(UroToday.com) The 2023 Society of Nuclear Medicine and Molecular Imaging (SNMMI) Annual Meeting held in Chicago, IL between June 24th and 27th, 2023 was host to a prostate cancer radiotracers session. Dr. Jens Kurth presented the results of an intra-individual analysis comparing the performance characteristics of [68Ga]Ga-RM2 and [68Ga]Ga-PSMA PET/CT in metastatic castrate-resistant prostate cancer (mCRPC) patients.
68Ga-PSMA-PET/CT is currently approved for the initial staging of unfavorable intermediate and high-risk prostate cancer patients, as well as for patients with evidence of biochemical failure following primary definitive therapy. However, in addition to PSMA, other cell surface membrane proteins have been identified as potential targets for PET-based imaging and theranostics. One such target is Gastrin-Releasing Peptide receptors (GRPr), which is a member of the bombesin family of receptors and has been shown to stimulate the growth of both androgen dependent and independent prostate cancer cells.1
RM2 is an anti-GRPr antibody that can be labeled with either 68Ga or 177Lu for theranostic purposes. 68Ga-RM2 PET/CT has demonstrated excellent diagnostic performance characteristics in patients with evidence of biochemically recurrent prostate cancer.2
The objective of this study was to perform a retrospective, intra-individual comparison of 68Ga-RM2 and 68Ga-PSMA-PET/CT for lesion detection in patients with mCRPC.
This study included 57 mCRPC patients who underwent both a 68Ga-RM2 and 68Ga-PSMA-PET/CT to identify RM2 or PSMA positivity as a prerequisite for radionuclide treatment with either 177Lu-RM2 or 177Lu-PSMA. Visual uptake and semiquantitative parameters (SUVmax, SUVpeak, and total tumor volume) were compared in all identified metastatic reference lesions.
Among the 57 mCRPC patients, 382 metastatic lesions were identified:
- Local: 7
- Lymph nodes: 132
- Bone: 212
- Liver: 23
- Lung: 7
- Other: 7
Of the 57 patients, three did not have evidence of PSMA- or RM2-positive lesions on visual assessment. The remaining 54 patients had evidence of PSMA uptake of any grade. Among these 54 patients, 37 had evidence of RM2 uptake (9 higher, 28 lower) and 17 had no evidence of RM2 uptake. There were discordant PET/CT imaging findings (i.e., PSMA+/RM2- or PSMA-/RM2+) in 4/57 patients. Overall, SUVmax and SUVpeak were significantly higher for PSMA-PET/CT, compared to RM2-PET/CT (p<0.001). When stratified by recurrent disease site, there was evidence of more intense uptake, as quantified by SUVmax and SUVpeak, in locally recurrent (p=0.051), lymph nodal (p<0.001), skeletal (p<0.001), and hepatic lesions (p=0.026).
Based on these results, Dr. Kurth concluded that the majority of mCRPC patients present with a higher number of PSMA-positive lesions and higher uptake on 68Ga-PSMA-PET/CT, compared to 68Ga-RM2-PET/CT. Simultaneous high uptake of PSMA and RM2 was uncommon, suggesting a fundamental difference in underlying tumor biology. Patients with absent or weak PSMA uptake, but high RM2 uptake, may benefit from the use of [68Ga]Ga-RM2 for restaging purposes and as a theranostic approach with additional [177Lu]Lu-RM2 therapy.
Presented by: Jens Kurth, PhD, Department of Nuclear Medicine, Rostock University Medical Centre, 18057 Rostock, Germany.
Written by: Rashid K. Sayyid, MD, MSc – Society of Urologic Oncology (SUO) Clinical Fellow at The University of Toronto, @rksayyid on Twitter during the 2023 Society of Nuclear Medicine and Molecular Imaging (SNMMI) Annual Meeting, Chicago, IL, Sat, June 24 – Tues, June 27, 2023.
References:- Baratto L, et al. Prostate Cancer Theranostics Targeting Gastrin-Releasing Peptide Receptors. Mol Imaging Biol, 2018;20(4):501-9.
- Mapelli P, et al. 68Ga-PSMA and 68Ga-DOTA-RM2 PET/MRI in Recurrent Prostate Cancer: Diagnostic Performance and Association with Clinical and Histopathological Data. Cancers (Basel), 2022;14(2):334.