Effect of zoledronic acid and an antibody against bone sialoprotein II on MDA-MB-231^GFP breast cancer cells in vitro and on osteolytic lesions induced in vivo by this cell line in nude rats

The aim of this study was to investigate the combined effect of zoledronic acid and an antibody against bone sialoprotein II (BSPII) on proliferation and osteolytic activity of MDA-MB-231^GFP breast cancer cells. For this purpose, the cells were exposed to zoledronic acid (10–20 μg/ml [25–50 μM]) and an anti-BSPII IgY (10–100 μg/ml) for up to 5 days alone or in combination. The combined treatment showed synergistic antiproliferative effects at the higher dose of zoledronic acid. Following inoculation of 1 × 10⁵ MDA-MB-231 ^GFP breast cancer cells into a branch of the femoral artery of nude rats, lytic lesions developed in the tibia, femur or fibula of the injected hind leg after approximately 30 days. The appearance and development of these lesions were monitored radiographically. Rats with lytic lesions were treated with zoledronic acid (60 μg/kg/week sc × 8; n = 10), zoledronic acid and an anti-BSPII IgY antibody (60 μg/kg/week sc × 8 + 10 mg/kg/week sc × 8; n = 10), or left untreated (n = 20). In addition, rats were treated for 4 weeks (n = 10) with both regimens starting right after tumor cell inoculation. Finally, ten rats were treated with zoledronic acid for 2 weeks before tumor cell inoculation (60 μg/kg/week sc × 2). The antiosteolytic effect of zoledronic acid was high as shown by inhibition of osteolytic growth. Addition of the anti-BSPII IgY further decreased the incidence of femoral osteolytic lesions (40% reduction), indicating remineralization, and reduced periosteal defects of cortical bone (20% reduction). These observations favor using the IgY-antibody in addition to zoledronic acid in order to stimulate osteoblast-induced remineralization.