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 castration-resistant 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.

Methods: 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/212Pb-generator. 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.

Keywords: 212Pb; 224Ra/212Pb-liquid generator; NG001; PSMA-617; TCMC; metastatic castration-resistant prostate cancer; targeted alpha therapy.

Fig. (1)

The decay chain of ²²⁸Th to stable ²⁰⁸Pb. Alpha-emission energies (E_α), beta-emission energies (E_β), recoil energies (E_R) and the half-lives for ²²⁸Th and daughter nuclides are specified [21]. (A higher resolution / colour version of this figure is available in the electronic copy of the article).

Fig. (2)

The decay of pure ²¹²Bi (A) and ²¹²Pb (B) and ingrowth/decay of their progenies. (C) Ingrowths and decays of ²¹²Pb and other progenies from ²²⁴Ra in a liquid generator system starting with a pure ²²⁴Ra source [22]. Grey areas present time frames for the suitable clinical use of the radiopharmaceuticals. (A higher resolution / colour version of this figure is available in the electronic copy of the article).

Fig. (3)

A simplified scheme comparing the ²¹²Pb production and labelling of chelated ligands or monoclonal antibodies (mAb) for the end user using (A) a standard ²²⁴Ra/²¹²Pb-generator [19] and (B) a ²²⁴Ra/²¹²Pb-liquid generator solution [17]. (A higher resolution / colour version of this figure is available in the electronic copy of the article).

Fig. (4)

Radiochemical purity (RCP) values as mean ± SD of ²¹²Pb and ²¹²Bi after the radiolabelling of different concentrations of NG001 in the ²²⁴Ra-solution in transient equilibrium with progeny. (A higher resolution / colour version of this figure is available in the electronic copy of the article).

Fig. (5)

Radiochemical purity (RCP) values (top) and cell binding fraction (bottom) expressed as mean ± SD of ²¹²Pb-labelled NG001 in ²²⁴Ra-solution containing no scavengers, 2% L-ascorbic acid, 2% HSA or both scavengers when incubated at activity concentrations of 2.8-5 MBq/ml up to 48 hours (at room temperature, n=2-3). (A higher resolution / colour version of this figure is available in the electronic copy of the article).

Fig. (6)

Percentage of injected activity per gram of tissue (%ID/g) ± SD of [²¹²Pb]PbCl₂, [²¹²Pb]Pb-NG001 and [²¹²Pb]Pb-PSMA-617 (top) and radium-224 (bottom) at 2 hours after intravenous injection of ²²⁴Ra-solution containing [²¹²Pb]PbCl₂, [²¹²Pb]Pb-NG001 or [²¹²Pb]Pb-PSMA-617 in athymic mice bearing human prostate C4-2 cancer xenografts (n=3-5 mice per group). (A higher resolution / colour version of this figure is available in the electronic copy of the article).