Imaging of 212Pb in mice with a clinical SPECT/CT.

INTRODUCTION: 212Pb is a promising radionuclide for targeted alpha therapy. Here, the feasibility of visualising the tumour uptake and biodistribution of 212Pb-NG001 in mice with a clinical SPECT/CT scanner was investigated. METHODS: A mouse phantom with 212Pb was imaged with a clinical- and a preclinical SPECT/CT scanner. Different acquisition and reconstruction settings were investigated on the clinical system (Siemens Symbia Intevo Bold). Two athymic nude mice carrying PC-3 PIP prostate cancer tumours of 235-830 µl received 1.44 MBq of 212Pb-NG001 and were imaged 2, 6, and 24 h post-injection on the clinical SPECT/CT with a Medium Energy collimator and a 40% energy window centred on 79 keV. All acquisition times were 30 min, except the mouse imaging 24 h post-injection which was 60 min. After the final imaging, the organs were harvested and measured on a gamma counter to give an indication of how much activity was present in organs of interest at the last imaging time point. RESULTS: Four volumes in the mouse phantom of ~ 300 µl with 246-303 kBq/ml of 212Pb were distinguishable on images acquired with the clinical SPECT/CT with a high number of reconstruction updates. With the preclinical SPECT, the same volumes were easily distinguished with 49 kBq/ml of 212Pb. Clinical SPECT/CT images of the mice revealed uptake in tumours and bladders 2 h after injection and in tumours containing down to approximately 15 kBq/ml at 6 and 24 h after injection. CONCLUSION: Although the preclinical scanner should be used preferentially in biodistribution studies in mice, the clinical SPECT/CT confirmed uptake in small volumes (e.g. ~ 300 µl volume with ~ 250 kBq/ml). Regardless of system, the resolution and sensitivity limits should be carefully determined, otherwise false negative or too low uptakes can be wrongly interpreted.

Targeted alpha therapy with the 224Ra/212Pb-TCMC-TP-3 dual alpha solution in a multicellular tumor spheroid model of osteosarcoma.

Osteosarcoma patients with overt metastases at primary diagnosis have a 5-year survival rate of less than 20%. TP-3 is a murine IgG2b monoclonal antibody with high affinity for an epitope residing on the p80 osteosarcoma cell surface membrane antigen. The tumor-associated antigen p80 is overexpressed in osteosarcomas, and has very low normal tissue expression. We propose a novel dual alpha targeting solution containing two radionuclides from the same decay chain, including the bone-seeking 224Ra, and cancer cell-surface seeking 212Pb-TCMC-TP-3 for the treatment of osteoblastic bone cancers, circulating cancer cells and micrometastases. In this in vitro study, the cytotoxic effects of 212Pb-TCMC-TP-3 (single alpha solution) and 224Ra/212Pb-TCMC-TP-3 (dual alpha solution) were investigated in a multicellular spheroid model mimicking micrometastatic disease in osteosarcoma. OHS spheroids with diameters of 253 ± 98 µm treated with 4.5, 2.7, and 3.3 kBq/ml of 212Pb-TCMC-TP-3 for 1, 4, and 24 h, respectively, were disintegrated within 3 weeks. The 212Pb-TCMC-TP-3 induced a 7-fold delay in spheroid doubling time compared to a 28-times higher dose with the non-specific 212Pb-TCMC-rituximab. The 224Ra/212Pb-TCMC-TP-3 completely disintegrated spheroids with diameters of 218-476 µm within 3 and 2 weeks after 4 and 24 h incubation with 5 kBq/ml, respectively. Treatment with 1 kBq/ml of 224Ra/212Pb-TCMC-TP-3 for 24 h caused an 11.4-fold reduction in spheroid viability compared with unconjugated 224Ra/212Pb. The single and dual alpha solutions with TP-3 showed cytotoxicity in spheroids of clinically relevant size, which warrant further testing of the dual alpha solution using in vivo osteosarcoma models.

Factors Influencing the Therapeutic Efficacy of the PSMA Targeting Radioligand 212Pb-NG001.

This study aimed to determine the influence of cellular PSMA expression, radioligand binding and internalization, and repeated administrations on the therapeutic effects of the PSMA-targeting radioligand 212Pb-NG001. Cellular binding and internalization, cytotoxicity, biodistribution, and the therapeutic efficacy of 212Pb-NG001 were investigated in two human prostate cancer cell lines with different PSMA levels: C4-2 (PSMA+) and PC-3 PIP (PSMA+++). Despite 10-fold higher PSMA expression on PC-3 PIP cells, cytotoxicity and therapeutic efficacy of the radioligand was only 1.8-fold better than for the C4-2 model, possibly explained by lower cellular internalization and less blood-rich stroma in PC-3 PIP xenografts. Mice bearing subcutaneous PC-3 PIP xenografts were treated with 0.2, 0.4, and 0.8 MBq of 212Pb-NG001 that resulted in therapeutic indexes of 2.7, 3.0, and 3.5, respectively. A significant increase in treatment response was observed in mice that received repeated injections compared to the corresponding single dose (therapeutic indexes of 3.6 for 2 × 0.2 MBq and 4.4 for 2 × 0.4 MBq). The results indicate that 212Pb-NG001 can induce therapeutic effects at clinically transferrable doses, both in the C4-2 model that resembles solid tumors and micrometastases with natural PSMA expression and in the PC-3 PIP model that mimics poorly vascularized metastases.