Dosimetry of an iodine-123-labeled tropane to image dopamine transporters.
UNLABELLED: N-(3-Iodopropen-2-yl)-2 beta-carbomethoxy-3 beta-(4-chlorophenyl)tropane (IPT) is an analog of cocaine that selectively binds the presynaptic dopamine transporter. The present study sought to measure the radiation dosimetry of IPT in seven healthy human volunteers. METHODS: Dynamic renal scans were acquired immediately after the intravenous administration of 165 +/- 16 MBq (4.45 +/- 0.42 mCi) of [123I]IPT. Between 7 and 12 sets of whole-body scans were acquired over the next 24 hr. The 24-hr renal excretion fractions were measured from conjugate emission scans of 7-11 discreet voided urine specimens. The fraction of the administered dose in 11 organs and each urine specimen was quantified from the attenuation-corrected geometric mean counts in opposing views. Subject-specific residence times were calculated for each subject independently by fitting the time-activity curves to a multicompartmental model. The radiation doses were estimated with the MIRD technique from the residence times for each subject individually before any results were averaged. RESULTS: The findings showed that IPT was excreted rapidly by the renal system. There were no reservoirs of retained activity outside the basal ganglia, where SPECT images in these subjects showed that the mean ratio of caudate to calcarine cortex averaged 25:1 at 3 hr after injection (range 19.6-32 hr). The basal ganglia received a radiation dose of 0.028 mGy/MBq (0.10 rad/mCi). The dose-limiting organ in men was the stomach, which received an estimated 0.11 mGy/MBq (0.37 rad/mCi). In women, the critical organ was the urinary bladder at 0.14 mGy/MBq (0.51 rad/mCi). CONCLUSION: Relatively high-contrast images of the presynaptic dopamine transporters in the basal ganglia can be acquired with 185 MBq (5 mCi) of [123I]IPT. The radiation exposure that results is significantly less than the maximum allowed by current safety guidelines for research volunteers.