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

Abstract

Hepatocellular carcinoma (HCC) is a significant cause of morbidity and mortality worldwide with limited therapeutic options for advanced disease. Targeted alpha therapy (TAT) is an emerging class of targeted cancer therapy in which alpha-particle-emitting radionuclides, such as thorium-227, are specifically delivered 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 227Th-labeled GPC3 targeting antibody conjugate (227Th-octapa-αGPC3) for treatment of HCC in an orthotopic murine model. METHODS The chelator p-SCN-Bn-H4octapa-NCS (octapa) was conjugated to a GPC3 targeting antibody (αGPC3) for subsequent 227Th radiolabeling (octapa-αGPC3). Conditions were varied to optimize radiolabeling of 227Th. In vitro stability was evaluated by measuring percentage of protein-bound 227Th by gamma-ray spectroscopy. An orthotopic athymic Nu/J murine model using HepG2 cells was developed. Biodistribution and blood clearance of 227Th-octapa-αGPC3 was evaluated in tumor bearing mice. Efficacy of 227Th-octapa-αGPC3 was assessed in tumor bearing animals with serial measurement of serum alpha-fetoprotein at 23 days after radionuclide injection. RESULTS Octapa-conjugated αGPC3 provided up to 70% 227Th labeling yield in 2 h at room temperature. In the presence of ascorbate, ≥97.8% of 227Th was bound to αGPC3-octapa after 14 d in phosphate buffered saline. In HepG2 tumor-bearing mice, highly specific GPC3 targeting was observed, with significant 227Th-octapa-αGPC3 accumulation in the tumor over time and minimal accumulation in normal tissue. 23 days after treatment, significant reduction in tumor burden was observed in mice receiving 500 kBq/kg 227Th-octapa-αGPC3 by tail vein injection. No acute off-target toxicity was observed and no animals died prior to termination of the study. CONCLUSION 227Th-octapa-αGPC3 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.