SYNERGISTIC GROWTH INHIBITORY ACTIVITY OF COMBINED MEK/MTOR PATHWAY BLOCKADE IN PTEN-NULL CANCERS

Aim: We have recently shown that induction of PTEN expression plays an integral role in MEK inhibitors' antitumor activity. Here we hypothesize that PTEN status critically influences the functional outcome of combined MEK and PI3K/mTOR inhibition. Methods: Single and combined MEK (trametinib, T) and mTOR (everolimus, E) blockade were assessed in a panel of cancer cell lines and in patient-derived lung and colon cancer stem cells (L- or C-CSC). Pharmacologic interactions were analyzed by conservative isobologram analysis. PTEN role was assessed by shRNA-mediated silencing or overexpression by stable transfection. Treatment-induced changes in the phosphoproteome were analyzed by antibody microarrays. Results: The presence of a wt-PTEN was the only significant predictor of sensitivity to T (p=0.001) in cancer cell lines. Combined MEK and mTOR inhibition resulted in a striking growth inhibitory synergism only in cells lacking PTEN expression (CI: 0.4-0.0005). PTEN silencing or its overexpression dramatically altered both sensitivity to T and the nature of its interaction with E (synergism vs antagonism), indicating PTEN as a fundamental determinant of functional response to combined MEK/mTOR inhibition. Similar results were obtained when either the double PI3K/mTOR kinase inhibitor PF-5212384 or the AKT allosteric inhibitors MK-2206 were substituted for E in combination with T. Concomitant T+E treatment was also highly synergistic in 3/5 L-CSC in vitro and, most importantly, in a C-CSC-derived model in vivo. Proteomic analysis indicated that a greater modification of protein expression/phosphorylation profiles in response to MEK or combined MEK/mTOR inhibition occurs in cells lacking a functional PTEN; preliminary analysis suggests that AXL, JAK1, and MAPK14 maybe be crucial mediators of synergistic growth-inhibitory interactions occurring with combined treatment. Conclusions: PTEN loss may constitute a suitable genetic/molecular marker of synergistic activity interactions between MEK/ERK and PI3K/mTOR pathway inhibitors and could be proposed as a potential patient selection biomarker for trials exploring such horizontal combinations. Disclosure: All authors have declared no conflicts of interest.