In situ Generated ²¹²Pb-PSMA Ligand in a ²²⁴Ra-Solution for Dual Targeting of Prostate Cancer Sclerotic Stroma and PSMA-positive Cells

Abstract

Background: New treatments combating bone and extraskeletal metastases are needed for patients with metastatic castrationresistant prostate cancer. The majority of metastases overexpress prostate-specific membrane antigen (PSMA), making it an ideal candidate for targeted radionuclide therapy. Objective: The aim of this study was to test a novel liquid 224Ra/212Pb-generator for the rapid preparation of a dual-alpha targeting solution. Here, PSMA-targeting ligands are labelled with 212Pb in the 224Ra-solution in transient equilibrium with daughter nuclides. Thus, natural bone-seeking 224Ra targeting sclerotic bone metastases and 212Pb-chelated PSMA ligands targeting PSMA-expressing tumour cells are obtained. Method: Two PSMA-targeting ligands, the p-SCN-Bn-TCMC-PSMA ligand (NG001), specifically developed for chelating 212Pb, and the most clinically used DOTA-based PSMA-617 were labelled with 212Pb. Radiolabelling and targeting potential were investigated in situ, in vitro (PSMA-positive C4-2 human prostate cancer cells) and in vivo (athymic mice bearing C4-2 xenografts). Results: NG001 was rapidly labelled with 212Pb (radiochemical purity >94% at concentrations of ≥15 µg/ml) using the liquid 224Ra/212Pbgenerator. The high radiochemical purity and stability of [212Pb]Pb-NG001 were demonstrated over 48 hours in the presence of ascorbic acid and albumin. Similar binding abilities of the 212Pb-labelled ligands were observed in C4-2 cells. The PSMA ligands displayed comparable tumour uptake after 2 hours, but NG001 showed a 3.5-fold lower kidney uptake than PSMA-617. Radium-224 was not chelated and, hence, showed high uptake in bones. Conclusion: A fast method for the labelling of PSMA ligands with 212Pb in the 224Ra/212Pb-solution was developed. Thus, further in vivo studies with dual tumour targeting by alpha-particles are warranted.

Preclinical and Clinical Status of PSMA-Targeted Alpha Therapy for Metastatic Castration-Resistant Prostate Cancer

“…Here, a 224 Ra solution in transient equilibrium with daughter radionuclides was used for in situ labeling of a PSMA-targeting ligand, i.e., 212 Pb is complexed by the ligand in the presence of 224 Ra [95,184,185]. The resulting solution has dual-targeting properties; natural bone-seeking 224 Ra will target osteoblastic metastatic lesions, and the 212 Pb-labeled PSMA ligand will target extraskeletal metastases by selective binding to the surface of PSMA-expressing cells [95]. Thus, the dual-alpha approach is providing a bone-seeker and a PSMA-seeker in one radiopharmaceutical solution.…”

“…Several research groups have developed PSMA-targeting ligands specifically designed for 212 Pb chelation that show promising tumor-targeting in preclinical models [ 91 , 92 , 94 , 95 ]. The urea-based PSMA ligands NG001, labeled with 212 Pb, and CA012 and L2, labeled with the surrogate 203 Pb, show high tumor uptake (8–25%ID/g) 1–2 h post-injection in mice bearing PSMA-positive tumors, which is comparable with clinically used 177 Lu-PSMA-617 [ 91 , 92 , 94 , 130 , 148 ].…”

“…An ideal PSMA-TAT for mCRPC must combat the entire spectrum of metastases present in the patients. A dual-alpha approach that uses the high LET ionizing energy of 224 Ra and daughter radionuclides to target various mCRPC lesions have been presented [ 95 ]. Here, a 224 Ra solution in transient equilibrium with daughter radionuclides was used for in situ labeling of a PSMA-targeting ligand, i.e., 212 Pb is complexed by the ligand in the presence of 224 Ra [ 95 , 184 , 185 ].…”

“…A dual-alpha approach that uses the high LET ionizing energy of 224 Ra and daughter radionuclides to target various mCRPC lesions have been presented [ 95 ]. Here, a 224 Ra solution in transient equilibrium with daughter radionuclides was used for in situ labeling of a PSMA-targeting ligand, i.e., 212 Pb is complexed by the ligand in the presence of 224 Ra [ 95 , 184 , 185 ]. The resulting solution has dual-targeting properties; natural bone-seeking 224 Ra will target osteoblastic metastatic lesions, and the 212 Pb-labeled PSMA ligand will target extraskeletal metastases by selective binding to the surface of PSMA-expressing cells [ 95 ].…”

“…Thus, the dual-alpha approach is providing a bone-seeker and a PSMA-seeker in one radiopharmaceutical solution. The accumulation of 224 Ra in bone and 212 Pb-labeled PSMA ligand in tumor sites was verified in C4-2 tumor-bearing mice and warrants further investigation in vivo [ 95 ].…”