Plasma quantitative drug metabolism

What is the underlying cause for these interspecies differences For equal doses, differences in plasma AUC values simply indicate differences in total body clearance. Renal and metabolic elimination processes are the major contributors to total body clearance. When allometric scaling is used as described in Chapter 30, renal clearance tends to exhibit only small differences across species, whereas there are many examples of interspecies differences in metabolism. Further, across many drug categories, metabolism is quantitatively more important than is renal elimination. Therefore, more emphasis on inter species differences in drug metabolism could improve Phase I studies. The next two sections provide specific examples of the impact of monitoring metabolism during early human studies. 

Experts in drug metabolism carry out time-course studies with radioactive samples. They give a variety of doses and, at intervals, measure plasma radioactivity rather than therapeutic effect. Sensitive instruments aUow for quantitative measurements for example, of the time and concentration at which radioactivity reaches a maximum. To lay a basis for seiecting the animal species used for long-term toxicology studies, these pilot probes of duration may employ several mammalian species. Different species metabolize a single drug dissimilarly, so a suitable animal species handles it as the human one does. 

One of major tasks of drug metabolism in drug development is to determine the exposure of drug metabolites in humans and toxicological species, especially the assessment of whether human circulating metabolites are present in the plasma of toxicology species at appropriate levels. The task usually is accomplished by qualitative and quantitative profiling in human and animal... 

Table 15.1). As a multiple-task instrument, the QTRAP may serve as the LC—MS platform of choice in certain DMPK and bioanalysis laboratories. Specific examples of these laboratories would include (1) bioanalytical laboratories where detection of plasma metabolites or in vitro ADME screening are also performed (2) drug metabolism laboratories where metabolite identification and/or in vitro ADME screening are conducted, and (3) small bioanalysis and drug metabolism laboratories where quantitative and qualitative analyses of drugs and/or metabolites are routinely conducted with limited numbers of LC—MS instruments and scientists. 

Christensen, H., Baker, M., Tucker, G.T. and Rostami-Hodjegan, A. (2006) Prediction of plasma protein binding displacement and its implications for quantitative assessment of metabolic drug-drug interactions from in vitro data. Journal of Pharmaceutical Sciences, 95, 2778-2787. 

The in vitro approaches feature human liver microsome incubations that contain drug candidates at a range of concentrations that span the anticipated maximum steady state plasma levels. The microsomal incubations also contain a specific probe substrate where the concentration closely approximates that Km value for the reaction under investigation. Quantitative analysis of the specific marker metabolites and internal standards using MRM provides a simple assessment of the potential inhibitory effects drug candidates have on the metabolism of specific CYP probe substrates. 

The use of SRM methods for quantitative bioanalysis represents increased dimensions of mass spectrometry analysis. SRM methods that use APCI-LC/MS/MS for the quantitative analysis of an antipsychotic agent, clozapine, in human plasma were described by Dear and co-workers (Dear et al., 1998). Preclinical development studies of clozapine in rats and dogs used HPLC with fluorescence detection (FLD). With this method, a better limit of quantitation (LOQ) of 1 ng/mL was obtained. As the compound moved into the clinical stages of development, a more sensitive method of analysis was required to obtain rapid metabolic information in support of drug safety evaluation studies. A standard LC/MS/MS method is used for the quantitative analysis of clozapine (I) and four metabolites (II-V) in human plasma (Figure 6.34). 

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