Mirdametinib (PD0325901) is an orally active, selective and non-ATP-competitive MEK inhibitor with an IC50 of 0.33 nM. Mirdametinib exhibits a Kiapp of 1 nM against activated MEK1 and MEK2. Mirdametinib suppresses the expression of p-ERK1/2 and induces apoptosis. Mirdametinib has anti-cancer activity for a broad spectrum of human tumor xenografts.