Dealing with resistance to mTOR inhibitors in cancer : new ways to improve therapeutic strategies in breast and bladder cancers

Abstract

The Phosphoinositide 3-Kinase (PI3K)/ Protein Kinase B (Akt)/ mammalian Target of Rapamycin (mTOR) pathway is one of the most dysregulated signalling cascades in human cancer. As this pathway is critically involved in cell proliferation, survival and angiogenesis, mTOR appears as a promising target for cancer therapy. Although rapamycin and derivatives (rapalogs) have demonstrated antiproliferative properties in a wide spectrum of pre-clinical models, they achieve only modest efficacy in clinical trials. This work, by exploring the antitumoral efficacy of rapalogs in two common human cancers, highlighted novel insights in the mTOR pathway understanding. The first part investigated the antitumoral activity of mTOR inhibitors in bladder cancer and, by integrating clinical and experimental approaches, deconstructed the resistance mechanisms developed by PTEN-deficient tumors against these agents. In a phase II trial, we evaluated everolimus in monotherapy in patients with advanced bladder cancer. Despite the frequent mTOR pathway alterations reported in bladder cancer, everolimus achieved limited clinical efficacy. In an exploratory way, we performed a retrospective immunohistochemical analysis of the PI3K/ Akt/ mTOR pathway in the pre-treated tumor samples, focusing on the expression of PTEN, a tumor suppressor protein that negatively regulates PI3K. Interestingly, loss of PTEN was observed only in patients insensitive to everolimus, contrasting with pre-clinical studies that showed that PTEN-deficient tumors could be more sensitive to mTOR inhibitors due to an unrepressed PI3K/ Akt activation and a subsequent constitutive stimulation of mTOR. Using several bladder cancer cell lines with different PTEN status, we observed that the absence of PTEN was indeed associated with resistance to rapamycin. We then demonstrated that PTEN-deficient tumors were not able to abrogate the Akt activation induced by mTOR inhibition, resulting in a sustained stimulation of multiple Akt-dependent survival pathways that counteract the anti-proliferative effects of rapamycin. Furthermore, we showed that the addition of a PI3K inhibitor to rapamycin could reverse this resistance induced by PTEN loss. In regards of these data, we are currently conducting a phase II trial evaluating the efficacy of BEZ-235, an inhibitor of both mTOR and PI3K catalytic activity in patients with advanced bladder cancer. The second part of this work explored the influence of hypoxia, commonly found in breast cancer, on the antitumoral efficacy of rapamycin. We observed in vitro that in a subset of breast cancer cells, hypoxia could decrease the efficacy of rapamycin through a strong stimulation of autophagy and that inhibition of autophagy by chloroquine (CQ) could reverse this resistance developed in hypoxia. Interestingly, this benefit induced by this association was not observed in our in vivo model when treating tumors at early stage of development. This was explained by the fact that rapamycin, by improving the oxygenation within the tumor, could render tumor cells less dependent of autophagy and thus less sensitive to CQ. Supporting this hypothesis, a robust antitumoral effect was observed when initiating this combined treatment in late-stage hypoxic and autophagy-dependent tumors. In conclusion, this work improves the current understanding of the mTOR pathway and reveals two additional potential mechanisms of resistance to rapalogs and attracting ways to improve efficacy of mTOR inhibitors.(MED - Sciences médicales) -- UCL, 201