A Guide to Ex Vivo Biodistribution Studies with Radiotracers in Rodent Models

Preclinical biodistribution studies are required at an early stage of radiopharmaceutical development to determine tracer pharmacokinetics in clinically relevant animal models of human diseases. When combined with quantitative analysis from non-invasive imaging, biodistribution experiments provide essential data on the uptake, retention, binding specificity, metabolism, and clearance of radiotracers in both target and non-target tissues. Most research groups have developed in-house protocols to perform these studies in a reproducible manner. However, there is a general lack of consistency in how different groups carry out biodistribution experiments. In addition to practical differences that occur during tissue collection (for example, washing and blotting dry the tissue, perfusion, sampling site for a given tissue, etc.), other aspects of biodistribution experiments, which often vary include the methods used for calibrating the injected activity, the processes used to calculate mass normalized tissue uptake (i.e. percentage of injected dose per gram [%ID g^-1] or percentage of injected activity per gram of tissue [%IA g^-1]) values, differences in data processing and statistical analyses (particularly error propagations and calculation of tissue contrast ratios), and variations in how the methods and data are reported and interpreted. This variability hinders the direct comparison of datasets produced at different laboratories. Here, we present a comprehensive guideline for conducting ex vivo biodistribution experiments with radiotracers in rodent models. An open source, freely accessible online biodistribution calculator and associated spreadsheet are provided which can be employed to compute the percent of injected dose per gram of tissue (%ID g^-1), standardized uptake value (SUV by mass), and target-to-background tissue contrast ratios. Finally, advice concerning biodistribution data presentation and statistical analysis are given to help the reader harness the full power of ex vivo biodistribution studies in radiotracer development.