The role of radioisotope applications in regulatory affairs concerning registration of new drugs

Although the original interest in labeled compounds was in order to conduct fundamental studies in pharmacology, a more compelling reason for their use soon became evident with the need to obtain metabolic and pharmacokinetic data required for registration of new drugs. A number of aspects of the protocol for registration of new drugs such as whole-body autoradiography, plasma and tissue protein binding etc. require the use of labeled drugs. The labeling of drugs for this purpose involves a number of problems of which the more important are : the choice of radioisotope, the choice of labeled position, the number of steps to obtain the relevant labeling position, the specific activity of the labeled compound the radiochemical and chemical purity and stability, the purification techniques as well as the time required and cost of performing the synthesis of labeled drugs. A newer application of labeled compounds is that of the synthesis of labeled chemicals to be used for Positron Emission Tomography (PET), a very useful technique not only in neurology but also in metabolic and pharmacokinetic studies.