Zoledronic acid alters hematopoiesis and generates breast tumor-suppressive bone marrow cells

Background: The bone-targeting agent zoledronic acid (ZOL) increases breast cancer survival in subsets of patients, but the underlying reasons for this protective effect are unknown. ZOL modulates the activity of osteoclasts and osteoblasts, which form hematopoietic stem cell niches, and therefore may affect hematopoietic cells that play a role in breast cancer progression. Method: Immunocompetent and immunocompromised strains of mice commonly used for breast cancer research were injected with a single, clinically relevant dose of ZOL (100 μg/kg) or vehicle control. The effects of ZOL on the bone marrow microenvironment (bone volume, bone cell number/activity, extracellular matrix composition) were established at various time points following treatment, using micro-computed tomography (μCT) analysis, histomorphometry, ELISA and immunofluorescence. The effects on peripheral blood and bone marrow hematopoietic progenitor populations were assessed using a HEMAVET® hematology analyzer and multicolor flow cytometry, respectively. Tumor support function of bone marrow cells was determined using an in vivo functional assay developed in our laboratory. Results: Using multiple mouse strains, we observed transient changes in numbers of hematopoietic stem cells, myeloid-biased progenitor cells, and lymphoid-biased cells concurrent with changes to hematopoietic stem cell niches following ZOL administration. Importantly, bone marrow cells from mice treated with a single, clinically relevant dose of ZOL inhibited breast tumor outgrowth in vivo. The ZOL-induced tumor suppressive function of the bone marrow persisted beyond the time point at which numbers of hematopoietic progenitor cells had returned to baseline. Conclusions: These findings provide novel evidence that alterations to the bone marrow play a role in the anti-tumor activity of ZOL and suggest possibilities for capitalizing on the beneficial effects of ZOL in reducing breast cancer development and progression.