radiolabeled compound are often conducted in preclinical and early clinical
stages, especially when a toxic dose and the therapeutically effective dose range
have been identified. These studies will define the rate and route of excretion of
the compounds. In addition, these studies will provide definite in vivo
metabolism comparison with subsequent metabolite profiling and identifica-
tion as well as limited data on absorption and distribution of the compound.
Metabolites can also be isolated fromin vitroand animal samples and these
metabolites can be used as reference standards for quantification in the clinical
and nonclinical studies. At the same time, complementary tissue distribution in
rats and bile collection from bile duct cannulated (BDC) animals will provide
tissue distribution and biliary elimination profiles of the drug candidate.
Sometimes, metabolism studies may be needed with animals of disease states or
to investigate toxic lesion discovered in toxicity studies. Metabolism studies
can also validate the animal species used in toxicology studies. Later, complete
quantitative tissue distribution studies in pregnant rats will be needed to assess
placenta transfer, and milk excretion in lactating rats will also need to be
evaluated.
The procedures given in this chapter are general recommendations. Since
each test compound and its metabolites have their own unique physiochemical
characteristics, discretion, and modifications of the procedures are sometimes
necessary when handling different test compounds.
18.2 Study Designs
18.2.1 Choice of Radiolabel
Radiolabeled drugs have been widely used in mass balance studies because the
radioactivity can easily be detected and quantified using liquid scintillation
techniques and disposition of drugs can be assessed. The choice of the
radioisotope, the position of the radiolabel in the drug compound, radio-
chemical purity, and the specific activity are important parameters in designing
the ADME studies. These parameters can have an effect on the metabolic,
chemical, and radiochemical stability of the drug, metabolite formation and
detection, and the recovery of radioactivity. Radioisotopes used in ADME
studies have included^3 H(Heggie et al., 1987),^14 C(Chando, 1998),^32 P(Boado
et al., 1995),^35 S(Lu et al., 1982) and^131 I(Wafelman et al., 1997).^14 Cis the
isotope of choice in most of the ADME studies. The labeled part of the drug
molecule should not be lost in metabolite formation. Tritium-labeled drugs are
also used commonly, but the risk of tritium exchange chemically or
metabolically needs to be considered. Data from the preclinical studies
performed with the radiolabeled drug can provide information to make the
choice of the radioisotope and its position in the drug molecule.
The tritium- or carbon-14-labeled drug at the metabolically stable position
should have a radiochemical purity of98%, and in special cases95% may
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