MTOR and cancer: The in vitro effects of metformin have been extensively studied in cancer cells where, for example, it inhibits the tumorigenic activity of the mammalian target of rapamycin (mTOR) [4], as well as in the immune components of the tumour microenvironment (TME), where it reduces the amounts of myeloid-derived suppressor cells, enhances the pro-inflammatory features of tumour-associated macrophages [5], downregulates regulatory T cell functions, enhances NK cell anti-tumour activity [6] and decreases the expression levels of immune checkpoints [7].