Abstract
Angiosarcoma is a biologically aggressive vascular malignancy with a high metastatic potential. In the era of targeted medicine, knowledge of specific molecular tumor characteristics has become more important. Molecular imaging using targeted ultrasound contrast agents can monitor tumor progression non-invasively. Secreted frizzled related protein 2 (SFRP2) is a tumor endothelial marker expressed in angiosarcoma. We hypothesize that SFRP2-directed imaging could be a novel approach to imaging the tumor vasculature. To develop an SFRP2 contrast agent, SFRP2 polyclonal antibody was biotinylated and incubated with streptavidin-coated microbubbles. SVR angiosarcoma cells were injected into nude mice, and when tumors were established the mice were injected intravenously with the SFRP2 -targeted contrast agent, or a control streptavidin-coated contrast agent. SFRP2 -targeted contrast agent detected tumor vasculature with significantly more signal intensity than control contrast agent: the normalized fold-change was 1.6±0.27 (n = 13, p = 0.0032). The kidney was largely devoid of echogenicity with no significant difference between the control contrast agent and the SFRP2-targeted contrast agent demonstrating that the SFRP2-targeted contrast agent was specific to tumor vessels. Plotting average pixel intensity obtained from SFRP2-targeted contrast agent against tumor volume showed that the average pixel intensity increased as tumor volume increased. In conclusion, molecularly-targeted imaging of SFRP2 visualizes angiosarcoma vessels, but not normal vessels, and intensity increases with tumor size. Molecular imaging of SFRP2 expression may provide a rapid, non-invasive method to monitor tumor regression during therapy for angiosarcoma and other SFRP2 expressing cancers, and contribute to our understanding of the biology of SFRP2 during tumor development and progression.
Highlights
Angiosarcoma is a biologically aggressive vascular malignancy with a high metastatic potential and subsequent mortality [1]
Since angiosarcomas have been reported to represent the signaling abnormalities of pathogenic angiogenesis [4], we speculated that secreted frizzled related protein 2 (SFRP2) would be expressed in human angiosarcomas, which we confirmed by immunohistochemistry [5]
The apparent molecular weight of the biotinylated SFRP2 antibody was increased in the presence of streptavidin, verifying its ability to bind to streptavidin-coated contrast agent
Summary
Angiosarcoma is a biologically aggressive vascular malignancy with a high metastatic potential and subsequent mortality [1]. It originates from endothelial cells of small blood vessels and may affect a variety of organs, including the retroperitoneum, skeletal muscle, subcutis, liver, heart and breast. The DeMore laboratory has recently discovered a novel angiogenesis factor involved in angiosarcoma growth. SFRP2 acts as a novel stimulator of angiogenesis in vivo and in vitro by stimulating endothelial cell migration, protecting against apoptosis, and is required for and stimulates angiosarcoma tube formation [5]. We recently reported the generation of a murine monoclonal antibody to SFRP2 that inhibits angiosarcoma allograft and breast cancer xenograft growth in vivo [6]. SFRP2 is a novel therapeutic target for angiosarcoma and other tumors
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