Dosimetry of a D2/D3 dopamine receptor antagonist that can be used with PET or SPECT

FIDA-2 (R-(+)-2,3-dimethoxy-5-iodo-N-[(1-4'-fluorobenzyl)-2-pyrrolidinyl) methyl] benzamide) is a simultaneously fluorinated and iodinated D2/D3 dopamine receptor antagonist. The purpose of this study was to measure its biodistribution and radiation dosimetry in humans.

Whole-body emission scans were sequentially acquired in eight healthy volunteers 24-43 hr after the intravenous administration of 101-150 MBq 123I FIDA-2. Regions of interest (ROIs) were placed on the initial set of conjugate emission images and transposed as a single set onto all the other scans without manipulating any of the regions for solid organs independently. The counts in each ROI were corrected for attenuation with transmission scans and compared to the net counts in images of the injection syringe containing the administered dose. The radiation doses were estimated with the MIRD formalism from the residence times for both the 18F- and 123I-labeled ligands.

There were no subjective or objective pharmacological effects of the tracer on any of the subjects. The findings showed that the dose-limiting organ for the 123I-labeled product was the thyroid gland in this sample. If the 18F-labeled product had been used, then the urinary bladder would have received 0.086 mGy/MBq (0.32 rads/mCi) and become the dose-limiting organ. The effective dose equivalents were 0.025 mSv/MBq (0.092 rem/mCi) for both the 123I- and the 18F-labeled versions of the tracer.

The data suggest that FIDA-2 can be used to produce relatively high contrast images of the D2/D3 dopaminergic system with substantially less than the maximum allowable radiation dose for research volunteers.