Doxorubicin induces bone loss and modifies multiple cell populations in vivo – Implications for modelling of bone metastasis

Doxorubicin (DOX), commonly used to treat breast cancer, is associated with cardiotoxicity and has negative effects on other organ systems, including the skeleton. DOX-induced bone damage has been demonstrated in murine models; however, results are conflicting due to the use of different doses, schedules, and rat/mouse strains. As DOX is used to limit tumour progression in models of skeletal metastasis, it is paramount to determine how the agent affects the bone microenvironment in the relevant mouse strains, to enable correct interpretation of DOX effects in tumour studies. We have therefore investigated the effects of DOX on bone structure and a range of bone and bone marrow cell populations, comparing immunocompetent and immunocompromised mice. Groups of 7-week-old female BALB/c and BALB/c Nude mice were treated with either saline (control), 4 or 6 mg/kg DOX weekly for four weeks. Effects on bone volume and structure was determined using <i>ex vivo</i> µCT, a panel of bone marrow cell populations were quantified by flow cytometry and osteoblast/osteoclast numbers were assessed using bone histomorphometry. DOX caused trabecular bone loss, with immunocompetent BALB/c mice being more sensitive to DOX than the immunocompromised BALB/c nude counterparts. The 6 mg/kg dose of DOX altered the ratio of bone marrow immune and haematopoietic cell populations in both groups, increasing the numbers of hematopoietic cells and progenitors, decreasing B cells and increasing the number of neutrophils. Bone marrow macrophage and monocyte numbers were increased following DOX treatment in BALB/c nude mice only. Our data demonstrate that DOX impacts a number of cell types in the bone microenvironment, highlighting the importance of considering treatment-induced bone effects when using DOX in models of bone metastasis.