Toxicology of Industrial Compounds

208 USE OF LONG-TERM CULTURES OF HEPATOCYTES IN TOXICITY TESTING
– CYP3A4 is a major catalyst in the activation of aflatoxins, pyrrolizidine
alkaloids and polycyclic hydrocarbon dihydrodiols (Shimada et al.,
1989a; Shimada and Guengerich, 1989).
The balance between the rates of formation of reactive metabolites and
detoxication will greatly determine the potential toxic response of a
chemical. Variables, known to affect normal biotransformation, such as
enzyme induction and inhibition can also change the bioactivation rate of
chemicals. A clear example is the metabolism of halogenated biphenyls
after treatment with arochlor 1254 (Borlakoglu and Wilkins, 1993).
Consequently, the toxicity of chemical products often depends upon their
specific biotransformation, the presence, absence, induction and inhibition
of specific phase 1 and phase 2 enzymes involved in their metabolism.
Towards an in vitro approach of risk assessment
Nearly all toxicological studies on chemical products, including industrial
chemicals, agrochemicals, pharmaceuticals, additives, materials in contact
with food and cosmetics, have been carried out in vivo using experimental
animals, in particular small vertebrates (News and Views, 1993).
Important scientific, technological, ethical and economic considerations,
however, justify the actual search for in vitro alternatives, replacing or
improving existing in vivo methods and reducing the number of animals
involved (Frazier and Goldberg, 1990; Roberfroid, 1991).
Since hepatocytes can be isolated from different species (Guguen-
Guillouzo et al., 1982; Green et al., 1986; van’t Klooster et al., 1992)
including man (Guguen-Guillouzo et al., 1982; Rogiers, 1993), they can
represent a powerful tool for short-term risk assessment studies when used
as suspensions of freshly isolated cells (up to 3–4 h) or as short-term
cultures (up to 2 days) (Klaassen and Stacey, 1982; Guillouzo, 1986). They
can be useful in biotransformation, cytotoxicity, hepatotoxicity and
genotoxicity studies, in species selection and in mechanistic studies
(Blaauboer, 1994). Long-term cultures of hepatocytes (up to several weeks)
represent a somewhat different approach in risk assessment. Such systems
are of interest for the assessment of long-term toxicity of xenobiotics
including the occurrence of enzyme induction, the effects on xenobiotic
biotransformation, lipid peroxidation, accumulation of triglycerides,
changes in glutathione content, interaction between compounds and the
hepatoprotection afforded by certain molecules. Consequently, long-term
cultures have been particularly applied in the development of
pharmaceuticals, in pharmaco-toxicological studies (Guillouzo, 1986,
1992; Rogiers and Vercruysse, 1993; Skett, 1995). As far as chemical
products other than pharmaceuticals and in particular industrial chemicals
and agrochemicals are concerned, the practical needs during development