Microbial conversion of methyl- and methoxy-substituted derivatives of 5H-indolo[2,3-b]quinoline as a method of developing novel cytotoxic agents

In furtherance of our structure-activity relationship studies on the antitu mor activity of indolo[2,3-b]quinolines, novel cytotoxic derivatives bearin g methyl groups at N-5, C-II, C-2 and/or C-9, as well as methoxy-groups at C-2 and/or C-9, were synthesized by the modified Graebe-Ullmann reaction. T o elucidate the metabolic pathways of these compounds, zygomycete fungus Cu nninghamella elegans ATCC 9245 (which is known to produce drug metabolites that are also for-med in mammals) was used as a mimetic organism. Simultane ously, biotransformation of the same substrates was carried out with a micr osomal fraction of rat liver: Three forms of microbial conversion were obse rved: hydroxylation of the aromatic ring or hydroxylation of the methyl gro up, and O-demethylation. The reaction proceeded regioselectively, and only positions C-2 and C-9 were affected in the indolo[2,3-b]quinoline system. T he products formed were found to be identical with the metabolites generate d by rat liver microsomes. The metabolites obtained displayed a cytotoxic a ctivity in vitro against colon adenocarcinoma SW-707 and lung carcinoma A-5 49 (ID50 in the range 0.27-3.04 mu M), which was as strong as that of the s ubstrates. In the course of the further metabolic pathway study of indolo[2 ,3-b]quinolines we found that metabolites with a hydroxyl group in the arom atic system were transformed to noncytotoxic polymeric products by multicop per oxidases: human ceruloplasmin or fungal laccase (used as mimetic enzyme ), whereas metabolites with a hydroxymethyl group did not undergo such bioc onversion. The last mentioned compounds can be regarded as a novel type of cytotoxic indolo[2,3-b]quinoline derivatives formed in metabolic processes.