Targeted alpha therapy of [212Pb]VMT-α-NET in metastatic neuroblastoma model

Introduction: Neuroblastoma (NB) is the most common extracranial solid tumor of childhood, accounting for 10% of all pediatric solid tumors. Although current multimodality treatment strategies are achieving excellent results for some children, over 50% of high-risk NB patients experience bone marrow and/or liver metastases and relapse within 4 years (with overall survival rates < 10%). NB tumors almost universally express somatostatin receptor subtype 2 (SSTR2), which establishes the feasibility of peptide-receptor radionuclide therapy. The elementally identical pair, ^203/212Pb, is ideal for diagnosis and targeted alpha therapy, when labeled to the novel SSTR2 peptide analogue peptide, PSC-PEG2-TOC (VMT-α-NET). Emerging evidence suggests that alpha-emitter therapy is significantly more potent than traditional beta-emitter therapy. In this study, we aim to develop a high-risk NB mouse model bearing both liver and bone marrow metastatic lesions and evaluate the efficacy of targeted alpha therapy using [²¹²Pb]VMT-α-NET.

Methods: Binding affinity of VMT-α-NET was determined by competitive receptor replacement assay on NB cell lines of IMR32 and IMR32-luc. VMT-α-NET was radiolabeled with ²⁰³Pb or ²¹²Pb according to the established method. Biodistribution was conducted by intravenous administration of [²⁰³Pb]VMT-α-NET in mice bearing subcutaneous IMR32 tumor xenografts. 2×10⁶ of IMR32-luc cells were implanted via intracardiac (IC) implantation in immunocompromised NSG mice. Mice were monitored for metastases formation by weekly imaging of luciferase signal using the IVIS imaging system. The metastases were further confirmed by overlaying with the SPECT imaging using [²⁰³Pb]VMT-α-NET and IHC staining against SSTR2 of dissected organs. For our therapy study, at about 3 weeks post IC inoculation, mice were treated with escalating levels of [²¹²Pb]VMT-α-NET. Efficacy was evaluated by luciferase signal by weekly imaging. Systemic toxicity was monitored by body weight change and main organ histopathological analyses at the time of euthanasia, hematological toxicity was assessed by complete blood count, and renal toxicity by urine biomarkers panel analyses. Studies involving animals were conducted under IACUC-approved protocols.

Results: VMT-α-NET demonstrated high binding affinity to IMR32 and IMR32-luc cells with IC₅₀ of 9.2 nm and 13.0 nm, respectively. Biodistribution profiles showed high tumor uptake and fast renal clearance. IC inoculation of IMR32-luc cells into NSG mice developed a continual increase of luciferase signal in liver and bone marrow by a magnitude of 10⁵ over 9 weeks of observation. IVIS imaging overlaid with SPECT imaging of [²⁰³Pb]VMT-α-NET indicated metastatic locations mostly in liver and bone marrow. Starting at 7 weeks, mouse death due to the large tumor burden was observed. IHC demonstrated SSTR2 positivity in liver. Efficacy study started 3 weeks post IC inoculation, is ongoing, and results will be presented. A previous pilot study suggested effective control of tumor development and regression.

Conclusions: VMT-α-NET is a novel SSTR2 analogue with high binding affinity to IMR32 and IMR32-luc cells. High-risk metastatic NB mouse model was established by IC inoculation of IMR32-luc cells to NSG mice and monitored for tumor progression. Technical steps to develop this model will be presented. Final data from the ongoing efficacy study will provide valuable evidence for targeted alpha therapy of [²¹²Pb]VMT-α-NET in treating high-risk pediatric NB patients. SPECT Imaging of [²⁰³Pb]VMT-α-NET will provide unique advantages in diagnosis of tumor lesions as well as dosimetry calculation for precise personalized targeted therapy of [²¹²Pb]VMT-α-NET.