Simultaneous measurement of metabolic stability and metabolite identification of 7-methoxymethylthiazolo[3,2-a]pyrimidin-5-one derivatives in human liver microsomes using liquid chromatography/ion-trap mass spectrometry

A liquid chromatography/mass spectrometry (LC/MS) method using an atmospheric pressure chemical ionisation source was used to measure the metabolic stability and metabolite identification of 7‐methoxymethylthiazolo[3,2‐a]pyrimidin‐5‐one derivative (1) in human liver microsomes. After 15 min incubation with human liver microsomes, compound 1 exhibited metabolic turnover of 44%. Data‐dependent tandem mass spectrometry (MS/MS) scanning was used to generate product ion spectra from the protonated ions of the compound and its metabolites. An unusual metabolite at m/z 407 corresponding to the [M–24+H]+ ion was identified for compound 1. Interestingly, the formation of the [M–24+H]+ ion was not observed in the analogues wherein the fused thieno double bond was substituted (2) and the thieno group replaced by a fused benzo derivative (3). Compounds 2 and 3 exhibited metabolic turnovers of 24 and 30%, yielding oxidative metabolites corresponding to [M+16] and [M+32]+, respectively. Based on these facts the mechanism for [M–24]+ formation in compound 1 through an initial epoxide formation on the double bond of the fused thieno ring followed by hydrolytic ring opening and deacylation is envisaged. Copyright © 2005 John Wiley & Sons, Ltd.