# Effect of Systemic Zoledronic Acid Dosing Regimens on Bone Regeneration in Osteoporotic Rats
## 4 Discussion
All patients receiving ZA infusion are advised to drink two glasses of water before and after IV infusion to ensure adequate hydration when receiving ZA in order to prevent adverse effects. The rats in the current study were administered 0.04 mg/kg body weight ZA IV every week for a duration of, respectively, 4 (ZA-Pre), 6 (ZA-Post), and 10 weeks (ZA-Pre+Post). No specific measures were taken for the hydration of the rats of before and after infusion. The posology and method of administration for the ZA-Pre and ZA-Post groups was based on experience from previous studies [37, 52]. However, the dosage for the ZA-Pre+Post group was obtained by combining the doses of the ZA-Pre and ZA-Post groups. It is known that the serum concentrations of ZA after IV infusion decrease rapidly due to the high affinity of ZA for bone mineral [53]. Additionally, the effect of ZA on osteoclasts is reported to be determined by ZA dosage and administration frequency [53, 54]. This was confirmed by a clinical study involving patients with [PERSON]'s disease, which reported a reduced therapeutic effect with ZA doses above 200 ug [55]. The serum alkaline phosphatase (SAP) level, which is an indicator for the therapeutic efficacy of ZA treatment, was significantly higher in patients treated with a high dose of ZA (>200 ug) compared with patients treated with a lower dose of ZA (<200 ug). The histomorphometric analysis of our bone specimens revealed no significant difference in BA% between the ZA-Pre+Post and the saline control treatment groups. Consequently, we suppose that the lack of effect of ZA infusion on BA% with this dosing regimen was due to an issue with the total administered dose. As previous work has shown that high doses of ZA (i.e., 66 mg/kg body weight; 3 times weekly intraperitoneally during the 3 weeksprior to surgery) drastically decrease implant osseointegration [56], it seems likely that already the ZA-Pre+Post dosing regimen in the current study was too high. Therefore, we recommend that in future studies with ZA (as well as other BPs), the SAP level in the experimental animals is determined to assess the efficacy of the treatment. The beneficial effect of BPs, including ZA, on bone regeneration in an osteoporotic condition corroborates the existing literature [57]. The effect of these agents is known to be independent of the route of administration (i.e., systemically or locally) [58]. Additionally, several animal studies showed that BP therapy can also enhance bone volume and density in healthy animals. In these studies, bone defects in healthy animals were grafted with bone substitutes, followed by the administration of ZA either locally or systemically [10; 11]. Multiple studies reported that ZA treatment of a bone graft, locally and systemically, increased osteogenesis of the graft material and enabled bone formation [59]. Local ZA administration was found to be a highly efficient method of concentrating BPs at the site of bone graft placement and supported early bone formation as well as facilitated the biomechanical fixation of the bone substitute to the native bone [11; 60]. Further, [PERSON] et al. used their rat bone chamber model to investigate the beneficial effects of systemic ZA administration [61]. They showed that a single subcutaneous injection of 0.1 mg/kg ZA in rats significantly increased the volume of cancellous bone grafts installed in the bone chamber and protected the bone grafts against resorption [61]. Also, experiments demonstrated that just the soaking of an allograft in a 4 ug/kg ZA solution increased bone volume and bone mineral density significantly over a 6-week healing period compared with a sodium chloride-soaked allograft [11; 60]. Our data from the ZA-Pre and ZA-Post groups are in agreement with all these previous studies. This observation may be explained by the capacity of ZA to promote bone metabolism and prevent osteocl activity. In addition, BPs can increase the proliferation of osteoblasts and the synthesis of collagen and osteoclastin by bone cells at the cellular level [62].