In vitro metabolism of deuterium labeled 5 in sodium phosphate-methanol results in no washout of label. Panel A shows the family of M+H ions corresponding to the mixture of non-, mono-, di-, and tri-deuterated isomers of 5, at m/z 178, 179, 180, and 181 respectively, obtained using LC-ESI/MS operating in the positive ion mode (plotted at 7× actual abundance). Panel B shows the mass spectrum of the mono-N-oxide metabolites generated by in vitro metabolism of deuterated 5 by NADPH:cytochrome P450 reductase (0.6 units/mL), NADPH (3 mM), in phosphate buffer 50 mM, pH 7, containing methanol (2 M), under anoxic conditions, obtained using LC-ESI/MS operating in the positive ion mode (plotted at 2× actual abundance). The relative intensity of M+H ions for the resulting mixture of non-, mono-, di-, and tri-deuterated isomers of 8, at m/z 162, 163, 164, and 165 respectively, is not significantly altered from that observed in the starting mixture of deuterated di-N-oxides 5.