Legal Information Sold for research purposes under agreement from Pfizer Inc. Biochem/physiol Actions PD 0325901 is a potent MKK1 (MEK1) and MKK2 (MEK2) inhibitor. The Ki is 1.1 and 0.79 nM for MEK1 and MEK2, respectively. PD 0325901 was inactive against a panel of 27 other kinases. PD 0325901 inhibited C26 tumor pERK by 75% when dosed at 25 mg/kg in mice.