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.
