Abstract
Abstract Alpha-amanitin blocks RNA polymerase II activity and is the most specific and most potent inhibitor of eukaryotic transcription. While widely used as a tool in molecular biology, only a few attempts have been made to employ alpha-amanitin as a therapeutic agent in oncology. The reason for this may be the high liver toxicity caused by the transport of amanitin into hepatocytes by the organic-anion transporter 1B3 (OATP1B3) present on hepatocytes, but not on other cell types. In this study we show that a direct impact on the transcription machinery can provide a novel approach in tumor therapy, provided that the amanitin can be targeted to tumor cells. This was achieved by using tumor-specific antibodies of high affinity armed with amanitin via suitable linker moieties. Using the HER2 specific monoclonal antibody Herceptin (Her) as a carrier, the antibody-drug conjugate (ADC) Her-DSC-30.0134 exhibits antiproliferative activity in HER2-expressing human tumor cell lines like SK-OV-3, SK-BR-3 and NCI-N87 in picomolar concentrations, indicating an up to 100.000-fold higher toxicity of the conjugate over non-conjugated alpha-amanitin. On cells lacking the HER2 receptor, e.g. HEK293 cells, toxicity of Her-DSC-30.0134 was not detected up to micromolar concentrations. This is also observed in HEK293 cells transfected with OATP1B3 (which incorporate amanitin similar as hepatocytes) indicating that amanitin-antibody-conjugates are no longer substrates for OATP1B3. These data suggest that a low hepatotoxicity of amanitin conjugates is expected. Subsequently, antitumoral activity of Her-DSC-30.0134 was evaluated in vivo in a s.c. xenograft mouse model using Herceptin-sensitive SK-OV-3 and Herceptin-resistant JIMT-1 cells. The ADC is well tolerated in nude mice and shows complete tumor remission in 7 out of 7 animals after a single i.v. injection in the SK-OV-3 and JIMT-1 models. By contrast, Herceptin shows only moderate antitumoral activity in the SK-OV-3 and no activity in the resistant JIMT-1 model, even after multiple injections. These data encourages further examination of amanitin-based antibody conjugates as highly potent and selective compounds for the treatment of various maligancies. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3616. doi:10.1158/1538-7445.AM2011-3616
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