used for toxicological testing, (2) genotoxicity studies that consist of onein vitro
assay to detect point mutations and another to detect chromosomal
aberrations, (3) embryo–fetal development studies for a drug candidate that
is interned for use in women of childbearing potential, and (4) carcinogenicity
studies. The guidance also suggests these safety studies be completed and
the study reports be submitted to the agency before beginning large-scale
Phase III trials if toxicity studies of a human metabolite are warranted.
However, since safety testing of drug metabolites is very complex, there is no
clear cut-off criteria for if direct safety testing of a metabolite is necessary. Both
members from regulatory agencies and scientists from the pharmaceutical
industry have emphasized that these complex issues should be treated on a
case-by-case basis (Davis-Bruno and Atrakchi, 2006).
Unique Human Metabolites The FDA metabolite safety testing guidance
indicates that the unique human metabolite(s) should be considered for safety
assessment in animals, especially those that have pharmacological activity or
associated with a structural alert. However, there are some situations where a
full set of safety studies for unique human metabolites may not be necessary,
such as if the metabolite is very minor in human plasma or it is only found in
human feces.
Major Metabolites in Human Circulation The FDA draft guidance suggests a
major metabolite in human circulation (>10% of drug-related material), which
is not present at comparable or higher levels in animals, to be considered for
direct safety evaluation. On the contrary, direct safety assessment is not
necessary for a major human plasma metabolite that is present at sufficient
levels in animal circulation.
Major Metabolites in Human Excreta The FDA draft guidance suggests
considering direct safety evaluation for a major human excreted metabolite
(>10% of dose) if it is not formed at comparable or higher levels in
toxicological species. The concept of the percentage of dose reflects the total
body burden of a metabolite. For example, an excretory metabolite that
represents 15% of dose (200 mg/subject) has a total body burden 30 mg. The
percentage of dose is a useful measurement of exposure to a metabolite that
represents a major metabolic pathway, but may not be present at significant
levels in plasma.
Conjugated Metabolites Glucuronide and sulfate conjugates are usually
pharmacologically inactive, and they are rarely associated with a toxic
response. These conjugates are more water soluble and more readily eliminated
than oxidative metabolites. Therefore, the treatment of conjugated metabolites
with respect to metabolite safety testing may warrant a less stringent safety
testing paradigm than that of oxidative metabolites (Davis-Bruno and
Atrakchi, 2006).
METABOLISM STUDIES RELEVANT TO METABOLITE SAFETY ASSESSMENT 217