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VOL. 73 (1), 5-26, 2007 IN VITRO INVESTIGATION OF DRUG METABOLISM AND TOXICITY...

calcium participates in many cellular functions and its levels should
be perfectly regulated to obtain a proper cell function. Many
substances can interfere in intracellular calcium homeostasis control,
thus leading to cell malfunction and death (50, 57). 5) Oxidative
stress is produced by compounds able to undergo repeated oxidation
and reduction cycles within the cell (58). This redox cycling causes
the continuous production of reactive oxygen species (e.g. superoxide
anion) and depletion of GSH and nicotinamide nucleotide pools,
with a concomitant increase in lipid peroxidation and intracellular
Ca2+ accumulation. Finally, biotransformation of xenobiotics can also
result in the formation of intermediates capable of covalently binding
to cell macromolecules (proteins, DNA and RNA) to form stable drug
adducts (58).

Cytotoxicity end-points (cell viability assays: MTT, neutral red
uptake tests, etc; cell membrane permeability alteration: enzyme
leakage; etc.) represent a first approach to assess hepatotoxicity, but
evaluation of these parameters alone may leave out of consideration
xenobiotics that impair cell function without causing cell death. This
may not be critical for the hepatocyte itself, but can be of toxicological
significance for the whole organism (59). By examining the effects on
hepatocyte-specific metabolism, it is possible to find out whether
relevant hepatic specific functions become altered by the presence of
a xenobiotic. Currently, several metabolic parameters, representative
of the liver’s most characteristic functions, should be evaluated,
namely, gluconeogenesis, glycogen metabolism, ureogenesis, plasma
protein synthesis, synthesis of VLDL, etc. In general, metabolic
parameters are more sensitive to the toxic effect of hepatotoxins than
cytotoxicity indicators (60). Concentrations to which cells are exposed
for cell metabolism studies should not cause perceptible cell death
(they should be up to the MNTC).

The ultimate goal of in vitro experiments is to generate the type
of scientific information needed to identify compounds that are
potentially toxic to man for which purpose not only the design of
experiments but also the interpretation of results are essential. Even
simple parameters for assessing cell toxicity have yielded promising
results when comparing in vitro effects with human toxicity as false
negatives are infrequent in compounds that are toxic without
biotransformation.

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