An agent comprising, as an active ingredient, a xanthine oxidase inhibitor such as 2-[3-cyano-4-(4-fluorophenoxy)phenyl]-4-hydroxythiazolo[5,4-d]pyrimidine potassium salt, 2-[3-cyano-4-(4-fluorophenoxy)phenyl]thiazolo[5,4-d]pyrimidine, 2-(3-cyano-4-phenoxyphenyl)-4-hydroxythiazolo[5,4-d]pyrimidine, 2-(3-cyano-4-phenoxyphenyl)-4-hydroxythiazolo[5,4-d]pyrimidine potassium salt, 2-(3-cyano-4-phenoxyphenyl)thiazolo[5,4-d]pyrimidine, TMX-67, and FYX-051, which can be used as an agent for overcoming anti-cancer agent resistance.一種抗癌劑之抗性克服劑，其特徵為使用含有2－〔3－氰基－4－(4－氟苯氧基)苯基］－4－羥基噻唑〔5,4－d］嘧啶 鉀鹽、2－〔3－氰基－4－(4－氟苯氧基)苯基］噻唑〔5,4－d］嘧啶、2－(3－氰基－4－苯氧基苯基)－4－羥基噻唑〔5,4－d］嘧啶、2－(3－氰基－4－苯氧基苯基)－4－羥基噻唑〔5,4－d］嘧啶 鉀鹽、2－〔3－氰基－4－苯氧基苯基)噻唑〔5,4－d］嘧啶、TMX－67及FYX－051等之黃嘌呤氧化酵素抑制劑(Xanthine Oxidase inhibitor)作為有效成份的藥劑。
