Background: Zoledronic acid is an important osteotropic compound used in combination with anticancer agents to reduce the incidence of hypercalcemia and skeletal morbidity in patients with advanced breast cancer and bone metastases. Ineffectiveness of anticancer drugs during chemotherapy is a frequently observed situation in cancer chemotherapy. The resistance of tumor cells to more than one cytotoxic drugs is defined as multidrug resistance. Drug resistance may be caused by altered gene expression levels and altered activities of proteins related to drug transport or cell death.
Materials and methods: To investigate the potential development of zoledronic acid resistance in breast cancer, parental MCF-7 cells were selected by increasing doses of zoledronic acid. MTT cytotoxicity assays, RT-PCR and Western blot were performed. The anticancer drugs paclitaxel, docetaxel, vincristine and doxorubicin were tested in combination to assess their combined antiproliferative effects and cross-resistance profiles.
Results: Results demonstrated that the drug-adapted cells are resistant to zoledronic acid compared to parental MCF-7 and de novo expression of resistance genes, such as BCRP and LRP, were found. Up-regulation of Bcl-2 gene expression in resistant cells was also found. Synergistic cytotoxic effects of the combination of zoledronic acid with paclitaxel, docetaxel and vincristine were confirmed by fractional inhibitory indices, and zoledronic acid resistant cells were also found to be cross-resistant to these agents.
Conclusion: Zoledronic acid may cause resistance in MCF-7 cells. Overexpression of BCRP and LRP genes and an increase in Bcl-2 gene expression may have roles in the development of zoledronic acid resistance in the MCF-7 cell line. On the other hand, MDR1 and MRP1 genes do not seem to contribute to the zoledronic acid resistance significantly.