In vitro biotransformation of thiazopyr by rat liver microsomes: oxidative cleavage of a carboxylic methylester by monooxygenases.

1. Thiazopyr was metabolized by liver microsomes from male Sprague-Dawley rats to a previously unidentified metabolite. 2. The new metabolite was identified by coelution with an authentic standard in hplc and by electrospray lc/ms as the corresponding carboxylic acid. 3. Formation of the carboxylic acid metabolite was inhibited in the presence of mono-oxygenase inhibitors including piperonyl butoxide, 1-aminobenzotriazole, metyrapone and tetcyclacis. 4. Transformation of thiazopyr to its carboxylic acid by rat liver microsomes is mediated by mono-oxygenases and not hydrolases.