ABSTRACT
II conjugation pathways (e.g., zidovudine) and therefore would be metabolically stable when incubated with microsomes. In hepatocytes, however, the metabolic stability of a drug would be a factor of phase I oxidation, phase II conjugation, or other pathway (e.g., esterase activity). Freshly isolated and cryopreserved human hepatocytes are found to be competent in the metabolism of xenobiotics via P450 pathways, non-P450 oxidative pathways, glucuronidation, sulfation, O-methylation, and N-acetylation (Li et al., 1999a, b; Song et al., 1998). Therefore, metabolic stability values obtained with hepatocytes should be more relevant than those obtained using liver microsomes. The major limitation in the use of hepatocytes is the limited extent of metabolism. The assay is similar to that for microsomes except that cofactors are not required and that the test compound concentration is lower (5 µM) than that used for microsomes to compensate for the lower amount of metabolizing enzyme activity. A typical screening assay for metabolic stability in human hepatocytes is shown in Table 3. For new chemicals with unknown metabolic pathways, hepatocytes represent the most appropriate system for metabolic stability evaluation. The major reason is that in hepatocytes all the drugmetabolizing enzyme pathways are present, whereas in microsomes only the oxidative pathways are present under normal experimental conditions (with NADPH as cofactor). The use of microsomes to screen for metabolic stability toward oxidative metabolism will generate results that may or may not be relevant to humans in vivo. A compound may not be oxidized in vivo but would be oxidized in vitro by microsomes. An example of this is our recent study
