Zoledronate treatment has different effects in mouse strains with contrasting baseline bone mechanical phenotypes

Abstract

Introduction Bisphosphonates are highly effective in reducing fracture risk yet some individuals treated with these agents still experience fracture. The goal of this study was to test the hypothesis that genotype influences the effect of zoledronate on bone mechanical properties. Methods Skeletally mature male mice from genetic backgrounds known to have distinct baseline post-yield properties (C57/B6, high post-yield displacement; A/J, low post-yield displacement) were treated for 8 weeks with saline (VEH) or zoledronate (ZOL, 0.06 mg/kg subcutaneously once every four weeks) in a 2×2 study design. Ex vivo µCT and mechanical testing (4-pt bending) were conducted on the femur to assess morphological and mechanical differences. Results Significant drug and/or genotype effects were found for several mechanical properties and significant drug × genotype interactions were found for measures of strength (ultimate force) and brittleness (total displacement, strain to failure). Treatment with ZOL affected bone biomechanical measures of brittleness (total displacement (−25%) and strain to failure (−23%)) in B6 mice significantly differently than in A/J mice. This was driven by unique drug × genotype effects on bone geometry in B6 animals yet likely also reflected changes to the tissue properties. Conclusion These data may support the concept that properties of the bone geometry and/or tissue at the time of treatment initiation play a role in determining the bone’s mechanical response to zoledronate treatment.

Raloxifene Improves Bone Mechanical Properties in Mice Previously Treated with Zoledronate

“…Male mice were chosen to build on previous data from our lab and others using this strain and sex [21–23]. Animals received a zoledronic acid (n=60; ZOL, subcutaneous injection of 0.06 mg/kg/body weight (BW)) or saline vehicle (n=20; VEH) at 16 weeks of age, then another at 20 weeks of age (Figure 1).…”

“…At 20 weeks of age, animals in designated treatment groups received raloxifene (RAL; 5 days/week, subcutaneous injection) at one of two doses: low dose (0.5 mg/kg/BW (n=20; ZOL + RAL-L)) or high dose (2.0 mg/kg/BW (n=20; ZOL + RAL-H)). The dose and duration of ZOL was chosen as it has shown efficacy in suppressing remodeling and affecting mechanical properties in this genetic strain [21]. The lower dose of RAL has been used previously and shown efficacy in vivo [24]; a higher dose was arbitrarily chosen as 4× higher to determine if dose-responses could be quantified.…”

“…Right femora were thawed and scanned with microCT (Skyscan 1176) to obtain trabecular and cortical bone morphology as previously described [21]. Scans were conducted using a 9 micron voxel size.…”

“…Whole bone mechanical properties were assessed in four-point bending as previously described [21] and in accordance with recommendations [26]. The anterior surface of each femur was placed on two lower supports with a span length of 9 mm and an upper span length of 3 mm.…”

“…The goal of this study was to test the hypothesis that raloxifene treatment could reverse the compromised mechanical properties that occur following zoledronate treatment. To test this hypothesis, we utilized a mouse model that has previously been shown to manifest bisphosphonate-induced reductions in bone mechanical properties [21]. …”