Bone is the preferred site of metastases in women with breast cancer, which can cause skeletal-related events (SRE¡¦s) such as pathologic fractures. Bisphosphonates are the current standard of care for treatment of meta-static bone disease by preventing further bone destruction. Photodynamic therapy (PDT) has been applied successfully as a non-radiative treatment for malignancies. In PDT, light is delivered to a tumour after the administration of a photosensitiser. Earlier pre-clinical studies in a metastatic rat model have shown that PDT reduced the tumour burden in the vertebrae. The goal of this investigation was to study the effect of PDT on bisphosphonate pre-treated cancer in-vitro.

Human breast cancer cells, MT-1, were cultured until confluent. The following groups were formed: no treatment; incubation with zoledronic acid (24h; 10 ƒÝmol) only; PDT treatment only and incubation with zoledronic acid and PDT treatment. Prior to light application 1 microg/ml of the photosensitiser BPD-MA was added. PDT was performed with a light dose of 1J and 10 J. The cells were stained with a live/dead stain and analyzed by fluorescence microscope and flowcytometry.

Incubation of the MT-1 carcinoma cells with bisphosphonate zoledronic acid resulted in a significantly higher number of dying cells following PDT treatment when compared cells that were not treated by zoledronic acid (p< 0.05). When comparing cell groups that did not undergo PDT treatment the incubation with zoledronic acid alone did not have a statistically significant effect on cell survival twenty-four hours following zoledronic acid administration.

In-vitro, breast cancer cells appear more susceptible to PDT after they have been incubated with the zoledronic acid. Zoledronic acid, a potent bisphosphonate, inhibits farsenylpyrophosphate (FPP) which is involved in farsenylation of cell membrane proteins. The inhibition of FPP may cause a reduced effect of PDT on cell rescue. The treatment with bisphosphonates seems to have a synergistic effect with PDT treatment. As such, light dosimetry in PDT treatment may need to take into account potential therapeutic interactions between PDT and current medical therapies in the treatment of skeletal metastatic burden.