The effect of methimazole on thioamide bioactivation and toxicity.

The hepatotoxicity induced by administration of ethionamide and thionicotinamide (TNA) was shown to be decreased by pre-administration of methimazole (MMI). Pre-administration of MMI was also shown to decrease the levels of excretion of TNA S-oxide. This indicates that thioamide S-oxidation, mediated by the flavin-containing mono-oxygenase, may be linked to the initiation of hepatotoxicity induced by these thioamides. SK&F-525-A, the cytochrome P-450 inhibitor, did not affect either thioamide-induced toxicity or levels of excretion of TNA S-oxide; however, the role of the P-450 isoenzymes cannot be totally ruled out.