The present invention provides a method of enhancing an antitumor effect by a compound strongly inhibiting ribonucleotide reductase (RNR) or a salt thereof.
A combination formulation involving combined administration of a sulfonamide compound represented by Formula (I) [In the formula, X 1 represents an oxygen atom or the like; X 2 represents an oxygen atom; X 3 represents -NH-; X 4 represents a hydrogen atom or the like; R 1 represents -C(R 11 ) (R 12 )- or the like; R 11 and R 12 are the same or different and each represents a hydrogen atom or the like; R 2 represents an optionally substituted C6-C14 aromatic hydrocarbon group or the like; R 3 represents an optionally substituted C6-C14 aromatic hydrocarbon group or the like; R 4 represents a hydrogen atom or the like] or a salt thereof, having RNR inhibitory activity, and other antitumor agent(s).