An objective of the present invention is to provide compounds that exhibit strong MEK-inhibiting activity and are stable in vivo and soluble in water, which can be used as preventive or therapeutic agents for proliferative diseases. The compounds of the present invention and pharmaceutically acceptable salts thereof are represented by the following formula (1): [where R 1 , R 2 , R 3 , R 4 , and X are the same as defined in the present patent application].本發明係提供具有強烈的MEK抑制活性、可穩定存在於活體內並可溶於水中之化合物,其可作為用於增生性病變之預防性或治療性之製劑。;本發明之化合物及其藥學上可接受之鹽類係由下列化學式(1)所代表:(其中,R1、R2、R3、R4以及X均與本專利申請案中所定義者相同)。