In Vivo Probes for Studying Induction and Inhibition of Cytochrome P450 Enzymes in Humans
The discovery of cytochrome P450 (EC18.104.22.168, CYP) in the early 1960s resulted in an explosion of knowledge about the monooxygenase system that still continues today. However, even before this critical event, it was apparent that the oxidative metabolism of drugs often exhibits large interindividual variability; moreover, drug-metabolizing activity may be modulated by environmental, pathophysiological, and genetic factors (1). Research during the subsequent four decades has largely focused on determining the mechanisms involved in such variability and, in the case of drug metabolism in humans, its clinical significance and importance. In certain situations, genotyping with respect to the presence of allelic variants can be of some value in accounting for this interindividual variability, especially, if a strong genetic determinant is involved (2,3). However, even when genetic polymorphism is present, considerable variability is often present within a phenotypic group (2); moreover, genotyping cannot take into account the modulation of catalytic activity by environmental and disease-state factors. In vitro approaches using tissue preparations, e.g., liver microsomes and recombinant expressed enzymes, have considerable merit in this
regard (see Chaps. 2, 3, and 7). However, the application of such invasive procedures to the clinical situation is obviously limited, especially when studying healthy subjects. Accordingly, so-called "noninvasive" procedures, utilizing readily available fluids, such as plasma and saliva, or excretions, such as urine and expired air, form the basis for measuring in vivo metabolizing ability. These measures are generally applied to two related types of experimental questions: What is the basal level of catalytic activity in an individual subject, i.e., phenotyping? What are the determinants of interindividual variability within or between populations, e.g., the effects of drug and environmental interactions, genetics, and disease states? The use of "model" compounds or, as currently termed, in vivo probes, has been extensively applied for these purposes since its conception some 30 years ago (4). This chapter considers the rationale, development, validation, and application of currently useful in vivo probes to assess the catalytic activity of specific human CYP isoforms in individual subjects.