PT - JOURNAL ARTICLE AU - Chungang Gu AU - Richard J. Lewis AU - Andrew S. Wells AU - Per H. Svensson AU - Vinayak P. Hosagrahara AU - Eskil Johnsson AU - Gösta Hallström TI - Lipid Peroxide–Mediated Oxidative Rearrangement of the Pyrazinone Carboxamide Core of Neutrophil Elastase Inhibitor AZD9819 in Blood Plasma Samples AID - 10.1124/dmd.115.065136 DP - 2015 Oct 01 TA - Drug Metabolism and Disposition PG - 1441--1449 VI - 43 IP - 10 4099 - http://dmd.aspetjournals.org/content/43/10/1441.short 4100 - http://dmd.aspetjournals.org/content/43/10/1441.full SO - Drug Metab Dispos2015 Oct 01; 43 AB - This study focused on the mechanistic interpretation of ex vivo oxidation of a candidate drug in blood plasma samples. An unexpected lipid peroxide–mediated epoxidation followed by a dramatic rearrangement led to production of a five-membered oxazole derivative from the original six-membered pyrazinone-carboxamide core of a human neutrophil elastase inhibitor, 6-(1-(4-cyanophenyl)-1H-pyrazol-5-yl)-N-ethyl-5-methyl-3-oxo-4-(3-(trifluoromethyl)phenyl)-3,4-dihydropyrazine-2-carboxamide (AZD9819). The rearranged oxidation product 2-(1-(4-cyanophenyl)-1H-pyrazol-5-yl)-5-(N-ethylacetamido)-N-(3-(trifluoromethyl)phenyl)oxazole-4-carboxamide was characterized by accurate-mass tandem mass spectrometry fragmentations, by two-dimensional NMR and X-ray crystallography of an authentic standard, and by incorporation of an 18O atom from molecular 18O2 to the location predicted by our proposed mechanism. The lipid peroxide–mediated oxidation was demonstrated by using human low-density lipoprotein (LDL) in pH 7.4 phosphate buffer and by inhibiting the oxidation with ascorbic acid or l-glutathione, two antioxidants effective in both plasma and the LDL incubation. A nucleophilic mechanism for the epoxidation of AZD9819 by lipid hydroperoxides explains the prevention of its ex vivo oxidation by acidification of the plasma samples. The discovery of the lipid peroxide–dependent oxidation of an analyte and the means of prevention could provide valuable information for biotransformation and bioanalysis.