Effect of zoledronic acid treatment on MCF-7 cell viability. (a) MCF-7 cells treated with increasing concentrations of zoledronic acid (0.1–100 μM) for 24 and 72 hours to determine a 50% growth inhibition (IC50) value. (b) MCF-7 cells treated with 20 μM zoledronic acid for increasing time of incubation to determine the time dependence of zoledronic acid (ZOL) toxicity. Results are expressed as the percentage of control or absolute cell numbers. *Statistically significant values compared with control. Ctl, control.