Combined Therapy of Local and Metastatic 4T1 Breast Tumor in Mice Using SU6668, an Inhibitor of Angiogenic Receptor Tyrosine Kinases, and the Immunostimulator B7.2-IgG Fusion Protein

Abstract

The therapeutic efficacy of combined antiangiogenic and immune therapy was tested against weakly immunogenic and highly metastatic 4T1 breast tumor using SU6668, an angiogenesis inhibitor and recombinant murine (rm) B7.2-IgG fusion protein, an immunostimulator. SU6668 inhibits the tyrosine kinase activity of three angiogenic receptors VEGFR2 (Flk-1/KDR), PDGFRβ, and FGFR1, which play a crucial role in tumor-induced vascularization. rmB7.2-IgG is a fusion protein of the extracellular domain of B7.2, and the hinge and constant domains of murine IgG2a. Intermolecule disulfide linkages are maintained so that it forms a dimer. Our studies showed that three weekly immunizations of BALB/c mice bearing 0.5–0.8 cm 4T1 breast tumors with rmB7.2-IgG and irradiated 4T1 tumor cells (B7.2-IgG/TC) resulted in a significant inhibition of tumor growth and formation of pulmonary metastases. T-cell depletion experiments revealed that both CD4⁺ and CD8⁺ T lymphocytes are required for stimulation of the antitumor and antimetastatic immune response by B7.2-IgG/TC. Treatment of mice with SU6668 substantially inhibited tumor vascularization but did not inhibit tumor infiltration by T lymphocytes or the T-cell response to rmB7.2-IgG therapy. On the contrary, tumors in mice immunized with B7.2-IgG/TC and treated with SU6668 had higher numbers of tumor infiltrating T cells than tumors of other groups. SU6668 treatments initiated either on day 3 or day 10 after inoculation of 4T1 tumor cells resulted in a significant inhibition of tumor growth. Similarly, three weekly immunizations with B7.2-IgG/TC starting either on day 7 or 12 inhibited growth of 4T1 tumors. However, the most potent antitumor effects were observed in mice treated with a combination of SU6668 and B7.2-IgG/TC. Analysis of pulmonary metastases revealed that combined therapy also had a more potent antimetastatic effect than each modality alone. These results indicate that antiangiogenic and immune therapies using SU6668 and B7.2-IgG/TC are compatible, and manifest complementary antitumor and antimetastatic effects. Combined antiangiogenic and immune therapy might represent a new strategy for cancer treatment.