Although anti-angiogenic agents hold promise as cancer therapies, one challenge of this approach is to disrupt the tumour vasculature without affecting normal blood vessels. In the April issue of The Journal of Clinical Investigation, Greenberger et al. describe a gene-therapy approach that specifically targets the endothelial cells of the growing tumour vasculature.

Tumour blood vessels make good therapeutic targets, not only because the tumour depends on their delivery of oxygen and nutrients for growth, but also because the vasculature comprises endothelial cells that are more genetically stable and, therefore, less prone to drug resistance. But how does one destroy the blood vessels of a tumour without affecting those of normal tissues? Greenberger and colleagues created an adenoviral vector that expressed a chimeric death receptor — composed of elements of the Fas receptor and tumour-necrosis factor receptor 1 (Tnfr1) — specifically in vascular endothelial cells. This receptor was designed to activate the Fas-induced apoptotic pathway following binding to Tnf-α, which is abundant in the tumour microenvironment. Furthermore, the gene encoding this receptor was placed under transcriptional control of the modified endothelial-cell-specific pre-proendothelin-1 ( PPE-1 ) promoter. PPE-1 is a vasoconstrictor and smooth muscle cell mitogen that is synthesized by endothelial cells. The PPE-1 promoter contains a hypoxia-responsive element that increases gene expression only under hypoxic conditions, such as in the tumour microenvironment. For extra specificity, Greenberger et al. engineered their therapeutic vector to contain three copies of the endothelial-cell regulatory elements of this promoter.

The vector lived up to expectations — it induced gene expression only in angiogenic vessels, and apoptosis only in cultured endothelial cells in the presence of TNF-α. But did it work in tumours? The authors injected the chimeric Fas-expressing vector into the tail vein of mice carrying Lewis lung carcinomas and observed that transcription of the gene encoding the chimeric Fas receptor was restricted to the tumour-bearing lung. Furthermore, the treatment resulted in a 56% decrease in the weight of the lung metastases, compared with controls. Lung surfaces of mice treated with the therapeutic vector were free of tumours or partly covered by small, underdeveloped metastases, whereas the lungs of control animals were almost completely replaced by tumour tissue. The Fas chimeric vector was also effective in slowing tumour growth in mice with pre-existing B16 melanomas, and the authors showed that these tumours succumbed to necrosis.

Histological analysis revealed that the tumour vasculature was specifically targeted in mice that received the therapeutic vector — tumour blood vessels were damaged and had lost their endothelial layer. Antivascular effects were not observed, however, in normal, non-angiogenic tissues, and no humoral immune response against the transgene product was observed. Importantly, expression of the chimeric receptor did not cause liver damage in mice, which was a concern because Fas ligand administration has been previously shown to destroy hepatocytes. The authors believe that the chimeric features of the receptor allowed Tnf-α to only activate Fas signalling and apoptosis within the tumour micrenvironment.