68Ga-RM2 Compared to 68Ga-PSMA-617 PET/CT for Prostate Cancer Imaging According to Various Metastatic Risks

Approximately 15% of men with prostate cancer have high-risk disease at diagnosis. For these patients the accuracy of initial staging is of critical importance for treatment decision-making. Recommended imaging modalities for initial staging include computerized tomography (CT) scan, bone scan, and Magnetic Resonance Imaging (MRI). In addition to initial work-up, 18F-Choline Positron Emission Tomography coupled with scanner (PET-CT) may be proposed in some high-risk patients but its sensitivity for lymph node detection remains limited. Nowadays, new radiotracers are becoming available for prostate cancer imaging. Among them, PET-CT imaging with radiolabeled ligands of prostate specific membrane antigen (PSMA) could be more sensitive and more specific for the detection of lymph node metastasis in high-risk cancers, as it is the case with radiolabeled PSMA-617 which has demonstrated very promising results in men with metastatic prostate cancer in recent studies. Therefore, PET imaging with 68Ga-PSMA-617 may participate to optimize work-up in the staging of high-risk patients. Another family of radiopharmaceuticals aimed to target the Gastrin-Releasing Peptide Receptor (GRP-R) which is overexpressed in low-grade prostatic carcinomas. GRP-R expression is correlated with androgen receptor expression and with better outcomes. Various radiolabeled GRP analogues have been developed and one of them, 68Ga-RM2, has shown very promising preclinical results. A study in 14 patients with prostate cancer showed encouraging results as related to the detection of primary prostate cancer and metastatic lymph nodes as well as in detection of local recurrence in the prostate bed and nodal relapse. However, 68Ga-RM2 failed to show some bone metastases in hormone-refractory patients. 68Ga-RM2 has also been recently used and compared to 68Ga-PSMA-11 for targeting biochemically recurrent prostate cancer. These radiotracers may offer complementary performances in lymph nodes detection due to their distinct pharmacokinetics. Since 68Ga-RM2 and 68Ga-PSMA-617 target different cell populations, combining these two radiopharmaceuticals in patients could be of additional value. The aim of this pilot study is to compare 68Ga-PSMA-617 PET/CT to 68Ga-RM2 PET/CT in 24 patients with prostate cancer of various progression risks to better understand how they could performed a metastatic risk mapping and how they could be used (or combined) in clinical practice.