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 radiat ion dosimetry of Tc-99m-RP527, a promising radioligand for the visualizatio n of gastrin-releasing peptide (GRP) receptor-expressing human malignancies . Methods: Whole-body scans were obtained up to 48 h after intravenous inje ction of 555 MBq Tc-99m-RP527 in each of 6 subjects. Blood samples were tak en at various times up to 48 h after injection. Urine was collected up to 4 8 h after injection for calculation of renal clearance and whole-body clear ance. Time-activity curves were generated for the thyroid, heart, breasts i n women, testes in men, and liver by fitting the organ-specific geometric m ean 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 ser ial whole-body images showed rapid hepatobiliary excretion, resulting in lo w 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 kid neys and to a lesser extent by the gastrointestinal tract. The mean excreti on in the urine (+/- SD) at 48 h after injection was 58.3 +/- 5.4 percentag e of the injected activity corrected for decay to the time of injection. Th e highest absorbed doses were received by the excretory organs (i.e., the u rinary bladder and gallbladder wall). The average effective dose of Tc-99m- RP527 was estimated to be 0.0095 mSv/MBq. Conclusion: The biodistribution o f Tc-99m-RP527 revealed low lung, myocardial, and liver uptake, which allow ed early imaging of the supradiaphragmatic region with a favorable dosimetr y (including effective dose) for administered activities required for SPECT imaging.