Synthesis and Preclinical Evaluation of Novel 203Pb-PSMA Radioligands as Leads for the Development of 203/212Pb-PSMA Theranostics.

Abstract

PURPOSE: This study aims to synthesize and characterize novel radioligands [203Pb]Pb-PSC-PSMA-617 and [203Pb]Pb-macropa-PSMA-617 as next-generation PSMA-targeted radiopharmaceuticals. We evaluated their radiolabelling efficiency, radiochemical stability, PSMA-specific binding in vitro, and in vivo biodistribution, tumour targeting, and clearance kinetics, with comprehensive SPECT/CT imaging to establish a preclinical foundation for the development and testing of 203/212Pb radiotheranostics. METHODS: Chemical synthesis of PSC-PSMA-617 and macropa-PSMA-617 conjugates was followed by radiolabelling with 203Pb2+ at room temperature. In vitro competitive binding assays were performed using LNCaP cells with [18F]PSMA-1007 as radioligand to determine IC50 values. Cellular internalization studies assessed radioligand uptake over 2 h. Ex vivo biodistribution studies were conducted in male athymic nude mice bearing LNCaP xenografts at 24 h post-injection, with PSMA specificity confirmed by blocking studies using 2-PMPA. SPECT/CT imaging was performed in Fox Chase CB17 severe combined immunodeficient (SCID) male mice with LNCaP tumours at 1, 24, 48, and 72 h post-injection to visualize tumour targeting and clearance kinetics. RESULTS: Both [203Pb]Pb-PSC-PSMA-617 and [203Pb]Pb-macropa-PSMA-617 were synthesized with high radiolabelling efficiencies (> 99%) within 10 min and demonstrated excellent in vitro stability in PBS over up to 72 h. The novel PSMA ligands exhibited strong PSMA-binding affinity, with IC50 values of 8 nM (natPb-PSC-PSMA-617) and 5 nM (natPb-macropa-PSMA-617). Cellular internalization reached 75-85% by 120 min. Biodistribution studies revealed high tumour uptake at 24 h post-injection for both radioligands: 11.94 ± 1.38%ID/g ([203Pb]Pb-PSC-PSMA-617) and 5.72 ± 1.04%ID/g ([203Pb]Pb-macropa-PSMA-617). SPECT/CT imaging demonstrated clear tumour visualization with distinct temporal patterns for both radioligands. [203Pb]Pb-PSC-PSMA-617 showed moderate tumour uptake at 1 h (SUV = 0.64 ± 0.28) that remained stable at 24 h (SUV = 0.56 ± 0.21), followed by slow washout to SUV = 0.54 ± 0.04 at 48 h and SUV = 0.41 ± 0.03 at 72 h. In contrast, [203Pb]Pb-macropa-PSMA-617 exhibited higher initial tumour accumulation at 1 h (SUV = 1.07 ± 0.21) that increased to a peak of SUV = 1.48 ± 0.18 at 24 h, followed by significant washout with SUV = 1.14 ± 0.09 at 48 h and SUV = 0.70 ± 0.08 at 72 h. Although image-derived tumour SUVs were higher for the macropa conjugate, ex vivo biodistribution demonstrated greater tumour uptake for the PSC analogue, likely reflecting methodological differences between SUV and %ID/g quantification together with the greater non-specific uptake and slower clearance of the macropa radioligand. The macropa radioligand exhibited higher lipophilicity (LogD7.4 = - 1.44) than the PSC analogue (LogD7.4 = - 2.54), consistent with its slower clearance and greater non-specific uptake. PSMA-specificity was rigorously confirmed through blocking studies with 2-PMPA (10 mg/kg), which resulted in significant reduction in tumour uptake for both radioligands at 1 h (PSC: 67%; macropa: 23%) and 24 h (PSC: 75%; macropa: 40%). CONCLUSIONS: Next-generation Pb chelators PSC and macropa, when conjugated to PSMA-617, produce 203Pb-labeled radioligands with efficient radiolabelling, robust stability, and PSMA-specific tumour targeting, along with biodistribution profiles well suited for 203/212Pb theranostic applications. Notably, [203Pb]Pb-PSC-PSMA-617 demonstrates somewhat greater PSMA specificity compared to [203Pb]Pb-macropa-PSMA-617. The mild labelling conditions, strong binding affinity, and favorable pharmacokinetic properties provide a solid preclinical basis for advancing 212Pb-based targeted alpha therapy in advanced prostate cancer.

Abstract as supplied by the indexing source. © the authors / publisher.