PD-0325901 is an orally bioavailable inhibitor of mitogen-activated protein kinase kinases (MAPK/ERK kinases or MEK) with potential antineoplastic activity. MEK inhibitor PD325901, a derivative of MEK inhibitor CI-1040, selectively binds to and inhibits MEK, which may result in the inhibition of the phosphorylation and activation of MAPK/ERK and the inhibition of tumor cell proliferation. PD-0325901 is tested in clinical trials against non-small cell lung cancer, neurofibromatosis, melanoma and breast cancer.