Novel ⁶⁸Ga-labeled peptide probe for PET/CT imaging of the cholecystokinin-2 receptor expression

Introduction: Medullary thyroid cancer (MTC) is a challenging malignancy with about 50% of the patients showing persistent or recurrent disease and 10% showing distant metastases at the time of diagnosis. The lack of sensitive imaging modalities and adjuvant therapies in this patient group has driven our scientific interest in developing a ⁶⁸Ga-labeled peptide probe allowing for high sensitivity PET imaging of the cholecystokinin-2 receptor (CCK2R) expression. This receptor is expressed at high incidence in MTC, as well as other tumor entities, such as small cell lung cancer and gastro-entero-pancreatic neuroendocrine tumors.

Methods: A novel DOTA-conjugated minigastrin analog (DOTA-MGS5) with two amino acid substitutions in the C-terminal receptor specific sequence displaying enhanced cell uptake and improved targeting profile in the preclinical evaluation was selected for clinical translation. The preparation of the ⁶⁸Ga-labeled peptide probe was validated using a cassette based synthesis module as well as direct labelling of a kit formulation. First PET/CT studies in patients with advanced MTC were carried out at an injected activity of 120-200 MBq. Image acquisition was carried out at 1 and 2 hours after tracer injection.

Results: ⁶⁸Ga-labeling using a synthesis module or a kit formulation yielded in a final product with total radioactivity of up to 1024 MBq, radionuclide incorporation of 98.7±0.7% and radiochemical purity of 94.4±1.8%. Within a shelf-life of 3 h a radiochemical purity of 92.0±0.9% was met. The injection of 120-200 MBq corresponding to a total peptide mass of 15 µg was well tolerated by the first patients examined. High physiological receptor-mediated uptake was observed in stomach together with predominant renal excretion. Comparative PET/CT in a 60-year old patient with advanced MTC using ⁶⁸Ga-DOTA-MGS5 and ⁶⁸Ga-DOTA-TOC at consecutive days, revealed several liver lesions with both tracers. However, visualization of liver metastases was clearly improved with ⁶⁸Ga-DOTA-MGS5 in comparison with ⁶⁸Ga-DOTA-TOC, due to a very low physiological liver uptake resulting in a higher tumor-to-background ratio.

Conclusions: ⁶⁸Ga-DOTA-MGS5 with its low physiological liver uptake allows for high contrast imaging of liver metastases. In addition, based on the favorable biodistribution profile this novel MG analog seems highly promising also for targeted radiotherapy when labeled with a beta emitting radiometal such as lutetium-177. Acknowledgements: This study was financially supported by the Austrian Science Fund (P 27844).