(567n) Novel Enzyme Prodrug for Targeting and Treatment of Solid Breast Tumors

A novel therapy for solid tumors is under development to treat primary tumors and distant metastases via an enzyme prodrug system targeted specifically to solid tumor vasculature. The L-methioninase enzyme is linked to 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 endothelial cells of tumor vasculature and that annexin V binds specifically to PS. Intravenous administration allows binding of the FP to the tumor vasculature, followed by delivery of the prodrug selenomethionine and conversion to cytotoxic methylselenol by L-methioninase. It is expected that this enzyme prodrug therapy will lead to the elimination of tumors because of a cutoff of the tumor's blood supply, as well by the cytotoxic effect of methylselenol on cancer cells and by methionine depletion of methionine-dependent cancer cells.

In vitro characterization of the FP showed a dissociation constant (K_d) of approximately 2 nM for human endothelial cells and MCF-7 and MDA-MB-231 breast cancer cells, indicating strong specific binding of annexin V to PS. In vitro cytotoxicity tests of enzyme prodrug treatment showed significant cell killing of the endothelial cells and the two breast cancer cell lines over a period of 3 days. Preliminary in vivo data will also be presented regarding FP pharmacokinetics and enzyme prodrug therapy of nude mice carrying breast tumor xenografts.