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Preclinical and preliminary clinical evidence indicates that radiolabeled somatostatin (sst) receptor antagonists perform better than agonists in detecting neuroendocrine tumors (NETs). We performed a prospective phase I/II study to evaluate the sst receptor antagonist 68 Ga-OPS202 ( 68 Ga-NODAGA-JR11; NODAGA = 1,4,7-triazacyclononane,1-glutaric acid-4,7-acetic acid and JR11 = Cpa-c(dCys-Aph(Hor)-dAph(Cbm)-Lys-Thr-Cys)-dTyr-NH 2 )) for PET imaging. Here, we report the results of phase I of the study. Methods: Patients received 2 single 150-MBq intravenous injections of 68 Ga-OPS202 3-4 wk apart (15 μg of peptide at visit 1 and 50 μg at visit 2). At visit 1, a dynamic PET/CT scan over the kidney was obtained during the first 30 min after injection, and static whole-body scans were obtained at 0.5, 1, 2, and 4 h after injection; at visit 2, a static whole-body scan was obtained at 1 h. Blood samples and urine were collected at regular intervals to determine 68 Ga-OPS202 pharmacokinetics. Safety, biodistribution, radiation dosimetry, and the most appropriate imaging time point for 68 Ga-OPS202 were assessed. Results: Twelve patients with well-differentiated gastroenteropancreatic (GEP) NETs took part in the study. 68 Ga-OPS202 cleared rapidly from the blood, with a mean residence time of 2.4 ± 1.1 min/L. The organs with the highest mean dose coefficients were the urinary bladder wall, kidneys, and spleen. The calculated effective dose was 2.4E-02 ± 0.2E-02 mSv/MBq, corresponding to 3.6 mSv, for a reference activity of 150 MBq. Based on total numbers of detected malignant lesions, the optimal time window for the scan was between 1 and 2 h. For malignant liver lesions, the time point at which most patients had the highest mean tumor contrast was 1 h. 68 Ga-OPS202 was well tolerated; adverse events were grade 1 or 2, and there were no signals of concern from laboratory blood or urinalysis tests. Conclusion: 68 Ga-OPS202 showed favorable biodistribution and imaging properties, with optimal tumor contrast between 1 and 2 h after injection. Dosimetry analysis revealed that the dose delivered by 68 Ga-OPS202 to organs is similar to that delivered by other 68 Ga-labeled sst analogs. Further evaluation of 68 Ga-OPS202 for PET/CT imaging of NETs is therefore warranted., (© 2018 by the Society of Nuclear Medicine and Molecular Imaging.)