Temsirolimus

Temsirolimus, sold under the brand name Torisel, is an intravenous drug for the treatment of renal cell carcinoma (RCC), developed by Wyeth Pharmaceuticals and approved by the U.S. Food and Drug Administration (FDA) in May 2007, and was also approved by the European Medicines Agency (EMA) in November 2007. It is a derivative and prodrug of sirolimus. Temsirolimus is a specific inhibitor of mTOR and interferes with the synthesis of proteins that regulate proliferation, growth, and survival of tumor cells. Though temsirolimus shows activity on its own, it is also known to be converted to sirolimus (rapamycin) in vivo; therefore, its activity may be more attributed to its metabolite rather than the prodrug itself (despite claims to the contrary by the manufacturer). Treatment with temsirolimus leads to cell cycle arrest in the G1 phase, and also inhibits tumor angiogenesis by reducing synthesis of VEGF. mTOR (mammalian target of rapamycin) is a kinase enzyme inside the cell that collects and interprets the numerous and varied growth and survival signals received by tumor cells. When the kinase activity of mTOR is activated, its downstream effectors, the synthesis of cell cycle proteins such as cyclin D and hypoxia-inducible factor-1a (HIF-1a) are increased. HIF-1a then stimulates VEGF. Whether or not mTOR kinase is activated, determines whether the tumor cell produces key proteins needed for proliferation, growth, survival, and angiogenesis. mTOR is activated in tumor cells by various mechanisms including growth factor surface receptor tyrosine kinases, oncogenes, and loss of tumor suppressor genes. These activating factors are known to be important for malignant transformation and progression. mTOR is particularly important in the biology of renal cancer (RCC) owing to its function in regulating HIF-1a levels. Mutation or loss of the von Hippel Lindau tumor-suppressor gene is common in RCC and is manifested by reduced degradation of HIF-1a.