OBJECTIVES: An attenuated dosing (AD) sunitinib regimen of 37.5 mg daily has been proposed to ameliorate frequent dose modifications caused by the toxicity observed with the approved dosing regimen to metastatic renal cell carcinoma (mRCC) patients. The aim of this study was to characterize the population pharmacokinetic (PK) properties of sunitinib and SU12662, the active metabolite, in patients receiving the AD regimen and to identify covariates that explain the variability in exposure following oral administration. METHODS: