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4348 Journal of Medicinal Chemistry, 2004, Vol. 47, No. 18 Perspectivematerial required to obtain a 1D carbon spectra in areasonable acquisition time has dropped from more thana hundred milligrams to submilligram quantities overthe past few years. Probe design has gone through ametamorphosis, and a wide selection of probes is nowavailable for various applications. Small-volume probes,such as high-resolution magic angle spinning nanoprobes,95 flow probes for LC NMR (400-100 µL), andcapillary probes (10-25 µL), 96-99 have made a tremendousimpact on metabolite and impurity identificationbecause of their high signal to noise performance. Therecent introduction of cryoprobes promises a further 3-to 4-fold boost in signal to noise ratios. 97,100-102A prelude to the advances in probe design was thedevelopment of solvent suppression pulse sequences. 103Solvent suppression sequences have been automated foreffortless incorporation into any NMR experiment. Thecombination of the gain in sensitivity due to probedesign and the ease of solvent suppression is the maindriver for the improvements in the identification ofsmall quantities of metabolites and in the developmentof methods that couple NMR with other analyticaltechniques such as liquid chromatography (LC/NMR)and mass spectrometry (LC/NMR/MS). 104-109 As a result,LC/NMR in combination with mass spectrometryhas become a major contributor of metabolite identification.Flow or direct injection NMR was introduced in themid 1990s for 96-well plate analyses. Shortly after that,flow-probe NMR technology was integrated with LC andMS as necessitated by the need for structural characterizationof low-level metabolites. This integrationallowed the determination of the exact molecular structureof metabolites in one HPLC injection. To increasethe sensitivity of LC/NMR further, an automated solidphaseextraction has been incorporated into an LCNMR system. 110-112 Recently, with the introduction ofcapillary probes, capillary electrophoresis-NMR is alsobecoming a reality. 98One-dimensional proton LC/NMR often providesenough information about the structure of metabolites.Various 2D NMR experiments can be used to confirmthe structure or for identification of more complexstructure. 92,93In summary, advances in NMR instrumentation andtechniques have made the characterization of metabolitestructures a routine part of the drug discovery anddevelopment process.ConclusionsThe numerous examples discussed above demonstratethat biological transformation can be a useful approachfor discovering drugs. It is therefore useful to screendrug candidates for possible conversion to active metabolitesduring the discovery stage. Active metabolitesmay show superior pharmacology, pharmacokinetics,and safety profiles in comparison to their parent molecules,and the inherent benefits that metabolites oftendisplay make their study a worthwhile endeavor. Asdiscussed above, a number of active metabolites ofmarketed drugs have already been developed as drugsin their own right. Also discussed in this paper areseveral examples where active metabolites were identifiedduring the discovery phase and either became leadcompounds or led to the structural alterations of thechemotype. Consequently, the discovery of drugs throughbiological transformation can be an integral part of thedrug discovery process and it may therefore be beneficialto have a process in place to screen drug candidates insearch of active metabolites.There are a number of experimental observationsduring the preclinical pharmacokinetic and pharmacodynamicevaluations of drug candidates that can triggerthe search for conversion of drug candidates to activemetabolites. Approaches such as the rapid bioassayguided method described herein can be used to generateand detect active metabolites. Different biological methodsare now available for production of active metabolites.Large-scale synthesis of metabolites by biologicalmethods is now feasible because of the recent advancesin biotechnology, particularly in the area of CYP bioreactorsand microbial methods. Recent advances in LC/MS and LC/NMR technologies have also expedited thestructural characterization of metabolites with highsensitivity, specificity, and speed. These advances havemade the routine identification and evaluation of metabolitesa reality.Acknowledgment. We gratefully acknowledge Dr.David Rodrigues for his critical review of this manuscript.BiographiesAberra Fura received his Ph.D. in Chemistry in 1992 fromCornell University. After 2 years of postdoctoral fellowship atthe Department of Chemistry at University of California,Berkeley, he moved to Cytomed inc., a biopharmaceuticalcompany in Cambridge, MA, where he began his career in drugmetabolism, pharmacokinetics, and bioanalytical sciences. Hemoved to Bristol-Myers Squibb, Pharmaceutical ResearchInstitute in 1997, where he is currently a Principal Scientistin the Metabolism and Pharmacokinetics Department. Hisinterest is in the application of drug metabolism and pharmacokineticsto drug discovery, in vitro and in vivo correlations,and LC/MS and new technology in drug metabolism. Heis a co-inventor of several patents in the drug design areabased on biological transformation methods, active metabolites,and structure-metabolism relationship.Yue-Zhong Shu received his Ph.D. degree in PharmaceuticalScience from Toyama Medical and Pharmaceutical University(Japan) in 1988 on the chemistry and biotransformationof bioactive natural products. He went on for postdoctoralappointments at University of Saskatchewan and VirginiaTech on the metabolism and synthesis of central nervoussystem drugs and naturally occurring heterocyclic mutagens.He started his industrial career at the Drug MetabolismDepartment of Pfizer Central Research Division in 1991. Later,he joined Bristol-Myers Squibb, Pharmaceutical ResearchInstitute, where he is currently Associate Director of DiscoveryBiotransformation. His research is centered around the mammalianand microbial metabolism of synthetic and naturalproduct drug candidates through application of bioorganic,biochemical, and bioanalyitcal methods, aiming at addressingpotential safety and efficacy issues related to drug metabolism.Mingshe Zhu received his B.S. degree in Biochemistry andhis M.S. degree in Chemistry. He received his Ph.D. degreein Analytical Toxicology from SUNY at Albany in 1994. Hecompleted 2 years of postdoctoral training in drug metabolismwith Professor Sidney Nelson at the University of Washington.He is currently a Senior Research Investigator at Bristol-Myers Squibb, Pharmaceutical Research Institute and hasbeen involved in the drug metabolism studies to support drugdevelopment and registration. His current main responsibilityis the development and implementation of new technologies