Dual-mechanism ERK1/2 inhibitors exploit a distinct binding mode to block phosphorylation and nuclear accumulation of ERK1/2

The RAS-regulated RAF-MEK1/2-ERK1/2 signalling pathway is frequently deregulated in cancer due to activating mutations of growth factor receptors, RAS or BRAF. Both RAF and MEK1/2 inhibitors are clinically approved and various ERK1/2 inhibitors (ERKi) are currently undergoing clinical trials. To date ERKi display two distinct mechanisms of action (MoA); catalytic ERKi solely inhibit ERK1/2 catalytic activity, whereas dual mechanism ERKi additionally prevent the activating phosphorylation of ERK1/2 at its T-E-Y motif by MEK1/2. These differences may impart significant differences in biological activity because T-E-Y phosphorylation is the signal for nuclear entry of ERK1/2, allowing them to access many key transcription factor targets. Here, we characterised the MoA of five ERKi and examined their functional consequences in terms of ERK1/2 signalling, gene expression and anti-proliferative efficacy. We demonstrate that catalytic ERKi promote a striking nuclear accumulation of p-ERK1/2 in KRAS mutant cell lines. In contrast, dual mechanism ERKi exploit a distinct binding mode to block ERK1/2 phosphorylation by MEK1/2, exhibit superior potency and prevent the nuclear accumulation of ERK1/2. Consequently, dual-mechanism ERKi exhibit more durable pathway inhibition and enhanced suppression of ERK1/2-dependent gene expression compared to catalytic ERKi, resulting in increased efficacy across BRAF and RAS mutant cell lines.