68Ga-DOTA-Bombesin (68Ga-RM2 or 68Ga-Bombesin) PET versus 68Ga-PSMA PET: A pilot prospective evaluation in patients with biochemical recurrence of prostate cancer.

Abstract

331 Background: Detection of prostate cancer (PC) after biochemical recurrence (BCR) remains a great challenge. RM2 is a synthetic bombesin receptor antagonist, which targets gastrin-releasing peptide receptors (GRPr). PSMA is a membrane antigen expressed on prostate cancer cells. Here we present a comparison of 2 novel PET tracers, 68Ga-RM2 and 68Ga-PSMA, in patients with BCR with prior non-contributory conventional imaging. Methods: Ten men (67-83 year-old; mean ± SD: 74.3 ± 5.9) with BCR PC were imaged with 68Ga-RM2 and 7 of them also had 68Ga-PSMA PET. The SUVmax and SUVmean measurements were recorded in normal tissues and areas of uptake outside the expected physiologic biodistribution. Results: All 7 patients who had both scans had rising PSA (range: 3.5-36.5 ng/mL; mean±SD: 13.5 ± 11.5) and non-contributory conventional imaging (CT, MRI, 18F FDG PET/CT, 18F NaF PET/CT, 99mTc MDP bone scan). 68Ga-PSMA had the highest physiologic uptake in the salivary glands and small bowel, with hepatobiliary and renal clearance noted, while 68Ga-RM2 had the highest physiologic uptake in the pancreas, with renal clearance noted. Uptake values uptake outside the expected physiologic biodistribution were not statistically different between 68Ga-PSMA and 68Ga-RM2. 68Ga-PSMA localized in a lymph node and seminal vesicle in one patient with no abnormal 68Ga-RM2 uptake. In 2 patients abdominal peri-aortic lymph nodes were more easily visualized by 68Ga-RM2 due to lack of interference by radioactivity accumulation in the small intestine. Conclusions: 68Ga-PSMA and 68Ga-RM2 have distinct biodistribution in this small cohort of patients with BCR PC. High uptake in multiple areas outside the expected physiologic biodistribution suggests that both 68Ga-PSMA and 68Ga-RM2 are promising PET radiopharmaceuticals for localization of disease in patients with BCR PC and non-contributory conventional imaging. The differences in target density, biodistribution and clearance pathways may result in the superiority of one of the two tracers in different patients. 68Ga-RM2 shows similar sensitivity to 68Ga-PSMA and has the potential for being FDA-approved in the future. Clinical trial information: NCT02440308.