Glypican-3 targeted thorium-227 alpha therapy reduces tumor burden in an orthotopic xenograft murine model of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is a significant cause of morbidity and mortality worldwide, with limited therapeutic options for advanced disease. Targeted alpha-therapy is an emerging class of targeted cancer therapy in which alpha-particle-emitting radionuclides, such as Th-227, are delivered specifically to cancer tissue. Glypican-3 (GPC3) is a cell surface glycoprotein highly expressed on HCC. In this study, we describe the development and in vivo efficacy of a Th-227-labeled GPC3-targeting antibody conjugate (Th-227-octapa-alpha GPC3) for treatment of HCC in an orthotopic murine model. Methods: The chelator p-SCN-Bn-H(4)octapa-NCS (octapa) was conjugated to a GPC3-targeting antibody (alpha GPC3) for subsequent Th-227 radiolabeling (octapa-alpha GPC3). Conditions were varied to optimize radiolabeling of Th-227. In vitro stability was evaluated by measuring the percentage of protein-bound Th-227 by gamma-ray spectroscopy. An orthotopic athymic Nu/J murine model using HepG2-Red-FLuc cells was developed. Biodistribution and blood clearance of Th-227-octapa-alpha GPC3 were evaluated in tumor-bearing mice. The efficacy of Th-227-octapa-alpha GPC3 was assessed in tumor-bearing animals with serial measurement of serum alpha-fetoprotein at 23 d after injection. Results: Octapa-conjugated alpha GPC3 provided up to 70% Th-227 labeling yield in 2 h at room temperature. In the presence of ascorbate, at least 97.8% of Th-227 was bound to alpha GPC3-octapa after 14 d in phosphate-buffered saline. In HepG2-Red-FLuc tumor-bearing mice, highly specific GPC3 targeting was observed, with significant Th-227-octapa-alpha GPC3 accumulation in the tumor over time and minimal accumulation in normal tissue. Twenty-three days after treatment, a significant reduction in tumor burden was observed in mice receiving a 500 kBq/kg dose of Th-227-octapa-alpha GPC3 by tail-vein injection. No acute off-target toxicity was observed, and no animals died before termination of the study. Conclusion: Th-227-octapa-alpha GPC3 was observed to be stable in vitro; maintain high specificity for GPC3, with favorable biodistribution in vivo; and result in significant antitumor activity without significant acute off-target toxicity in an orthotopic murine model of HCC.