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.