Bisphosphonate (BP) drugs target rapidly to bone and are the gold standard treatment to inhibit osteoclastic resorption in patients with metastatic bone disease. However, BPs also have anti-cancer effects outside the skeleton; in some preclinical models they reduce soft tissue tumour growth and increased survival in some clinical trials of adjuvant BP therapy in breast cancer and myeloma patients. The exact mechanisms underlying these anti-cancer effects are not known since BPs are considered to only affect osteoclasts in vivo. To directly answer this question, we determined the cell types capable of internalising fluorescently-labelled BP in murine 4T1 mammary tumours in live mice. Within minutes of tail vein injection, intravital 2-photon imaging revealed the flow of BP into mammary tumours via the disorganised tumour vasculature, with slow diffusion and retention in tumour tissue. 2hr after injection, fluorescent BP was visible within F4/80+ tumour-associated macrophages. Flow cytometric analysis of the tumours 24hr later confirmed that uptake occurred predominantly by CD11b+F4/80+ macrophages, but not tumour cells. Furthermore BP did not accumulate in normal mammary tissue.
These studies provide conclusive evidence that BPs can be rapidly internalised by myeloid cells completely outside the skeleton. The disorganised vascular pattern of tumours may facilitate the local diffusion of BP and endocytic uptake by macrophages. Given the important role of tumour-associated macrophages in promoting tumour progression and metastasis, our studies suggest that the anti-tumour activity of BPs occurs via effects on these cells.