Hepatotoxicity Testing in Larval Zebrafish - Zebrafish: Methods for Assessing Drug Safety and Toxicity - page 89

United States (Ostapowicz et al., 2002). It is also the most common adverse drug
response that leads to the failure of otherwise promising drug candidates during
preclinical or clinical development and also the withdrawal or restriction of pre-
scription drugs use after initial approval (Navarro and Senior, 2006; Abboud and
Kaplowitz, 2007). Idiosyncratic DILI induced by a single drug is a rare event
occurring in less than 1 per 10,000-100,000 of subjects who take the medication
and as such the risk factors and pathogenesis are poorly understood (Navarro and
Senior, 2006; Abboud andKaplowitz, 2007; Uetrecht, 2007). Likewise, the symptoms
exhibited by DILI sufferers can be diverse and unpredictable: acute liver disease,
overt acute or subacute liver failure, prolonged jaundice and disability, or alternatively
predominantly asymptomatic. A necessity therefore exists to devise ways to help
identify these compounds.
Conventional cytotoxicity assays have less than 25% sensitivity for the detection
of hepatotoxins (O'Brien et al., 2003) and even the newest high-content cell assays
still fail in detecting the more complex hepatotoxic mechanisms, such as cholestasis
and those of an idiosyncratic nature (O'Brien et al., 2006). Therefore, assessment of
organ toxicity frequently involves full histopathological assessment in rodents and
commonly requires the use of higher order species such as macaque monkeys or even
human subjects before their toxic liabilities can be fully ascertained. It can therefore
be concluded that the traditional approaches for identifying hepatotoxicants are
insufficient and new technologies and models need to be developed.
8.5 PHENOTYPIC-BASED LARVAL ZEBRAFISH
HEPATOTOXICITY SCREENS
Recently, the zebrafish has been evaluated as a potential model vertebrate to be used to
identify compounds with hepatotoxic liability early in the lead optimization stage of
drug discovery, a time when large compound scale-up and whole animal studies have
not commenced (Hill, 2008a, 2008b). This is possible because these larval studies can
be performed with single milligrams of compound in microtiter plates, essentially
allowing high-content in vivo information to be gathered in an in vitro format. In
addition, as a whole organism, the zebrafish larva should also be able to capture
toxicity associated with toxic metabolites that are unlikely to be synthesized in vitro
(Hill, 2008a). In this way, pharmaceutical companies should be able to advance lead
candidate drugs for further development with a low likelihood of toxicity after having
made only a small investment in compound manufacture and typical ADME in vitro
screens.
As described previously, one of the major advantages of this model is that the
zebrafish larvae are virtually transparent. Therefore, the intention of recent studies has
been to concentrate on creating a medium-throughput assay based on morphological
endpoints of toxicity that are assessable using transmitted light without the need for
dissection. When viewed dorsolaterally with a stereomicroscope, the liver of a 5-6dpf
larvae is situated posterior to the pericardium and predominantly anterior to the gut,
although they tend to overlap in this view and in transverse section liver tissue can