The introduction of atmospheric pressure ionization (API) techniques, as the interface between high performance liquid chromatography (HPLC) and mass spectrometry (MS), has established LC-MS as an important characterization and quantification tool in the pharmaceutical industry. Specifically, LC-MS has dramatically changed the approaches necessary in assessing the metabolism of a novel drug candidate. For example, from a simple LC-MS analysis, the metabolic fate of a novel drug in both in vitro and/or in vivo systems can be assessed. LC-MS spectra, acquired in the full scan mode, combined with targeted searches (precursor ion and neutral loss scans) for possible metabolites, can provide information on the metabolic pathway of a novel drug. Among such metabolic modifications, oxidation of carbon (hydroxylation), nitrogen (N-oxides & hydroxyl amines) or sulfur (sulfoxides) atoms are very common and results in the shift of the LC-MS detected mass of metabolites by 16 Da from that of the parent drug. Distinguishing between hydroxylation, N-oxidation and/or sulfoxidation are important due to the N-oxide metabolites being associated with toxicity. Previously, we have shown that LC-MS, coupled with atmospheric pressure chemical ionization (APCI), can be used to distinguish N-oxides from hydroxyl and sulfoxide metabolites based on the thermal instability of N-oxides relative to other oxygenated metabolites. In this presentation, we will also show that H/D exchange, in conjunction with MS/MS, can also be used to distinguish various forms of oxide metabolites.
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Back to The 37th Middle Atlantic Regional Meeting (May 22-25, 2005)