# Effect of Systemic Zoledronic Acid Dosing Regimens on Bone Regeneration in Osteoporotic Rats
## 1 Introduction  
Skeletal conditions such as osteoporosis, which further accentuate bone resorption and healing, are a major challenge for bone regenerative processes. The physiological process of bone healing is more complex in situations associated with metabolic disorders compared to healthy condition [1, 2]. Osteoporosis decreases both cortical and trabecular bone mineralization and is often diagnosed in postmenopausal women. It is characterized by decreased bone mass and strength, altered bone microstructure, and reduced regenerative capacity [3, 4]. Medication is prescribed for osteoporosis treatment to improve bone health. The mostly prescribed drugs are bisphosphonates (BPs), which decrease bone turnover by inhibiting osteoclastic cells [5, 6] through the disruption of the intracellular processes required for osteoclast function [7]. The chemical structure of BPs is characterized by two phosphate groups covalently linked to carbon [8]. Further, the phosphorus-carbon-phosphorus backbone contains two side chains (called R1 and R2), which determine the mode of action and strength of BPs. Considering the R-groups, two classes of BPs can be discerned, i.e., non-nitrogenous (no nitrogen in R2) and nitrogenous (nitrogen in R2). Non-nitrogenous BPs affect cellular energy metabolism and initiate osteoclast apoptosis, resulting in a reduction of bone breakdown. Nitrogenous BPs affect osteoclastogenesis and cytoskeletal dynamics and inhibit the formation of the ruffled border. Non-nitrogenous-BPs are associated with increased negative effects compared with nitrogenous BPs. Zoledronic acid (ZA) is a nitrogenous BP, generally administered by intravenous (IV) infusion [9]. It is the most potent BP, demonstrating a very high affinity for hydroxyapatite crystals and the longest retention into bone mineral in comparison to other BPs [10]. It has been hypothesized that ZA might interfere in bone turnover, which can result in improvement of bone healing around bone substitutes through reduction of the hyperactivity of osteocl cells in osteoporosis. Recent animal studies have investigated the effects of IV and subcutaneously ZA administration in a healthy animal model [7; 11; 12; 13], but there are no published data regarding the long-term effect of systemically administered ZA on bone healing in bone defects grafted with a bone substitute in an osteoporotic animal model. Besides, BP drugs can cause a rare but serious condition called bisphosphonate-related osteonecrosis of the jaw (BRONJ) [14]. Despite the risk posed by BP administration of development of a jaw disease, studies have also shown that BPs can promote bone regeneration [6; 15; 16]. In dental clinics, bone regeneration is commonly carried out by several bone grafting procedures to restore alveolar bone loss [17]. Bone grafting involves using natural or synthetic bone substitutes to stimulate bone healing through three different mechanisms: osteogenesis, osteoconduction, and osteoinduction [18]. However, the regeneration of alveolar bone is still a major clinical challenge in dentistry. Alveolar bone loss can result from trauma, pathology, chronic/acute dental infections, or severe periodontal disease. The most frequently reported cause of clinical discrepancies in dedentulous residual alveolar ridges is the loss of mechanical function associated with the early loss of teeth and a lack of immediate replacement. This physiological bone loss after tooth extraction has been corroborated both in clinical and in experimental studies, which reported both vertical and horizontal bone resorption of the residual alveolar ridge [19; 20; 21]. Bone defect healing after dental extraction occurs through regeneration and reconstruction of lost bone tissue and is essentially similar to fracture healing [20]. However, new bone formation could be impaired or delayed due to several external factors such as decreased blood supply, surgical site infection, and deficiency of essential micronutrients such as calcium and phosphorous [22].