mTOR inhibitors: facing new challenges ahead.

The enzyme mammalian target of rapamycin (mTOR) is a master kinase that regulates several critical intracellular processes. It is now well established that this enzyme has a key role in cancer and its inhibition as therapeutic anti-cancer strategy is well recognised. Several clinical trials using mTOR inhibitors have been and are currently being performed. A huge scientific literature exists not only reporting the results of these trials but also discussing the reasons for the limited efficacy of strategies used so far and the need for new strategies to overcome the problem of resistance. The aim of this review is mainly to reflect on how the complexity of the mTOR-dependent signalling pathway and our difficulty to untangle it can ultimately affect the development of proper strategies to fully exploit the potential benefits of mTOR inhibition as anti-cancer strategy.