Benefits of combined treatments using antiresorptive agents and cytotoxic drugs

Breast cancer patients often receive a combination of different therapies, but our understanding of how best to utilise such combinations to achieve maximal benefit for the patients is incomplete. We have investigated the ability of the antiresorptive agent zoledronic acid (Zol) and the commonly used chemotherapy agents paclitaxel (Pac) and doxorubicin (Dox) to induce apoptotic breast cancer cell death in vitro.

Hormone-dependent (MCF7) and hormone-independent (MDA-MB-436) breast cancer cells were treated with increasing doses of Zol, alone and in sequence or combination with a low dose of Pac (2 nM) or Dox (0.05 μM) for 1–72 hours. The following treatment groups were used: (A) untreated controls, (B) each drug given as a single agent, (C) the drugs given simultaneously, (D) chemotherapy agent followed by Zol, and (E) Zol followed by the chemotherapy agent. In some cases Zol was given together with GGOH, a downstream component of the mevalonate pathway targeted by Zol. The effects of the different treatments on both apoptotic and necrotic cell death were determined at 72 hours, by evaluation of nuclear morphology following staining with Hoechst and PI. The effects of the various treatments on the cell cycle distribution were also determined.

Our data show that exposing breast cancer cells to the chemotherapy agent prior to Zol results in a synergistic increase in tumour cell death, compared with when the drugs are used as single agents. This was seen both for paclitaxel and doxorubicin, and the effect was found to be associated with changes in the cell cycle distribution following pretreatment with the cytotoxic drug. The synergistic increase in tumour cell death could be reversed by addition of GGOH, a compound that counteracts the effects of Zol on a key metabolic pathway, supporting an essential role of Zol in the toxic effects of the combined treatments. We also show that these effects are significant using clinically achievable doses and exposure times, suggesting that sequential treatments may be relevant also in a clinical setting.

We have shown that combining chemotherapy agents and the antiresorptive drug Zol results in a synergistic increase in breast cancer cell death in vitro. We are currently investigating whether the same is seen using more complex in vivo model systems. Our data suggest that in order to achieve maximum benefit from combined treatments, the order and timing of the combinations are crucial.