Biodistribution and dosimetry of iodine-123-IBF: a potent radioligand for imaging the D2 dopamine receptor.

Iodine-123-labeled iodo-benzofuran (IBF) is a potent D2 dopamine receptor antagonist that has been developed as a potential SPECT imaging agent. This report documents its biodistribution and radiation dosimetry in seven healthy humans. Approximately 100 MBq of IBF were administered to each volunteer. Urine was collected to measure the fraction of the activity that was voided by the renal system. Conjugate images were serially acquired over 24 hr to determine the fraction of activity in the other organs. Standard image analysis techniques were used to measure the geometric mean count rates in the brain, GI tract, heart, liver, lungs and thyroid at each time point. Corrections for attenuation were made with 123I transmission scans. Multicompartmental modeling was used to stimulate and predict the biokinetic behavior of 123I-IBF in the rest of the body. The absorbed doses for 24 organs were then estimated with the MIRD formalism. Rapid biological washout minimized the absorbed dose to most tissues. The excretory organs were exposed to the most radiation. The lower large intestine received about 0.13 mGy/MBq (0.48 rad/mCi), and the urinary bladder received 0.11 mGy/MBq. This low radiation burden will allow more than 370 MBq (10 mCi) to be administered to healthy research subjects during each study of the D2 receptor. Since high quality images of the brain can be obtained with half this amount, the findings suggest that 123I-labeled IBF has a large margin of radiation safety in humans. Its stability in vivo and its high target-to-background contrast ratio in the human brain may make it a useful SPECT imaging agent.