Human studies quantitative drug metabolism

Steven R. Tannenbaum, a member of the Institute of Medicine, has a Ph.D. in food science and technology from the Massachusetts Institute of Technology, where he is currently the codirector and Underwood-Prescott Professor, Division of Bioengineering and Environmental Health, and professor of chemistry, Department of Chemistry. Dr. Tannenbaum s research interests include the chemistry and pathophysiology of nitric oxide, the quantitative measurement of human exposure to carcinogens, and tissue-based microsensors for toxin detection and drug metabolism. He has been a member of the NRC Board on Environmental Studies and Toxicology and has served on several NRC committees. 

Cryopreserved hepatocytes in suspension were successfully applied in short-term metabolism studies and as metabolizing system in mutagenicity assays (Hengstler 2000), providing qualitative metabolic information and quantitative pharmacokinetic parameters from key animal species and human at the early stage of drug discovery and drug development. 

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. 

Hydrolytic Lability. Hydrolytic lability is also of considerable relevance therefore, a quantitative structure-metabolism relationship (QSMR) study was undertaken recently to identify a structure-based QSMR equation that is not limited to congener series (241). An equation accounting for 80% of the variability of the in vitro human blood logt, of seven different drug classes containing a total of 67... 

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. 

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). 

Dapsone (4,4 -diaminodiphenylsulfone) has been widely used for phenotyping with respect to acetylation by NAT-2 however, the drug is also N-hydroxylated. Formation of the hydroxylamine metabolite by human liver microsomes was found to be selectively mediated by CYP3A (286) this led to the development of a zero- to eight-hour urinary metabolic recovery ratio approach [dapsone hydroxylamine (dapsone + dapsone hydroxylamine)] to quantitatively assess this pathway of metabolism (287,288). Subsequently, the trait measure has been applied as part of a cocktail approach (35) in a number of studies investigating the putative role of CYP3A as a risk factor in cancer (289-291) and other disease states (288,292,293). 

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