MTOR and cancer: The use of rapamycin (Sirolimus) and its analogues (mTOR inhibitors (mTORi)) in preclinical and clinical investigations has revealed that mTORC1 pathway is involved in a network of signaling cross-talk and feedback mechanisms and that mTORC1 inhibition results in loss of negative feedback loop and in the activation of AKT and MAPK pathway, reducing the effectiveness of mTORi in cancer treatment [19–21].