Detection of Recurrent Prostate Cancer Using ⁶⁸Ga-RM2 PET/MRI in Patients with Negative Conventional Imaging

Objectives: ⁶⁸Ga-labeled DOTA-4-amino-1-carboxymethyl-piperidine-D-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH2 (⁶⁸Ga-RM2, formerly known as ⁶⁸Ga-Bombesin or BAY86-7548) is a synthetic bombesin receptor antagonist that targets gastrin-releasing peptide receptors (GRPr). GRPr are highly overexpressed in several human tumors, including prostate cancer (PC). Because of their low expression in BPH and inflammatory prostatic tissues, imaging GRPr has potential advantages over current choline- and acetate-based radiotracers.

Methods: We enrolled 31 men with BCR PC, 59-83 year-old (mean±SD: 68.7±6.8) in an IRB-approved prospective study. Imaging started at 40-69 minutes (mean±SD: 50.2±7.1) after injection of 3.6-4.1 mCi (mean±SD: 3.8±0.2) of ⁶⁸Ga-RM2 using a time-of-flight enabled simultaneous PET/MRI scanner. MRI sequences consisted of T1-weighted, T2-weighted and DWI. SUV_max and SUV_mean measurements were recorded in normal tissues and areas of uptake outside the expected physiologic biodistribution.

Results: All patients had rising PSA (range: 0.3-36.4 ng/mL; mean±SD: 7.4±8.1) and non-contributory CI (CT, MRI, ^99mTc MDP bone scan). High ⁶⁸Ga-RM2 uptake (SUV_max: 12.7 ± 7.8 [range: 2.6 - 33.5], SUV_mean: 5.7 ± 2.5 [range: 1.7 - 10.8]) corresponded to pelvic lymph nodes (8 patients), retroperitoneal lymph nodes (5 patients), prostate bed (3 patients), seminal vesicle (2 patients), supraclavicular lymph node (2 patients), mesenteric lymph nodes (1 patient), mediastinal lymph node (1 patient), liver (1 patient), lung (1 patient) and bone marrow (1 patient). ⁶⁸Ga-RM2 PET findings were compatible with recurrent prostate cancer in 21 of the 31 participants. MRI identified findings compatible with recurrent prostate cancer in 10 of the 31 patients (lymph nodes in 7 patients, prostate bed in 1 patient, lung in 1 patient and bone marrow in 1 patient). Findings were confirmed by biopsy in 7/31 cases (22.6%) or clinical follow-up in 24/31 cases (77.4%).

Conclusion: ⁶⁸Ga-RM2 is a promising PET radiopharmaceutical for localization of disease in participants with BCR PC and non-contributory conventional imaging. Research Support: Piramal Imaging, GmbH

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