Abstract P1-13-09: Marked Inhibition of Both Invasion and Proliferation by Zoledronic Acid and the Epidermal Growth Factor Receptor Inhibitor Gefitinib in a Bone-Seeking Clone of Breast Cancer

Abstract

Abstract Background: Zoledronic acid (Zol) is the most potent inhibitor of bone resorption in bisphosphonates. In order to examine whether the inhibitory effect is involved in the mechanism of bone metastasis in breast cancer, a bone-seeking clone was examined. It was established by repeated sequential passages of metastatic cells from bone in nude mice and in vitro, and it exclusively metastasizes to bone with larger osteolytic lesions than MDA-MB-231 parental cells. Recent findings indicate that epidermal growth factor (EGF) signaling is also an important mediator of bone metastasis. We examined the synergistic effects of gefitinib on the proliferation and invasion of a bone-seeking clone compared with MDA-MB-231. Materials and Methods: The cellular proliferation was estimated using WST-1. Migration in the matrigel in a Boyden chamber was applied to the invasion assay. Results: Treatment with 1 µM of Zol reduced 51% of cell proliferation in the bone-seeking clone; however, the inhibited rate was 37% in MDA-MB-231. The migration was also inhibited with 10 µM of Zol (bone-seeking clone: 69% reduction, MDA-MB-231: 54% reduction). The reduction rates of both assays by Zol treatment were significantly higher in the bone-seeking clone than in MDA-MB-231. Treatment with 10 µM of Zol plus 1 µM of gefitinib resulted in a proliferation reduction rate of 27% compared to that with 10 µM Zol treatment in the bone-seeking clone and no reduction in MDA-MB-231. The migration reduction rate was 88% in the bone-seeking clone and no reduction in MDA-MB-231. There was a significant difference between the bone-seeking clone and MDA-MB-231 in the reduction rates of both cellular proliferation and migration. Although the effect of gefitinib was synergistic with that of Zol in the bone-seeking clone, it was not clear in MDA-MB-231. Conclusion: There appear to be at least two independent signaling pathways that occur in osteolytic bone metastasis. One is involved in the bone resorption of bisphosphonates, and the other is an EGFR signaling pathway. Clinically, the combination of Zol and gefitinib is suggested to be a strong therapy specific to bone metastasis. Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P1-13-09.