The mammalian target of rapamycin (mTOR) is a protein that is intricately involved in signaling pathways controlling cell growth. Rapamycin is a natural product that binds and inhibits mTOR function by interacting simultaneously with its FKBP-Rapamycin-Binding (FRB) domain and the proline cis-trans isomerase FKBP12. Several attempts have been made to design molecules that separate the FKBP12 binding activity from the mTOR inhibitory activity of Rapamycin, but to date no such viable compounds have been reported that are efficient against mTOR. However, we found that Rapamycin binds the FRB domain and inhibits the kinase activity of mTOR even in the absence of FKBP12 albeit at much higher concentrations (low micromolar versus the low nanomolar inhibition observed in presence of FKBP12). Hence, we propose an unbiased NMR- fragment-based approach to develop novel high affinity chemical probes that interfere with the function of mTOR by targeting its FRB domain. These ligands could become very useful in deciphering the complex regulation of mTOR in the cell and in validating the FRB domain as a possible target for the development of novel therapeutic compounds. The chemical structures, SAR data and range of biochemical activities of the resulting compounds will provide a framework onto which to develop potentially novel anti-cancer therapies. ? ? ?
