ADME studies with radiolabeled material. Comparative metabolic profiling
data, especially metabolite profiles in plasma across species, have very
important implications for drug development, including:
(1) Selection and/or validation of the most appropriate toxicological animal
species to be used for drug safety testing (Guengerich, 2006), such as
carcinogenicity studies (ICH, 1995b);
(2) Selection of metabolites for pharmacological and/or biological activity
testing (Baillie et al., 2002);
(3) Selection of metabolites for monitoring by a validated bioanalytical
method in selected nonclinical toxicology studies and clinical studies
(Baillie et al., 2002; Humphreys and Unger, 2006);
(4) Dtermination of whether the safety of a particular human metabolite
need to be directly evaluated in toxicology studies (FDA, 2005b).In general, metabolite profiles of a nonradiolabeled therapeutic agent in liver
tissues from humans and animals are obtained in drug discovery, which are used
for selection of animal species for pre-IND safety evaluation. Later on,in vitro
metabolite profiling of a radiolabeled drug is often conducted to gain better
quantitative and qualitative analysis evaluation of metabolites. Radiolabeled
ADME studies provide the ultimate assessment of the similarity of biotrans
formation between humans and the toxicology species. Animal ADME studies
are usually conducted from late drug discovery to early stages of drug
development. Human ADME studies, which require far more resources and
follow more rigorous regulatory requirements, are carried out from late Phase I
trials to the early Phase III studies. In some cases, the exposure to a specific
metabolite is further analyzed using a validated bioanalytical assay in selected
clinical and toxicological studies in which radiolabeled drugs are not suitable
(FDA, 2001a).
Finding a unique and/or major metabolite in human circulation late in drug
development could slow down the drug development process. Therefore, the
FDA has recommended ‘‘the in vivo metabolic evaluation in humans be
performed as early as feasible’’ (FDA, 2005b).
7.3.2 In vitroMetabolite Profiling Studies
In vitrometabolite profiles are often obtained for early assessment of the
similarity of drug metabolism across species (FDA 2005b and 2006a). Liver
microsomes and hepatocytes are the most usedin vitromodels for this purpose
although several otherin vitromodels, including liver slits, liver S9 fractions,
and recombinant metabolizing enzymes, are available. Liver microsomes have
higher CYP enzyme activities, while hepatocytes contain a more comprehen-
sive set of metabolizing enzymes, transporters, and cofactors. If analytical
sensitivity is not an issue, hepatocytes should be the first choice for
212 REGULATORY CONSIDERATIONS OF DRUG METABOLISM AND DRUG