Human dose estimation of a novel SPECT imaging agent using animal experimental data and Monte Carlo simulation

Undoubtedly, the precise measurement and determination of the optimum delivered dose to patients during diagnostic and therapeutic procedures is essential. This study estimated the human absorbed dose of the novel GRPR-targeting single-photon emission computed tomography (SPECT) imaging agent [^113mIn]In-RM2 (GRPR - gastrin-releasing peptide receptor) for potential clinical application in diagnosing GRPR-positive tumors, particularly prostate cancer, using animal experimental data and Monte Carlo simulation. The radiolabeled compound was prepared under optimized conditions using an in-house developed ¹¹³Sn/^113mIn generator, with a molar activity at least 43 GBq/µmol. The radiochemical purity of the final product was evaluated using radio-thin layer chromatography (RTLC) and high-performance liquid chromatography (HPLC), revealing a purity > 98%. The complex demonstrated high stability for at least three hours post-incubation. The biodistribution of the radiotracer in mice showed rapid blood clearance, with the primary excretion route being the urinary tract. Additionally, this SPECT agent showed high accumulation in GRPR-expressing organs at various time points. The time-integrated activity of animal source organs was extrapolated to human source organs using a mass-scaling method. Finally, the absorbed dose to human organs was calculated using the MCNPX software (Version 2.6.0) and a voxel-based human phantom developed by Oak Ridge National Laboratory (ORNL). The results showed that the pancreas and kidneys received the highest absorbed doses of 0.008 ± 0.0007 and 0.0036 ± 0.0003 mGy/MBq, respectively, while all other organs received negligible doses. Moreover, this novel SPECT radiotracer exhibited lower absorbed doses in nearly all organs compared to similar radiopharmaceuticals. Overall, [^113mIn]In-RM2 can be considered a safe and promising agent for SPECT imaging.