PSMA-targeted radioligand therapy is a promising approach for the treatment of advanced prostate cancer; however, the clinical efficacy of [177Lu]Lu-PSMA-617 (Pluvicto®) is limited by the relatively low cytotoxic potency of the β-emitting radionuclide 177Lu (t1/2: 6. 65 d). This has driven high interest in α-emitting radionuclides, such as 212Pb (t1/2: 10.64 h) and 225Ac (t1/2: 9.92 d), which deliver high Linear Energy Transfer (LET) and cause more potent tumor cell killing. In this work, we assessed the radiolabeling and in vitro characteristics of 212Pb- and 225Ac-labeled PSMA-617 compared with [177Lu]LuPSMA-617, using two different PSMA-positive prostate cancer cell lines, LNCaP-AR and DU145-PSMA, along with paired negative controls.
The radioligands were synthesized with an isolated radiochemical yield of > 95% for [177Lu]Lu-PSMA-617 and about 45% for [212Pb]Pb-PSMA-617 and [225Ac]Ac-PSMA-617. The molar activity after Sep-Pak purification was about 37.0 MBq/nmol for [177Lu]Lu-PSMA-617, 4.4-11.1 MBq/nmol for [212Pb]Pb-PSMA-617, and 0.22-0.55 MBq/nmol for [225Ac]Ac-PSMA-617. In vitro stability studies in PBS, human serum, and whole blood revealed ≥ 90% stability for [177Lu]Lu-PSMA-617 (up to 5 days) and [212Pb]Pb-PSMA-617 (24 h), whereas [225Ac]Ac-PSMA-617 exhibited ~ 72% stability in PBS, > 94% in serum, and > 86% in whole blood for 5 days. The uptake of [177Lu]Lu-PSMA-617 in LNCaP-AR cells was 7.5 ± 0.6%, with about 33% of the cell-bound activity internalized at 4 h. The uptake and internalization were significantly higher in the PSMA-overexpressing cell line DU145-PSMA (31.1 ± 0.6%, 65% internalized). Compared to [177Lu]Lu-PSMA-617, [212Pb]Pb-PSMA-617 showed about 2-fold higher uptake and increased internalization in LNCaP-AR cells at 4 h (14.2 ± 0.0%, 47.1% internalized), but a similar uptake in DU145-PSMA cells (29.3 ± 0.7%, 62% internalized). In contrast, [225Ac]Ac-PSMA-617 exhibited lower uptake in both cell lines, with 1.6 ± 0.2% in LNCaP-AR cells and 4.6 ± 0.2% in DU145-PSMA cells after 4 h incubation. For all 3 radioligands, the uptake could be fully blocked by co-incubation with unlabeled PSMA-617 (300 nM), confirming the specificity of binding to PSMA, and the uptake was minimal in the paired PSMA-negative cell lines CWRR1-EnzR and DU145. In saturation binding assays, the three radioligands exhibited comparable binding affinities (KD) in LNCaP-AR and DU145-PSMA cell lines, with 5.9 ± 0.7 nM and 2.3 ± 0.3 nM for [177Lu]Lu-PSMA-617, 5.7 ± 0.5 nM and 5.8 ± 0.9 nM for [225Ac]Ac-PSMA-617, and 2.7 ± 0.5 nM and 8.0 ± 1.1 nM for [212Pb]Pb-PSMA-617, respectively.
[212Pb]Pb-PSMA-617 showed similar or better uptake in PSMA-positive cell lines compared to [177Lu]Lu-PSMA-617. While [225Ac]Ac-PSMA-617 also showed selective and specific uptake in the two PSMA-positive cell lines, the uptake levels were substantially lower, likely due to the low molar activities achievable for [225Ac]Ac-PSMA-617 compared to [212Pb]Pb-PSMA-617 or [177Lu]Lu-PSMA-617.
EJNMMI radiopharmacy and chemistry. 2026 May 20 [Epub ahead of print]
Abhijit Bera, Graham Ragland, Yuhan Zhang, Patricia G Madel, Jasmine B'Lanton, Atchimnaidu Siriki, Chin-Tu Chen, Russell Z Szmulewitz, Satish K Chitneni
Department of Radiology, The University of Chicago, Chicago, IL, USA., Department of Medicine, The University of Chicago, Chicago, IL, USA., Department of Radiology, The University of Chicago, Chicago, IL, USA. .