Delivery of bone morphogenetic proteins for orthopedic tissue regeneration

Molecular Trojan Horses for treating lysosomal storage diseases

Lysosomal storage diseases (LSDs) are caused by monogenic mutations in genes encoding for proteins related to the lysosomal function. Lysosome plays critical roles in molecule degradation and cell signaling through interplay with many other cell organelles, such as mitochondria, endoplasmic reticulum, and peroxisomes. Even though several strategies (i.e., protein replacement and gene therapy) have been attempted for LSDs with promising results, there are still some challenges when hard-to-treat tissues such as bone (i.e., cartilages, ligaments, meniscus, etc.), the central nervous system (mostly neurons), and the eye (i.e., cornea, retina) are affected. Consistently, searching for novel strategies to reach those tissues remains a priority. Molecular Trojan Horses have been well-recognized as a potential alternative in several pathological scenarios for drug delivery, including LSDs. Even though molecular Trojan Horses refer to genetically engineered proteins to overcome the blood-brain barrier, such strategy can be extended to strategies able to transport and deliver drugs to specific tissues or cells using cell-penetrating peptides, monoclonal antibodies, vesicles, extracellular vesicles, and patient-derived cells. Only some of those platforms have been attempted in LSDs. In this paper, we review the most recent efforts to develop molecular Trojan Horses and discuss how this strategy could be implemented to enhance the current efficacy of strategies such as protein replacement and gene therapy in the context of LSDs.

Novel scaffold platforms for simultaneous induction osteogenesis and angiogenesis in bone tissue engineering: a cutting-edge approach

Despite the recent advances in the development of bone graft substitutes, treatment of critical size bone defects continues to be a significant challenge, especially in the elderly population. A current approach to overcome this challenge involves the creation of bone-mimicking scaffolds that can simultaneously promote osteogenesis and angiogenesis. In this context, incorporating multiple bioactive agents like growth factors, genes, and small molecules into these scaffolds has emerged as a promising strategy. To incorporate such agents, researchers have developed scaffolds incorporating nanoparticles, including nanoparticulate carriers, inorganic nanoparticles, and exosomes. Current paper provides a summary of the latest advancements in using various bioactive agents, drugs, and cells to synergistically promote osteogenesis and angiogenesis in bone-mimetic scaffolds. It also discusses scaffold design properties aimed at maximizing the synergistic effects of osteogenesis and angiogenesis, various innovative fabrication strategies, and ongoing clinical studies.

Two-phase matrices for the controlled release of therapeutic proteins

Controlled and sustained delivery of therapeutic proteins is crucial for achieving desired effects in wound healing applications. Yet, this remains a challenge in growth factor delivery for bone tissue engineering. Current delivery systems can lead to negative side effects, such as ectopic bone growth and cancer, due to the over administration of growth inducing proteins. Here, we have developed a two-phase system for the controlled release of therapeutic proteins. The system consists of protein-loaded poly(methacrylic acid)-based nanoparticles conjugated to chitosan scaffolds. The effect of co-monomer hydrophilicity and crosslinking density on nanoparticle properties was evaluated. It was found that the release kinetics of model therapeutic proteins were dependent on nanoparticle hydrophilicity. The chitosan scaffold, chosen for its biocompatibility and osteogenic properties, provided additional barriers to diffusion and promoted nanoparticle retention, leading to more sustained protein delivery. Additionally, the ability of MC3T3-E1 pre-osteoblast cells to proliferate on scaffolds with and without conjugated nanoparticles was evaluated and all scaffolds were shown to promote cell growth. The results demonstrate that the two-phase scaffold system presents a superior strategy for the sustained and controlled release of therapeutic proteins for bone tissue engineering applications.

