Synergistic Interaction between an Anti-p185HER-2 Pseudomonas Exotoxin Fusion Protein [scFv(FRP5)–ETA] and Ionizing Radiation for Inhibiting Growth of Ovarian Cancer Cells That Overexpress HER-2

Objectives. The HER-2 proto-oncogene is overexpressed in 15–30% of ovarian cancers, providing a target for therapy in a fraction of patients. We previously described the ability of a recombinant single-chain anti-p185HER-2–Pseudomonas exotoxin A fusion protein, scFv(FRP5)–ETA, to inhibit growth of cancer cells in culture and tumor xenografts in vivo. We have also described synergistic interaction between an anti-p185HER-2-ricin A chain immunotoxin and ionizing radiation for inhibiting growth of ovarian and breast cancer cells that overexpress HER-2. Our objective in this report was to evaluate the in vitro and in vivo effects of scFv(FRP5)–ETA against SKOv3 ovarian cancer cells that have been transfected to express >106 p185HER-2 receptors per cell (clone-9002-18). Methods. Inhibition of clonogenic growth was measured in vitro by limiting dilution analysis. Inhibition of tumor growth was measured in vivo using heterografts established with the same ovarian cancer cell lines. Results. Clone-9002-18 cells were substantially more sensitive than parental SKOv3 cells to the cytotoxic effects of scFv(FRP5)–ETA. Exotoxin fusion protein induced apoptosis in clone-9002-18 cells, but did not affect parental SKOv3 cells. Treatment of clone-9002-18 cells with 200- to 2000-cGy external beam irradiation in combination with scFv(FRP5)–ETA produced synergistic elimination of clonogenic tumor cells in culture. In vivo, subcutaneous or intraperitoneal growth of tumor xenografts in nu/nu mice was significantly inhibited (P = 0.004) by treatment for 10 days with scFv(FRP5)–ETA or with 131I-labeled-520C9 anti-p185HER-2 radionuclide conjugate, but additive effects were not observed with combined treatment. Conclusion. ScFv(FRP5)–ETA deserves further evaluation for intraperitoneal therapy of ovarian cancers that overexpress p185HER-2.