Current status of mammalian target of rapamycin inhibitors in lung cancer.
The mammalian target of rapamycin (mTOR) is a serine/threonine kinase that plays a critical role in the control of cell growth and proliferation. The mTOR integrates mitogenic signals and intracellular nutrient levels to activate eukaryotic initiation factor 4E-binding protein-1 and the 40S ribosomal protein S6 kinase, which controls protein translation and cell cycle progression. Abnormal activation of signaling pathways proximal and distal to mTOR appears to occur frequently in human cancer, making mTOR an attractive target for anticancer drug development. Inhibitors of mTOR, including the naturally occurring inhibitor rapamycin as well as newer agents against this target, are currently in clinical development for cancer treatment. In preclinical studies, these agents have shown significant effects against a variety of preclinical models of cancer. In early clinical studies, mTOR inhibitors have been well tolerated, resulted in plasma levels able to inhibit mTOR in normal and tumor tissues of patients treated with the drug, and resulted in antitumor responses in patients with different tumor types including lung cancer. These agents are now in late phases of clinical development. As with other targeted agents, the key issues in the future will be to elucidate the molecular factors predicting a favorable response to the drugs as well as the rational integration with other targeted agents with activity in lung cancer, such as inhibitors of the epidermal growth factor receptor tyrosine kinase.