Rationale: Sunitinib is an oral multi-targeted tyrosine kinase inhibitor used for first-line systemic therapy in metastatic renal cell carcinoma. It is metabolised to a pharmacologically active metabolite, SU012662, which is of equal potency to the parent compound. At a standard 50mg daily dose, variability in plasma levels between patients is approximately ten-fold. In clinical trials, over 30% of patients require a dose reduction due to toxicity. However, some patients can tolerate up to 100mg without excessive toxicity. It is unknown if sunitinib clearance changes with time. Pre-clinical experiments observed tyrosine kinase inhibition at a plasma concentration of 50-100ng/ml.