An Artificial Silk Elastin-Like Protein Modifies the Polarization of Macrophages

A silk elastin-like protein (SELP) is an artificial compound with silk fibroin-like and elastin-like tandem repeats. The objective of this study is to evaluate the influence of SELP on the polarization of mouse bone marrow-derived macrophages. When the macrophages of inflammation-type (M1) were cultured with different concentrations of SELP solution, the secretion of a pro-inflammatory cytokine, tumor necrotizing factor (TNF)-α, was significantly suppressed at the higher concentrations. In addition, the secretion of an anti-inflammation cytokine, interleukin (IL)-10, was significantly enhanced from the macrophage of an original type (M0). By the incubation with soluble SELP, the morphology of M0- and M1-type macrophages changed to be of a round shape with a large size. Following incubation with the sponge of SELP, the M0-type macrophages secreted IL-10 with time. When injected into an air pouch of mice subcutis which had been prepared by the injection of air, the SELP sponge and 5 wt % of SELP solution induced IL-10 secretion to a significantly high extent compared with the saline injection. Cells isolated from the air pouch 24 h after the injection were stained by the CD206 of a M2 marker. It is concluded that the SELP itself in solution has an ability to induce the anti-inflammation M2-type macrophages.

Advances in bone regeneration with growth factors for spinal fusion: A literature review

Bone tissue is regenerated via the spatiotemporal involvement of various cytokines. Among them, the bone morphogenetic protein (BMP), which plays a vital role in the bone regeneration process, has been applied clinically for the treatment of refractory orthopedic conditions. Although BMP therapy using a collagen carrier has shown efficiency in bone regeneration over the last two decades, a major challenge-considerable side effects associated with the acute release of high doses of BMPs-has also been revealed. To improve BMP efficiency, the development of new carriers and biologics that can be used in conjunction with BMPs is currently underway. In this review, we describe the current status and future prospects of bone regeneration therapy, with a focus on BMPs. Furthermore, we outline the characteristics and molecular signaling pathways involving BMPs, clinical applications of BMPs in orthopedics, clinical results of BMP use in human spinal surgeries, drugs combined with BMPs to provide synergistic effects, and novel BMP carriers.

Comparison of synthetic ceramic products formulated with autologous blood coagulum containing rhBMP6 for induction of bone formation

PURPOSE

Osteogrow, an osteoinductive device containing recombinant human Bone Morphogenetic Protein 6 (rhBMP6) in autologous blood coagulum, is a novel therapeutic solution for bone regeneration. This study aimed to evaluate different commercially available calcium phosphate synthetic ceramic particles as a compression-resistant matrix (CRM) added to Osteogrow implants to enhance their biomechanical properties.

METHODS

Osteogrow implants with the addition of Vitoss, ChronOs, BAM, and Dongbo ceramics (Osteogrow-C, where C stands for ceramics) were evaluated in the rodent subcutaneous ectopic bone formation assay. Osteogrow-C device was prepared as follows: rhBMP6 was added to blood, and blood was mixed with ceramics and left to coagulate. Osteogrow-C was implanted subcutaneously in the axillary region of Sprague-Dawley rats and the outcome was analyzed 21 days following implantation using microCT, histology, morphometric analyses, and immunohistochemistry.

RESULTS

Osteogrow-C implants with all tested ceramic particles induced the formation of the bone-ceramic structure containing cortical bone, the bone between the particles, and bone at the ceramic surfaces. The amount of newly formed bone was significant in all experimental groups; however, the highest bone volume was measured in Osteogrow-C implants with highly porous Vitoss ceramics. The trabecular number was highest in Osteogrow-C implants with Vitoss and ChronOs ceramics while trabeculae were thicker in implants containing BAM and Dongbo ceramics. The immunological response and inflammation were comparable among ceramic particles evaluated in this study.

CONCLUSION

Osteogrow-C bone regenerative device was effective with a broad range of commercially available synthetic ceramics providing a promising therapeutic solution for the regeneration of long bone fracture nonunion, large segmental defects, and spinal fusion surgeries.

Carrier systems for bone morphogenetic proteins: An overview of biomaterials used for dentoalveolar and maxillofacial bone regeneration

Different types of biomaterials have been used to fabricate carriers to deliver bone morphogenetic proteins (BMPs) in both dentoalveolar and maxillofacial bone regeneration procedures. Despite that absorbable collagen sponge (ACS) is considered the gold standard for BMP delivery, there is still some concerns regarding its use mainly due to its poor mechanical properties. To overcome this, novel systems are being developed, however, due to the wide variety of biomaterial combination, the heterogeneous assessment of newly formed tissue, and the intended clinical applications, there is still no consensus regarding which is more efficient in a particular clinical scenario. The combination of two or more biomaterials in different topological configurations has allowed specific controlled-release patterns for BMPs, improving their biological and mechanical properties compared with classical single-material carriers. However, more basic research is needed. Since the BMPs can be used in multiple clinical scenarios having different biological and mechanical needs, novel carriers should be developed in a context-specific manner. Thus, the purpose of this review is to gather current knowledge about biomaterials used to fabricate delivery systems for BMPs in both dentoalveolar and maxillofacial contexts. Aspects related with the biological, physical and mechanical characteristics of each biomaterial are also presented and discussed.

