Abstract 5857: HER2-targeted thorium-227 conjugate (HER2-TTC): Efficacy in a HER2 positive orthotopic bone model

In 2015, the estimated incidence of new breast cancer (Ca) cases in the US was 234.190 and number of deaths 40.730. Human epidermal growth factor receptor 2 (HER2) is encoded by the proto-oncogene c-erbB-2 and initiates downstream signaling pathways leading to cell proliferation and tumorigenesis. HER2 is overexpressed in several cancer types and has emerged as one of the most strongly validated targets for the treatment of breast and gastric cancer serving as both a prognostic and predictive biomarker. Given that 20% of breast Ca patients are HER2 positive and 70% of patients with metastatic disease will develop bone metastases and associated morbidities, there is still an unmet medical need for improved therapies targeting HER2. Radium-223 (Ra-223) is a novel targeted alpha therapeutic for treatment of patients with castration-resistant prostate cancer and bone metastases. Localized high energy alpha particle emission induces double-stranded DNA breaks and cellular apoptosis. Thorium-227 (Th-227) is the immediate precursor for Ra-223 which, in contrast to Ra-223, can be complexed by chelating agents at high affinity, allowing targeted delivery to tumor cells via antibodies. We describe herein the generation of a novel HER2-targeted Th-227 conjugate (HER2-TTC). HER2-TTC consists of the humanized HER2 targeting IgG1 antibody trastuzumab covalently linked via an amide bond to a 3,2-hydroxypyridino-based chelator moiety, enabling efficient radiolabeling with the alpha particle emitting radionuclide Th-227. Anonymized samples of consenting breast cancer patients were analyzed by Immunohistochemistry (IHC). The IHC data demonstrated HER2 positive expression in breast tumor and matched bone metastases, supporting the preclinical evaluation of the anti-tumor efficacy of HER2-TTC in the BT-474 orthotopic bone mouse model. HER2-TTC was prepared at high radiochemical yield and purity. When tested for binding to recombinant HER2, HER2-TTC was shown to retain comparable binding affinity to trastuzumab. In vitro cytotoxicity experiment of HER2-TTC demonstrated target mediated in vitro cytotoxicity in the pM-range on breast cancer cell line BT-474 (430 000 mAbs bound/ cell as determined by FACS). Anti-tumor efficacy of HER2-TTC was evaluated at 250 and 500 kBq/kg at a protein dose of 0.14 mg/kg. X-ray imaging, serum bone formation marker PINP, micro CT 3D reconstruction imaging and histological analysis demonstrated significantly reduced bone lesions and tumor induced bone remodeling. The promising preclinical anti-tumor activity supports the development of the HER2-TTC as a novel targeted alpha therapeutic for the treatment of patients with HER2 positive bone metastatic disease. Citation Format: Jenny Karlsson, Urs B. Hagemann, Christoph Schatz, Derek Grant, Christine Ellingsen, Alexander Kristian, Dessislava Mihaylova, Steinar R. Uran, Mari Suominen, Roger M. Bjerke, Olav B. Ryan, Carl F. Nising, Dominik Mumberg, Alan Cuthbertson. HER2-targeted thorium-227 conjugate (HER2-TTC): Efficacy in a HER2 positive orthotopic bone model [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5857. doi:10.1158/1538-7445.AM2017-5857