Radioiodinated Small Molecule Tyrosine Kinase Inhibitor for HER2 Selective SPECT Imaging

Abstract

Objectives: One of the most clinically relevant molecular aberrations in breast cancer is the overexpression of human epidermal growth factor receptor type 2 (HER2). We aimed to develop a radiolabeled tyrosine kinase inhibitor (TKI) for HER2 targeted breast cancer imaging. In this study, a radioiodinated analog (125/¹³¹I-IBA-CP) of the HER2 selective inhibitor CP724,714 was prepared and evaluated in HER2 positive or negative subcutaneous human breast cancer xenografts. Methods: The CP724,714 analog IBA-CP was synthesized and assayed for its inhibitory activities against HER2 and six other tyrosine kinases. 125/¹³¹I-IBA-CP was prepared using a copper-mediated radioiodination method with enhanced labeling yield and molar activity. In vitro biological activities, including specific and nonspecific binding of ¹³¹I-IBA-CP to its HER2 kinase target were assessed in different cell lines. In vivo microSPECT imaging with ¹²⁵I-IBA-CP and biodistribution studies were conducted in mice bearing HER2-positive, HER2-negative or epidermal growth factor receptor (EGFR)-positive tumors. Nonradioactive IBA-CP and the EGFR inhibitor erlotinib were employed as blocking agents to investigate the binding specificity and selectivity of 125/¹³¹I-IBA-CP toward HER2 in vitro and in vivo. Additionally, 125/¹³¹I-ICP was prepared by direct radioiodination of CP724,714 for comparison with 125/¹³¹I-IBA-CP. Results: IBA-CP displayed good in vitro inhibitory activity (IC50 = 16 nM) and selectivity for HER2 over six other cancer-related tyrosine kinases. 125/¹³¹I-IBA-CP was prepared in a typical radiochemical yield of about 65% (decay-corrected), radiochemical purity of >98%, and molar activity of 42 GBq/μmol at the end of synthesis. SPECT imaging revealed significantly higher uptake of ¹²⁵I-IBA-CP than ¹²⁵I-ICP in the HER2-positive MDA-MB-453 tumor. Uptake in the HER2-negative MCF-7 tumor was much lower. Binding of ¹²⁵I-IBA-CP in the MDA-MB-453 tumor was blocked by co-injection with an excess amount of IBA-CP, but not by erlotinib. Conclusion: The radiolabeled HER2 selective inhibitor 125/¹³¹I-IBA-CP is a promising probe for in vivo detection of HER2-positive tumors.