Use of radioactive compounds and autoradiography to determine drug tissue distribution

Radioactivity has been used in drug discovery and development for several decades because it offers researchers a highly sensitive way to quantitatively assess the absorption, distribution, metabolism, and/or excretion (ADME) of chemical entities by incorporating a radioactive isotope into the structure of the drug molecule. Regulatory agencies around the world require drug makers to characterize the ADME properties of prospective new drugs as one way to help ensure that patients are not exposed to dangerous drug and/or drug metabolite levels before they can be approved for human use. Radiolabeled compounds have consistently proved to be the most efficient tool for determining that information, even though attempts have been made to use nonradioactive techniques. The techniques of quantitative whole-body autoradiography (QWBA) and microautoradiography (MARG), which rely on the use of radiolabeled drugs, are two techniques that are routinely used to examine tissue distribution of drugs in discovery and development. These techniques provide drug researchers with quantitative tissue concentration data and a visual location of those concentrations in intact organs, tissues, and cells of laboratory animals. It is important for readers to realize that these techniques visualize total radioactivity, which can include the parent molecule along with its metabolites, and/or degradation products or impurities. This requires investigators to treat the quantitative data with caution unless the identity of the radioactivity is determined using some type of other bioanalytical techniques, such as mass spectroscopy and/or radio-HPLC, which can be easily performed on the tissue obtained from the animals used for QWBA and/or MARG. Nevertheless, these data are used in drug discovery and development to answer questions related to tissue penetration, fetal/placental transfer, tissue retention, routes of elimination, drug-drug interactions, enzyme induction/inhibition, formulation comparisons, in vivo compound solubility, differential metabolite distribution, interspecies comparisons, and to predict human exposure to parent drugs, metabolites, and radiation during clinical studies. This review will consider the strategic use of WBA, QWBA, and MARG in the pharmaceutical industry. Case studies and anecdotal information will also be presented; however, readers should realize that these are general examples and that some details have been omitted for brevity and/or because the data is proprietary and could not be presented at this time. Nevertheless, the images and discussions are provided to demonstrate how the techniques can and have been used to examine in situ tissue distribution of therapeutic compounds.