to be qualified. MDF-treated ion chromatograms may have provided the first
useful LC/MS method for estimation of amounts of metabolites when
metabolite standards are not available to develop a quantification method.
18.5 Conclusions and Path Forward
Radioactivity profiling in plasma, urine, bile, and feces of appropriate animals
and human subjects from mass balance studies enable a quantitative evaluation
of the absorption, tissue distribution, and excretion of a drug candidate. The
data can be supplemented with tissue distribution information from whole-
body analysis and the further defining the sites of absorption and excretion in
surgically prepared animals.
Identification of major elimination pathway will help design renal or hepatic
impairment clinical study, and identification of major metabolic clearance
pathways will help design reaction-phenotyping of major metabolic pathways
for potential drug–drug interaction studies. For major circulating metabolites,
pharmacological and toxicological activities should be tested and they may
need to be monitored in clinical and toxicological studies. Selection of animal
species for long-term toxicological studies may need to be reevaluated if the
metabolite exposures or major metabolic pathways are significantly different.
The ADME study results may help investigate the metabolite and toxicity
correlation in animals and humans.
Recently, accelerated mass spectrometry (AMS) have been used in mass
balance studies in animals and humans (Garner et al., 2002; Mauthe et al.,
1998; Rickert et al., 2005). AMS only requires a small amount of radioactivity
(nCi) with a normal dose (not considered as a radioactive study in some
countries). However, this technique has limited applications in metabolite
identification by connecting LC/MS/MS analysis and radioactivity detection.
Acknowledgments
We would like to thank Lifei Wang, Mingshe Zhu and W. Griff Humphreys
for helpful discussions.
APPENDIX A
Rat Tissue Distribution and Dosimetry Calculation
The objectives of rat tissue distribution studies are to (1) obtain radioactivity
distribution in rat tissues after single doses of^14 Cor^3 Hlabeled compounds, (2)
obtain the pharmacokinetic parameter of radioactivity (^14 Cor^3 H) in rat major
tissues or organs, and (3) calculate dosimetry for human ADME study.
Animals used for this study are pigmented male Long–Evan rats,250 g body
weight, 6–7 weeks. The numbers of rats used for a tissue distribution studies
CONCLUSIONS AND PATH FORWARD 597