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

compound undergoes oxidation to a more polar substance.
Cytochrome P-450 (P450)-depending monooxygenases and flavin
monooxygenases, are major role players in the oxidative metabolism
of xenobiotics and endogenous compounds. This process is followed
by the Phase II reactions in which metabolites are further conjugated
by hepatocytes with endogenous molecules by glucoronidation,
sulfation, methylation, acetylation and mercapture formation,
rendering derivatives that are much more soluble, thus facilitating
their elimination (1-3). Biotransformation reactions generally follow
a detoxification process rendering metabolites inactive. Nevertheless,
many drug intermediary products generated during metabolism
are highly reactive and toxic, causing hepatotoxicity (4-6).
Metabolism is the major determinant of drug clearance and
interindividual pharmacokinetic differences, and the indirectly
determinant of the clinical efficacy and toxicity of drugs. Many
potential drug candidates are rejected in drug discovery given the
undesirable pharmacokinetics that can result in an inadequate
concentration of the drug at the site of action and/or great variations
in clinical response and adverse effects.

Drug regulatory agencies, such as the Food and Drug
Administration (FDA) and the European Agency for the Evaluation
of Medicinal Products (EMEA) (7), have issued different guidelines
to enhance the importance of ADME studies in the drug development
process. The pharmaceutical industry is required to market safer
drugs with fewer side effects, predictable pharmacokinetic properties
and quantifiable drug-drug interactions. Therefore, companies are
increasingly interested in optimizing these properties during early
drug development phases (Figure 1). Rapid biotransformation,
resulting in a short exposure to the pharmacologically active parent
compound and the formation of active or toxic metabolites, is not
usually welcomed. Alternatively, an extremely stable drug may pose
a potential problem of drug-drug interactions and toxicity. Medicinal
chemists are primarily concerned in designing molecules that will
not only offer the desired activity, but suitable potency and duration
of action which are influenced by pharmacokinetic properties.
Consequently over the past decade, in vitro-based strategies in lead
optimization screening in conjunction with ADMET screening studies
have been incorporated earlier in the drug discovery phase (8, 9).

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