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Strategies for bone formation and regeneration are a major concern in dentistry.

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Topical delivery of bone morphogenetic proteins (BMPs) allows rapid bone formation.

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BMPs requires proper carrier system to allow controlled and sustained release.

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Carrier should also fulfill mechanical requirements of bone defect sites.

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By using complex composites, it would be possible to develop new carriers for BMPs.

Synthesis and characterization of growth factor free nanoengineered bioactive scaffolds for bone tissue engineering

BACKGROUND

To address the obstacles that come with orthopedic surgery for biological graft tissues, including immune rejections, bacterial infections, and weak osseointegration, bioactive nanocomposites have been used as an alternative for bone grafting since they can mimic the biological and mechanical properties of the native bone. Among them, PCL-PEG-PCL (PCEC) copolymer has gained much attention for bone tissue engineering as a result of its biocompatibility and ability for osteogenesis.

METHODS

Here, we designed a growth factor-free nanoengineered scaffold based on the incorporation of Fe₃O₄ and hydroxyapatite (HA) nanoparticles into the PCL-PEG-PCL/Gelatin (PCEC/Gel) nanocomposite. We characterized different formulations of nanocomposite scaffolds in terms of physicochemical properties. Also, the mechanical property and specific surface area of the prepared scaffolds, as well as their feasibility for human dental pulp stem cells (hDPSCs) adhesion were assessed.

RESULTS

The results of in vitro cell culture study revealed that the PCEC/Gel Fe₃O₄&HA scaffold could promote osteogenesis in comparison with the bare scaffold, which confirmed the positive effect of the Fe₃O₄ and HA nanoparticles in the osteogenic differentiation of hDPSCs.

CONCLUSION

The incorporation of Fe₃O₄ and HA with PCEC/gelatin could enhance osteogenic differentiation of hDPSCs for possible substitution of bone grafting tissue.

Percutaneous-Reinforced Osteoplasty: A Review of Emerging Treatment Strategies for Bone Interventions

Percutaneous-reinforced osteoplasty is currently being investigated as a possible therapeutic procedure for fracture stabilization in high-risk patients, primarily in patients with bone metastases or osteoporosis. For these patients, a percutaneous approach, if structurally sound, can provide a viable method for treating bone fractures without the physiologic stress of anesthesia and open surgery. However, the low strength of fixation is a common limitation that requires further refinement in scaffold design and selection of materials, and may potentially benefit from tissue-engineering-based regenerative approaches. Scaffolds that have tissue regenerative properties and low inflammatory response promote rapid healing at the fracture site and are ideal for percutaneous applications. On the other hand, preclinical mechanical tests of fracture-repaired specimens provide key information on restoration strength and long-term stability and enable further design optimization. This review presents an overview of percutaneous-reinforced osteoplasty, emerging treatment strategies for bone repair, and basic concepts of in vitro mechanical characterization.

Engineering immunomodulatory hydrogels and cell-laden systems towards bone regeneration

The well-known synergetic interplay between the skeletal and immune systems has changed the design of advanced bone tissue engineering strategies. The immune system is essential during the bone lifetime, with macrophages playing multiple roles in bone healing and biomaterial integration. If in the past, the most valuable aspect of implants was to avoid immune responses of the host, nowadays, it is well-established how important are the crosstalks between immune cells and bone-engineered niches for an efficient regenerative process to occur. For that, it is essential to recapitulate the multiphenotypic cellular environment of bone tissue when designing new approaches. Indeed, the lack of osteoimmunomodulatory knowledge may be the explanation for the poor translation of biomaterials into clinical practice. Thus, smarter hydrogels incorporating immunomodulatory bioactive factors, stem cells, and immune cells are being proposed to develop a new generation of bone tissue engineering strategies. This review highlights the power of immune cells to upgrade the development of innovative engineered strategies, mainly focusing on orthopaedic and dental applications.

