A class of cancer drugs called mTOR inhibitors may produce kidney toxicity in at least some patients. mTOR inhibitors, including rapamycin (sirolimus/Rapamune), temsirolimus (Torisel), everolimus (Afinitor), and ridaforolimus, are used to treat certain forms of breast and kidney cancer, and are under investigation for several other cancers. Researchers described four cases of patients treated with mTOR inhibitors developing severe acute kidney injury. They recommend that doctors carefully monitor kidney function in patients receiving these drugs. However, other experts emphasize that it is unclear whether the mTOR inhibitors were indeed the cause of the kidney injury. In at least one case, the patient was also taking other drugs known to cause kidney toxicity.