PET Using a GRPR Antagonist Ga-68-RM26 in Healthy Volunteers and Prostate Cancer Patients

This study was designed to analyze the safety, biodistribution, and radiation dosimetry of a gastrin-releasing peptide receptor (GRPR) antagonist PET tracer, Ga-68-RM26; to assess its clinical diagnostic value in prostate cancer patients; and to perform a direct comparison between GRPR antagonist Ga-68-RM26 and agonist Ga-68-BBN. Methods: Five healthy volunteers were enrolled to validate the safety of Ga-68-RM26 and calculate dosimetry. A total of 28 patients with prostate cancer (17 newly diagnosed and 11 posttherapy) were recruited and provided written informed consent. All the cancer patients underwent PET/CT at 15-30 min after intravenous injection of 1.85 MBq (0.05 mCi) per kilogram of body weight of Ga-68-RM26. Among them, 22 patients (11 newly diagnosed and 11 posttherapy) underwent Ga-68-BBN PET/CT for comparison within 1 wk. Tc-99m-MDP (methylene diphosphonate) bone scans were obtained within 2 wk for comparison. GRPR immunohistochemical staining of tumor samples was performed. Results: The administration of Ga-68-M26 was well tolerated by all subjects, with no adverse symptoms being noticed or reported during the procedure and at 2-wk follow-up. The total effective dose equivalent and effective dose were 0.0912 +/- 0.0140 and 0.0657 +/- 0.0124 mSv/MBq, respectively. In the 17 patients with newly diagnosed prostate cancer, Ga-68-RM26 PET/CT showed positive prostate-confined findings in 15 tumors with an SUVmax of 6.49 +/- 2.37. In the 11 patients who underwent prostatectomy or brachytherapy with or without androgen deprivation therapy, Ga-68-RM26 PET/CT detected 8 metastatic lymph nodes in 3 patients with an SUVmax of 4.28 +/- 1.25 and 21 bone lesions in 8 patients with an SUVmax of 3.90 +/- 3.07. Compared with Ga-68-RM26 PET/CT, GRPR agonist Ga-68-BBN PET/CT detected fewer primary lesions and lymph node metastases as well as demonstrated lower tracer accumulation. There was a significant positive correlation between SUV derived from Ga-68-RM26 PET and the expression level of GRPR (P < 0.001). Conclusion: This study indicates the safety and significant efficiency of GRPR antagonist Ga-68-RM26. Ga-68-RM26 PET/CT would have remarkable value in detecting both primary prostate cancer and metastasis. Ga-68-RM26 is also expected to be better than GRPR agonist as an imaging marker to evaluate GRPR expression in prostate cancer.