Non-small cell
lung cancers with activating mutations in the
epidermal growth factor receptor (EGFR) are highly responsive to EGFR
tyrosine kinase inhibitors (TKIs), such as
gefitinib and
erlotinib. Such
cancers are "addicted" to EGFR, and treatment with a TKI invariably leads to down-regulation of the PI3K-AKT-mTOR and
MEK-ERK signaling pathways, resulting in apoptosis. Using a dual PI3K-mTOR inhibitor,
NVP-BEZ235, we evaluated whether PI3K-mTOR inhibition alone induced apoptosis in these
cancers. In contrast to HER2-amplified breast
cancers, we found that PI3K-mTOR inhibition did not promote substantial apoptosis in the EGFR mutant
lung cancers. However, blocking both PI3K-mTOR and
MEK simultaneously led to apoptosis to similar levels as the EGFR TKIs, suggesting that down-regulation of these pathways may account for much of the apoptosis promoted by EGFR inhibition. In EGFR mutant
lung cancers, down-regulation of both intracellular pathways converged on the BH3 family of
proteins regulating apoptosis. PI3K inhibition led to down-regulation of Mcl-1, and
MEK inhibition led to up-regulation of BIM. In fact, down-regulation of Mcl-1 by
siRNA was sufficient to sensitize these
cancers to single-agent
MEK inhibitors. Surprisingly, an AKT inhibitor did not decrease Mcl-1 levels, and when combined with
MEK inhibitors, failed to induce apoptosis. Importantly, we observed that the combination of PI3K-mTOR and
MEK inhibitors effectively shrunk
tumors in a transgenic and xenograft model of EGFR T790M-L858R
cancers. These data indicate simultaneous inhibition of PI3K-mTOR and
MEK signaling is an effective strategy for treating EGFR mutant
lung cancers, including those with acquired resistance to EGFR TKIs.