Temsirolimus, an ester of sirolimus (rapamycin), selectively inhibits the kinase mammalian target of rapamycin and consequently blocks the translation of cell cycle regulatory proteins and prevents the over expression of angiogenic growth factors. Patients with advanced renal cell carcinoma and a poor prognosis who received a once-weekly intravenous infusion of temsirolimus 25 mg experienced significant survival benefits compared with patients receiving standard interferon-alpha (IFNalpha) therapy (3-18 MU subcutaneously three times weekly) in a large phase III clinical study. Median overall survival was 10.9 versus 7.3 months, progression-free survival was 5.5 versus 3.1 months. Objective response rates were 8.6% in temsirolimus recipients versus 4.8% in IFNalpha recipients. Temsirolimus monotherapy recipients experienced significantly fewer grade 3 or 4 adverse events and had numerically fewer withdrawals for adverse events than patients receiving IFNalpha.