Purpose: Bony metastases in vertebrae secondary to breast cancer can result in osteolysis and an increase in skeletal related events. Bisphosphonates (BP) are the current standard of care for breast cancer patients with skeletal disease. Photodynamic therapy (PDT) is a non-radiative treatment, which has been successfully applied to various malignancies and shown to successfully ablate vertebral human breast cancer (MT1) metastases in a murine model. Previous in-vitro study has shown that pre-treatment of MT-1 cells with the BP zoledronic acid (Zometa^®) renders them more susceptible to PDT. The aim of this study was to evaluate the influence of pre-treatment with BPs on the effect of PDT treatment on tumour ablation in metastatically involved vertebrae in vivo.

Method: Metastases were induced in fourteen 5–6 weeks old female athymic rats (Hsd:RH-Foxn1rnu) by intra-cardiac injection of 2x10^6 MT-1 cells. Four groups were formed:

control, no treatment;

Seven days after MT-1 injection 60 μg/kg of zoledronic acid was injected. PDT treatment was administered on day 14 using the photosensitizer BPD-MA (1.0 mg/kg; Visudyne). Fifteen minutes later, laser-light (690nm; 75J) was administered to the lumbar vertebrae. The rats were euthanized 7 days after PDT treatment. A total of 45 vertebrae were evaluated using a histomorphometric program (GENIE™, Aperio) to assess tumour burden. Statistical analyses were performed using a one-way ANOVA with a Tukey post hoc test. A p-value p< .05 was considered to be statistically significant..

Results: The total The total tumour burden within vertebrae of rats pre-treated with BP and/or PDT was significantly lower compared to the control rats (p< .001). In addition, the PDT alone treated group demonstrated significantly less tumour burden than the combined BP+PDT group. In the control and BP-only groups, large tumours were found to include regions of necrosis. The PDT treatment groups (PDT and BP+PDT) exhibited areas of necrosis throughout the entire vertebral bodies with adjacent formation of granulation tissue.

Conclusion: BP, PDT and combined BP+PDT treatments resulted in a lower overall tumour burden at day 21 post MT-1 cell injection compared to control rats. A surprising increased level of tumour burden was found in comparing the combined treatment group to the PDT-only group. These findings are in contrast to previous in-vitro results, where the pre-treatment with BPs made the cells more susceptible to PDT. Pre-treatment with BP affects both the bone and tumour cells, and as such may induce different cellular pathways in response to PDT treatment. However, the ability of PDT applied at day 14 to cause a similar reduction in tumour burden compared to BP treatment at day 7, suggests its ability to rapidly and effectively ablate the tumour within the bone, even in the presence of BP.