We are developing a new type of enzyme-prodrug therapy designed to selectively clot the vasculature of tumors and kill cancer cells. It has been shown that the anionic phospholipid phosphatidylserine (PS) is exposed almost exclusively on the external membrane layer of cancer cells and endothelial cells of the tumor vasculature. It is also known that the human protein annexin V binds specifically and strongly to PS. We have engineered a fusion protein (FP) containing annexin V linked to the enzyme methioninase, which has the ability to convert a biologically safe compound, selenomethionine, to a toxic compound, methylselenol. The annexin V part of the FP will specifically bind to the tumor vasculature, while the methioninase part will convert the selenomethionine prodrug to the toxic methylselenol leading to two mechanisms to kill the tumor: 1) death of the endothelial cells causing cloture of tumor vasculature, thus cutting off the oxygen supply of the cancer cells; and 2) methylselenol will be transported to tumor cells by diffusion and fluid permeation resulting in their death.

We have studied the binding of purified FP to PS immobilized on microtiter plates and to human endothelial cells in which PS has been induced to the on the cell surface by the addition of a low level of hydrogen peroxide. Data has also been obtained for the effect of adding seleonmethionine to endothelial cells in vitro in which the FP is bound on the cell surface. The cell viability is evaluated by performing the Alamar Blue assay, cell counting, and cell observation with a microscope.