4-substituted 1,5-dihydro-pyrido[3,2-b]indol-2-ones of formula (I): the N-oxides, salts, stereoisomeric forms, racemic mixtures, prodrugs, esters and metabolites thereof, wherein n is 1, 2 or 3; R1 is hydrogen, cyano, halo, substituted carbonyl, methanimidamidyl, N-hydroxy-methanimidamidyl, mono- or di(C1-4alkyl)methanimidamidyl, Het1 or Het2; X is NR2, O, S, SO, SO2; R2 is hydrogen, C1-10alkyl, C2-10alkenyl, C3-7cycloalkyl, which are optionally substituted; R2 is aryl substituted with —COOR4; or R2 is: —CpH2p—CH(OR14)—CqH2q—R15 (b-3); —CH2—CH2—(O—CH2—CH2)m—OR14 (b-4); —CH2—CH2—(O—CH2—CH2)m—NR5aR5b (b-5); -a1=a2-a3=a4- is —CH═CH—CH═CH—; —N═CH—CH═CH—; —CH═N—CH═CH—; —CH═CH—N═CH—; —CH═CH—CH═N—; wherein hydrogen atoms in (c-1)-(c-5) may be replaced by certain radicals; R3 is optionally substituted phenyl or a monocyclic or bicyclic aromatic heterocyclic ring system that can be substituted; R21 is halo, hydroxy, amino, carboxyl, C1-6alkyl, C3-7cycloalkyl, arylC1-6alkyl, formyl, C1-6alkylcarbonyl, C1-6alkyloxycarbonyl, C1-6alkylcarbonyloxy, —C(═O)—NR13R14; C1-6alkyloxy, arylC1-6alkyloxy, polyhaloC1-6alkyloxy, amino, mono or di(C1-6alkyl)amino, formylamino, C1-6alkylcarbonylamino, C1-6alkylsulfonylamino, mercapto, C1-6alkylthio, arylthio, aryloxy, arylC1-6alkylthio, C1-6alkylsulfynyl, C1-6alkylsulfonyl, aryl, arylamino, Het1, Het2.