Regression of Human Breast Carcinoma in Nude Mice after Adsflt Gene Therapy Is Mediated by Tumor Vascular Endothelial Cell Apoptosis

SIMPLE SUMMARY: Two vascular endothelial growth factor (VEGF) receptors, FLT-1 and KDR, are expressed preferentially in proliferating endothelium. There is increasing evidence that soluble VEGF receptor domains that compete for VEGF binding may inhibit tumor angiogenesis, growth and metastatic spread. We tested whether soluble FLT-1 (sFLT-1) could inhibit breast cancer tumor growth in mice by blocking angiogenesis. A viral vector carrying the sflt-1 cDNA (Adsflt) was used to overexpress the receptor in MCF-7 cells, which produce VEGF. After six weeks, tumors were treated either with Adsflt or a negative control virus. After six months, average tumor volume in the Adsflt-treated group was decreased by 91% and vascular density was reduced 50%, relative to negative controls (p < 0.05), yet mice treated with Adsflt showed no delay in healing cutaneous wounds. These results provide evidence that viral sFLT-1 overexpression may be an effective anti-angiogenic therapy for cancer without the risk of systemic anti-angiogenic effects. ABSTRACT: Two vascular endothelial growth factor (VEGF) receptors, FLT-1 and KDR, are expressed preferentially in proliferating endothelium. There is increasing evidence that recombinant, soluble VEGF receptor domains interfering with VEGF signaling may inhibit in vivo neoangiogenesis, tumor growth and metastatic spread. We hypothesized that a soluble form of FLT-1 receptor (sFLT-1) could inhibit the growth of pre-established tumors via an anti-angiogenic mechanism. A replication-deficient adenovirus (Ad) vector carrying the sflt-1 cDNA (Adsflt) was used to overexpress the sFLT-1 receptor in a breast cancer animal model. MCF-7 cells, which produce VEGF, were used to establish solid tumors in the mammary fat pads of female nude mice. After six weeks, tumors were injected either with Adsflt or a negative control virus (AdCMV.βgal). After six months, average tumor volume in the Adsflt-infected group (33 ± 22 mm(3)) decreased by 91% relative to that of the negative control group (388 ± 94 mm(3); p < 0.05). Moreover, 10 of 15 Adsflt-infected tumors exhibited complete regression. The vascular density of Adsflt-infected tumors was reduced by 50% relative to that of negative controls (p < 0.05), which is consistent with sFLT-1-mediated tumor regression through an anti-angiogenic mechanism. Moreover, cell necrosis and fibrosis associated with long-term regression of Adsflt–infected tumors were preceded by apoptosis of tumor vascular endothelial cells. Mice treated with Adsflt intratumorally showed no delay in the healing of cutaneous wounds, providing preliminary evidence that Ad-mediated sFLT-1 overexpression may be an effective anti-angiogenic therapy for cancer without the risk of systemic anti-angiogenic effects.