Whole Zebrafish Cytochrome P450 Assay for Assessing Drug Metabolism and Safety - Zebrafish: Methods for Assessing Drug Safety and Toxicity - page 100

9.2 BACKGROUND AND SIGNIFICANCE

9.2.1 CYPs Are Responsible for Drug Metabolism and
Drug Toxicity

Many clinically relevant drug-drug interactions are associated with inhibition or
induction of specific CYP enzymes (Gomez-Lechon et al., 2003; Daly, 2004; Luo et al.,
2004; Vermeir et al., 2005) and modification of CYP activity can profoundly affect
therapeutic efficacy leading to life-threatening toxicity. The U.S. Food and Drug
Administration (FDA) now requires CYP assessment prior to drug approval and it has
published industry guidance for performing studies to assess drug-drug interaction. In
well-described examples, a first drug inhibits a CYP enzyme that metabolizes a
second coadministered drug, slows clearance of the second drug, and causes toxic
accumulation. CYP-dependent drug-drug interaction can also occur when a first
drug induces CYP gene expression, leading to increased CYP enzyme activity,
accelerated clearance, and reduced drug efficacy (Wienkers, 2001; Yueh et al., 2005).
CYP3A4, CYP2D6, and CYP2C9, the most abundant hepatic P450 enzymes,
metabolize more than 70% of frequently prescribed drugs and safety profiling usually
focuses on these enzymes (Davidson, 2000; Gomez-Lechon et al., 2003). Examples of
costly, high-profile drug withdrawals due to CYP-related drug-drug interactions
include terfenadine and astemizole (antihistamines), mibefradil (antihypertensive),
and cerivastatin (statin). Although additional problematic drugs, including cimetidine
(antacid), are still commercially available, due to complications fromCYP-dependent
drug-drug interactions, many of these are rapidly losing market share.

9.2.2 Similarity of Drug Metabolism and CYPs in
Humans and Zebrafish

As a general defense against toxic chemicals, zebrafish exhibit mammalian-equiv-
alent mechanisms, including induction of xenobiotic enzymes and increase in reactive
oxygen species (ROS) (Wiegand et al., 2000; Dong et al., 2001; Carney et al., 2004).
Several zebrafish CYP genes, including CYP3A65, CYP1A1, A19, B19, 2K6, 3C1,
2J1, and CYP26D1, which have human homology, exhibit similar catalyzing reac-
tions (Miranda et al., 1993; Collodi et al., 1994; Trant et al., 2001; Bresolin et al.,
2005; Gu et al., 2005; Tseng et al., 2005; Wang-Buhler et al., 2005; Corley-Smith
et al., 2006; Rubinstein, 2006; Wang et al., 2007). In our recent studies, we confirmed
that CYP1A1, 4A4, A19, and B19 are expressed in zebrafish (Parng et al., 2002) and
we determined that, similar to effects in mice and humans, drug toxicity is dose
responsive (Semino et al., 2002). A human CYP3A ortholog, designated CYP3A65,
is expressed in zebrafish liver and intestine. Similar to the response of CYP3A4 to
these drugs in humans, CYP3A65 is upregulated in zebrafish by both rifampicin
and dexamethasone. TCDD, the environmental toxin, also upregulates zebrafish
CYP3A65 gene expression (Tseng et al., 2005). Similar to results in humans
(Bertilsson et al., 1998), another study found that pregnelone-16 a -carbonitrile, a