Accurate mass measurement Metabolism

A slightly different use of accurate mass measurement in drug metabolism studies has been reported [31]. In this investigation, the accurate masses of the ions derived from Glyburide (Figure 5.48) and some of its metabolites were used to calculate the difference in mass, and thus the elemental composition, between certain of the ions observed in the spectra of the various compounds. 

A similar approach using accurate mass measurements and predictive fragmentation sofiware was also applied for the examination of the human microsomal metabolism of nefazodone using a linear ion trap-orbitrap hybrid mass spectrometer. Based on a single LC-MS run, using data-dependant acquisition, 15 metabolites of nefazodone could be identified in MS and MS/MS with a mass accuracy better than 3 ppm. 

In many areas of science (chemistry, biochemistry, etc.), and particularly in drug metabolism studies, there is a need to determine the molecular formula of metabolites of interest. This is where the use of MS for accurate mass measurement has transformed the way in which scientists work. Today, it is very easy to obtain an exact mass measurement of a known or unknown substance. 

Similar experiments have previously been performed separately using a triple-quadrupole or an ion trap mass spectrometer for quantitation followed by accurate mass measurements with a Q-TOF mass spectrometer. With the introduction of the LTQ-FTMS instruments, all the data can be gathered quickly and easily with one experimental setup, and the variations in observed metabolic profiles introduced by... 

Peterman, S. M., Duczak, N., Jr., Kalgutkar, A. S., Lame, M. E., and Soglia, J. R. (2006). Application of a linear ion trap/orbitrap mass spectrometer in metabohte characterization studies Examination of the human liver microsomal metabolism of the non-tricyclic antidepressant nefazodone using data-dependent accurate mass measurements. J. Am. Soc. Mass Spectrom. 17 363-375. 

A. Deroussent, M. Re, H. Hoellinger, E. Vanquelef, O. Duval, M. Sonnier, T. Cresteil, In vitro metabolism of ethoxidine by human CYPlAl and rat microsomes identification of metabolites by LC—ESI-MS—AdS and accurate mass measurements by TOF-MS, Rapid Commun. Mass Spectrom., 18 (2004) 474. 

The newer MS experiments in a data-dependent acquisition mode provide the MS and MS" data from a single injection. Accurate mass measurements, software-assisted data acquisition, and processing methods have been very useful for metabolite detection and identification. In addition, when MS is combined with other analytical techniques such as derivatization, H/D exchange, and stable isotope labeling have been proven very useful for structural characterization of unusual, uncommon, and difficult metabolites. Further, the flexibility and broad applications of mass spectrometry have allowed for the creation of hybrid instruments and coupling to other powerful analytical techniques, most notably nuclear magnetic resonance (NMR), to further enhance the utility in the field of drug metabolism. 

The LTQ-FT mass spectrometer was introduced in late 2003 and, as expected, the main application discussed in the literature is for the analysis of proteins and peptides (Johnson et al., 2004 Syka et al., 2004). A recent book chapter (van der Greef et al., 2004) and a review article (Brown et al., 2005) discussed the application of the LTQ-FT to metabolomics. FTMS applications to dmg metabolism are still very new and dmg discovery research laboratories which have recently purchased the instmment are still in the process of developing and validating methods and approaches. A recent publication describes the depth and flexibility of the experimental setup utilizing accurate mass data-dependent exclusion MS" measurements with a LTQ-FT (Tozuka et al., 2005). We have reported several integrated approaches for determination of metabolic stability, characterization of metabolites and metabolic... 

The quantitative abilities of the LC-TOF, although limited in the linear dynamic range, often allow measurement of metabolic profiles and PK profiles from the same samples. Zhang and co-workers [37] provide such an example. Quantitative results for a co-administration of five compounds to rats, with assay limits of quantitation (LOQ) between 1 and 5 ng/mL, and an upper limit of quantitation (ULQ) at 100 ng/mL were described. In this study, precision and accuracy were better than 20% for all five analytes, and comparable to triple-quadrupole quantitative data. More importantly, in addition to following the levels of dosed compound, the authors were able to identify several metabolites from the same full-scan data using the accurate mass capabilities ofthe LC/TOF-MS. 

Accurate, precise isotope ratio measurements are important in a wide variety of applications, including dating, examination of environmental samples, and studies on drug metabolism. The degree of accuracy and precision required necessitates the use of special isotope mass spectrometers, which mostly use thermal ionization or inductively coupled plasma ionization, often together with multiple ion collectors. 

Special isotope ratio mass spectrometers are needed to measure the small variations, which are too small to be read off from a spectrum obtained on a routine mass spectrometer. Ratios of isotopes measured very accurately (usually as 0/00, i.e., as parts per 1000 [mil] rather than parts per 100 [percent]) give information on, for example, reaction mechanisms, dating of historic samples, or testing for drugs in metabolic systems. Such uses are illustrated in the main text. 

Accurate, precise isotope ratio measurements are used in a variety of applications including dating of artifacts or rocks, studies on drug metabolism, and investigations of environmental issues. Special mass spectrometers are needed for such accuracy and precision. 

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