Zoledronic Acid Inhibits Protein Prenylation in Plasmacytoma Tumors In Vivo and Enhances Survival in the INA-6 SCID Mouse Model.

Bisphosphonates such as zoledronic acid (ZOL) are effective at preventing osteolytic bone disease in patients with multiple myeloma. ZOL inhibits bone resorption by inhibiting FPP synthase and preventing the prenylation of small GTPases in osteoclasts. In vitro studies have demonstrated previously that ZOL can also directly affect the growth and viability of myeloma cells by inhibiting protein prenylation and therefore could, potentially, have a direct anti-tumour effect in vivo in addition to effects on osteoclasts. To examine this further, the effect of ZOL on six myeloma cell lines, including the IL-6 dependent INA-6 line, was investigated. ZOL caused cell cycle arrest and concentration-dependent growth inhibition in all six cell lines, with varying sensitivity (IC50= 30–285 μM).

The potential anti-tumour effect of ZOL on INA-6 cells in vivo was studied in a SCID mouse xenograft model, in which mice injected intraperitoneally with INA-6 cells develop plasmacytomas but do not develop significant osteolytic lesions. In several experiments involving more than 50 mice, ZOL was administered subcutaneously (sc) or intravenously (iv). ZOL, at a dose of 8 μg or 2 μg three times per week for two weeks after inoculation of INA-6 cells, significantly reduced tumour burden and increased survival of the mice (p= 0.002). The effect of ZOL on protein prenylation in plasmacytomas dissected from the mice was measured by western blotting to specifically detect the unprenylated form of the small GTPase Rap1A. Unprenylated Rap1A was virtually absent from tumour samples of untreated animals, while a single iv injection of 8 μg ZOL induced a marked accumulation of unprenylated Rap1A in the tumours after 24–72 hours.

These studies are the first to demonstrate that ZOL can inhibit protein prenylation in plasmacytomas in vivo. Together with the decreased tumour burden and increased survival, the data suggest that ZOL may have direct anti-tumour effects in this animal model. Thus, nitrogen-containing bisphosphonates such as ZOL may have therapeutic potential in multiple myeloma beyond the prevention of osteolytic lesions.