isoform that plays an important role in the metabolism of drugs and
xenobiotics in humans. In addition to the noninducible isoform CYP2D6,
which is capable of metabolizing approximately 25% of known prescription
drugs in humans, other inducible forms of CYP that are important in human
drug metabolism include CYP1A2, CYP2E1, members of the CYP2C family,
and CYP2B6. Limitations on the length of this chapter preclude an in-depth
discussion of the experimental approaches to investigating the potential
induction of all of these important CYP isoforms, or of the induction of
equally important human Phase II enzymes such as the UDP-glucuronosyl
transferases. This chapter will therefore focus on methods for routine screening
and initial determination of the potential of drugs or drug candidates to induce
what is arguably the most important CYP isoform, in terms of overall impact
on human therapeutics, namely, CYP3A4. It should be recognized that in the
case of CYP2C and CYP2B6, molecular pathways that mediate their induction
in humans are similar, but not identical to, those that mediate CYP3A4
induction (Eloranta et al., 2005; Pascussi et al., 2003). Therefore many of the
approaches that are used to examine the potential for CYP3A4 induction
might theoretically be adaptable to investigating potential induction of these
other isoforms.
In this chapter, three general approaches for screening for potential inducers
of CYP3A4 will be discussed, and specific methodologies will be described.
(1) Intact animals. In the past, prediction of the potential of a drug candidate
to induce CYP3A4 in humans was often based on animal studies. Studies using
common animal species are now known to be often poorly predictive of human
CYP3A4 induction, due to well documented species-dependent differences in
CYP3A induction between humans and common laboratory animal species.
More recently, transgenic ‘‘humanized’’ mice have been developed, which
express the human PXR receptor. These strains appear to provide a much more
accurate intact mouse model for predicting CYP3A4 induction in humans.
(2)In vitromodels.In vitromodels include primary human hepatocyte cultures,
immortalized and cryopreserved human hepatocytes, and the PXR reporter
gene assay. Primary human hepatocyte cultures provide an integrated model of
intact human liver, have the advantage of providing easily controlled exposure
to potential inducing agents. This model has major disadvantages of expense
and lack of ready availability. The PXR reporter gene assay, employing
cultured cells transfected with human signaling genes, appears to currently
provide the best balance between economy and efficiency in screening on one
hand, and accurate prediction of CYP3A4 induction in humans on the other.
These methods measure binding of putative inducers with human signaling
molecules in transfected cultured cells, and subsequent activation of gene
expression through pathways responsible for activation of CYP3A4 expres-
sion. These in vitro methods hold great promise for rapid, economical
screening, and accurate prediction of the potential of a drug candidate for
CYP3A4 induction in humans. (3) Direct measurement of induction in
humans. Potential inducing agents can be directly administered to volunteer
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