PD 0325901 is selective and non ATP-competitive MEK inhibitor with IC50 of 0.33 nM, roughly 500-fold more potent than CI-1040 on phosphorylation of ERK1 and ERK2.

The dual specific threonine/tyrosine kinase MEK is a key component of the RAS/RAF/MEK/ERK signaling pathway that is frequently activated in human tumors.PD 0325901 is a potent MEK inhibitor that suppresses phosphorylation of ERK in mouse colon 26 tumors with an IC50 value of 0.33 nM.Suppression of ERK activation with 1 µM PD 0325901 combined with 3 µM CHIR99021 (a glycogen synthase kinase 3 inhibitor) prevents cell differentiation and sustains self renewal of mouse embryonic stem cells for at least eight passages.