RT Journal Article SR Electronic T1 GRPr Antagonist 68Ga-SB3 PET/CT Imaging of Primary Prostate Cancer in Therapy-Naïve Patients JF Journal of Nuclear Medicine JO J Nucl Med FD Society of Nuclear Medicine SP 1517 OP 1523 DO 10.2967/jnumed.120.258814 VO 62 IS 11 A1 Ingrid L. Bakker A1 Alida C. Fröberg A1 Martijn B. Busstra A1 J. Fred Verzijlbergen A1 Mark Konijnenberg A1 Geert J.L.H. van Leenders A1 Ivo G. Schoots A1 Erik de Blois A1 Wytske M. van Weerden A1 Simone U. Dalm A1 Theodosia Maina A1 Berthold A. Nock A1 Marion de Jong YR 2021 UL http://jnm.snmjournals.org/content/62/11/1517.abstract AB The gastrin-releasing peptide receptor (GRPr) is overexpressed in prostate cancer (PCa) cells, making it an excellent tool for targeted imaging. The 68Ga-labeled GRPr antagonist SB3 has shown excellent results in preclinical and clinical studies and was selected for further clinical investigation. The aims of this phase I study were to investigate 68Ga-SB3 PET/CT imaging of primary PCa tumors and assess safety. More aims included an investigation of biodistribution and dosimetry and a comparison with pathology and GRPr expression. Methods: Ten therapy-naïve, biopsy-confirmed PCa patients planned for prostatectomy were included. A 3-h extensive PET/CT imaging protocol was performed within 2 wk before prostatectomy. Prostate tissue was evaluated for tumor localization and Gleason score, and in vitro autoradiography was performed to determine GRPr expression. Available MRI scans performed within 3 mo before the study were matched. For dosimetry, residence times were estimated and effective dose to the body as well as absorbed doses to organs were calculated using the IDAC dose model, version 2.1. Results: Administration of 68Ga-SB3 (187.4 ± 40.0 MBq, 40 ± 5 μg) was well tolerated; no significant changes in vital signs or laboratory results were observed. 68Ga-SB3 PET/CT showed lesions in 8 of 10 patients. Pathologic analysis revealed a total of 16 tumor lesions, of which PET/CT showed 14, resulting in a sensitivity of 88%. 68Ga-SB3 PET/CT imaging showed uptake in 2 large prostatic intraepithelial neoplasia foci, considered a precursor to PCa, resulting in an 88% specificity. Autoradiography of tumor lesions revealed heterogeneous GRPr expression and was negative in 4 patients. Both PET/CT-negative patients had a GRPr-negative tumor. In autoradiography-positive tumors, the level of GRPr expression showed a significant correlation to tracer uptake on PET/CT. Dosimetry calculations estimated the effective dose to be 0.0144 mSv/MBq, similar to other 68Ga-labeled radiopeptides. The highest absorbed dose was detected in the physiologic GRPr-expressing pancreas (0.198 mGy/MBq), followed by the bladder wall and kidneys. Conclusion: 68Ga-SB3 PET/CT is a safe imaging method and a promising tool for early PCa imaging.