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-hydroxymethanimidamidyl, mono- or di(C1-4alkyl)methanimidamidyl, Het1 or Het2; X is NR 2, O, S, SO, SO2; R2 is hydrogen, C 1-10alkyl, C2-10alkenyl, C 3-7cycloalkyl, which are optionally substituted; R2 is aryl substituted with -COOR4 ; or R2 is: (b-1); (b-2); -CpH2p-CH(OR14 )-CqH 2q-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 obtionally 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, C1-6alkylcarbonylo xy, -C(=O)-NR13R14; C1-6alkyloxy, arylC1-6alkyloxy, polyhaloC1-6alkyloxy, amino, mono or di(C1-6alkyl)amino, formylamino, C1-6alkylcarbonylamino, C1-6alkylsulfo nylamino, mercapto, C1-6alkylthio, arylthio, aryloxy, arylC1-6alkylthio, C1-6alkylsulfynyl, C1-6alkylsulfo nyl, aryl, arylamino, Het1, Het2.