A novel pan ERBB inhibitor PF-00299804 (dacomitinib) is currently in phase II clinical trials in glioblastoma multiforme (GBM) patients; however its pre-clinical efficacy in GBMs has not been tested. In this study, we evaluated the efficacy of dacomitinib alone or in combination with PI3K/mTOR dual inhibitor PF-05212384 in GBM and assessed the mechanisms of resistance and the molecular determinants of response. A panel of established and patient derived primary GBM lines that present different molecular profiles and also the GBM lines engineered to express EGFRvIII mutant or PTEN were treated with either dacomitinib, PF-05212384, or combination and assessed for their viability and changes in EGFR/PI3K/mTOR signaling. We show that dacomitinib significantly reduced phosphorylated EGFR in all the GBM lines but did not show a dose-dependent response on cell viability in a majority of the lines tested. Multiple lesions in the receptor tyrosine kinases (RTKs) pathway including PTEN mutation, co-activation of RTKs, and EGFRvIII mutation resulted in unaltered active status of PI3K/mTOR in the GBM lines even in the presence of EGFR inhibition. Blocking PI3K/mTOR dramatically inhibited cell proliferation in most GBM lines and enhanced dacomitinib induction of apoptosis in a GBM line that has both EGFR amplification and EGFR-independent PI3K activation. These data suggest molecular profiling of EGFR/PI3K/PTEN status to select GBM patients for EGFR or/and PI3K/mTOR targeted therapies.
Keywords: EGFR; PF-00299804; PF-05212384; PI3K; dacomitinib; glioblastoma (GBM); mTOR.