The Theranostic Optimization of PSMA-GCK01 Does Not Compromise the Imaging Characteristics of [(99m)Tc]Tc-PSMA-GCK01 Compared to Dedicated Diagnostic [(99m)Tc]Tc-EDDA/HYNIC-iPSMA in Prostate Cancer

Science and Research

PURPOSE: Radiolabeled PSMA-ligands play a major role in today's nuclear medicine. Since approval of [(177)Lu]Lu-PSMA-617 for therapy of metastatic prostate cancer, availability of (177)Lu became bottleneck of supply due to the high demand. Recently, a theranostic PSMA-ligand, PSMA-GCK01, was developed which can be labeled either diagnostically with (99m)Tc or therapeutically with (188)Re with both nuclides available from well-known generator systems. This novel tracer might aid to overcome aforementioned supply limitations. In this investigation, the biodistribution and general imaging characteristics of [(99m)Tc]Tc-PSMA-GCK01 were compared with the diagnostic reference compound [(99m)Tc]Tc-EDDA/HYNIC-iPSMA in patients with advanced stage prostate cancer. In addition, the binding of both ligands to PSMA was analyzed at the molecular level using molecular docking. PROCEDURES: Two cohorts (n = 19 vs. n = 21) of patients with metastatic castration-resistant prostate cancer matched for age, tumor stage, and Gleason score underwent a planar gamma camera imaging with [(99m)Tc]Tc-EDDA/HYNIC-iPSMA or [(99m)Tc]Tc-PSMA-GCK01 prior to PSMA-ligand therapy for PSMA-phenotyping. The imaging data were retrospective analyzed for salivary gland, kidney, liver, soft tissue, and tumor uptake on a semi-automated ROI-analysis using HERMES Medical Solutions AB (HMS, Sweden). RESULTS: The data sets were semi-automated quantified on a ROI-based analysis. The tumor-to-background presented equal results of [(99m)Tc]Tc-PSMA-GCK01 compared to [(99m)Tc]Tc-EDDA/HYNIC-iPSMA. The physiological PSMA-positive organs like salivary gland presented also equal uptake in counts/MBq (salivary gland median 9.48 [(99m)Tc]Tc-PSMA-GCK01 vs. median 9.11 [(99m)Tc]Tc-EDDA/HYNIC-iPSMA), while liver-to-kidney ratio presented a slight shift to the liver parenchyma using [(99m)Tc]Tc-PSMA-GCK01 (0.83) compared to [(99m)Tc]Tc-EDDA/HYNIC-iPSMA (0.55) with no statistical significance. This is in agreement with the results from the docking study revealing only a minor difference in the docking scores for both ligands. CONCLUSIONS: The novel theranostic tracer [(99m)Tc]Tc/[(188)Re]Re-PSMA-GCK01 demonstrates comparable general imaging characteristic with the reference compound [(99m)Tc]Tc-EDDA/HYNIC-iPSMA. These results pave the way for the PSMA-targeting imaging and theranostic agents for a broader, rather low-cost, generator applied radio-ligand therapy utilization.