A novel osteotropic biomaterial OG-PLG: Synthesis andin vitro release

Development of a simvastatin loaded injectable porous scaffold in situ formed by phase inversion method for bone tissue regeneration

INTRODUCTION:

The use of injectable scaffolds as a minimally invasive method is a good choice in tissue engineering applications. A critical parameter for the tissue engineering scaffolds is a suitable morphology with interconnected pores. We present the development of a simvastatin loaded scaffold that forms in situ and provides the porous structure with interconnected pores.

METHODS:

The formulation of these scaffolds includes a polymeric solution of poly lactic-co-glycolic acid (25 wt%) in N-methyl-2-pyrrolidone containing 6 wt% deionized water and porogen (mannitol, four times the weight of the polymer). We have grafted simvastatin to poly lactic-co-glycolic acid by the esterification reactions. Simvastatin or simvastatin-grafted poly lactic-co-glycolic acid in different levels was added to polymer solution and finally the solution was injected into phosphate buffered saline. The simvastatin-grafted poly lactic-co-glycolic acid was characterized by attenuated total reflection Fourier-transform infra-red and ¹H-nuclear magnetic resonance spectroscopy. The morphology, porosity, and biocompatibility of the scaffolds were evaluated. The in vitro simvastatin release from the various formulations was studied. Osteogenic differentiation of the adipose-derived stem cells was investigated using alkaline phosphatase activity assay and cell mineralization was evaluated using Alizarin red staining.

RESULTS:

The morphology results showed the resultant scaffold was porous with the interconnected pores. The scaffolds presented 91% porosity. Non-toxic doses of simvastatin in the scaffolds were determined by methyl-thiazolyl diphenyl-tetrazolium bromide assay. The released simvastatin from the scaffolds continues over 80 days. Alkaline phosphatase activity and Alizarin red results indicated that cell osteogenic differentiation is promoted.

CONCLUSION:

The results demonstrated that release of simvastatin from the injectable scaffolds can have positive effects on osteogenic differentiation of the adipose-derived stem cells.

Analysis of Bone Repair and Inflammatory Process Caused by Simvastatin Combined With PLGA+HA+βTCP Scaffold

PURPOSE

This study evaluated the tissue and inflammatory responses to the use of simvastatin and poly(lactic-co-glycolic acid) + hydroxyapatite + β-tricalcium phosphate (PLGA+HA+βTCP) scaffold for bone repair.

MATERIALS AND METHODS

Two defects of 5 mm in diameter were made in the calvaria of rats, which were shared into the following 6 groups: naive, sham, vehicle, PLGA+HA+βTCP scaffold, simvastatin (4 mg/mL), and simvastatin with the scaffold. Tissue samples were collected at 1, 7, 15, 30, and 60 days after surgery. Inflammation was evaluated by interleukin-1 beta and tumor necrosis factor alpha quantification and by a hemogram, whereas bone repair was evaluated using densitometry and scanning electron microscopy. Data were statistically analyzed using ANOVA followed by post hoc tests (P < 0.05).

RESULTS

There was an increased cytokine expression in the scaffold and simvastatin groups (P < 0.001 and P < 0.05, respectively) 1 day after surgery but no alterations on the hemogram were observed. It was found on bone tissue samples that 60 days after surgery all groups presented similar densitometry values and morphology characteristics, despite the occurrence of bone formation delay in the simvastatin group (P < 0.01).

CONCLUSION

The use of simvastatin and PLGA+HA+βTCP scaffold, associated or not, did not lead to improvement in bone repair.

Osteogenic effect of fluvastatin combined with biodegradable gelatin-hydrogel

The aim was to investigate the local osteogenic effect of fluvastatin incorporated into a biodegradable gelatin-hydrogel (GH) scaffold. The GH scaffolds were prepared through crosslinking by ultraviolet irradiation followed by freeze-drying. Two circular defects were surgically created on fifteen-week-old male rats calvaria. All defects of each rat were randomly filled with two of three treatments, specifically: fluvastatin incorporated into a GH disk (Flu-GH), distilled water incorporated into a GH disk, and no treatment. New bone formation was quantitatively analyzed after 7, 14, and 28 days using a micro-computed tomography (micro-CT) system, and histologically observed. Evaluation by micro-CT revealed a significant difference in new bone formation among the three kinds of defect. A highly osteogenic effect was observed in the Flu-GH group. The results showed that the fluvastatin incorporated into a biodegradable GH scaffold promoted osteogenesis in rat calvarial bone, indicating its potential for bone regeneration.

Highly efficient release of lovastatin from poly(lactic-co-glycolic acid) nanoparticles enhances bone repair in rats

Lovastatin exhibits higher thermal stability and lower degradation rate than simvastatin. However, the amount of research studying a lovastatin delivery device has been far less than similar research on simvastatin. As a consequence, a high lovastatin release rate system has not been developed. We hypothesized that highly efficient release of lovastatin from poly(lactic-co-glycolic acid) (PLGA) nanoparticles in a short-term release (7 days) could provide an effective delivery system for bone repair. This study optimized the emulsion (o/w) technique in the fabrication process for PLGA nanoparticles, thereby producing the first recorded case of a high release rate (97%) of lovastatin. We also calculated the calibration curve of lovastatin using a UV spectrometer. The results demonstrated that the ALPase activity in human osteoblasts could be significantly stimulated by lovastatin carried in PLGA nanoparticles, but was prominently decreased by free lovastatin with the concentration higher than 4 µg/ml. Animal studies showed that the amount of lovastatin contained in 1 mg PLGA was the optimum dosage. These results suggest the new lovastatin-releasing PLGA delivery device exhibits potential for clinical treatment of bony defects.

