AZD6244 (
ARRY 142886) is a potent and selective
mitogen-activated protein kinase (MAPK)/
extracellular signal-regulated kinase (ERK)
kinase (MEK) inhibitor currently in early clinical trials. We examined the activity of
AZD6244 in a panel of
non-small cell lung cancer and a panel of cell lines representing many
cancer types using in vitro growth assays.
AZD6244 induced G(0)-G(1) cell cycle arrest in sensitive cell lines that primarily included cells containing the BRAF V600E mutation. In these cells, G(0)-G(1) arrest is accompanied by the up-regulation of the cell cycle inhibitors p21(WAF1) and p27(Kip1) and down-regulation of
cyclin D1. In the majority of cell lines tested, including those with K-ras or non-V600E BRAF mutations,
AZD6244 induced the accumulation of phospho-
MEK, an effect not observed in the most sensitive BRAF V600E-containing cells. Accumulation of phospho-
MEK in non-V600E-containing cell lines is due to abrogation of negative feedback pathways. BRAF V600E disrupts negative feedback signaling, which results in enhanced baseline phospho-
MEK expression. Exogenous expression of BRAF V600E disrupts feedback inhibition but does not sensitize cells to
AZD6244. Specific suppression of endogenous BRAF V600E does not confer resistance to
AZD6244 but enhances sensitivity to
AZD6244. Thus, our findings show that BRAF V600E marks cells with an in vitro requirement for MAPK signaling to support proliferation. These cells are exquisitely sensitive to
AZD6244 (IC(50), <100 nmol/L), have high baseline levels of phospho-
MEK, and lack feedback inhibition between ERK and Raf. These data suggest an approach to identifying cells that may be sensitive to
AZD6244 and other
MEK inhibitors.