The skeletal impact of the chemotherapeutic agent etoposide

Effects of the chemotherapeutic agent etoposide on the skeleton were determined in mice. Numbers of bone marrow cells were reduced and myeloid cells were increased. Bone volume was significantly decreased with signs of inhibition of bone formation. Etoposide after pre-treatment with zoledronic acid still reduced bone but overall bone volume was higher than with etoposide alone.

Chemotherapeutics target rapidly dividing tumor cells yet also impact hematopoietic and immune cells in an off target manner. A wide array of therapies have negative side effects on the skeleton rendering patients osteopenic and prone to fracture. This study focused on the pro-apoptotic chemotherapeutic agent etoposide and its short- and long-term treatment effects in the bone marrow and skeleton.

Six- to 16-week-old mice were treated with etoposide (20-25 mg/kg) or vehicle control in short-term (daily for 5-9 days) or long-term (3×/week for 17 days or 6 weeks) regimens. Bone marrow cell populations and their phagocytic/efferocytic functions were analyzed by flow cytometry. Blood cell populations were assessed by CBC analysis. Bone volume and area compartments and osteoclast numbers were measured by microCT, histomorphometry, and TRAP staining. Biomarkers of bone formation (P1NP) and resorption (TRAcP5b) were assayed from serum. Gene expression in bone marrow was assessed using qPCR.

Flow cytometric analysis of the bone marrow revealed short-term etoposide reduced overall cell numbers and B220⁺ cells, with increased marrow apoptotic (AnnexinV⁺PI^-) cells, mesenchymal stem-like cells, and CD68⁺, CD45⁺, and CD11b⁺ monocyte/myeloid cells (as a percent of the total marrow). After 6 weeks, the CD68⁺, Gr1⁺, CD11b⁺, and CD45⁺ cell populations were still relatively increased in etoposide-treated bone marrow. Skeletal phenotyping revealed etoposide decreased bone volume, trabecular thickness, and cortical bone volume. Gene expression in the marrow for the leptin receptor and CXCL12 were reduced with short-term etoposide, and an increased ratio of RANKL/OPG mRNA was observed. In whole bone, Runx2 and osteocalcin gene expressions were reduced, and in serum, P1NP was significantly reduced with etoposide. Treatment with the antiresorptive agent zoledronic acid prior to etoposide increased bone volume and improved the etoposide-induced decrease in skeletal parameters.

These data suggest that etoposide induces apoptosis in the bone marrow and significantly reduces parameters of bone formation with rapid reduction in bone volume. Pre-treatment with an antiresorptive agent results in a preservation of bone mass. Preventive approaches to preserving the skeleton should be considered in human clinical studies.