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Abstract No.: TuP-039
Session: Drug Discovery and Development
Presentation date: Tue, Aug 29, 2006
Presentation time: 09:50 – 11:20

Analysis of Electrochemical Products and Their Fragmentations for Zotepine Assay Using On-line Electrochemistry/Electrospray Ionization Mass Spectrometry

Ryuichi Arakawa1, Kazuyoshi Nozaki2, Issey Osaka1, Sumihisa Kimura2, Akira Kagayama2

1 Kansai University, Suita, Japan
2 Astellas Pharmaceutical Inc., Osaka, Japan

Correspondence address: Ryuichi Arakawa, Kansai University, Applied Chemistry, 3-3-35 Yamate, Suita, 564-8680 Japan.

Keywords: Drug; Electrochemistry; Electrospray Ionization (ESI); MS/MS.

Novel aspect: Searching for electrochemical products of zotepine and their fragmentations to develop a sensitive assay using EC/ESI-MS/MS.

 

Zotepine has been clinically used as an antipsychotic drug with relatively few extrapyramidal side effects. Although the GC-MS method for zotepine was developed rather recently, the LC-MS/MS method has not yet been reported. This is in part because the fragment ion of zotepine has too small a mass-to-charge ratio (m/z) and too weak an ion intensity to develop a sensitive LC-MS/MS assay. In recent years, on-line hyphenation of electrochemistry with mass spectrometry (EC/MS) has been developed as a new tool for the determination or identification of problematic compounds. Here the formation of new fragment ions allowing sensitive quantification of zotepine by on-line EC/ESI-MS/MS is described. The combination of EC/MS and collision-induced dissociation (CID) appears to be useful for generating new fragments of zotepine. Chlorpromazine was also investigated as a chemical structural analog of zotepine. In addition, the comparison between the electrochemical oxidation products and phase I metabolites of zotepine is discussed.

The mass spectra were acquired using a triple stage quadrupole mass spectrometer TSQ 7000 equipped with an ESI interface in positive ion mode. The mass spectrometer was connected to a microflow coulometric cell. The electrolyses of sample solutions were performed at a flow rate of 5 µL/min by using a syringe pump, and the solutions were electrosprayed directly for the mass spectrometric analysis.

In the CID spectrum of zotepine with normal ESI-MS/MS, there was only one major fragment ion at m/z 72 which was derived from the cleaved side chain moiety. However, the electrochemical product ions of [M+16+H]+ (m/z 348), [M-H]+ (m/z 330) and [M-14+H]+ (m/z 318) were detected using an on-line EC/ESI-MS system. In addition, it was found that new fragment ions, such as m/z 315 or 286 ions, were observed in the CID spectrum of the intense electrochemical product [M-H]+ ion. As these fragments were relatively specific and had high ion intensities, EC/ESI-MS/MS is apparently a useful means of developing a sensitive LC-MS/MS assay for zotepine. On the other hand, the information on mass fragmentations of electrochemical products by EC/ESI-MS/MS will be useful for elucidating the structures of metabolites and analogs. The electrochemical products of zotepine generated by EC/ESI-MS were S-oxide and N-demethylated zotepine, i.e. a portion of the P450 metabolites of zotepine could be mimicked by this electrochemical system.