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Toxicological Testing: In Vivo and In Vitro Models Chapter | 9 151

VetBooks.ir models that can be used in toxicity testing, mainly in IN VITRO MODELS OF TOXICITY TESTING
carcinogenicity and mutagenicity, as well as for the study
Introduction
of xenobiotic metabolism. These in vivo systems permit
the study of toxicological effects of tested compounds on In vitro cellular models are relatively inexpensive and
foreign genes of human or other origin that have been
easy to maintain and manipulate compared to animal
genetically transferred to produce transgenic animals, by
models. In vitro methods allow the study of direct cellular
transgenesis or targeted gene modification. From the point
effects of toxins on specific cell or tissue types in a con-
of view of toxicity testing, it is considered that such mod-
trolled environment.
els will lead to faster results that are more representative
However, the main disadvantage of in vitro systems
of human response to xenobiotics. In addition, transgenic
over animal models is the lack of systemic effects such as
mice can bring significant animal welfare benefits,
an appropriate balance and supply of growth factors and a
because they are able to reduce the group size needed in
system of xenobiotic metabolism and elimination of tox-
experiments and replace testing in other species, including
ins. The former can be at least partially addressed by add-
nonhuman primates. Finally, these models are also excel-
ing appropriate growth factors and the latter can be
lent experimental systems that are able to address and addressed by the use of metabolic activation systems such
answer specific mechanistic questions in toxicology more as the introduction of nicotinamide adenine dinucleotide
efficiently (Valancius-Mangel and Doetschman, 1999). phosphate hydrogen-activated microsomes or a hepatic
However, the use of “humanized” animal models in toxi- cell line in a cell culture fitted with a filter insert
cological studies is an expensive process because it (Fig. 9.1). The toxin is introduced into the insert and a
involves higher costs for their development and breeding. mixture of metabolized and nonmetabolized toxin (but
There is also concern that the human genome, once trans- not the microsomes or the hepatic cells) diffuses through
ferred to laboratory animals, may express the same pro- the filter into the growth medium containing the target
teins, but it does not guarantee that the protein will have cells. Another disadvantage is that many cellular systems
similar function as that found in humans. In addition, there lack the complexity of cell cell interactions in tissue,
is limited background information on transgenic animals, although this can be addressed to varying degrees using
making the interpretation of data in toxicological studies coculture systems, a three-dimensional cell culture, or a
difficult (Valancius-Mangel and Doetschman, 1999).
tissue slice/organ culture, which are discussed later.
The main transgenic genotoxicity systems used in tox-
icology to follow up in vitro genotoxicity positives are
the Muta Mouse and Big Blue rat/mouse models, in which Types of Cell Culture System Used
the genome is “tagged” with the markers lacZ (β-galacto- in Toxicity Testing
sidase) and lacI, respectively (Gossen et al., 1994;
Winegar et al., 1994; Wahnschaffe et al., 2005a,b). Before discussing the assays used in the in vitro systems,
Recently, these assays have been validated and approved it would be useful to discuss the principles, strengths and
by OECD (2013). weaknesses of the main types of in vitro systems. All cell

Cell culture dish
insert, containing
Toxin added into insert hepatocyte cells or
activated microsomes

Cell culture
dish or well,
Growth containing
medium target cells

Cell
monolayer

Microporous barrier allows Toxin and metabolite(s)
passage of toxin and metabolites diffuse toward target cells

FIGURE 9.1 Metabolic activation in cell culture systems.