BCRP/ABCG2 inhibitor

A BCRP inhibitor comprising, as an active ingredient, an acrylonitrile derivative represented by formula (1): wherein one of R 1 and R 2 represents a cyano group and the other represents a hydrogen atom; Ar 1 represents a group selected from among the groups represented by formulas (2) to (4): wherein R 7 and R 8 , which are identical to or different from each other, each represent a hydrogen atom, a halogen atom, or a lower alkoxy group; A represents an oxygen atom, a sulfur atom, or NR 9 ; and R 9 represents a hydrogen atom or a lower alkyl group; Ar 2 represents a group selected from among the groups represented by formulas (5) to (15): wherein R 3 represents a hydrogen atom, an oxygen atom (as N-oxide), a lower alkyl group, a lower alkoxy group, a halogen atom, a nitro group, a methylsulfanyl group, a lower hydroxyalkyl group, an aromatic hydrocarbon group which may have a substituent, or NR 5 (R 6 ) R 5 and R 6 , which are identical to or different from each other, each represent a hydrogen atom, a lower alkyl group which may have a substituent, a lower hydroxyalkyl group, or an aromatic hydrocarbon group which may have a substituent or heterocyclic group; R 5 and R 6 may form, with their adjacent nitrogen atom, a heterocyclic ring which may have a substituent; and the hydroxyl group of the lower hydroxyalkyl group or the heterocyclic ring substituted by a hydroxyl group or a lower hydroxyalkyl group may form an ester bond with a phosphoric acid group or a salt thereof or with an acyl group which may have a substituent; R 4 represents a hydrogen atom, a lower alkyl group, a phenyl group which may have a substituent, or a benzyl group; X represents a carbon atom, CH, or a nitrogen atom, provided that when A is an oxygen atom, X is not a nitrogen atom; and A, R 7 , R 8 , and R 9 have the same meanings as defined above; or a salt thereof.