An objective of the present invention is to provide compounds that exhibitstrongMEK-inhibiting activity and are stable in vivo and soluble in water, which canbe used aspreventive or therapeutic agents for proliferative diseases.The compounds of the present invention and pharmaceutically acceptable saltsthereof arerepresented by the following formula (1):(see formula 1)[where R1, R2, R3, R4, and X are the same as defined in the present patentapplication].