ZOL inhibits RANKL-induced osteoclast differentiation without cytotoxicity. RAW264.7 cells were cultured with ZOL (0, 0.1, 1, 5, 15, 30 or 50 µM). (A and B) A Cell Counting Kit-8 assay was performed to determine cell viability at various time-points (24, 48 and 72 h). The results (A) without or (B) with 100 ng/ml RANKL were plotted as cellular growth curves. (C) RAW264.7 cells were incubated with different concentrations of ZOL (0, 0.1, 1 and 5 µM) in the presence or absence of 100 ng/ml RANKL for 5 days, and then stained using a TRAP staining kit. (D and E) The numbers and percentages of osteoclasts were determined. (F) RAW264.7 cells were cultured with or without 1 µM ZOL, and then stained for TRAP at 3, 5 and 7 days, respectively. (G and H) Osteoclast numbers and area percentage were counted at 3, 5 and 7 days, respectively. ##P<0.01 vs. the vehicle group; *P<0.05, **P<0.01 vs. the RANKL-only group. ZOL, zoledronic acid; RANKL, receptor activator of nuclear-κB ligand; TRAP, tartrate-resistant acid phosphatase.