Rapid and definitive identification of pharmaceutical drug metabolites using mass spectrometry

Low-energy collision-induced dissociation-tandem mass spectrometry (CID-MS/MS) is a well-established approach for identifying pharmaceutical drug metabolites. The technique fulfils many necessary requirements for this task, such as a low limit of detection, simple interfacing to chromatographic techniques, capability of fast analyses and automation, and high sensitivity, selectivity and accuracy. However, one of the main limitations of low-energy CID-MS/MS is that unambiguous assignment of the site of metabolism is often not possible, particularly for oxidised metabolites. Further, data interpretation can be time-consuming, thus producing a bottleneck to high-throughput analyses. The aim of the presented study was to identify structurally dependent dissociation pathways using low-energy CID-MS/MS that could facilitate rapid and definitive assignment of the sites of metabolism of new chemical entities.Chapter 4 details a specific loss of 50 m/z units in a model S-oxide that arises due to an ortho-effect. This loss could be used to definitively assign the site of oxidation and discriminate between multiple sulfur atoms in a parent compound. The 50 m/z unit loss was also shown to be a two-step process involving sequential radical losses; a rare observation for even-electron precursor ions under low-energy CID conditions. Chapter 5 discusses the experimental investigation of two unexpected rearrangements during the dissociation of a model S-oxide that could prevent correct assignment of the site of metabolism. Chapter 6 presents a rapid and definitive approach to the characterisation of dialkyl tertiary amine-N-oxides. The work also elucidated generic dissociation behaviour under low-energy CID conditions. Finally, chapter 7 considers an observation of site-specific intra-ionic hydrogen/deuterium exchange in the gas phase. Seven sets of compounds were analysed to investigate the substructures that facilitate the exchange. The work demonstrates a method by which a deuterium label can be inserted into the carbon skeleton of a small molecule without having to synthetically produce the compound, which could be useful in performing timely and cost-effective structural elucidation studies. In summary, the presented study provides two potentially useful approaches for the rapid and definitive identification of oxidised metabolites, as well as increasing the body of knowledge relating to ion-chemistry under low-energy CID conditions.