monitoring specific metabolites. In Fig. 9.3, two metabolites reach the
threshold of10% as per FDA draft guidelines, and since the monohydroxy-
lated metabolite had pharmacological activity and the N-demethylated
metabolite was relatively abundant in humans, a decision was made to
monitor both the metabolites in future studies.
To make decisions regarding monitoring of metabolites in humans and
animal species, it is critical to generate quantitative exposure information
regarding parent and metabolites. The quantitative information is generated by
dosing animals and humans with radiolabeled drug and following the fate of
the label in plasma and the excreta (urine, feces, or bile). In cases where
synthetic standards are available for metabolites, similar quantitative
information can be generated by monitoring for the metabolites by a specific
LC/MS/MS assay. Alternatively, when synthetic metabolite standards are not
available, radioactive metabolites generated in animals or in vitro sources
could be used as reference standards to estimate metabolite exposures in the
first-in-man and early toxicology studies (Zhang et al., 2007a).
9.5 IN VITRODRUG METABOLISM STUDIES
IN DRUG DEVELOPMENT
In vitrodrug metabolism studies are conducted, both at the discovery and
development stages, to help understand the potential safety and efficacy
liabilities of a drug. In drug discovery/early drug development where the drug
has not been tested in humans,in vitrometabolism studies provide an early
read on the metabolic profiles likely to be observed in humans. Generally, the
in vitro studies conducted are (1) metabolite profiling studies with liver
microsomes, S-9 fraction or hepatocytes to understand the metabolic profile
across different toxicology species and humans, (2) detailed reaction
phenotyping studies to identify the enzymes responsible for metabolism of
the drug, (3) inhibition studies to see if the drug is an inhibitor of CYP
enzymes, and (4) induction studies to see if the drug induces CYP enzymes. The
last three types of studies are particularly tailored toward identifying the CYP
enzymes involved, since most of the drugs in the market are metabolized by this
class of enzymes. The information generated from reaction phenotyping,
inhibition, and induction studies helps understand the potential for drug–drug
interaction and drives decisions regarding the clinical studies that should be
done to address these potential interactions (Davit et al., 1999). Described
below is a brief summary of the variousin vitrotools used in drug discovery
and development. Readers are encouraged to refer to the PhRMA position
paper by Bjornsson et al., recent FDA guidances and Chapters 5 and 7 of this
book for additional information on drug–drug interaction studies. (Bjornsson
et al., 2003). The PhRMA position paper also outlines the optimal substrates,
inhibitors or inducers of CYP enzymes that should be used in assessing the
potential of drug to cause drug–drug interaction.
274 ROLE OF DRUG METABOLISM IN DRUG DEVELOPMENT