Rapamycin: something old, something new, sometimes borrowed and now renewed.

The molecular target of rapamycin (mTOR) is central to a complex intracellular signaling pathway and is involved in diverse processes including cell growth and proliferation, angiogenesis, autophagy, and metabolism. Although sirolimus (rapamycin), the oldest inhibitor of mTOR, was discovered more than 30 years ago, renewed interest in this pathway is evident by the numerous rapalogs recently developed. These newer agents borrow from the structure of sirolimus and, although there are some pharmacokinetic differences, they appear to differ little in terms of pharmacodynamic effects and overall tolerability. Given the multitude of potential applications for this class of agents and the decrease in cost that can be expected upon the expiration of sirolimus patents, renewed focus on this agent is warranted.