Biodistribution and Radiation Dosimetry for ⁶⁸Ga-DOTA-CCK-66, a Novel CCK₂R-Targeting Compound for Imaging of Medullary Thyroid Cancer

Cholecystokinin 2 receptor (CCK2R) is a promising target for imaging and treatment of medullary thyroid cancer due to its overexpression in over 90% of tumor cells. ⁶⁸Ga-DOTA-CCK-66 is a recently introduced PET tracer selective for CCK₂R, which has shown favorable pharmacokinetics in vivo in preclinical experiments. In order to further investigate safety and suitability of this tracer in the human setting, whole-body distribution and radiation dosimetry were evaluated. Patients and Methods Six patients with a history of medullary thyroid cancer were injected intravenously with 169 ± 19 MBq of ⁶⁸Ga-DOTA-CCK-66. Whole-body PET/CT scans were acquired at 10 minutes, 1 hour, 2 hours, and 4 hours after tracer injection. Time-activity curves per organ were determined, and mean organ-absorbed doses and effective doses were calculated using OLINDA/EXM. Results Injection of a standard activity of 150 MBq of ⁶⁸Ga-DOTA-CCK-66 results in an effective dose of 4.5 ± 0.9 mSv. The highest absorbed organ doses were observed in the urinary bladder wall (40 mGy) and the stomach (15 mGy), followed by the kidneys (6 mGy), as well as the liver and the spleen (3 mGy each). CCK₂R-expressing tumor manifestations could be detected in 2 of the 6 patients, including lymph node, bone, and liver metastases. Conclusions ⁶⁸Ga-DOTA-CCK-66 exhibits a favorable dosimetry. Beyond physiologic receptor expression of the stomach, no other relevant tracer accumulation could be observed, rendering this organ at risk in case of subsequent radioligand therapy using ¹⁷⁷Lu-DOTA-CCK-66.