Chapter 6: Autoradiography in Pharmaceutical Discovery and Development

Autoradiography (ARG) is a powerful, high resolution, quantitative molecular imaging technique used to study the tissue distribution and pharmacokinetics of new radiolabeled chemical entities in biological models. In the classic technique, the radioactivity in a sample produces an image on a detection media such as X-ray film, or another type of media that contains a photographic emulsion. In most cases the specimen contains a compound that had been intentionally labeled with an isotope having low energy beta radioactivity and that is synthesized into the molecule without damaging its chemical characteristics and/or intended effect(s). Most often the isotope is ¹⁴Carbon, ³Hydrogen, or ¹²⁵Iodine, however, ³⁵Sulfur, ³³Phosphorus, and other beta emitters can and have been used. In pharmaceutical research, the specimens can be single cells, individual organs, organ systems, and/or the whole-body of lab animals, such as rats, mice, rabbits, dogs, and monkeys. ARG can be separated into two basic categories: macro-autoradiography and micro-autoradiography. Macro-autoradiography is imaging of thin sections obtained from intact organs, organ systems, and/or intact whole-bodies, and micro-autoradiography provides localization of radioactivity at the cellular level in a histological preparation. Whole-body autoradiography (WBA) is a general term given to a macro-autoradiography technique where a radioactive compound, which is administered to a lab animal, is visualized by obtaining autoradiographs from thin whole-body sections, which have been obtained from the intact frozen carcass of a lab animal. Micro-autoradiography results are obtained from much thinner cryosections obtained form individual tissues that have been dissected out from an animal and that have been exposed to photographic emulsion on a glass slide, which is used for typical light microscopy. The roles of WBA and MARG in drug discovery and development have changed over the years, and the use of WBA has increased dramatically due to the development of phosphor imaging technology. This chapter will discuss the history, validation, current applications, instrumentation, and, strengths and limitations of WBA and MARG in pharmaceutical discovery and development. It will also present some ideas on how these techniques might be improved in the future to better serve pharmaceutical research.