Dihydroorotic and acid dehydrogenase inhibitor

A method for suppressing immune system, or treating a rheumatism, a cancer, a graft rejection in organ transplantation, or a H. pylori infection, wherein the method comprises administering to a subject in need thereof an effective amount of a dihydroorotic acid dehydrogenase inhibitor,

• wherein the dihydroorotic acid dehydrogenase inhibitor comprises, as an active ingredient(s), one or two or more of compounds represented by formula (I):
• (wherein
• X represents a halogen atom,
• R1 represents a hydrogen atom,
• R2 represents a C1-7 alkyl group,
• R3 represents —CHO, and
• R4 represents —CH2—CH═C(CH3)—R0 (wherein R0 represents a C1-12 alkyl group which may have a substituent on the terminal carbon and/or on a non-terminal carbon, a C2-12 alkenyl group which may have a substituent on the terminal carbon and/or on a non-terminal carbon, or a C2-12 alkynyl group which may have a substituent on the terminal carbon and/or on a non-terminal carbon), or —CH═CH—(CH2)6—O—CO—CH(CH3)2),
• wherein the substituent for the C1-12 alkyl group, the C2-12 alkenyl group, and the C2-12 alkynyl group is selected from the group consisting of —COOH, —COORa, where Ra represents a C1-7 alkyl group, —CHO, —COOCH2CH(OH)CH2OH, —COO—CH2—Rb, where Rb represents a group obtained by removing one of the hydrogen atoms on the carbons of an aromatic hydrocarbon, —O—CO-Rc, where Rc represents a C1-7 alkyl group, —OH, —O-Rd, where Rd represents a C1-7 alkyl group, —O—CH2—O—CH3, —CH(OCH3)—CH2—CO—C(CH3)3, —CO—CH3, —CO—C(CH3)3, -HET, where HET represents a group obtained by removing one hydrogen atom from the carbon or nitrogen atom(s) of pyridine, furan, thiophene, pyran, pyranone, imidazole, 1,3-dioxolane, oxirane, or 3,3-dimethyloxirane, and —O-HET,
• optical isomers thereof and pharmaceutically acceptable salts thereof, as well as a pharmaceutically acceptable carrier.