RT Journal Article
SR Electronic
T1 Elucidation of a Novel Bioactivation Pathway of a 3,4-Unsubstituted Isoxazole in Human Liver Microsomes: Formation of a Glutathione Adduct of a Cyanoacrolein Derivative after Isoxazole Ring Opening
JF Drug Metabolism and Disposition
JO Drug Metab Dispos
FD American Society for Pharmacology and Experimental Therapeutics
SP 302
OP 311
DO 10.1124/dmd.110.036285
VO 39
IS 2
A1 Jian Yu
A1 James J. Folmer
A1 Valerie Hoesch
A1 James Doherty
A1 James B. Campbell
A1 Doug Burdette
YR 2011
UL http://dmd.aspetjournals.org/content/39/2/302.abstract
AB Studies on the biotransformation of isoxazole rings have shown that molecules containing a C3-substituted isoxazole or a 1,2-benzisoxazole can undergo a two-electron reductive ring cleavage to form an imine. In the absence of a C3 substituent, the isoxazole ring opens via deprotonation of the C3 proton followed by N–O bond cleavage to yield an α-cyanoenol analog. We report the identification of a novel bioactivation pathway of a 3,4-unsubstituted isoxazole in human liver microsomes. After the enzyme-catalyzed cleavage of the 3,4-unsubstituted isoxazole ring of N-((2-isopropyl-7-methyl-1-oxoisoindolin-5-yl)methyl)isoxazole-5-carboxamide (P) in human liver microsomes, the formed α-cyanoenol (M1) condenses with formaldehyde to generate an α,β-unsaturated Michael acceptor intermediate (a cyanoacrolein derivative, VII), which further reacts with the cysteinyl thiol of glutathione to yield a GSH adduct of a cyanoacrolein derivative (M3). The same adduct also is formed when M1, generated in 0.1 N NaOH aqueous solution, reacts with formaldehyde and GSH. 13C-labeled methanol was used to confirm that methanol from the drug stock solution was oxidized by liver microsomal enzymes to formaldehyde and the carbon atom from methanol was finally incorporated in the corresponding GSH adduct. The formation of isoxazole ring-opened products (M1 and M2) in human liver microsomes is NADPH-dependent. M1 and M2 were found in human liver microsomes preincubated with 1-aminobenzotriazole (1 mM) and NADPH (5 mM) at ∼10% of the levels found in the samples in the absence of 1-aminobenzotriazole, suggesting that this biotransformation pathway is primarily catalyzed by cytochrome P450. The formation of M3 also was inhibited by 1-aminobenzotriazole at a similar level.