**7. ATP-competitive inhibitors**

Although rapamycin is a potent allosteric mTORC1 inhibitor with clinical applications, a second-generation ATP-competitive inhibitor have been developed, including Torin1, Torin2, PP242, PP30, KU0063794, WAY-600, WYE-687, WYE-354, XL-388, INK-128, AZD-2014, AZD8055, and OSI-027 [148-153]. The ATP-competitive inhibitors of mTOR directly inhibit the mTOR kinase activity, affecting both mTORC1 and mTORC2 complexes simultaneously and suppress AKT activity.

ATP-competitive mTOR inhibitors represent a promising new approach to target the pathway with potentially grater tolerability and efficacy than rapamycin. It has been shown that ATP inhibitors displayed dramatic antiproliferative activity across a range of cancer cell lines [151,154,155].

Studies have been conducted with PP242 in colon cancer cells *in vitro* and *in vivo* showed decrease cell growth alone or in combination with MEK inhibitors [156]. Another ATP competitive inhibitor, Torin2, was developed to overcome the pharmacological limitations of Torin1 and it is a potent inhibitor of ATR, ATM, and DNA-PK [157,158]. Lung cancer cell treatment with Torin2 resulted in a prolonged block in negative feedback and consequent threonine-308 phosphorylation on AKT. These effects were associated with strong growth inhibition *in vitro* [159].

Studies conducted by Rodrik-Outmezguine and colleagues [160], comparing mTORC1 inhibition with rapamycin and AZD8055, revealed that rapamycin treatment led to an almost complete loss in the mTORC1 phosphorylation of S6K-1 (threonine-389) and increased phospho-AKT (serine-473). In contrast, AZD8055 treatment led to reductions in phospho-S6K-1 (threonine-389), phospho-4EBP1 (threonine-37/40, threonine-65, and threonine-70), and phospho-AKT (serine-473). Thereby, AZD8055 was a better inhibitor of mTORC1 in compar‐ ison to rapamycin. *In vivo* studies indicated that AZD8055 can inhibit tumor growth and AZD8055 showed promise as a therapeutic agent.

At present, there are several clinical trials focused on the examination of new agents, such as AZD-8055 (NCT00731263), OSI-027 (NCT00698243), and INK128 (NCT02142803), in a variety of human hematological malignancies and solid tumors, including breast cancer. Also some studies were conducted using GSK795 in patients with advanced platinum-resistant ovarian and showed interesting results as tumor regressions and CA125 decreases [161]. Phase I study are ongoing to evaluate the safety and toxicity profile of AZD2014 in combination with paclitaxel in patients with ovarian cancer (NCT02193633).

Despite the clinical improvements observed with the ATP-competitive inhibitor when compared to the rapalogs, the literature still acknowledges significant limitations that outcome from compensatory cellular events. With this regard, it has been found that loss of the feedback on PI3K results in compensatory activation of the MAPK/ERK cascade by mTOR downstream effectors, such as 4EBP1/eIF4E, maintaining cell proliferation [162]. Furthermore, it has been shown that chronic inhibition of mTORC2 induces the activation of AKT by its phosphoryla‐ tion mediated by PDK-1, even in the absence of the priming serine-473 phosphorylation. Altogether, the referred mechanisms ultimately drive the acquisition of the resistant pheno‐ type by the cancer cells [154,163].
