Recently, several drugs that contain the thiophene moiety have been removed from the market due to toxicity. It was postulated that the toxicity was caused
by the reactive intermediates formed from the thiophene moiety [9-12]. To understand the biactivation pathways of thiophene moiety, a model compound, 2-(4-methoxybenzoyl)thiophene, was incubated in rat or human liver microsomes. The major metabolites identified were an O-demethylated metabolite with (M + H)+ at m/z 205, and a monohydroxylated metabolite with (M + H)+ at m/z 235. Analysis of the incubation mixtures that contained 5-mM GSH revealed the formation of a GSH adduct at m/z 524 in both the human and rat liver microsomes. Figure 10.5 shows the product-ion MS/MS spectrum of the m/z 524 GSH adduct. Loss of glycine and pyroglutamic acid generated the product ions at m/z 449 and m/z 395, respectively. Further loss of NH3 from m/z 395 generated the product ion at m/z 378. The product ion at m/z 135 corresponded to the 4-methoxybenzaldehyde portion of the molecule.
An additional GSH adduct at m/z 526 was observed in the rat liver microsomes, which could be formed from the addition of GSH to reduced thiophene moiety, or the addition of GSH to a ring-opened thiophene moiety. The fragmentation pattern for the second GSH adduct (m/z 526) observed in the rat liver microsomes was very different from the spectrum obtained on previous GSH adduct. The major product ions observed in the MS/MS spectrum (Figure 10.6A) were at m/z 219 and m/z 308, which corresponded to the molecular mass of 2-(4-methoxybenzoyl)thiophene and GSH, respectively. LC-MS, with D2O (Figure 10.6B) substituted for H2O in the mobile phase,
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