Autoradiography with positron emitting isotopes in positron emission tomography tracer discovery.

Rapid development of labeling chemistry and generation of new chemical entities for biologic interactions via combinatorial chemistry and high throughput screening gives great potential for the development of new positron emission tomography (PET) tracer candidates. It must, however, be realized that a large fraction of these candidates will fail to characterize the desired biochemistry in vivo due to undesirable properties that are not relevant to providing a specific signal in vitro. A full characterization of a PET tracer is a lengthy and expensive procedure, and it is necessary to establish confidence via a number of assays that a tracer will provide useful data in the target species (generally human). These assays should also serve as a background to choose or to reject a tracer at an early time point, conserving valuable resources and time. Autoradiography performed as an ex vivo binding technique or as ex vivo recording of in vivo tracer distribution are, in this respect, very important tools. Autoradiography binding methods allow a range of frozen tissues to be sectioned and incubated with the PET tracer, and with due caution and controls with selective blockade of binding, quantitative values can be obtained with respect to tracer binding and its regional distribution. The method is easy to learn and set up, and should be included in all PET research and development labs. Ex vivo autoradiography of selected organs or whole animals gives qualitative images of a tracer's distribution with high resolution and is especially valid for 18F-labeled tracers. When tracer administration is not limited by mass due to specific radioactivity, 11C-tracers can also be readily used. This method is attractive to use as a complement to small animal imaging due to its high resolution and anatomical correlate. Living slice autoradiography is a more cumbersome method, but has an advantage of utilizing live tissue that retains certain metabolic functions. The use of these different methods in the validation of tracers and for supplying complementary information is illustrated.