Gas-Phase Rearrangement of the O-Glucuronide of Vildagliptin Forms Product-Ion Fragments Suggesting Wrongly an N-Glucuronide.

The O-glucuronide of vildagliptin, a dipeptidyl peptidase 4 inhibitor, is a major metabolite in monkeys and a minor metabolite in humans, rats, and dogs. Its product ion spectrum shows fragments that can be explained only by an N-glucuronide. Biotransformation using rat liver yielded milligram amounts of the O-glucuronide, and its structure was assigned unambiguously by nuclear magnetic resonance. The tandem mass spectra (MS/MS) of this compound was investigated in detail using MSn and accurate mass spectrometers and was identical to the animal metabolite. Thus, the MS/MS fragments suggesting an N-glucuronide had to be formed by gas-phase rearrangement. This gas-phase rearrangement can be observed on quadrupole time-of-flight and ion-trap mass instruments. The literature on gas-phase rearrangements is reviewed.