Synergistic effect of nanostructure and calcium ions on improving the bioactivity of titanium implants

Surface structure and composition play essential roles in the osseointegration of titanium implants. In the present study, a nanoscale surface structure incorporated with calcium ions was fabricated on a titanium surface by hydrothermal treatment. The characteristics of the surfaces were analysed, and the bioactivity of the samples was evaluated in vitro and in vivo. nm-Ti and nm/Ca-Ti surfaces were significantly more hydrophilic than control-Ti surfaces. nm/Ca-Ti samples showed much faster bone-like apatite precipitation in simulated body fluid than the other samples. The results of MC3T3-E1 cell tests demonstrated that both nm-Ti and nm/Ca-Ti surfaces accelerated cell adhesion and proliferation. The highest level of osteogenesis-related genes (Runx2, bone morphogenetic protein-2, osteopontin and osteocalcin) were observed in nm/Ca-Ti samples, followed by nm-Ti samples. Alizarin red staining experiment showed that the amount of extracellular matrix mineralized nodules in nm/Ca-Ti group was significantly higher than others. In animal experiments using SD rats, nm/Ca-Ti showed the highest value of new bone formation at two and four weeks. The present study suggested that the nanostructure and calcium ions showed synergetic effects on accelerating bone-like apatite precipitation and osteoblast cell growth and differentiation. Animal experiment further indicated that such surface could promote early osteogenesis.

Synergistic effect of nanostructure and calcium ions on improving the bioactivity of titanium implants

Surface structure and composition play essential roles in the osseointegration of titanium implants. In the present study, a nanoscale surface structure incorporated with calcium ions was fabricated on a titanium surface by hydrothermal treatment. The characteristics of the surfaces were analysed, and the bioactivity of the samples was evaluated in vitro and in vivo. nm-Ti and nm/Ca-Ti surfaces were significantly more hydrophilic than control-Ti surfaces. nm/Ca-Ti samples showed much faster bone-like apatite precipitation in simulated body fluid than the other samples. The results of MC3T3-E1 cell tests demonstrated that both nm-Ti and nm/Ca-Ti surfaces accelerated cell adhesion and proliferation. The highest level of osteogenesis-related genes (Runx2, bone morphogenetic protein-2, osteopontin and osteocalcin) were observed in nm/Ca-Ti samples, followed by nm-Ti samples. Alizarin red staining experiment showed that the amount of extracellular matrix mineralized nodules in nm/Ca-Ti group was significantly higher than others. In animal experiments using SD rats, nm/Ca-Ti showed the highest value of new bone formation at two and four weeks. The present study suggested that the nanostructure and calcium ions showed synergetic effects on accelerating bone-like apatite precipitation and osteoblast cell growth and differentiation. Animal experiment further indicated that such surface could promote early osteogenesis.

Long-term posterolateral spinal fusion in rabbits induced by rhBMP6 applied in autologous blood coagulum with synthetic ceramics

Autologous bone graft substitute (ABGS) containing rhBMP6 in autologous blood coagulum (Osteogrow) is a novel therapeutic solution for bone regeneration. This study is aimed to investigate the long-term outcome of ABGS with synthetic ceramics (Osteogrow-C) in rabbit posterolateral spinal fusion (PLF) model. Osteogrow-C implants were implanted bilaterally between rabbit lumbar transverse processes. We compared the outcome following implantation of ABGS with ceramic particles of different chemical composition (TCP and biphasic ceramics containing both TCP and HA) and size (500–1700 µm and 74–420 µm). Outcome was analyzed after 14 and 27 weeks by microCT, histology, and biomechanical analyses. Successful bilateral spinal fusion was observed in all animals at the end of observation period. Chemical composition of ceramic particles has impact on the PLF outcome via resorption of TCP ceramics, while ceramics containing HA were only partially resorbed. Moreover, persistence of ceramic particles subsequently resulted with an increased bone volume in implants with small particles containing high proportion of HA. ABGS (rhBMP6/ABC) with various synthetic ceramic particles promoted spinal fusion in rabbits. This is the first presentation of BMP-mediated ectopic bone formation in rabbit PLF model with radiological, histological, and biomechanical features over a time course of up to 27 weeks.

