(381d) Targeting the Vasculature of Solid Tumors with Enzyme/Prodrug and Methionine-Depletion Therapy

A new anticancer therapy for solid tumors is being developed to eliminate both the primary tumor and distant metastases. This approach uses both enzyme/prodrug and methionine-depletion therapy. The L-methioninase enzyme is linked to the human annexin V protein in a fusion protein (FP). This therapy takes advantage of the fact that phosphatidylserine (PS), an anionic phospholipid, is exposed almost exclusively on the outer membrane of cancer and tumor vasculature endothelial cells and that annexin V binds specifically to PS. After the FP administered to the bloodstream binds to the tumor vasculature, the prodrug selenomethionine is administered and converted by L-methioninase to a cytotoxic compound, methylselenol. Binding of the FP will lead to the following effects: (1) death of tumor endothelial cells from the methylselenol generated, causing clotting of the tumor vasculature and a cutoff of oxygen to the tumor, (2) death of tumor cells by the action of methylselenol, (3) death of methionine-dependent tumor cells by the depletion of methionine.

Using cells grown in vitro, binding assays were performed with the FP to determine the strength and specificity of binding, and cytotoxicity assays were carried out using the enzyme/prodrug treatment. The dissociation constant of the FP was found for both human endothelial cells and breast cancer cells, and the dissociation constant values indicated relatively strong binding. An in vitro binding stability test determined that the FP remained bound on the endothelial cells for at least 3 days. Cytotoxicity data for both endothelial and breast cancer cells was obtained for the effect of adding selenomethionine prodrug to cells for which the FP is bound on the cell surface. Cell viability was evaluated using the Alamar blue assay and cell counting.