DOSIMETRY OF AN IODINE-123-LABELED TROPANE TO IMAGE DOPAMINE TRANSPORTERS

N-(3-lodopropen-2-yl)-2 beta-carbomethoxy-3 beta-(4-chlorophenl)tropan e (IPT) is an analog of cocaine that selectively binds the presynaptic dopamine transporter. The present study sought to measure the radiati on dosimetry of IPT in seven healthy human volunteers. Methods: Dynami c renal scans were acquired immediately after the intravenous administ ration of 165 +/- 16 MBq (4.45 +/- 0.42 mCi of [I-123]IPT. Between 7 a nd 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 adm inistered dose in 11 organs and each urine specimen was quantified fro m the attenuation-corrected geometric mean counts in opposing views. S ubject-specific residence times were calculated for each subject indep endently by fitting the time-activity curves to a multicompartmental m odel. The radiation doses were estimated with the MIRD technique from the residence times for each subject individually before any results w ere averaged. Results: The findings showed that IPT was excreted rapid ly 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 19.632 hr). The basal ganglia received a ra diation 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 0.1 4 mGy/MBq (0.51 rad/mCi). Conclusion: Relatively high-contrast images of the presynaptic dopamine transporters in the basal ganglia can be a cquired with 185 MBq (5 mCi) of [I-123]IPT. The radiation exposure tha t results is significantly less than the maximum allowed by current sa fety guidelines for research volunteers.