Metabolism studies matrix effects

With these innovative sample introduction techniques, 96 samples can be analyzed in less than 20 min, which is compared to >2.5 h for traditional LC-MS/ MS. However, as in the case for the BioTrove s RapidFire system where no LC separation is involved, both techniques rely solely on the specificity of MS/ MS to differentiate different compounds. To take full advantage of FlashQuant and LDTD MS on complex system (e.g., hepatocyte incubation assay or in vivo studies), matrix effects, isobaric interferences, metabolic interferences, and MS cross-talk need to be carefully investigated prior to applying those techniques in samples analysis. 

Mei, H. Matrix effects causes and solutions, in Using Mass Spectrometry for Drug Metabolism Studies, CRC Press, Boca Raton, 2005, pp 103-150. 

Because the catabolic and metabolic pathways of biotech drugs are often poorly defined and sufficiently sensitive comparator assays are lacking, additional matrix effect tests by parallelism should be conducted with actual study samples. These are often performed on subject samples with aberrant PK profiles. A pool from several time points with sufficient analyte concentration of that subject is serially diluted. The observed concentration times the dilution factors should be within... 

Little JL, Wempe MF, Buchanan CM (2006) Liquid chromatography-mass spectrometry/mass spectrometry method development for drug metabolism studies examining lipid matrix ionization effects in plasma. J Chromatogr B 833 219-230... 

Organotypic models, available for all organs involved in ADME, are used to study food matrix effects, intestinal metabolism/stability, and regional differences in permeability. The latter is of particular importance since it has been shown that permeability to various marker molecules varies along the intestinal tract in general, permeability decreases in the order jejunum > ileum > colon [54]. The half-lives of organotypic models limits the duration of possible studies to 1-3 hours. 

While effective doses of 1 to 2 mg seem small, the total body pool of /3C can be estimated at 15 to 20 mg, with a total plasma pool of ateut 0.5 to 1 mg. In comparison with these pool sizes, a 2-mg effective dose is relatively large. In addition, the average daily effective dose of jSC from food sources is likely to be less than 1 mg, since matrix effects probably impair bioavailability of /3C to a larger extent than with supplements such as that used by van Vliet et al (1995). The minimum effective dose required to yield a significant TGR fraction response, taking into account the error associated with measurement, has not been determined, but is not likely to be much less than 1 mg. Consequently, use of unlabeled jSC, even coupled with use of the TGR fraction, is probably not well suited to study /3C uptake and metabolism at effective doses typically derived from dietary sources. 

A rather new technique is APPI, which has only been applied in two studies to determine PFOS [57] in river waters and FTOH and sulfonamido derivatives in biotic samples [58]. APPI is a very selective tool and, in stark contrast to ESI, is considered to be virtually imperceptible to matrix effects, which was confirmed in both studies. APCI and especially APPI are not recommended for metabolism studies of unknown compounds, since ionization is very delicate with these methods. Therefore, unknown compounds may not be discovered due to a lack of ionizability. ESI is the method of choice due to the wide range of ionizable compounds after LC separations. 

The nature and timing of DOM inputs influence both community organization and metabolism (see Chapter 15), which in turn affect DOM concentration and export at the ecosystem scale. The relationship of source heterogeneity to system function is one of the least studied aspects of DOM dynamics. For planktonic communities, source effects are direct and immediate. Attached communities also respond and adapt to local DOM inputs, but the capacity of biofilms to self-organize within a secreted polymer matrix... 

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