Antiangiogenic therapy of cerebral melanoma metastases results in sustained tumor progression via vessel co-option

Purpose: In the brain, tumors may grow without inducing angiogenesis, via co-option of the dense pre-existent capillary bed. The purpose of this study was to investigate how this phenomenon influences the outcome of antiangiogenic therapy. Experimental Design: Mice carrying brain metastases of the human, highly angiogenic melanoma cell line Mel57-VEGF-A were either or not treated with different dosages of ZD6474, a vascular endothelial growth factor (VEGF) receptor 2 tyrosine kinase inhibitor with additional activity against epidermal groeth factor receptor. Effect of treatment was evaluated using contrast-enhanced magnetic resonances imaging (CE-MRI) and (immuno)morphologic analysis. Results: Placebo-treated Mel57-VEGF-A brain metastases evoked an angiogenic response and were highlighted in CE-MRI. After treatment with ZD6474 (100 mg/kg), CE-MRI failed to detect tumors in either prevention or therapeutic treatment regimens. However, (immuno)histologic analysis revealed the presence of numerous, small nonangiogenic lesions. Treatment with 25 mg/kg ZD6474 also resulted in efficient blockade of vessel formation, but it did not fully inhibit vascular leakage, thereby still allowing visualization in CE-MRI scans. Conclusions: Our data show that, although angiogeniesis can be effectively blocked by ZD6474, in vessel-dense organs this may result in sustained tumor progression via co-option, rather than in tumor dormancy. Importantly, blocking VEGF-A may result in undetectability of tumors in CE-MRI scans, leading to erraneous conclusions about therapeutic efficacy during magnetic resonance imaging follow-up. The maintenance of VEGF-A- induced vessel leakage in the absence of neovascularization at lower ZD6474 doses may be exploited to improve delivery of chemotherapeutic agents in combined treatment regimens of angiogenic and chemotherapeutic compounds.