Value of 68Ga-labeled Bombesin Antagonist (RM2) in the Detection of Primary Prostate Cancer Comparing With [18F]fluoromethylcholine PET/CT and mpMRI – a Phase I/II Study
Authors: Beheshti Mohsen , Taimen Pekka , Kemppainen Jukka , Jambor Ivan , Müller Andre , Loidl Wolfgang , Kähkönen Esa , Käkelä Meeri , Berndt Mathias , Stephens Andrew W. , Minn Heikki , Langsteger Werner 
Published: September 2021

Abstract
Purpose: Overexperssion of Gastrin-releasing-peptide receptor (GRPr) in prostate carcinoma (PCa) suggests new means in the detection of prostate cancer foci. The bombesin derivative RM2 (DOTA-4-amino-1-carboxy­methyl­piperidine-D-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH2) is a GRPr antagonist with strong binding affinity. Based on promising results from a first-in-man study on PCa detection in patients with local disease, a Phase I/II study was initiated and the ability of [68Ga]Ga-RM2 PET/CT to detect PCa lesions was compared with [18F]fluoromethylcholine ([18F]FCH) PET/CT and multiparameteric prostate Magnetic Resonance Imaging (mpMRI).Methods: This Phase I/II study was conductedwith a pre-specified interim analysis following the enrollment of 30 biopsy-positive PCa subjects, stratified into low, intermediate and high pretreatment risk of extra-glandular metastases with reference to NCCN criteria. Each subject had PCa detected by transrectal ultrasound guided prostate biopsy and subjects were scheduled to undergo prostatectomy with pelvic lymph node (LN) dissection in intermediate and high risk patients. Following administration of an intravenous dose of 140 MBq of [68Ga]Ga-RM2,imaging was conducted at 60 min. p.i.. Twenty-five (25/30) subjects had concomitant [18F]FCH PET/CT imaging. All patients underwent mpMRI. Intra-prostatic and pelvic nodal PET/CT findings were correlated with histopathologic results.Results: High uptake of [68Ga]Ga-RM2 was seen in pancreas and the urinary system with very low background uptake in the rest of the abdomen or thorax. Despite of high bladder activity, focal intraprostatic uptake was readily well detectable. Of overall 312 analyzed regions, 120 regions (4 to 8 lesions per-patient) showed abnormal finding in the prostate gland. In a region-based analysis overall sensitivity and specificity of [68Ga]Ga-RM2 PET/CT in the detection of primary tumor were 74% and 90%, respectively; while it was 60% and 80% for [18F]FCH PET/CT and 72% and 89% for mpMRI. Although, the overall sensitivity of [68Ga]Ga-RM2 PET/CT was higher comparing to [18F]FCH PET/CT and mpMRI; however, the statistical analysis showed only significant difference between [68Ga]Ga-RM2 PET/CT and [18F]FCH PET/CT in intermediate-risk group (P=0.01) and [68Ga]Ga-RM2 PET/CT and mpMRT in high-risk group (p=0.03). [68Ga]Ga-RM2 PET/CT correctly detected 2 histopathologically verified LN metastases in 2 high risk patients; while 18F-FCH PET/CT only identified the LN lesion in 1 patient.Conclusion: [68Ga]Ga-RM2 is a promising new PET-tracer with a high detection rate for intraprostatic PCa. While index lesion detection rates were similar in both PET/CT studies, the improved specificity of [68Ga]Ga-RM2 for canver versus BPH renders it notably better than [18F]FCH in the detection of intraprostatic lesions. In addition, GRP-R-based imaging seems to play a complementary role to Choline-based imaging for full characterization of PCa extent, biopsy guidance in low and intermediate metastatic risk PCa patients and has the potential to discriminate them from whom of those at higher risks.Trial Registeration number: EudraCT-Nr.: 2014-003027-21, Date: 10 June, 2014