Abstract P6-14-03: Zoledronic Acid Reduces Breast Tumour Growth When Combined with Chemotherapy — Emerging Evidence of Anti-Tumour Effects Outside Bone

Abstract

Abstract Background: There is accumulating evidence that zoledronic acid (zol) causes increased anti-tumour effects when added to chemotherapy. In model systems, combining doxorubicin and zol causes a reduction of both subcutaneous1 and bone-residing2 breast tumour growth. This is supported by recent neo-adjuvant studies, where the addition of zol to standard chemotherapy results in decreased residual breast tumour volume3,4 and a reduced number of patients with disseminated tumour cells in the bone marrow4. Thus, zol may affect tumour cells both in the primary site and in bone, and here we use in vivo models to explore the potential underlying mechanisms of combination therapy. Methodology: In xenografts studies, MDA-MB-436 cells were implanted subcutaneously in immunocompromised mice, and animals were treated weekly for 6 weeks with saline, 2mg/kg dox or 100ug/kg zol, alone and in combination, and survival monitored over the following 5 months. Tumour size and levels of vascularisation, macrophage infiltration, apoptosis and proliferation were measured, and a comprehensive molecular analysis of tumours performed. Accumulation of unprenylated Rap1a in tumours was measured as a surrogate marker for zol uptake. The potential involvement of immune cells in determining response to combination therapy was investigated in a model of spontaneous mammary tumour growth in immuno-competent PyMT/FVB mice. Results: In both models there was a significant reduction in tumour burden in animals receiving dox followed by zol, compared to all other treatment groups. In xenografts this effect was associated with increased levels of tumour cell apoptosis, decreased levels of proliferation, reduced vascularisation and tumour macrophage infiltration. The effect was sustained in the 5 months following completion of therapy, suggesting that precursor cell migration required for tumour re-growth is impaired. A direct effect of zol on subcutaneous tumours, as demonstrated by accumulation of unprenylated Rap-1a, was seen only in the combined treatments group, indicating that dox may lead to increased tumour uptake of zol. Initial data from the spontaneous mammary tumour model revealed a significant reduction in tumour burden associated with increased survival following 6 weeks of dox-zol combination therapy from week 7. This ongoing study demonstrates that in combination with dox, zol mediates a reduction in primary tumour development, and affects tumour progression outside bone. Conclusions: Our data suggest that when combined with chemotherapy, zoledronic acid may inhibit breast tumour development and progression by various mechanisms; by direct effects on tumours, by affecting ‘dormant’ tumour cell residing in niches in the bone marrow, by modifying migration of bone marrow precursor cells required for tumour progression, and by altering the level of circulating pro-angiogenic cytokines.