An orally bioavailable, synthetic organic molecule targeting mitogen-activated protein kinase kinase (MAPK/ERK kinase 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. 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.