Real-Time Intravital Imaging Establishes Tumor-Associated Macrophages as the Extraskeletal Target of Bisphosphonate Action in Cancer

Recent clinical trials have shown that bisphosphonate drugs improve breast cancer patient survival independent of their anti-resorptive effects on the skeleton. However, since bisphosphonates bind rapidly to bone mineral, the exact mechanisms of their anti-tumour action, particularly on cells outside of bone, remain unknown. Here we used real-time intravital two-photon microscopy to show extensive leakage of fluorescent bisphosphonate from the vasculature in 4T1 mouse mammary tumours, where it initially binds to areas of small, granular microcalcifications that are engulfed by tumour-associated macrophages (TAMs), but not tumour cells. Importantly, we also observed uptake of radiolabeled bisphosphonate in the primary breast tumour of a patient and showed the resected tumour to be infiltrated with TAMs and to contain similar granular microcalcifications. These data represent the first compelling in vivo evidence that bisphosphonates can target cells in tumours outside the skeleton and that their anti-tumour activity is likely to be mediated via TAMs.