Treatment with the vascular disruptive agent OXi4503 induces an immediate and widespread epithelial to mesenchymal transition in the surviving tumor.

Epithelial to mesenchymal transition (EMT) is considered an important mechanism in tumor resistance to drug treatments; however, in vivo observation of this process has been limited. In this study we demonstrated an immediate and widespread EMT involving all surviving tumor cells following treatment of a mouse model of colorectal liver metastases with the vascular disruptive agent OXi4503. EMT was characterized by significant downregulation of E-cadherin, relocation and nuclear accumulation of ß-catenin as well as significant upregulation of ZEB1 and vimentin. Concomitantly, significant temporal upregulation in hypoxia and the pro-angiogenic growth factors hypoxia-inducible factor 1-alpha, hepatocyte growth factor, vascular endothelial growth factor and transforming growth factor-beta were seen within the surviving tumor. The process of EMT was transient and by 5 days after treatment tumor cell reversion to epithelial morphology was evident. This reversal, termed mesenchymal to epithelial transition (MET) is a process implicated in the development of new metastases but has not been observed in vivo histologically. Similar EMT changes were observed in response to other antitumor treatments including chemotherapy, thermal ablation, and antiangiogenic treatments in our mouse colorectal metastasis model and in a murine orthotopic breast cancer model after OXi4503 treatment. These results suggest that EMT may be an early mechanism adopted by tumors in response to injury and hypoxic stress, such that inhibition of EMT in combination with other therapies could play a significant role in future cancer therapy.

Spatial morphological and molecular differences within solid tumors may contribute to the failure of vascular disruptive agent treatments.

BACKGROUND: Treatment of solid tumors with vascular disrupting agent OXi4503 results in over 90% tumor destruction. However, a thin rim of viable cells persists in the tumor periphery following treatment, contributing to subsequent recurrence. This study investigates inherent differences in the microenvironment of the tumor periphery that contribute to treatment resistance. METHODS: Using a murine colorectal liver metastases model, spatial morphological and molecular differences within the periphery and the center of the tumor that may account for differences in resistance to OXi4503 treatment were investigated. H&E staining and immunostaining were used to examine vessel maturity and stability, hypoxia and HIF1α levels, accumulation of immune cells, expression of proangiogenic factors/receptors (VEGF, TGF-ß, b-FGF, and AT1R) and expression of EMT markers (ZEB1, vimentin, E-cadherin and ß-catenin) in the periphery and center of established tumors. The effects of OXi4503 on tumor vessels and cell kinetics were also investigated. RESULTS: Significant differences were found between tumor periphery and central regions, including association of the periphery with mature vessels, higher accumulation of immune cells, increased growth factor expression, minimal levels of hypoxia and increased evidence of EMT. OXi4503 treatment resulted in collapse of vessels in the tumor center; however vasculature in the periphery remained patent. Similarly, tumor apoptosis and proliferation were differentially modulated between centre and periphery after treatment. CONCLUSIONS: The molecular and morphological differences between tumor periphery and center may account for the observed differential resistance to OXi4503 treatment and could provide targets for drug development to totally eliminate metastases.

Alterations in vascular architecture and permeability following OXi4503 treatment.

OXi4503 retards tumor growth in a dose-dependent manner and improves survival in a murine model of colorectal liver metastases. This agent causes extensive vascular shutdown by selectively altering the tubulin cytoskeleton within the endothelial cells of tumor vessels. The destruction of tumor vessels is incomplete, however, and tumor revascularization occurs after the treatment. This study evaluates the pattern of microcirculatory changes and alterations to the ultrastructural properties of the tumor vasculature that result from OXi4503 treatment. Male CBA mice were induced with liver metastases via an intrasplenic injection of a murine-derived colorectal cell line. After administering a single intraperitoneal dose of OXi4503, changes in tumor perfusion, microvascular architecture and permeability were assessed at various time points. One hour after a 100-mg/kg dose of OXi4503, a significant decrease in the percentage of tumor perfusion (63.96+/-1.98 in controls versus 43.77+/-2.71 in treated mice, P<0.001) was observed, which was still evident 5 days after the treatment. Substantial tumor microvascular damage and minimal normal liver injury were observed. Tumor vascular permeability was significantly elevated 45 min after the OXi4503 treatment (67.5+/-3.60 in controls versus 80.5+/-2.24 microg/g, P<0.05). The findings suggest that OXi4503 selectively targets tumor vessels and causes immediate microvascular destruction. Even at the maximum tolerated dose, however, residual patent tumor vessels were still present after treatment, implying incomplete tumor destruction. A combination of OXi4503 with other chemotherapeutic modalities might achieve complete tumor eradication and improve long-term survival.

Effect of vascular targeting agent Oxi4503 on tumor cell kinetics in a mouse model of colorectal liver metastasis.

BACKGROUND: Oxi4503 has been shown to inhibit tumor growth and improve survival in an animal model of colorectal (CRC) liver metastases. This agent appears to selectively target the endothelial cytoskeleton with resultant vessel occlusion and tumor necrosis. MATERIALS AND METHODS: This study evaluated the pattern of tumor necrosis caused by Oxi4503, with particular emphasis on patterns of cell proliferation and apoptosis in a murine model of CRC liver metastases. RESULTS: A single dose of Oxi4503 caused immediate tumor vasculature collapse and subsequently tumor necrosis. There was widespread central necrosis with evidence of viable tumor cells at the periphery. Alterations in the number and spatial pattern of tumor cells undergoing apoptosis and the rate of cellular proliferation were also observed following treatment. Microvessel density was reduced following treatment, however patent vessels were still observed within the necrotic core. CONCLUSION: Although Oxi4503 caused significant tumor destruction, synergistic treatment with cytotoxic and/or anti-angiogenic agents should be considered in order to achieve complete tumor eradication and long-term survival.