properties of the parent. The typical administered radioactivity range is 60–
200 mCi/kg. In our experience, this is sufficient to generate metabolic profiles in
plasma at multiple time points with adequate radioactive sensitivity. The
duration of the study is determined by the terminal half-life of the parent (in
that species) and should be conducted for 5- plasma half-lives. For
compounds with extremely long half-lives, the duration of the study can be
based on the criteria that the study can be terminated when1% of the total
dose is excreted in a given 24-h interval in both urine and feces. Plasma, urine,
and fecal samples are collected during the duration of the study to analyze for
radioactivity and for metabolite profiling. If needed, additional tissue samples
are collected to determine the concentration and accumulation of drug and
drug-related component in a particular tissue.
The first set of information generated from the ADME studies are the
overall plasma profile of total radioactivity (TRA) versus time, which is
compared to the plasma profile of parent versus time measured by a validated
LC/MS/MS assay. For those drugs where the parent is the major component
at all time points in plasma, the total radioactivity profile usually parallels the
profile of the parent. Metabolic profiles of plasma samples generated at
different time points by HPLC analysis, followed by radioactivity and mass
spectrometric detection, provides exposure-related information for parent
and metabolites in humans and animal species. In addition, it provides
information about metabolites that humans are exposed to and how this
compares to exposures in animal species. This is illustrated in Fig. 9.3, which
shows the plasma profiles in rats, dogs, and humans after an oral dose of a
C-14-labeled drug. The plasma profiles clearly show that the metabolic
profiles are similar across species. TheFig. 9.3 table insert summarizes the
percent AUC of parent and two circulating metabolites as compared to AUC
of total radioactivity. Based on this data, it can be clearly concluded that the
monohydroxylated and N-demethylated metabolites are major circulating
metabolites in humans that are also represented in the animal species. These
comparative radioactivity profiles could be used to generate the absolute
concentration of the metabolites present in human and animal plasma
samples at the relevant doses and gives an estimate of the exposure margin for
the metabolites in these species.
Metabolic profiles of urine and fecal samples generated in ADME studies
provide information about the extent of metabolism and routes of excretion for
parent and metabolites. The excretion of radioactivity after administration of
[^14 C] gemopatrilat to healthy human volunteers shows that the compound is
excreted in both urine and feces with most of the radioactive dose excreted by
48 h postdose. Based on urinary excretion, at least 55% of the dose is absorbed
when administered orally. Figure 9.4 shows the comparative urinary and fecal
profiles of orally administered [^14 C] gemopatrilat in rat, dog, and human (Wait
et al., 2006). These types of data representations help understand the
quantitative and qualitative differences in metabolism across species. As can
be clearly seen in Fig. 9.4, the drug is extensively metabolized in all species,
270 ROLE OF DRUG METABOLISM IN DRUG DEVELOPMENT