Objective   This study aimed to investigate the inhibitory effect of zoledronic acid (ZOL) alone or the combined treatment of ZOL and paclitaxel (PTX) on the cell growth of lung cancer cell line in vitro.

  Methods   The effects of different concentrations of ZOL alone, 2 nM PTX, and combined treatment of ZOL and PTX on the growth of A-549 cell line were determined using methyl thiazolyl tetrazolium (MTT) method. The mechanism of the curative effect was analyzed by flow cytometry on the basis of apoptotic rate. AKT, phospho-AKT, ERK, phospho-ERK, and Bcl-2 expressions were determined by western blot analysis. AKT and ERK gene expressions were determined by RT-PCR.

  Results   MTT results showed that ZOL alone could inhibit the growth of lung cancer cell line A-549 in a dose-dependent manner. The combined therapy of ZOL and PTX could inhibit cell growth. This combined treatment is more effective than the single treatment with either ZOL or PTX alone. The synergistic inhibition rate is dependent on drug sequencing. Furthermore, maximum inhibition was induced by sequence order, i.e., initial treatment with PTX and then with ZOL. RT-PCR results demonstrated that the mRNA of ERK and AKT of the group treated with PTX and then ZOL were lower than that in other treatment groups. Western blot analysis results demonstrated that the ERK and AKT levels of the treated groups were parallel in the cell line. However, the lowest phospho-ERK, phospho-AKT, and Bcl-2 levels were observed in the PTX then ZOL group. The cell lines treated with PTX alone and ZOL alone ranked second and third among the lowest results, respectively. The highest level was observed in the control group.

  Conclusion   The combined ZOL and PTX treatment induced the downregulation of phospho-ERK, phospho-AKT, and Bcl-2 protein expressions in RAF/MEK/ERK and PI3K/AKT signaling pathway. This pathway could be one of the synergistic antitumor mechanisms of the two drugs.