Abstract LB-511: In vivo imaging of tumor vasculature with a novel near infra-red lectin agent

Proceedings: AACR 103rd Annual Meeting 2012‐‐ Mar 31‐Apr 4, 2012; Chicago, IL Tumors induce significant blood vessel development in order to support their aggressive growth and progression, and the extensive and disordered nature of tumor vasculature impairs drug delivery and efficacy. Destroying the tumor vasculature and/or inhibiting neo-vascularization, alone or in combination with traditional chemotherapies, has become a well-accepted and proven cancer treatment strategy. A well-known tool for studying tumor angiogenesis and measuring microvessel density is tomato (Lycopersicon esculentum) lectin, a single polypeptide glycoprotein that readily binds to sugar-containing proteins present on the endothelium. Our objective was to develop a near infra-red (NIR) tomato lectin agent in order to non-invasively assess tumor vasculature in vivo. A NIR fluorophore, VivoTag 680 XL (e=210,000 M/cm; abs/em max 665/688 nm), was conjugated to tomato lectin to produce an imaging agent, tLectin 680. The conjugation was carried out by addition of the fluorophore in a DMSO solution to lectin in aqueous sodium bicarbonate. Yields of greater than 95% were achieved, based on absorbance with an average loading of 2 dyes per lectin. In vitro, tLectin 680 preferentially labeled primary endothelial cells from human umbilical veins. Specificity of binding was confirmed by control experiments using free dye and competitive blockade with unlabeled excess tomato lectin. In vivo, we used tLectin 680, in combination with Fluorescence Molecular Tomography (FMT), for non-invasive imaging and quantification of tumor neo-vasculature in Lewis Lung Carcinoma tumors implanted in nude mice. FMT imaging quantified a statistically significant difference between the concentrations of localized tLectin 680 in tumors implanted in the flank of nude mice versus in control (non-tumor) contra-lateral flanks (50.96 +12 versus 2.32 +1 pmol), as early as 6 hours after intravenous delivery. Specific localization of the agent to the tumor vasculature was confirmed by fluorescence microscopy of frozen tumor sections and by comparison to FITC-labeled CD31. In addition vessel counts performed ex vivo in frozen sections of different tumor cell lines by fluorescence microscopy, showed a good correlation (R2= 0.99) between CD31 and tLectin 680 signal: Lewis Lung Carcinomas (27.7 vessels/sample tLectin vs. 32 vessels/sample CD-31), HT-29 (13.4 vessels/sample vs. 15.4 vessels/sample), and matrigel plugs (5.5 vessels/sample vs. 7 vessels/sample). In vivo tLectin 680 signal was also shown to correlate with ex vivo microscopy (R2= 0.90) in these tumors (Lewis Lung Carcinomas 177.6 + 15 nM, 27.7 vessel/sample, HT-29 118.1 + 6 nM, 13.4 vessel/sample), and matrigel plugs 73.6 + 9 nM, 5.5 vessel/sample). These results underscore the potential of tLectin 680 combined with FMT imaging in assessing vascularity in vivo and in real time, improving the early detection and monitoring of anti-angiogenic treatments in cancer. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr LB-511. doi:1538-7445.AM2012-LB-511