Drug metabolism studies structural elucidation

One approach to drug metabolism studies is therefore to predict the molecular weights of possible metabolites of the drug under consideration, to use reconstructed ion chromatograms to locate any components that have the appropriate molecular weights and then use MS-MS to effect fragmentation of the (M - - H)+ ions from these metabolites, and then to finally link the m jz values of the ions observed with ions of known structure from the parent drug or from other metabolites whose structures have been elucidated. 

Nassar, A. E., and Lee, D. Y. (2007b). Novel radio-HPLC detector for sensitive metabolite profiling and structural elucidation in support of drug metabolism studies. Drug Metab. Rev. 39(Suppl. 1) 73. 

Frequently, during organic synthesis or drug metabolism studies, unknown compounds are isolated and their structure must be confirmed or, if they are totally unknown, solved using a combination of analytical techniques. Exercises of this type use qualitative spectroscopy rather than the quantitative aspects discussed so far. Whole university courses and many textbooks and online tutorials have been produced to cover elucidation of chemical structure, and a detailed treatment of the subject is beyond the scope of this book. However, a brief overview of the major techniques and their limitations may be useful. 

Applications of radioisotopes to studies of ADME have been eomprehen-sively reviewed recently (Dalvie, 2000 Marathe et ah, 2004 Veltkamp, 1990). This chapter focuses on the recent advance in liquid radiochromatographic techniques and their application to metabolite detection and quantification in drug metabolism studies. In addition, the use of liquid radiochromatography coupled with mass spectrometry in metabolite profiling and structural elucidation is described. 

Although LC/MS/MS is the most widely used analytical technique for drug metabolism studies, this analytical platform may not provide complete molecular structure information in certain circumstances. As a complementary analytical tool, offline or online nulear magnetic resonance (NMR) is usually able to elucidate the precise structures of drug metabolites (Martinez-Granados et al., 2006 Lenz et al., 2007 Trivedi et al., 2009). NMR techniques are much less... 

G. S. Walker and T. N. O Connell, Comparison of LC-NMR and Conventional NMR for Structure Elucidation in Drug Metabolism Studies , Expert Opin. Drug. Met. Toxicol., 2008, 4, 1295. 

Nuclear magnetic resonance (NMR) spectroscopy in pharmaceutical research has been used primarily in a classical, organic chemistry framework. Typical studies have included (1) the structure elucidation of compounds [1,2], (2) investigating chirality of drug substances [3,4], (3) the determination of cellular metabolism [5,6], and (4) protein studies [7-9], to name but a few. From the development perspective, NMR is traditionally used again for structure elucidation, but also for analytical applications [10]. In each case, solution-phase NMR has been utilized. It seems ironic that although —90% of the pharmaceutical products on the market exist in the solid form, solid state NMR is in its infancy as applied to pharmaceutical problem solving and methods development. 

The degree of exposure of the fetus to a particular substance can be best assessed in human subjects, but concerns of fetal safety have restricted the use of this approach. Moreover, clinical studies cannot elucidate the various mechanisms that contribute to transplacental transport of a particular compound. There are many structural differences between the human placenta and the placenta of other mammalian species, which complicates extrapolation of data obtained from in vivo animal models to humans [7], Thus, several ex vivo and in vitro techniques have been developed to study the placental role in drug transfer and metabolism during pregnancy and there are some excellent articles that discuss these systems in detail [7], Both isolated tissues and various cell culture techniques are currently in use and these have been summarized below. 

As liquid chromatography plays a dominant role in chemical separations, advancements in the field of LC-NMR and the availability of commercial LC-NMR instrumentation in several formats has contributed to the widespread acceptance of hyphenated NMR techniques. The different methods for sampling and data acquisition, as well as selected applications will be discussed in this section. LC-NMR has found a wide range of applications including structure elucidation of natural products, studies of drug metabolism, transformation of environmental contaminants, structure determination of pharmaceutical impurities, and analysis of biofiuids such as urine and blood plasma. Readers interested in an in-depth treatment of this topic are referred to the recent book on this subject [25]. 

This review is intended to survey those uses of stable isotopes which are of particular importance in medicinal chemistry. These include their use in 1) structure elucidation, 2) studies of drug metabolism, 3) pharmacokinetic analysis and 4) drugs used in therapy. The use of stable isotopes for investigating intermediary metabolism, and their rapidly expanding use in clinical chemistry will not be reviewed since these topics have been covered elsewhere. Synthesis and biosynthesis with stable isotopes have also been previously surveyed >10 and many articles appear in each issue of the Journal of Labelled Compounds and a few have appeared in Biomedical Mass Spectrometry in the past year. In addition, one recent article has appeared on the use of plants to continously produce stable isotope labeled compounds of pharmacological interest. ... 

Metabolism studies are an integral part of drug discovery and development, and LC—MS/MS has now become the most powerful tool for the rapid detection and structure elucidation of drug metabolites from biological fluids. 

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