Imaging GRPr Expression in Metastatic Castration-Resistant Prostate Cancer with [ 68 Ga]Ga-RM2-A Head-to-Head Pilot Comparison with [ 68 Ga]Ga-PSMA-11.

BACKGROUND: The gastrin-releasing peptide receptor (GRPr) is highly overexpressed in several solid tumors, including treatment-naïve and recurrent prostate cancer. [68 Ga]Ga-RM2 is a well-established radiotracer for PET imaging of GRPr, and [177 Lu]Lu-RM2 has been proposed as a therapeutic alternative for patients with heterogeneous and/or low expression of PSMA. In this study, we aimed to evaluate the expression of GRPr and PSMA in a group of patients diagnosed with castration-resistant prostate cancer (mCRPC) by means of PET imaging.

METHODS: Seventeen mCRPC patients referred for radio-ligand therapy (RLT) were enrolled and underwent [68 Ga]Ga-PSMA-11 and [68 Ga]Ga-RM2 PET/CT imaging, 8.8 ± 8.6 days apart, to compare the biodistribution of each tracer. Uptake in healthy organs and tumor lesions was assessed by SUV values, and tumor-to-background ratios were analyzed.

RESULTS: [68 Ga]Ga-PSMA-11 showed significantly higher uptake in tumor lesions in bone, lymph nodes, prostate, and soft tissues and detected 23% more lesions compared to [68 Ga]Ga-RM2. In 4/17 patients (23.5%), the biodistribution of both tracers was comparable.

CONCLUSIONS: Our results show that in our cohort of mCRPC patients, PSMA expression was higher compared to GRPr. Nevertheless, RLT with [177 Lu]Lu-RM2 may be an alternative treatment option for selected patients or patients in earlier disease stages, such as biochemical recurrence.