Site-Directed Immobilization of an Engineered Bone Morphogenetic Protein 2 (BMP2) Variant to Collagen-Based Microspheres Induces Bone Formation In Vivo

For the treatment of large bone defects, the commonly used technique of autologous bone grafting presents several drawbacks and limitations. With the discovery of the bone-inducing capabilities of bone morphogenetic protein 2 (BMP2), several delivery techniques were developed and translated to clinical applications. Implantation of scaffolds containing adsorbed BMP2 showed promising results. However, off-label use of this protein-scaffold combination caused severe complications due to an uncontrolled release of the growth factor, which has to be applied in supraphysiological doses in order to induce bone formation. Here, we propose an alternative strategy that focuses on the covalent immobilization of an engineered BMP2 variant to biocompatible scaffolds. The new BMP2 variant harbors an artificial amino acid with a specific functional group, allowing a site-directed covalent scaffold functionalization. The introduced artificial amino acid does not alter BMP2′s bioactivity in vitro. When applied in vivo, the covalently coupled BMP2 variant induces the formation of bone tissue characterized by a structurally different morphology compared to that induced by the same scaffold containing ab-/adsorbed wild-type BMP2. Our results clearly show that this innovative technique comprises translational potential for the development of novel osteoinductive materials, improving safety for patients and reducing costs.

Optimization of a Tricalcium Phosphate-Based Bone Model Using Cell-Sheet Technology to Simulate Bone Disorders

Bone diseases such as osteoporosis, delayed or impaired bone healing, and osteoarthritis still represent a social, financial, and personal burden for affected patients and society. Fully humanized in vitro 3D models of cancellous bone tissue are needed to develop new treatment strategies and meet patient-specific needs. Here, we demonstrate a successful cell-sheet-based process for optimized mesenchymal stromal cell (MSC) seeding on a β-tricalcium phosphate (TCP) scaffold to generate 3D models of cancellous bone tissue. Therefore, we seeded MSCs onto the β-TCP scaffold, induced osteogenic differentiation, and wrapped a single osteogenically induced MSC sheet around the pre-seeded scaffold. Comparing the wrapped with an unwrapped scaffold, we did not detect any differences in cell viability and structural integrity but a higher cell seeding rate with osteoid-like granular structures, an indicator of enhanced calcification. Finally, gene expression analysis showed a reduction in chondrogenic and adipogenic markers, but an increase in osteogenic markers in MSCs seeded on wrapped scaffolds. We conclude from these data that additional wrapping of pre-seeded scaffolds will provide a local niche that enhances osteogenic differentiation while repressing chondrogenic and adipogenic differentiation. This approach will eventually lead to optimized preclinical in vitro 3D models of cancellous bone tissue to develop new treatment strategies.

Influence of reducing agents on in situ synthesis of gold nanoparticles and scaffold conductivity with emphasis on neural differentiation

BACKGROUND

Recorded advancements in nerve tissue regeneration have still not provided satisfactory results, and complete physiological recovery is not assured. The engineering of nanofibrous scaffolds provides a suitable platform for stem cell transplantation by controlling cell proliferation and differentiation to replace lost cells. In this study, a conductive scaffold was fabricated by in situ synthesis of gold nanoparticles (Au-NPs) on electrospun polycaprolactone/chitosan nanofibrous scaffolds and its effect on neural differentiation of mesenchymal stem cells was investigated.

METHOD

The conductive scaffold was prepared using polycaprolactone/chitosan solution containing soluble Au ions by electrospinning approach. In situ synthesis of Au-NPs was conducted using two reducing agents, Tetrakis(hydroxymethyl)phosphonium chloride (THPC) as an organophosphorus compound and formaldehyde, and also different reduction times. Morphology and distribution of the Au-NPs on the nanofibrous scaffolds were assessed using field emission scanning electron microscopy (FE-SEM) and energy dispersed X-ray spectroscopy (EDX). The hydrophilicity and biocompatibility of the scaffolds were determined by water contact angle and MTT assays respectively. The characterization of the scaffolds was proceeded by testing the porosity, tensile strength and electrical conductivity. Also, the scaffold's ability to support neural differentiation of mesenchymal stem cells was evaluated by immune-staining/blotting of Beta tubulin III.

