studies in human liver microsomes as a better representative of thein vivo
situation.
Clinical inhibition potential can be predicted based on inhibition type andin
vitroinhibition data with various degree of success as described in Chapter 5
(Bjornsson et al., 2003; Zhang and Wong, 2005; Zhou et al., 2004, 2005).
9.5.4 Evaluation of CYP induction
Induction of enzyme refers to increase in an enzyme activity through over
expression of the protein. Induction of drug-metabolizing enzyme by
administration of a xenobiotic occurs through multiple mechanisms.
Increasing gene transcription by modulating the activity of the transcription
factors is the most common mechanism by which enzyme inducers cause
induction. Induction of CYP enzymes has potential for clinical drug–drug
interaction (Luo et al., 2004) and FDA guidelines require that drugs in
development be tested for their induction potential of CYP P450 enzymes (US
Food and Drug Administration). There are number of different ways in which
enzyme induction potential can be tested. Since induction of CYP3A4 is
considered most relevant and common for drug–drug interaction, in the drug
discovery stage high throughput reporter-based assays are used for screening
compounds for their induction potential of CYP3A4 (see Chapter 6) (Sinz
et al., 2006). FDA considers data from cell-based system more relevant toin
vivosituation. An FDA accepted method is to evaluate the enzyme induction
potential of a compound in primary hepatocytes from three individual donors
after treatment with the drug of interest for 3 days (see Chapter 17 for details).
This is compared against known inducers as a positive control and with solvent
as a negative control. A greater than twofold increase in probe enzyme activity
compared to solvent control or40% of activity compared to positive control
is considered an enzyme inducer. The rationale for selection of various
concentrations of a drug in the induction assay is similar to the one used for
CYP inhibition. Typically, at least two indicators of enzyme induction are
looked at: namely, enzyme activity toward known substrate and mRNA levels
or protein expression by Western immunoblotting. It is important to look at
two indicators of enzyme induction since an inducer could also be an inhibitor
of enzyme activity and in the absence of mRNA or protein expression data this
could be misleading. Ritanovir, for example, is both an inhibitor and an
inducer of CYP3A4, and in primary cultured human hepatocyte measured for
only CYP3A4 activity, incubation with this compound would lead to an
erroneous conclusion, that is, no induction (Luo et al., 2002).
9.6 EXAMPLES OF ROLE OF DRUG METABOLISM
TO ADDRESS SAFETY ISSUES
When metabolites are involved in eliciting toxicity in animal species it is
important to establish the relevance of these findings to humans. If a
278 ROLE OF DRUG METABOLISM IN DRUG DEVELOPMENT