Bone Regeneration by Controlled Release of Bone Morphogenetic Protein-2: A Rabbit Spinal Fusion Chamber Molecular Study

Enhanced effects of slowly co-released TGF-β3 and BMP-2 from biomimetic calcium phosphate-coated silk fibroin scaffolds in the repair of osteochondral defects

Bioactive agents have demonstrated regenerative potential for cell-free bone tissue engineering. Nevertheless, certain challenges persist, including ineffective delivery methods and confined therapeutic potency. Here, we demonstrated that the biomimetic calcium phosphate coating system (BioCaP) could effectively uptake and slowly release the incorporated bioactive agents compared to the surface absorption system via osteoclast-mediated degradation of BioCaP coatings. The release kinetics were determined as a function of time. The release rate was stable without remarkable burst release during the first 1 day, followed by a sustained release from day 7 to day 19. Then, we developed the bi-functional BioCaP-coated silk fibroin scaffolds enabling the effective co-delivery of TGF-β3 and BMP-2 (SFI-T/SFI-B) and the corresponding slow release of TGF-β3 and BMP-2 exhibited superior potential in promoting chondrogenesis and osteogenesis without impairing cell vitality in vitro. The SFI-T/SFI-B scaffolds could improve cartilage and bone regeneration in 5 × 4 mm rabbit osteochondral (OC) defect. These findings indicate that the biomimetic calcium-phosphate coated silk fibroin scaffolds with slowly co-released TGF-β3 and BMP-2 effectively promote the repair of OC defects, hence facilitating the future clinical translation of controlled drug delivery in tissue engineering.

Improved intervertebral fusion in LLIF rabbit model with a novel titanium cage

BACKGROUND CONTEXT

There is no established small animal approach model for the strict simulation of lateral lumbar interbody fusion (LLIF) surgery.

PURPOSE

This study aims to establish a reliable LLIF rabbit model that strictly simulates the procedure and to preliminarily evaluate the differences in fusion outcomes with different graft materials.

STUDY DESIGN

A controlled laboratory.

METHODS

Fifty-four 4-month-old white New Zealand female and male rabbits were selected and divided into five groups: Group A (dissection group) consisted of 9 rabbits, Group B (normal approach group) consisted of 9 rabbits, Group C (autogenous iliac bone group) consisted of 12 rabbits, Group D (BMP-2 carrier material group) consisted of 12 rabbits, and Group E (allograft bone group) consisted of 12 rabbits. Based on data from Group A, a novel titanium metal fusion device was designed. Postoperatively, at the 12-week mark, manual palpation was employed to compare the interbody fusion status among Groups B, C, D, and E. Specimens from Groups C, D, and E were subjected to Micro-CT scanning to compare various parameters such as trabecular bone volume (BV), bone volume fraction (BV/TV, BVF), and bone surface area (BS). Furthermore, a tissue histopathological examination was performed to observe the structure and morphology of newly formed bone within the fusion mass as well as the remodeling of the graft in each group.

RESULTS

Based on the measurements obtained from the dissection group, we designed a U-shaped interbody fusion device with dimensions of 10 mm in length, 2.5 mm in width, and 1.3 mm in height. In Group B, 9 cases exhibited intervertebral mobility. In Group C, 1 case showed nonfusion. In Group D, all cases achieved fusion. In Group E, 4 cases did not achieve fusion. Additionally, the Micro-CT results showed that the interbody fusion index scores were 4.64±0.50 in Group C, 4.33±0.65 in Group D, and 3.36±0.81 in Group E. There was no statistically significant difference in fusion index scores between Groups C and D (p=.853). Notably, Groups C and D had higher scores than Group E (p<.001). The trabecular bone volume (BV) in Groups C and D also showed no significant difference but was significantly higher than in Group E (p<.001). Furthermore, the histopathological results revealed that the specimens from Group E had less newly formed cartilage and bone compared to Groups C and D.

CONCLUSIONS

This study successfully established a strict simulation of the clinical LLIF procedure in a rabbit model. Moreso, we conducted a preliminary validation indicating that the BMP-2 carrier material achieved interbody fusion outcomes similar to autogenous iliac bone.

CLINICAL SIGNIFICANCE

The findings of this investigation from animal models provide a theoretical basis for the clinical use of BMP-2 to promote early spinal fusion in LLIF procedures. Importantly, the study provides a small animal model foundation for research related to LLIF surgery.

