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SUBSTITUENT EFFECT ON THE REDUCTIVE N-DEARYLATION OF 3-(INDOL-1-YL)-1,2-BENZISOXAZOLES BY RAT LIVER MICROSOMES

Department of Drug Metabolism & Basic Chemistry, Merck Research Laboratories, Rahway, New Jersey

The reductive metabolism of a series of 3-(indol-1-yl)-1,2-benzisoxazoles was examined in vitro using rat liver microsomes. 3-(Indol-1-yl)-1,2-benzisoxazole was reduced to the corresponding amidine (resulting from N–O bond cleavage) under anaerobic conditions. The reaction required viable microsomes and NADPH and was inhibited by carbon monoxide, air, and ketoconazole, suggesting the involvement of cytochrome P450 enzymes. The amidine was subsequently nonenzymatically hydrolyzed to 1-salicylindole, which in turn was hydrolyzed to indole. Addition of electron-withdrawing substituents (Cl^-, MeSO₂^-) at the 6-position of the benzisoxazole ring resulted in a significant increase in the rate of substrate reduction. Introduction of electron-withdrawing substituents on the indole ring likewise increased the rate of substrate consumption but caused a substituent-dependent shift of the site of bond cleavage from the 1,2-isoxazole N–O bond to the C–N bond linking the 1,2-benzisoxazole to the indole moiety. In the case of 3-(2-chloro-3-methanesulfoxylindol-1-yl)-1,2-benzisoxazole, C–N bond cleavage was nearly quantitative, and products resulting from N–O bond reduction were not observed. The overall rates of 3-(indol-1-yl)-1,2-benzisoxazoles reduction were found to be substrate concentration-dependent and observed Michaelis-Menten-type behavior. The apparent V_max of substrate reduction by rat liver microsomes correlated negatively with the free energy of the lowest unoccupied molecular orbitals (E_LUMO) calculated semiempirically using a parameterized model 3 (PM3), and suggested that the initial electron transfer was rate-determining and that the E_LUMO could be used as an indication of the susceptibility of 1,2-isoxazoles to undergo reductive metabolism.

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