validated bioanalytical assay in selected clinical and nonclinical studies.
However, metabolite monitoring in clinical and nonclinical studies for the
purpose of metabolite safety testing is not discussed in detail in the FDA draft
guidance. Smith and Obach have proposed a decision-making process for
selecting human plasma metabolites for monitoring in toxicological and
clinical studies based on their absolute concentrations in the circulation rather
than relative abundance and pharmacological activity (Smith and Obach,
2006). The question of what metabolites to be monitored in what studies will
continue to be debated (Humphreys and Unger, 2006; Prueksaritanont et al.,
2006). In general, radiolabeled human and animal ADME studies after a single
dose of radiolabeled drug can provide the necessary information on what are
the major or unique human metabolites. This consideration is consistent with
the MIST document and the FDA metabolite safety draft guidance, in which
radioactive metabolite profiling data are considered to be the key information
resource for the assessment of metabolite exposure. If the exposure of
metabolites in humans and toxicological species is evaluated properly in
radiolabeled ADME studies, metabolite monitoring by a validated analytical
assay in additional clinical and nonclinical studies may be not necessary.
Quantification of selected metabolites by a validated LC/MS assay can provide
more accurate or confirmatory assessment of metabolite exposure, which is
especially useful in the measurement of the exposure to metabolites in special
populations or after multiple dosing.
7.3.5.4 Selection of Metabolite(s) for Monitoring in Toxicokinetic Studies The
primary objective of toxicokinetic studies is to evaluate system exposure
achieved in animals and its relationship to the dose level in the toxicity studies
(Table 7.1). The assessment of the systemic exposure in the toxicity studies is
mainly carried out by measurement of drug concentrations in plasma or other
body fluids using a validated bioanalytical assay. However, in some cases the
measurement of metabolite concentrations in plasma or excreta is important,
particularly when the metabolite is the primary active entity of a prodrug or is
the pharmacologically or toxicologically active compound (ICH S3A). In
addition, significant plasma metabolites that are present at the levels equivalent
to or higher than those of the parent drugs in circulation are often monitored in
the toxicokinetic studies. Metabolic profiling data from animal ADME studies
are essential in selecting appropriate analytes to be measured in certain
matrixes. For example, a unique toxic metabolite in animal species or a
metabolite that is a major in animal plasma and minor in human plasma may
be considered for monitoring for toxicokinetic evaluation, but not necessarily
for formal metabolite safety testing.
7.3.5.5 Recommended Studies for Metabolite Safety Testing The FDA
metabolite safety testing draft guidance outlines four kinds of safety studies
for assessing the safety of unique or major human metabolites. These include
(1) general toxicity studies with multiple dosing of the metabolite to species
216 REGULATORY CONSIDERATIONS OF DRUG METABOLISM AND DRUG