Long‐Term Protective Effects of Zoledronic Acid on Cancellous and Cortical Bone in the Ovariectomized Rat

Current bisphosphonate therapies effectively prevent bone loss in postmenopausal women. We studied the effect of a single intravenous dose of ZOL in ovariectomized rats. Protection from bone loss was dose dependent, lasting for up to 32 weeks, supporting the rationale for an annual intravenous dosing regimen of ZOL for treatment of postmenopausal osteoporosis. Introduction: Once‐yearly dosing with zoledronic acid (ZOL) 5 mg can increase BMD and reduce fracture rate in postmenopausal women with low BMD. The primary objective of this study was to determine the duration of bone protective effects of a single dose of ZOL in ovariectomized rats, an animal model of postmenopausal osteopenia. Secondary objectives were to determine the effects on bone turnover and mechanical properties. Materials and Methods: Female Wistar rats (10 per group) received single intravenous doses of ZOL 0.8, 4, 20, 100, or 500 μg/kg, alendronate 200 μg/kg, or isotonic saline 4 days before bilateral ovariectomy. Sham‐operated controls were pretreated with saline. Mass and density of cancellous and cortical bone (pQCT) were measured at 4‐wk intervals for 32 wk. Bone architecture (μCT), bone formation dynamics (fluorochrome label‐based histomorphometry), and biomechanical strength in compression testing were also assessed at 32 wk. Results: Ovariectomy‐associated BMD loss was significantly attenuated for 32 wk by ZOL ≥4 μg/kg for total BMD, ZOL ≥20 μg/kg for cortical BMD, and ZOL ≥4 μg/kg for cancellous BMD (p < 0.01 versus ovariectomized controls). Alendronate 200 μg/kg was of equivalent potency to ZOL 20 μg/kg. Ovariectomy‐associated decreases in trabecular architectural parameters were dose‐dependently attenuated by ZOL. Alendronate 200 μg/kg was equivalent to ZOL 20 μg/kg. The bone resorption marker TRACP5b indicated transient suppression of elevated osteoclast activity by ZOL relative to OVX‐rats even at the lowest dose of 0.8 μg/kg, whereas at 100–500 μg/kg, the effect was significant relative to the OVX control for the entire duration of the study of 32 wk. Bone formation parameters were not significantly affected by ZOL 20 μg/kg but were significantly reduced by ZOL 100–500 μg/kg. Alendronate 200 μg/kg was equivalent to ZOL 100 μg/kg. ZOL produced dose‐related improvements in bone strength parameters after ovariectomy. Alendronate 200 μg/kg was of similar potency to ZOL 20 μg/kg. Conclusions: The duration and magnitude of the bone‐protecting effect of a single intravenous dose of