Toxicology of Industrial Compounds

214 USE OF LONG-TERM CULTURES OF HEPATOCYTES IN TOXICITY TESTING
that 1-naphthol UDP-GT activity is maintained well in co-cultures whereas
this is not true for morphine UDP-GT, although for both a steady state
situation was reached. These data point to a shift towards a more
differentiated pattern since 1-naphthol is considered to be more specific for
the late foetal form of UDP-GT and morphine for the neonatal form. The
maintenance of 1-naphthol UDP-GT has been confirmed by Utesch and
Oesch (1992). Other studies on preservation of phase 2 enzymatic activity
in co-cultures have dealt with the identification of the metabolites formed
when drugs are added to the culture medium. In human co-culture it could
be shown that the glucuronide metabolite of ketotifen was still present
after 3 weeks whereas it became undetectable after 6 days in mono-culture
(Bégué et al., 1984b). Similar observations have been made for other drugs
such as caffeine and theophylline (Ratanasavanh et al., 1990).
Conclusions
At present no ideal culture system for hepatocytes can be proposed. In all
models reported in the literature, phenotypic changes occur, affecting the
various components of phase 1 and phase 2 xenobiotic metabolism to a
different extent. An interesting conclusion, however, remains from the
observation that, when co-cultures and mono-cultures of hepatocytes are
compared, cocultures exhibit higher biotransformation capacities which are
better and preserved for longer than is the case for mono-cultures. The
inducibility by common inducers is fairly well maintained and seems, to a
certain extent, comparable with the in vivo situation. In addition, hormonal
regulation of phase 1 and phase 2 key enzymes seems to be well maintained
and comparable with the in vivo situation. Co-cultures of hepatocytes with
rat liver epithelial cells are therefore already of importance as an alternative
model for risk assessment. In particular, when long-term effects of a
chemical are to be expected.
Some experience already exists concerning the application of co-cultured
hepatocytes for the study of pharmaceuticals. As far as chemical products
other than pharmaceuticals are concerned, experience is lacking although
interesting results are to be expected particularly in those cases where
chemicals can interfere with the human organism via the food chain or by
occupational exposure. In vitro exploration of this new field in toxicology
is a challenge for the coming years.
Notes
1a. 88/379/EEC
Directive du Conseil du 7 juin 1988 concernant le rapprochement des
dispositions législatives, réglementaires et administratives des Etats membres