Page 182 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

Toxicological Testing: In Vivo and In Vitro Models Chapter | 9 149

VetBooks.ir duration of a toxic compound administration and the of the above approaches are suggested to be used individ-
ually to predict genotoxicity. Negative or positive results
observation period.
should always be confirmed first by in vitro or nonmam-
The albino rabbit has been until recently the animal
model of choice because of the high permeability and sen- malian mutagenicity tests.
sitivity to toxic agents exhibited by its skin, which some-
times led to overprediction showing little relevance to
Carcinogenicity Tests
human irritation (Auletta, 2004). Currently, albino rats
and occasionally guinea pigs are considered preferable In vivo assays of carcinogenicity examine the possibility
species to assess local irritation. Traditionally, guinea that a tested agent might cause tumors and other chemi-
pigs and mice have been used also to perform sensitiza- cally related effects in one or more animal species.
tion tests (EPA, 2003; OECD, 2010). On the other hand, Currently available is the rodent carcinogenicity test
long-term cutaneous toxicity studies usually require the (OECD, 2009a). This test runs for up to 2 years and
use of a rodent (albino rat and mouse) as well as a nonro- involves the use of three different concentrations, one
dent model. Minipigs have been proven to be a reliable administration route and both sexes. The results are based
nonrodent species because their skin demonstrates many on clinical chemistry, gross and histopathological analysis
physiological similarities to those of humans and pigs of more than 40 tissues and organs in order to determine
(Auletta, 2004). Other nonrodent models are dog and non- the site, the number and type of tumors (OECD, 2009a,b).
human primates, commonly used to test the metabolism One serious disadvantage of using mice in carcinoge-
of toxic agents and the safety of newly designed recombi- nicity testing is their tendency to present high incidence
nant pharmaceutical products (Vail et al., 1998; deBlois of spontaneous liver or lung tumors in some strains, lead-
and Horlick, 2001). ing occasionally to inconclusive results (Gad, 1998).
Furthermore, because of the long duration of carcinoge-
nicity tests and therefore the old age of laboratory
Genotoxicity Testing
animals, it means that the natural occurrence of tumors
The aim of genotoxicity testing is to detect gene damage increases, making it difficult to distinguish between a
induced by the test compound, by measuring chromosome real treatment and a background carcinogenic effect.
aberration and breakage, point mutation, and other DNA In addition, the most common mechanisms of tumor
and chromosomal effects in vivo. The host-mediated development in rodents may not be relevant to human
assay is based on the inoculation of a microorganism into carcinogenicity, posing limitations to any attempt to use
a rodent such as a mouse, rat or hamster and a subsequent in vivo data for carcinogen risk assessment.
assessment of the point mutations found in the microor- Tumorigenic tests using dogs are limited because of
ganisms, after certain treatment of the rodents with the the high cost and the long duration of the studies (up to 7
potential mutagen (Gabridge and Legator, 1969; Dhillon years). Furthermore, dogs demonstrate high susceptibility
et al., 1995; Loomis and Hayes, 1996). On the other hand, to aromatic amines and therefore care should be taken
to identify chromosome breakage, male rodents are when using these animals for the evaluation of potential
treated with the test compounds and, after mating with carcinogenic compounds of this category (Loomis and
untreated females, fetal mortality and survival are Hayes, 1996).
recorded (OECD, 1984; EPA, 1998c).
The mouse spot test is capable of detecting somatic
Neurotoxicity
gene mutations and is based on in utero exposure to the
tested chemical. This in vivo mutation test works by mon- The neurotoxic potential of chemical compounds can be
itoring the appearance of colored spots in the coat of the assessed by determining relevant effects on the autonomic
animal that may appear due to an altered or lost specific or central nervous system (CNS), not only in adult but
wild-type allele in a pigment precursor cell (EPA, 1998h). also in developing animals. Clinical signs including
A popular in vivo test to identify genetic risks is the changes in behavior (e.g., movement, motor coordination
rodent bone marrow micronucleus test, although it is not or reflexes, paralysis, tremor, learning, and memory),
the most sensitive test and it does not improve predictivity neurochemical (e.g., activity of enzymes associated with
of rodent genotoxicity when combined with in vitro tests neuropathies, cell signaling pathways, synthesis, release
(OECD, 1997a; Zeiger, 1998). However, this in vivo test and uptake of neurotransmitters), neurophysiological
is widely used because it is relatively easy to perform. (e.g., electroencephalography and nerve conduction veloc-
Another in vivo test that has been validated and recom- ity) and neuroanatomical effects are commonly explored.
mended by the Organisation for Economic Cooperation Rodents are the most frequently used animal models
and Development (OECD, 1997b) is the rat liver unsched- and there are a considerable number of available guide-
uled DNA synthesis test. It is worth mentioning that none lines to examine the neurotoxicity of xenobiotics on them