Therapeutic Efficacy of Pb-214-labeled Trastuzumab in a Preclinical Model of Ovarian Cancer

93 Introduction: Targeted ɑ-particle therapy (TAT) may be a clinical solution to bypass resistance to traditional treatments. The study reports the first preclinical TAT using Pb-214-TCMC-trastuzumab for treatment of HER2-positive ovarian cancer. Pb-214 (half-life=26.8 min), which decays through a series of rapid ɑ and β- emissions, is produced from a new Rn-222/Pb-214 generator system. Methods: Pb-214 was separated from its grandparent radionuclide, Rn-222, in a novel generator system and eluted in 0.1M HCl. Trastuzumab antibody and isotype-matched control antibody (IgG) were conjugated with 2-(4-isothiocyanotobenzyl)-1,4,7,10-tetraaza-1,4,7,10-tetra-(2- carbamoylmethyl)-cyclododecane (TCMC:Ab; 6:1 molar ratio), radiolabeled with Pb-214, and G25 spin-column purified. Pb-214 bound to Ab (purity) was measured by iTLC. Binding assays used 96-well break-apart plates coated with ErbB2/Fc Chimera (n=7). Clonogenic survival assays used luciferase-positive SKOV-3 cells treated with 10 μCi of Pb-214-labeled antibodies. Female nude mice (8 wks) implanted intraperitoneally (IP) with 100,000 luciferase-positive SKOV-3 cells 3 weeks earlier were randomly assigned to 3 equal groups based on bioluminescence signal (n=5-6/group), injected 1X IP with 20 μCi Pb-214-TCMC-Trastuzumab (G1), 20 μCi Pb-214-TCMC-IgG (G2), or nothing (G3-untreated control). Cells and mice were imaged over time with an IVIS Spectrum. Results: Both antibodies were radiolabeled and purified in 25 minutes, in high yield and purity (>97%), specific activity ranged from 1-3 μCi/μg. Pb-214-TCMC-Trastuzumab retained high-affinity, specific binding to ErbB2. For clonogenic assays, at day 10 after Pb-214-TCMC-Trastuzumab or Pb-214-TCMC-IgG treatments, the mean imaging signals were 10+2% and 25+5%, respectively, relative to signal in untreated wells. Mice at 3 weeks after treatment showed the mean tumor signal for G1 decreased to 37% of starting signal. The mean tumor signal in G2 and G3 increased to 127% and 223%, respectively, compared with starting signal. There was no toxicity as determined by weight loss with all animals surviving. Conclusions: The ɑ-particle treatment of HER2-positive ovarian cancer with Pb-214-TCMC-Trastuzumab was effective in vitro and in a mouse model. The Rn-222/Pb-214 generator can be eluted hourly, at low cost which leads to better efficiency compared with alternative methods for ɑ-particle therapy.