The present invention provides a method for enhancing an antitumor effect by a compound that strongly inhibits ribonucleotide reductase (RNR) or a salt of said compound. A combination formulation in which a sulfonamide compound that has RNR inhibitory activity and is represented by formula (I) (in the formula, X1 represents an oxygen atom, etc.； X2 represents an oxygen atom； X3 represents -NH-； X4 represents a hydrogen atom, etc.； R1 represents -C(R11)(R12)-, etc.； R11 and R12 are the same or different and represent a hydrogen atom, etc.； R2 represents an optionally substituted C6-C14 aromatic hydrocarbon group, etc.； R3 represents an optionally substituted C6-C14 aromatic hydrocarbon group, etc.； and R4 represents a hydrogen atom, etc.), or a salt thereof, is administered in combination with another antitumor agent.本發明提供一種利用較強地抑制核糖核苷酸還原酶(RNR)之化合物或其鹽增強抗腫瘤效果之方法。本發明之組合製劑係將式(I)[式中，X1表示氧原子等；X2表示氧原子；X3表示-NH-；X4表示氫原子等；R1表示-C(R11)(R12)-等；R11與R12相同或不同，表示氫原子等；R2表示可經取代之C6-C14芳香族烴基等；R3表示可經取代之C6-C14芳香族烴基等；R4表示氫原子等]所表示之具有RNR抑制活性之磺醯胺化合物或其鹽與其他抗腫瘤劑併用投予。
