Page 13 - 73_01

VOL. 73 (1), 5-26, 2007 IN VITRO INVESTIGATION OF DRUG METABOLISM AND TOXICITY...

compared to those of untreated cells (Figure 3). Different
concentrations of the drug, covering a large range of concentrations,
are recommended to assess the inductive potential of a new chemical.

Generally, if a negative results from in vitro inhibition/induction
assays is obtained (no identified interaction), the study of potential
clinical interactions is not needed. In vitro experiments should
be conducted at similar concentrations to the relevant in vivo
concentration for the optimization of the clinical assays.

FIGURE 3. Effects of model inducers on P450 enzymes in cultured human
hepatocytes. After 24 h in culture, human hepatocytes were exposed to 2 µM
3-methylcholanthrene (MC), 1 mM phenobarbital (PB), 100 mM ethanol (ETH),
1 mM clofibric acid (CLF), 50 µM rifampicin (RIF), 1 µM dexamethasone (DEX),
50 µM omeprazole (OMP), or 1 nM ecteinascidin (ECT), and P450 activities

and mRNA levels were measured 48 h later. Activities were determined in
three different hepatocyte preparations by the use of selective substrates:
7-methoxyresorufin (CYP1A2) and testosterone (CYP3A4). Specific CYP mRNA
levels were quantified by quantitative RT-PCR in two different cultures.
Results are expressed as fold increases over corresponding activity or mRNA

values in controls (untreated cells).

15