Dehydrogenation of indoline by cytochrome P450 enzymes: a novel "aromatase" process

Indoline derivatives possess therapeutic potential within a variety of drug candidates. In this study, we found that indoline is aromatized by cytochrome P450 (P450) enzymes to produce indole through a novel dehydrogenation pathway. The indole products can potentially be bioactivated to toxic intermediates through an additional dehydrogenation step. For example, 3-substituted indoles like 3-methylindole and zafirlukast [4-(5-cyclopentyloxy-carbonylamino-1-methyl-indol-3-ylmethyl)-3-methoxy-N-o-tolylsulfonylbenzamide] are dehydrogenated to form 3-methyleneindolenine electrophiles, which react with protein and/or DNA nucleophilic residues to cause toxicities. Another potentially significant therapeutic consequence of indoline aromatization is that the product indoles might have dramatically different therapeutic potency than the parent indolines. In this study, indoline was indeed efficiently aromatized by human liver microsomes and by several P450s, but not by flavin-containing monooxygenase (FMO) 3. CYP3A4 had the highest aromatase activity. Four additional indoline metabolites [2,3,4,7-tetrahydro-4,5-epoxy-1H-indole (M1); N-hydroxyindole (M2), N-hydroxyindoline (M3), and M4 ([1,4,2,5]dioxadiazino[2,3-a:5,6-a']diindole)] were characterized; none was a metabolite of indole. M1 was an arene oxide from P450 oxidation, and M2, M3, and M4 were produced by FMO3. Our data indicated that indoline was oxidized to M3 and then to an intermediate indoline nitrone, which tautomerized to form M2, and subsequently dimerized to a di-indoline. This dimer was immediately oxidized by FMO3 or atmospheric oxygen to the final product, M4. No evidence was found for the P450-mediated production of an aliphatic alcohol from indoline that might dehydrate to produce indole. Therefore, P450 enzymes catalyze the novel "aromatase" metabolism of indoline to produce indole. The aromatase mechanism does not seem to occur through N-oxidation or dehydration of an alcohol but rather through a formal dehydrogenation pathway.