Cell-permeable bone morphogenetic protein 2 facilitates bone regeneration by promoting osteogenesis

The use of the FDA-approved osteoinductive growth factor BMP2 is widespread for bone regeneration. However, its clinical application has been hindered by limitations in cell permeability and a short half-life in circulation. To address this issue, we have developed a modified version of BMP2, referred to as Cell Permeable (CP)-BMP2, which possesses improved cell permeability. CP-BMP2 incorporates an advanced macromolecular transduction domain (aMTD) to facilitate transfer across the plasma membrane, a solubilization domain, and recombinant human BMP2. Compared to traditional rhBMP2, CP-BMP2 exhibits enhanced cell permeability, solubility, and bioavailability, and activates Smad phosphorylation through binding to BMP receptor 2. The effectiveness of CP-BMP2 was evaluated in three animal studies focusing on bone regeneration. In the initial study, mice and rabbits with critical-size calvarial defects received subcutaneous (SC) injections of CP-BMP2 and rhBMP2 (7.5 mg/kg, 3 injections per week for 8 weeks).Following 8 weeks of administration, CP-BMP2 demonstrated a remarkable 65 % increase in bone formation in mice when compared to both the vehicle and rhBMP2. Moreover, rabbits exhibited faster bone formation, characterized by a filling pattern originating from the center. In a subsequent study involving injured horses, hind limb bones treated with CP-BMP2 exhibited an 85 % higher bone regeneration rate, as evidenced by Micro-CT results, in contrast to horses treated with the vehicle or rhBMP2 (administered at 150 μg/defect, subcutaneously, once a week for 8 weeks, without a scaffold). These results underscore the potential of CP-BMP2 to facilitate rapid and effective healing. No noticeable adverse effects, such as ectopic bone formation, were observed in any of the studies. Overall, our findings demonstrate that CP-BMP2 holds therapeutic potential as a novel and effective osteogenic agent.

Improving the handling properties and long-term stability of polyelectrolyte complex by freeze-drying technique for low-dose bone morphogenetic protein 2 delivery

A variety of controlled release carriers for bone morphogenetic protein 2 (BMP-2) delivery have been developed and tested in animal models. An alginate-based polyelectrolyte complex (PEC) for controlled release of low-dose BMP-2 has shown promising results in preclinical research. However, the poor handling properties and long-term stability of PEC need to be improved for translational applications. This study aimed to address these limitations of alginate-based PEC by employing a freeze-drying technique. The size and structure of freeze-dried PEC (FD-PEC) were maintained with the addition of a cryoprotectant, trehalose. The release profile of BMP-2 from FD-PEC was similar to that of freshly prepared PEC. In vitro bioactivity analysis of the released BMP-2 showed that the carrier performance of PEC was not compromised by freeze-drying up to three-month storage at room temperature. BMP-2-bound FD-PEC induced comparable bone formation to that using freshly prepared regular PEC in a rat posterolateral spinal fusion model. These results suggest that FD-PEC is capable of delivering low-dose BMP-2 and could be developed as an off-the-shelf product for translational applications. The simplicity of this preservation method provides promise for the translational application of PEC.

Synergistic Effect of NELL-1 and an Ultra-Low Dose of BMP-2 on Spinal Fusion

Bone morphogenetic protein 2 (BMP-2) is widely used in spinal fusion but it can cause adverse effects such as ectopic bone and adipose tissue in vivo. Neural epidermal growth factor like-like molecule-1 (NELL-1) has been shown to suppress BMP-2-induced adverse effects. However, no optimum carriers that control both NELL-1 and BMP-2 releases to elicit long-term bioactivity have been developed. In this study, we employed polyelectrolyte complex (PEC) as a control release carrier for NELL-1 and BMP-2. An ultra-low dose of BMP-2 synergistically functioned with NELL-1 on bone marrow mesenchymal stem cells osteogenic differentiation with greater mineralization in vitro. The osteoinductive ability of NELL-1 and an ultra-low dose of BMP-2 in PEC was investigated in rat posterolateral spinal fusion. Our results showed increased fusion rate, bone architecture, and improved bone stiffness at 8 weeks after surgery in the combination groups compared with NELL-1 or BMP-2 alone. Moreover, the formation of ectopic bone and adipose tissue was negligible in all the PEC groups. In summary, dual delivery of NELL-1 and an ultra-low dose of BMP-2 in the PEC control release carrier has greater fusion efficiency compared with BMP-2 alone and could potentially be a better alternative to the currently used BMP-2 treatments for spinal fusion. Impact Statement In this study, polyelectrolyte complex was used to absorb neural epidermal growth factor like-like molecule-1 (NELL-1) and bone morphogenetic protein 2 (BMP-2) to achieve controlled dual release. The addition of NELL-1 significantly reduced the effective dose of BMP-2 to 2.5% of its conventional dose in absorbable collagen sponge, to produce solid spinal fusion without significant adverse effects. This study was the first to identify the efficacy of combination NELL-1 and BMP-2 in a control release carrier in spinal fusion, which could be potentially used clinically to increase fusion rate and avoid the adverse effects commonly associated with conventional BMP-2.