Abstract 3635: Improved efficacy of HER2 targeted-immunotoxins using photochemical internalization (PCI)

Abstract

Abstract Human epidermal growth factor receptor 2 (HER2) is overexpressed in several cancers (breast, ovary, stomach, colon and esophagus) and is an attractive target for delivery of cancer therapeutics. However, both primary and acquired resistance to current anti-HER2 therapies is of major clinical concern and calls for more effective HER2 targeted treatment strategies. We here present photochemical internalization (PCI) of HER2 targeted immunotoxins (ITs) as a novel treatment strategy for HER2 positive cancers. PCI has been shown to result in release of macromolecular therapeutics entrapped in endocytic vesicles. The method is based on photosensitizers (PSs) which localize to the membranes of endocytic vesicles, and subsequent membrane rupture upon light exposure so that the drugs can escape into the cytosol. The purpose of this study was to evaluate photochemical internalization (PCI) of HER2 targeted immunotoxins (ITs) with various binding sites and affinities. ITs were constructed by chemically coupling the monoclonal HER2 antibody, trastuzumab, to a type I ribosome inactivating protein (RIP), saporin, through a biotin-streptavidin linkage, or by genetically fusing human single-chain antibodies (scFv) to recombinant gelonin, another type I RIP. The cytotoxic effect of PCI of the ITs was assessed against HER2 positive Zr-75-1 and BT-20 cells and HER2 negative MDA-MB231 cells. The present data confirm synergistic cytotoxicity of PCI of trastuzumab-based and HER2 scFv-based ITs compared to treatment with ITs and photodynamic therapy (PDT) alone (PS+light). At LD50-PDT a subtoxic dose of 100pM of the different ITs was sufficient to kill 75-85% of Zr-75-1 cells. LD95 of PCI of the recombinant fusions was detected between 1 and 10nM of all 4 ITs (subtoxic dosage) at a photodynamic dose reducing the viability of the BT-20 cells to 50%. PCI of the ITs was shown to be much more effective compared to PCI of the respective toxins alone, as no significant increase in toxicity was found by PCI of 0.1nM of the toxins. Animal model studies to assess the ability of PCI to augment the efficacy of immunotoxins are ongoing. These studies suggest that the selectivity of HER2-based ITs may be further improved by the addition of an independent, selective strategy such as PCI which augments efficacy only in tumor cells. Research Conducted, in part, by the Norwegian Cancer Society and the Clayton Foundation for Research. 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 3635. doi:10.1158/1538-7445.AM2011-3635