The chemotherapy of malignant tumours is greatly restricted by the generally slight differentiation of the available cytostatic agents between normal and malignant tissue. In order to achieve an improvement of the selectivity in cancer therapy, novel prodrugs have been developed from 6-hydroxy-2,3-dihydro-1H-indolene, 5-hydroxy-1,2-dihydro-3H-pyrrolo[3,2-e]indolene and 5-hydroxy-1,2-dihydro-3H-benzo[e]indolene as well as from 6-hydroxy-1,2,3,4-tetrahydro-benzo[f]-quinolines, that may be used within the framework of the ADEP therapy (antibody directed enzyme prodrug therapy). The new prodrugs are characterised by a high difference in toxicity between the prodrug and underlying drug and by a very high efficacy of the drug. After splitting off of the glycosidic and/or acetal group on the phenolic hydroxy groups of the prodrugs, a spirocyclopropacyclohexadiene is formed which, being a highly toxic group, effects an alkylation of the DNA or RNA.