Abstract 928: Glypican-3 targeted thorium-227 alpha therapy reduces tumor burden in an orthotopic xenograft model of hepatocellular carcinoma

Abstract The purpose of this study is to develop a thorium-227 (227Th) antibody radioimmunoconjugate targeting glypican-3 (GPC3) and to test its therapeutic efficacy in a hepatocellular carcinoma (HCC) orthotopic xenograft model. GPC3 targeting antibody (αGPC3) was conjugated to bifunctional chelator p-SCN-Bn-H4octapa (octapa), and αGPC3-octapa binding affinity for GPC3 was evaluated by flow cytometry. 227Th radiolabeling of this conjugate was optimized, and in vitro stability of 227Th-αGPC3-octapa was evaluated in PBS with and without free radical scavenging agent over 14 days. For in vivo evaluation, an orthotopic xenograft model was generated by hepatic subcapsular injection of human HCC HepG2 cells. In vivo biodistribution was assessed in blood, tumor and organs at 1, 7 and 21 days after tail vein injection of 227Th-αGPC3-octapa (500 KBq/kg). To test therapeutic efficacy, tumor-bearing animals were injected with 227Th-αGPC3-octapa (250 kBq/kg or 500 kBq/kg) and compared to an irrelevant control antibody, 227Th-αBHV1-octapa (500 kBq/kg), and to a no-treatment control. Tumor burden was assessed with serial serum alpha-fetoprotein (AFP) measurements, a marker of tumor burden validated in this model. Toxicity to 227Th-αGPC3-octapa was measured by serum comprehensive metabolic panel obtained 21 days after injection. GPC3 binding affinity was highest after conjugation of αGPC3 with 10 equivalents of octapa. The protein recovery from the conjugation process was >85%, and mass spectral analysis indicated an average of 3.3 octapa moieties per molecule of αGPC3-octapa. After two weeks, >98% of αGPC3-octapa and αBHV1-octapa had 227Th bound in the presence of scavenging agent. αGPC3-octapa maintained high affinity for GPC3 as measured by flow cytometry, indicating the octapa conjugation reaction did not alter immunoreactivity of αGPC3. In vivo, 227Th-αGPC3-octapa accumulated in the tumor over time and cleared from normal tissues with a %ID/g of <5% by 21 days after injection. Significant antitumor activity was observed after treatment with 227Th-αGPC3-octapa (500 kBq/kg) with mean AFP level of 10,696 ± 19,754ng/mL compared to 308,175 ± 362,372ng/mL and 145,154 ± 166,780ng/mL in the 227Th-αBHV1-octapa and no-treatment control group, respectively. Reductions in serum AFP was observed in all but one tumor-bearing mouse after 500kBq/kg 227Th-αGPC3-octapa therapy. Organ-specific toxicity was not observed after treatment with 227Th-αGPC3-octapa (500 KBq/kg) compared to no-treatment control. In conclusion, we report the development of a GPC3 targeted thorium conjugate and demonstrate its in vivo therapeutic efficacy in a murine orthotopic xenograft model of hepatocellular carcinoma. Citation Format: Kevin P. Labadie, Donald K. Hamlin, Aimee Kenoyer, Sara K. Daniel, Alan F. Utria, Andrew D. Ludwig, Heidi L. Kenerson, Delphine L. Chen, Johnnie Orozco, Raymond S. Yeung, Lily Li, Chris Orvig, Yawen Li, D Scott Wilbur, James O. Park. Glypican-3 targeted thorium-227 alpha therapy reduces tumor burden in an orthotopic xenograft model of hepatocellular carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 928.