribute to improve toxicity management and to identify sunitinib underexposure. So we plan to conduct a randomized study to see whether individualization of dose based on the sunitinib plasma levels would lead to better tolerability and improved efficacy in mRCC patients.