Biodistribution and radiation dosimetry of [¹¹C]raclopride in healthy volunteers

This study reports on the whole-body biodistribution and radiation dosimetry of [¹¹C]raclopride, a dopamine D₂ receptor antagonist.

In three healthy male volunteers, whole-body scans were performed up to 2 h following i.v. injection of 320±65 MBq [¹¹C]raclopride. Transmission scans (3 min per step, eight or nine steps according to the height of the subject) in 2D mode were used for subsequent attenuation correction of emission scans. Emission scans (1 min per step, eight or nine steps) were acquired over 2 h. Venous blood samples and urine were collected up to 2 h after injection of the radiotracer. For each subject, the percentage of injected activity measured in regions of interest over brain, intestine, lungs, kidneys and liver was fitted to a mono-exponential model, as an uptake phase followed by a mono-exponential washout, for urinary bladder to generate time–activity curves. Using the MIRD method, several source organs were considered in estimating residence time and mean effective radiation absorbed doses.

Blood pressure and ECG findings remained unchanged after tracer injection. The analysed blood and urine pharmacological parameters did not change significantly after [¹¹C]raclopride injection. The primary routes of clearance were renal and intestinal. Ten minutes after injection, high activities were observed in the gall-bladder, kidneys and liver. High activity was observed in the gall-bladder during the whole study. The kidneys, urinary bladder wall, liver and gall-bladder received the highest absorbed doses. The average effective dose of [¹¹C]raclopride was estimated to be 6.7±0.4 μSv/MBq.

The amount of [¹¹C]raclopride required for adequate dopamine D₂ receptor imaging results in an acceptable effective dose equivalent, permitting two or three repeated clinical PET imaging studies, with the injection of 222 MBq for each study.