Biodistribution and internal radiation dosimetry of a novel probe for thymidine phosphorylase imaging, [ 123 I]IIMU, in healthy volunteers

Objective We evaluated the radiation dosage, biodistribution, human safety, and tolerability of the injection of a single dose of [ 123 I] 5-iodo-6-[(2-iminoimidazolidinyl)methyl]uracil (IIMU), a new radiotracer targeting thymidine phosphorylase (TP), in healthy volunteers. Methods Potential participants were tested at our hospital to confirm their eligibility. Two healthy male adults passed the screening tests. They were injected with 56 and 111 MBq of [ 123 I]IIMU, respectively. Safety assessments were performed before and at 1, 3, 6, 9, 24, 48 h, and 1-week post-injection. Whole-body emission scans were conducted at 1, 3, 6, 24, and 48 h post-injection. Regions of interest were manually drawn to enclose the entire body at each time point, identifying high-uptake organs to obtain the time-activity curves. Urine and blood samples were collected at 1, 2, 3, 4, 5, 6, 9, 24, and 48 h post-injection. The radiation dose for each organ and the effective doses were estimated using OLINDA/EXM 1.1 software. Results No adverse events were observed as of the follow-up visit > 1-week post-injection. In both subjects, the highest uptake of [ 123 I]IIMU occurred in the liver, with peak injected activity (%IA) values of 17.7% and 15.1%, respectively. The second highest uptake was in the thyroid (0.35% and 0.66% IA). The %IA decreased gradually toward the end of the study (48 h) in all organs except the liver and thyroid. By the end of the study, 52.5% and 51.5% of the injected activity of [ 123 I]IIMU had been excreted via the subjects’ renal systems. The estimated mean effective doses of [ 123 I]IIMU were 9.19 μSv/MBq and 10.1 μSv/MBq, respectively. Conclusion In this preliminary study, [ 123 I]IIMU was safely administered to healthy adults, and its potential clinical use in TP imaging was revealed.