Metabolite identification in drug discovery
Recent developments in the technologies and approaches to identify metabolites in a drug discovery environment are reviewed. Samples may be generated using either in vitro systems - typically, but not exclusively, liver subcellular fractions, such as microsomes, or whole cells, such as hepatocytes. Alternatively, metabolites are generated in vivo using excreta obtained following dosing in preclinical species. Recombinant drug metabolizing enzymes or microorganisms may offer alternate vectors. New techniques, such as the use of solid-phase microextraction, have found application in the isolation of metabolites from biological matrices. However, this is still dominated by the use of preparative chromatography, which has advanced through the use of mass-directed detection. Detection and structural elucidation by mass spectrometry have improved markedly with increases in sensitivity, allowing lower abundance metabolites to be detected, and increases in selectivity, with the use of high-resolution time-of-flight and quadrupole-time-of-flight instruments. Finally, higher field strength magnets coupled with novel probe designs and increased use of liquid chromatographic hyphenation techniques continue to drive the capabilities of nuclear magnetic resonance spectroscopy as the definitive structural elucidation tool.