The identification of in vitro metabolites of bupropion using ion trap mass spectrometry

We have identified in vitro metabolites of bupropion (Wellbutrin®) from incubations with human liver S9 fraction and human liver microsomes based on molecular weight information from full scan experiments using a liquid chromatograph coupled to a quadrupole ion trap mass spectrometer capable of multi‐stage operation (LC/MS n ). Preliminary experiments have shown that this instrument provides comparable sensitivity to conventional LC‐coupled triple quadrupole instruments for metabolic studies, while allowing detailed structural studies using MS n experiments and routine on‐line coupling with high performance liquid chromatography via an external atmospheric pressure chemical ionization (APCI) source. The LC/MS analysis of human S9 showed the presence of three isomeric monohydroxylated metabolites of bupropion. These were further characterized in a series of MS/MS experiments which gave characteristic spectra for the three isomers. A minor dihydroxylated species was also identified in the human S9 sample and further characterized in a series of MS n experiments. Detailed structural information was generated by the use of on‐line LC/MS n type experiments. We have followed the fragmentation pathways of several molecular ion species in a series of sequential LC/MS n experiments, extending as far as MS 6 with scan cycle times of less than 1.5 s. Such experiments have provided insights into the structure of specific fragment ions. Additional metabolic products were identified in the rat liver microsomes incubation sample.