Abstract 4507: Discovery of a novel single chain ribosome inactivating protein that selectively kills human melanoma cells

Few treatment options exist for patients with metastatic melanoma. The standard treatment, dacarbazine (DTIC), shows low response rates ranging from 15% to 25 % with an 8-month median survival time. The development of targeted therapeutics with novel mechanisms of action may improve patient outcome. Ribosome-inactivating proteins (RIPs) such as Shiga-like Toxin 1 (SLT-1) represent powerful scaffolds for developing selective anticancer agents. Here we report the discovery and properties of a single chain, ribosome-inactivating protein (scRIP), termed SLT-1A IYSNKLM that selectively targets and kills human melanoma cells. SLT-1A IYSNKLM was identified following the purification and screening of cytotoxic A chain mutants derived from a combinatorial SLT-1A library This scRIP was shown to selectively kill 7 out of 8 human melanoma cell lines while being ineffective at killing non-melanoma cell lines. Biodistribution and imaging studies of radiolabeled SLT-1A IYSNKLM in SCID mice bearing a human melanoma xenograft indicate that it readily accumulates at the tumor site. Furthermore, the co-administration of SLT-1A IYSNKLM [i.v.] with DTIC [i.p.] resulted in tumor regression and greatly increased animal survival in this mouse xenograft model in comparison to DTIC or SLT-1A IYSNKLM treatments alone. SLT-1A IYSNKLM is stable in serum and binds to human melanoma 518-A2 cells with a dissociation constant of 18 nM. The intravenous administration of SLT-1A IYSNKLM resulted in modest immune responses following repeated i.v. injections in immunocompetent CD1mice. These results suggest that scRIP templates can be evolved to target cancer cells and in the case of SLT-1A IYSNKLM can specifically kill human melanoma cells. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4507.