Differential Inhibition of Invasion and Proliferation by Bisphosphonates: Anti Metastatic Potential of Zoledronic Acid in Prostate Cancer

Abstract

Objectives: To determine the mode of action of Zoledronic acid in the inhibition of metastasis in prostate cancer and the reduction of prostate cancer bone metastases. Methods: Benign and malignant primary prostatic epithelial cells (PEC) and the PC-3 prostate cancer cell line were studied in co-culture using human bone marrow stroma in the presence of escalating doses of EDTA, Clodronate, Pamidronate and Zoledronic acid. PEC binding and colony growth in bone marrow stroma was measured using standardised quantitative techniques. PEC cellular invasion through Matrigel and an endothelial monolayer was measured either in invasion chambers or by the measurement of endothelial monolayer permeability to fluorescent dextran. Co-culture supernatants were assayed for specific cytokine levels. Bone marrow cellular toxicity was assessed using a standard Mix assay. Results: Treatment of PEC with up to 100 μM bisphosphonate did not affect their ability to bind to bone marrow endothelium or stroma. Bone marrow endothelial permeability was reduced by 100 μM Zoledronic acid by 3.8% ( p =0.03856). Both Pamidronate (40% at 100 μM, p ≤0.05) and Zoledronic acid inhibited PEC invasion, with Zoledronic acid being the most potent (40% at 10 μM, p ≤0.05 rising to 91% at 100 μM, p ≤0.001). Zoledronic acid inhibits malignant PEC proliferation in bone marrow stroma co-culture (26.5% at 10 μM rising to 66.5% at 40 μM). This was accompanied by changes within the cytokine milieu with a >800% rise in TIMP-2. Conclusion: Zoledronic acid is a potent inhibitor of PEC invasion across bone marrow endothelium and colony formation with the bone marrow stroma, affecting the MMP: TIMP-2 balance to favour MMP inhibition.