Anabolic agents and the bone morphogenetic protein pathway

A major unmet need in the medical field today is the availability of suitable treatments for the ever-increasing incidence of osteoporosis and the treatment of bone deficit conditions. Although therapies exist which prevent bone loss, the options are extremely limited for patients once a substantial loss of skeletal bone mass has occurred. Patients who have reduced bone mass are predisposed to fractures and further morbidity. The FDA recently approved PTH (1-34) (Teriparatide) for the treatment of postmenopausal osteoporosis after both preclinical animal and clinical human studies indicated it induces bone formation. This is the only approved bone anabolic agent available but unfortunately it has limited use, it is relatively expensive and difficult to administer. Consequently, the discovery of low cost orally available bone anabolic agents is critical for the future treatment of bone loss conditions. The intricate process of bone formation is co-ordinated by the action of many different bone growth factors, some stored in bone matrix and others released into the bone microenvironment from surrounding cells. Although all these factors play important roles, the bone morphogenetic proteins (BMPs) clearly play a central role in both bone cartilage formation and repair. Recent research into the regulation of the BMP pathway has led to the discovery of a number of small molecular weight compounds as candidate bone anabolic agents. These agents may usher in a new wave of more innovative and versatile treatments for osteoporosis as well as orthopedic and dental indications.

Synthesis and characterization of a fluvastatin-releasing hydrogel delivery system to modulate hMSC differentiation and function for bone regeneration

Increases in bone formation have been demonstrated in mice and rats treated with statins, a group of molecules that increase the production of bone morphogenetic proteins-2 (BMP2) by stimulating its promoter. However, clinical use of statins (e.g., fluvastatin) is limited by the lack of a suitable delivery system to localize and sustain release. To harness the therapeutic effect of statins in orthopedic applications, a fluvastatin-releasing macromer was synthesized. When copolymerized with a dimethacrylated poly(ethylene glycol) solution, this fluvastatin-containing molecule was covalently incorporated into hydrogel networks, and hydrolysis of lactic acid ester bonds resulted in the release of the pendantly tethered fluvastatin from the hydrogel into the surrounding solution. The rate of fluvastatin release was controlled by the length of lactic acid spacer (2-6 repeats), and the dose was controlled by the initial comonomer composition (5-500 microg fluvastatin/gel). Released fluvastatin increased human mesenchymal stem cell (hMSC) gene expression of CBFA1, ALP, and COL I by 34-fold, 2.6-fold, and 1.8-fold, respectively, after 14 days of in vitro culture. In addition, treating hMSCs with the released fluvastatin resulted in an average of 2.0- and 1.5-fold greater BMP2 production whereas mineralization increased an average of 3.0-fold and 2.5-fold for 0.01 and 0.1 microM fluvastatin, respectively, over the 2 week culture period. Therefore, fluvastatin-releasing hydrogels may be useful in bone tissue engineering applications, not only for triggering osteogenic differentiation of hMSCs, but also by modulating their function.

Statins and osteoporosis: new role for old drugs

Osteoporosis is the most common bone disease, affecting millions of people worldwide and leading to significant morbidity and high expenditure. Most of the current therapies available for its treatment are limited to the prevention or slowing down of bone loss rather than enhancing bone formation. Recent discovery of statins (HMG-CoA reductase inhibitors) as bone anabolic agents has spurred a great deal of interest among both basic and clinical bone researchers. In-vitro and some animal studies suggest that statins increase the bone mass by enhancing bone morphogenetic protein-2 (BMP-2)-mediated osteoblast expression. Although a limited number of case-control studies suggest that statins may have the potential to reduce the risk of fractures by increasing bone formation, other studies have failed to show a benefit in fracture reduction. Randomized, controlled clinical trials are needed to resolve this conflict. One possible reason for the discrepancy in the results of preclinical, as well as clinical, studies is the liver-specific nature of statins. Considering their high liver specificity and low oral bioavailability, distribution of statins to the bone microenvironment in optimum concentration is questionable. To unravel their exact mechanism and confirm beneficial action on bone, statins should reach the bone microenvironment in optimum concentration. Dose optimization and use of novel controlled drug delivery systems may help in increasing the bioavailability and distribution of statins to the bone microenvironment. Discovery of bone-specific statins or their bone-targeted delivery offers great potential in the treatment of osteoporosis. In this review, we have summarized various preclinical and clinical studies of statins and their action on bone. We have also discussed the possible mechanism of action of statins on bone. Finally, the role of drug delivery systems in confirming and assessing the actual potential of statins as anti-osteoporotic agents is highlighted.