Dual Inhibition of Phosphatidylinositol 3-Kinase and Mammalian Target of Rapamycin: a Therapeutic Strategy for Acute Leukemias

The phosphatidylinositol 3-kinase (PI3K) and the mammalian target of rapamycin (mTOR) are two major signaling molecules in the PI3K/Akt/mTOR signal transduction cascade. This pathway is a key regulator of a wide range of physiological cell processes which include proliferation, differentiation, survival, metabolism, exocytosis, motility, and autophagy. However, aberrantly upregulated PI3K/Akt/mTOR signaling characterizes many types of cancers where it negatively influences response to therapeutic treatments. Therefore, targeting PI3K/Akt/mTOR signaling with small molecule inhibitors could improve cancer patient outcome. The PI3K/Akt/mTOR signaling network is activated in acute leukemias of both myelogenous and lymphoid lineage, where it correlates with poor prognosis and enhanced drug-resistance. The catalytic sites of PI3K and mTOR share a high degree of sequence homology. This feature has allowed the synthesis of ATP-competitive compounds that targeted the catalytic site of both PI3K and mTOR (e.g. PI-103, NVP-BEZ235). In preclinical settings, dual PI3K/mTOR inhibitors displayed a much stronger cytotoxicity against leukemic cells than either PI3K inhibitors or allosteric mTOR inhibitors, such as rapamycin and its derivatives (rapalogs). At variance with rapamycin/rapalogs, dual PI3K/mTOR inhibitors targeted both mTOR complex 1 and mTOR complex 2, and inhibited the rapamycin-resistant phosphorylation of eukaryotic initiation factor 4E-binding protein 1, resulting in a marked inhibition of oncogenetic protein translation in leukemic cells. Hence, they strongly reduced the proliferation rate and induced an important apoptotic response. Here, we reviewed the evidence documenting that dual PI3K/mTOR inhibitors represent a promising option for future targeted therapies of leukemic patients.