Chapter 30. Tandem Mass Spectrometry for the Identification of Drug Metabolites

Abstract

The combination of gas chromatography and mass spectrometry (GC/MS) has become a principal technique for the separation and structural elucidation of drug metabolites. Drug metabolite identification methods are based on the incorporation of radioactive or stable isotopes into the drug molecule, and tracking the tracer by some form of chromatography in physiological samples from test animals or humans. Selected fractions are then identified by mass spectrometry. Tandem mass spectrometry (MS/MS) offers such a method. MS/MS can eliminate the need for chromatography and provide additional capabilities not obtainable with the relatively slow chromatographic techniques in which sample components elute one at a time. MS/MS is capable of rapidly (2–3 hrs.) identifying metabolites in physiological samples with minimal sample preparation, and can be used to postulate new metabolite structures without the use of reference standards. This chapter provides an overview of the application of MS/MS to detection and identification of drug metabolites, and illustrates the use of tandem quadrupole MS/MS (TQMS). Described here is the development of MS/MS for mixture analysis, and the unique characteristics and special operating modes of a tandem mass spectrometer that make the system particularly well suited to the rapid detection and identification of drug metabolites. Characteristics of a tandem quadrupole mass spectrometer have been reviewed and its operational modes discussed. Premise of the MS/MS method have been delved upon and a summary of the MS/MS procedure provided. Metabolite identification by MS/MS involving both old and new metabolites and conjugated drug metabolites with details on major metabolites of the antiepileptic drugs primidone, cinromide, and phenytoin identified in urine and plasma by MS/MS, normal and daughter spectra of pure zonisamide. and neutral loss, and parent spectra of enzyme- hydrolyzed urine extract.