An antiangiogenic urokinase-derived peptide combined with tamoxifen decreases tumor growth and metastasis in a syngeneic model of breast cancer.

Expression of urokinase (uPA) and its receptor (uPAR) is associated with increased tumor-cellinvasion and metastasis in several malignancies including breast cancer. An 8-mer peptide derived from the nonreceptor-binding domain of urokinase (A6) has been shown to have antiangiogenic and proapoptotic effects to block the progression of breast cancer in vivo. In the present study, we evaluated the effects of A6 and the antiestrogen tamoxifen (TAM) alone and in combination on estrogen-receptor-positive Mat B-III rat breast cancer cells in vitro and in vivo. Treatment of Mat B-III cells with A6 and TAM resulted in a dose-dependent decrease in tumor-cell invasion through Matrigel; these effects were more marked when A6 and TAM were tested in combination. In addition, treatment of Mat B-III cells with either A6 or TAM resulted in a significant reduction of vascular endothelial growth factor receptor (flk-1) expression and in transforming growth factor beta activity, effects that were significantly higher after combined treatment with A6 and TAM. For in vivo studies, female Fischer rats were inoculated with Mat B-III cells (1 x 10(6)) into the mammary fat pad. These orthotopic tumors were staged to 30-40 mm(3) in volume and then treatment was initiated with A6 (75 mg/kg/day) and TAM (3 mg/kg/day) alone or in combination. Both A6 and TAM caused a significant reduction in tumor volume; however, these antitumor effects were significantly greater in animals receiving both A6 and TAM, which demonstrated a 75% reduction in tumor growth as compared with control animals. The number of macroscopic tumor foci was significantly reduced in A6-treated animals, whereas TAM failed to exhibit any antimetastatic effects. Histological analysis of primary tumors from different groups showed a decrease in new blood-vessel density and increased tumor-cell death in A6- and TAM-treated animals, and these effects were greater in experimental animals receiving A6 and TAM in combination. Collectively, these studies demonstrate that the addition of novel antiangiogenic/antimetastatic agents like A6 to hormone therapy can enhance the antitumor effects of hormone therapy through increased inhibition of angiogenesis and induction of tumor-cell death.