Biodistribution and dosimetry of Tc-99m-RP527, a gastrin-releasing peptide (GRP) agonist for the visualization of GRP receptor-expressing malignancies

The aim of this study was to determine the human biodistribution and radiation dosimetry of Tc-99m-RP527, a promising radioligand for the visualization of gastrin-releasing peptide (GRP) receptor-expressing human malignancies. Methods: Whole-body scans were obtained up to 48 h after intravenous injection of 555 MBq Tc-99m-RP527 in each of 6 subjects. Blood samples were taken at various times up to 48 h after injection. Urine was collected up to 48 h after injection for calculation of renal clearance and whole-body clearance. Time-activity curves were generated for the thyroid, heart, breasts in women, testes in men, and liver by fitting the organ-specific geometric mean counts, obtained from regions of interest, on the respective images as a function of the time after injection. The MIRD formulation was applied to calculate the absorbed radiation dose for various organs. Results: The serial whole-body images showed rapid hepatobiliary excretion, resulting in low background and potentially high-contrast imaging of the thoracic region. Imaging of abdominal tumors may prove problematic, however, because of the extensive bowel activity. Tc-99m-RP527 was predominantly cleared by the kidneys and to a lesser extent by the gastrointestinal tract. The mean excretion in the urine (+/- SD) at 48 h after injection was 58.3 +/- 5.4 percentage of the injected activity corrected for decay to the time of injection. The highest absorbed doses were received by the excretory organs (i.e., the urinary bladder and gallbladder wall). The average effective dose of Tc-99m-RP527 was estimated to be 0.0095 mSv/MBq. Conclusion: The biodistribution of Tc-99m-RP527 revealed low lung, myocardial, and liver uptake, which allowed early imaging of the supradiaphragmatic region with a favorable dosimetry (including effective dose) for administered activities required for SPECT imaging