In vivo bone augmentation in an osteoporotic environment using bisphosphonate-loaded calcium deficient apatite
Introduction
Osteoporosis has been defined as “a systemic disease characterized by low bone mass and micro-architectural deterioration of bone tissue, with consequent increase in bone fragility and susceptibility to fracture” [1]. After the age of fifty, 40% of women suffer from osteoporosis and one in three women will experience osteoporotic fractures [2]. At 80 years of age or older, 70% of osteoporotic women present at least one fracture [3]. Fractures commonly associated with osteoporosis are preferentially located at the proximal femur, vertebral bodies and distal radius (wrist). Patients with a history of a prior fracture have an increased risk of future fractures [4]. Fractures of the hip and vertebrae are the most serious health risks associated with the disease, resulting in prolonged hospital care, reduced autonomy, a decreased quality of life and an increased mortality rate [5], [6]. For example, the percentage of deaths one year after a femoral fracture is estimated at 20–30%, with 50% remaining handicapped.
With the increasing life expectancy of the population, osteoporotic fractures may have a serious economic impact on society and on patient quality of life. In fact, fractures are associated with significant pain and the loss of both mobility and autonomy. Considering all these data, preventing fractures would be of huge benefit.
Bisphosphonates are the preferred therapy for preventing and treating osteoporosis. Bisphosphonates are carbon-substituted analogues of pyrophosphate that act as powerful inhibitors of osteoclastic activity. These compounds show high affinity for the mineralized bone matrix where they can be retained for many years, resulting in potent pharmacological effects on target tissue [7]. The most potent nitrogen-containing bisphosphonates such as alendronate, risedronate and zoledronate act by inhibiting enzymes of the mevalonate pathway involved in cholesterol synthesis [8]. This inhibition leads to a reduced level of farnesyl diphosphate and geranylgeranyldiphosphate, both of which are required for prenylation of guanine triphosphate binding proteins (such as RHO, Rab and Cdc42) [9]. Given that this pathway is essential for both activity and survival of osteoclasts, bisphosphonates inactivate osteoclasts and promote apoptosis, resulting in reduced bone resorption [10], [11]. Clinically, bisphosphonates effectively increase bone density, prevent bone loss and reduce the risk of vertebral and non-vertebral fractures [12], [13], [14]. The prevention of fracture in osteoporotic patients is the ultimate goal of bone resorption inhibitor therapy [15]. However, this can be hampered by the required high compliance from the patient throughout long-term treatment. Considerable constraints related to administration modalities and adverse events may indeed negatively impact medication compliance, which has been reported to decrease dramatically (by 58%) within the first year [16]. Consequently, some studies have demonstrated an increased risk of fracture as a consequence of poor persistence and compliance with treatment [17], [18], [19], [20]. For example, Siris et al. have shown that patients who take more than 80% of their osteoporosis medications have a 26% reduction in fractures compared to other patients with lower long-term compliance [21]. An alternative consisting of a weekly or monthly administration has been proposed to improve long-term compliance with therapy [22]. However, despite these strategies, compliance still remains suboptimal [23].
In this context, we propose an innovative local approach by associating calcium phosphate biomaterials with BP. CaP biomaterials have been largely used as bone substitutes in humans [24], [25], [26]. Implanting such BP-loaded CaP biomaterial could not only limit the excessive activity of osteoclastic resorption but could also promote new bone ingrowth and reinforce the trabeculae structures that are weakened by osteoporosis, thereby preventing fractures.
Previous material design studies have described different modes associating CaP matrices and BP [27], [28], [29]. Calcium deficient apatite (CDA) has been shown to be effective for loading and releasing BP at doses that can inhibit excessive bone resorption without affecting osteoblasts [30]. Moreover, in a rat model, osteointegration of HA-coated titanium implants was found to be dependent on the amount of BP previously loaded onto the HA coating [31], [32]. The optimal concentration of zoledronate related to the treated bone volume was defined in order to generate the best bone density distribution around the implant leading to the highest mechanical stability of the implant [31], [33]. Various strategies to prevent hip fractures and consequent hip fracture surgery have been introduced over the last few decades [34], [35], [36], [37], [38], [39]. However, these preventive approaches have not yet led to a reduction in the incidence of sequential hip fracture [40].
Recently results have been published [41], [42], [43] concerning femoroplasty-cement reinforcement of the proximal femur as a means of fracture prevention (PMMA, using Silicone as an inert material). The authors showed on cadaver femurs that the cement led to a reinforcement of the implanted site, but no clinical use was considered because the temperature associated with the cement curing was too high, possibly causing osteonecrosis.
Therefore, the aim of the present study was to determine the effect of an injectable BP-loaded biomaterial on osteoporotic femur bone structure. For ethical reasons, the rat model was first used to prove the feasibility of the approach and then osteoporotic ewes were used to receive the chosen drug device combination within their proximal femurs. Bone modifications after implantation were determined using 3D-μCT, histology analysis and SEM observations.
Section snippets
Synthesis of zoledronate-loaded calcium phosphate materials
Calcium deficient apatite (CDA) was obtained by alkaline hydrolysis of 40 g of dicalcium phosphate dihydrate (DCPD), in 506 mL of 0.3 m aqueous ammonia for 4 h at 90 °C, as previously described [44]. The Ca/P ratio of the obtained powder was checked on a Philips PW 1830 diffractometer, from the X-ray diffraction powder pattern of the corresponding calcined phases, in accordance with the literature [45], [46]. According to the described process, 24 g of CDA were obtained. Zoledronate loading
The rat model (Fig. 2Table 2)
Using μ-CT images, it was established that the position of implants in the femurs was similar for all animals. Three weeks after implantation, a significant decrease in relative bone content (−60.1%) and an alteration of micro-architectural parameters were observed in the distal femur of osteoporotic animals receiving implants of pure CDA. When comparing femurs treated with CDA-Zol versus those treated with CDA alone in ovariectomized rats, μ-CT histomorphometric measurements showed significant
Discussion
Our main purpose in this study was to locally reinforce osteoporotic sites in vivo by increasing the relative bone content and improving the trabecular micro-architecture. To achieve such a goal, a zoledronate-combined calcium-deficient apatite (CDA-Zol) was developed that is able to promote new bone formation and to release bisphosphonates in the treated site [28], [29]. A model predicting zoledronate adsorption/desorption from CDA was developed [30] and specific in vitro dose-response
Conclusion
All these results revealed the ability of CDA-ZOL to form new bone and to reinforce existing trabeculae in an osteoporotic site. A local combined effect of calcium phosphate particles and bisphosphonate was demonstrated in the proximal femurs of osteoporotic ewes. By this local approach, we expect to reinforce specific osteoporotic bone sites (such as the proximal femur, spine or wrist) by implanting CaP materials that can promote new bone ingrowth and, by releasing BP, which can control the
Acknowledgments
This work was supported by the ANR (MIADROS, RNTS 05-00401) and Graftys SA (Aix en Provence, France).
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