Metabolic Activation of a Novel 3-Substituted Indole-Containing TNF-α Inhibitor: Dehydrogenation and Inactivation of CYP3A4

SPD-304 is a recently discovered small-molecule TNF-alpha antagonist. However, SPD-304 contains a potentially toxic 3-alkylindole moiety. Previous studies on 3-methylindole and the 3-alkylindole-containing drugs zafirlukast and MK-0524 structural analogues found that they were bioactivated by cytochrome P450s through a dehydrogenation process to form 3-methyleneindolenine intermediates that are electrophilic alpha,beta-unsaturated iminium species. These electrophiles could react with protein and/or DNA nucleophilic residues to cause toxicities. In the present study, we found that SPD-304 was bioactivated through a similar dehydrogenation mechanism to produce a similar electrophilic 3-methyleneindolenine intermediate. The electrophile was trapped with nucleophilic glutathione and identified by LC/MS/MS. The iminium or another reactive intermediate also was a mechanism-based inactivator of CYP3A4. The inactivation parameters were K I = 29 microM and k inact = 0.047 min (-1). In addition, SPD-304 was metabolized through hydroxylation, N-dealkylation, and epoxidation pathways, and several metabolites and glutathione adducts were characterized by tandem mass spectrometry. The metabolism profile was also evaluated by in silico molecular docking of SPD-304 into the active site of CYP3A4, which predicted that the dehydrogenation reaction was initiated by 3-methylene C-H atom abstraction at the trifluoromethylphenyl-1 H-indol-3-ylmethyl portion of SPD-304. Hydroxylation of the 6'-methyl of the dimethylchromone portion of SPD-304 was the other major predicted metabolic pathway. The molecular models correlated precisely with experimental metabolic results. In summary, dehydrogenation of SPD-304 may cause toxicities through the formation of electrophilic intermediates and cause drug-drug interactions through CYP3A4 inactivation.