Pharmacodynamics, tissue distribution, toxicity studies and antitumor efficacy of the vascular targeting fusion toxin VEGF121/rGel

Targeting of Tumor Neovasculature with GrB/VEGF121, a Novel Cytotoxic Fusion Protein

Angiogenesis is a critical process in numerous diseases, and intervention in neovascularization has therapeutic value in several disease settings, including ocular diseases, arthritis, and in tumor progression and metastatic spread. Various vascular targeting agents have been developed, including those that inhibit growth factor receptor tyrosine kinases, blocking antibodies that interfere with receptor signal transduction, and strategies that trap growth factor ligands. Limited anti-tumor efficacy studies have suggested that the targeted delivery of the human pro-apoptotic molecule Granzyme B to tumor cells has significant potential for cancer treatment. Here, we review biological vascular targeting agents, and describe a unique vascular targeting agent composed of Granzyme B and the VEGF receptor ligand VEGF₁₂₁. The fusion protein GrB/VEGF₁₂₁ demonstrates cytotoxicity at nanomolar or sub-nanomolar levels, excellent pharmacokinetic and efficacy profiles, and has significant therapeutic potential targeting tumor vasculature.

Non-invasive monitoring of tumor-vessel infarction by retargeted truncated tissue factor tTF-NGR using multi-modal imaging

The fusion protein tTF-NGR consists of the extracellular domain of the thrombogenic human tissue factor (truncated tissue factor, tTF) and the peptide GNGRAHA (NGR), a ligand of the surface protein CD13 (aminopeptidase N), upregulated on endothelial cells of tumor vessels. tTF-NGR preferentially activates blood coagulation within tumor vasculature, resulting in tumor vessel infarction and subsequent tumor growth retardation/regression. The anti-vascular mechanism of the tTF-NGR therapy approach was verified by quantifying the reduced tumor blood-perfusion with contrast-enhanced ultrasound, the reduced relative tumor blood volume by ultrasmall superparamagnetic iron oxide-enhanced magnetic resonance imaging, and by in vivo-evaluation of hemorrhagic bleeding with fluorescent biomarkers (AngioSense(680)) in fluorescence reflectance imaging. The accumulation of tTF-NGR within the tumor was proven by visualizing the distribution of the iodine-123-labelled protein by single-photon emission computed tomography. Use of these multi-modal vascular and molecular imaging tools helped to assess the therapeutic effect even at real time and to detect non-responding tumors directly after the first tTF-NGR treatment. This emphasizes the importance of imaging within clinical studies with tTF-NGR. The imaging techniques as used here have applicability within a wider scope of therapeutic regimes interfering with tumor vasculature. Some even are useful to obtain predictive biosignals in personalized cancer treatment.

The functionalized human serine protease granzyme B/VEGF₁₂₁ targets tumor vasculature and ablates tumor growth

The serine protease granzyme B (GrB) induces apoptosis through both caspase-dependent and -independent multiple-cascade mechanisms. VEGF₁₂₁ binds to both VEGF receptor (VEGFR)-1 and VEGFR-2 receptors. We engineered a unique GrB/VEGF₁₂₁ fusion protein and characterized its properties in vitro and in vivo. Endothelial and tumor cell lines showed varying levels of sensitivity to GrB/VEGF₁₂₁ that correlated closely to total VEGFR-2 expression. GrB/VEGF₁₂₁ localized efficiently into VEGFR-2-expressing cells, whereas the internalization into VEGFR-1-expressing cells was significantly reduced. Treatment of VEGFR-2(+) cells caused mitochondrial depolarization in 48% of cells by 48 hours. Exposure to GrB/VEGF₁₂₁ induced apoptosis in VEGFR-2(+), but not in VEGFR-1(+), cells and rapid caspase activation was observed that could not be inhibited by treatment with a pan-caspase inhibitor. In vivo, GrB/VEGF₁₂₁ localized in perivascular tumor areas adjacent to microvessels and in other areas in the tumor less well vascularized, whereas free GrB did not specifically localize to tumor tissue. Administration (intravenous) of GrB/VEGF₁₂₁ to mice at doses up to 40 mg/kg showed no toxicity. Treatment of mice bearing established PC-3 tumor xenografts with GrB/VEGF₁₂₁ showed significant antitumor effect versus treatment with GrB or saline. Treatment with GrB/VEGF₁₂₁ at 27 mg/kg resulted in the regression of four of five tumors in this group. Tumors showed a two-fold lower Ki-67-labeling index compared with controls. Our results show that targeted delivery of GrB to tumor vascular endothelial cells or to tumor cells activates apoptotic cascades and this completely human construct may have significant therapeutic potential.