Radionuclide tracer technique is an important method in the drug research. With its advantage of high-sensitivity, good-reliability, wide limit of detection and the test results that are not affected by endogenous substance, it has become the preferred pharmacokinetic study method for the large molecules including proteins and peptides.
The reason why radionuclide tracer technique can be applied to drug research is that both theory and practice have confirmed that the labelled compound has the similar physical & chemical properties and the biological behavior with labelling precursor (i.e. test article). The in-vivo ADME of the labelled compound presents the in-vivo ADME of the labelling precursor. The most commonly used radionuclide is 125I. It is because the half-life and decay mode (electron capture) are preferable. On the other hand, it is due to the increasing maturity of 125I labelling technique.