Determine effect of dual-pathway inhibition with mTORC1/2 and MEK inhibitors on glioblastoma neurosphere cell lines.

The PI3K/mTOR signaling pathway is frequently hyper-activated in glioblastoma, making it the focus of active scientific and drug development efforts in brain tumors. Unfortunately, small molecule targeted therapies against EGFR and its downstream effector pathways, while promising in the pre-clinical setting, have been disappointing in clinical trials. Despite demonstrable target inhibition, their development has been limited by both suboptimal antitumor efficacy as well as significant toxicity.

Seven glioblastoma neurosphere lines were evaluated for sensitivity to the mTORC1/2 inhibitor sapanisertib (MLN0128) alone or in combination with the MEK1/2 inhibitor trametinib (GSK1120212) using cell growth assays and immunoblots as indicated.

The dual mTORC1/2 inhibitor sapanisertib inhibits mTOR signaling and cell growth in neurosphere lines, but only partially inhibits phosphorylation of the downstream target ribosomal protein S6 (phospho-S6) in this model. Activation of RPS6 is also mediated by phosphorylation via ERK through p90-RSK. We therefore tested the hypothesis that combined inhibition, using both an allosteric MEK inhibitor, trametinib, and sapanisertib, would result in enhanced activity as well as a reduction in the dose of each required for antitumor activity. Alone, trametinib inhibited ERK phosphorylation in all neurosphere lines, though downstream targets and cell growth were only inhibited in some. When combined together, sapanisertib and trametinib showed enhanced suppression of downstream phospho-S6, as well as additive growth inhibition and induction of apoptosis in some neurosphere lines. We further investigated the mechanisms underlying enhanced growth inhibition via convergence of mTOR and ERK signaling on RPS6 in this model.

The combination of sapanisertib and trametinib may have significant antitumor activity in glioblastoma neurospheres by inhibiting a common downstream effector pathway.