RESULTS & CONCLUSION

FE-SEM and EDX results demonstrated the uniform distribution of Au-NPs on electrospun nanofibers made of a combination of polycaprolactone and chitosan (PCL/CS). We found that electrical conductivity of the scaffolds fabricated using THPC for 4 days and formaldehyde for 7 days was in the range of electrical conductivity of the scaffolds suitable for nerve regeneration. Contact angle measurements showed the effect of Au-NPs on the hydrophilic properties of the scaffolds, where the scaffold showed the porosity of 50% in the presence of Au-NPs. Au-NPs decoration on the scaffold decreased the mechanical properties with the ultimate strength of 14 (MPa). In vitro assessment demonstrated the potential of the fabricated conductive scaffold to enhance the attachment and proliferation of fibroblast cells, and differentiation potential of mesenchymal stem cells toward neuron-like cells. This designed scaffold holds promise as a future carrier and delivery platform in nerve tissue engineering.

Shape-Memory ECM-Mimicking Heparin-Modified Nanofibrous Gelatin Scaffold for Enhanced Bone Regeneration in Sinus Augmentation

Biomaterials with clinical maneuverability and predictable bone regeneration are needed in the field of maxillary sinus augmentation. Herein, gelatin was chemically modified with heparin that specifically interacted with and stabilized bone morphogenetic protein-2 (BMP-2). We then introduced thermally induced phase separation to form the injectable, shape-memory, highly porous scaffold for bone regeneration in sinus augmentation. The hydrated heparin-modified nanofibrous gelatin scaffolds (NH-GS) were demonstrated with high resilience and shape-memory property, both macroscopically and microscopically, making them injectable scaffolds and expected to be applied in sinus augmentation. This novel scaffold was verified to be biocompatible and an excellent matrix to support cell attachment, proliferation, and infiltration. Further, the growth factor-loaded NH-GS showed sustained release kinetics of BMP-2 through affinity-based scaffold-growth factor interaction, compared with BMP-2 loaded gelatin sponge (GS) and nanofibrous gelatin scaffold (NF). Both in vitro and in vivo experiments demonstrated that the BMP-2-loaded NH-GS exhibited the highest osteogenesis among the other groups. Taken together, this study introduces a new regenerative strategy in maxillary sinus augmentation, which is injectable with a predefined shape and structure and promotes bone regeneration through a more sustained BMP-2 release.

A BMP/Activin A Chimera Induces Posterolateral Spine Fusion in Nonhuman Primates at Lower Concentrations Than BMP-2

BACKGROUND

Supraphysiologic bone morphogenetic protein (BMP)-2 concentrations are required to induce spinal fusion. In this study, a BMP-2/BMP-6/activin A chimera (BV-265), optimized for BMP receptor binding, delivered in a recombinant human collagen:CDHA [calcium-deficient hydroxyapatite] porous composite matrix (CM) or bovine collagen:CDHA granule porous composite matrix (PCM), engineered for optimal BV-265 retention and guided tissue repair, was compared with BMP-2 delivered in a bovine absorbable collagen sponge (ACS) wrapped around a MASTERGRAFT Matrix (MM) ceramic-collagen rod (ACS:MM) in a nonhuman primate noninstrumented posterolateral fusion (PLF) model.

METHODS

In vivo retention of 125I-labeled-BV-265/CM or PCM was compared with 125I-labeled-BMP-2/ACS or BMP-2/buffer in a rat muscle pouch model using scintigraphy. Noninstrumented PLF was performed by implanting CM, BV-265/CM, BV-265/PCM, or BMP-2/ACS:MM across L3-L4 and L5-L6 or L3-L4-L5 decorticated transverse processes in 26 monkeys. Computed tomography (CT) images were acquired at 0, 4, 8, 12, and 24 weeks after surgery, where applicable. Manual palpation, μCT (microcomputed tomography) or nCT (nanocomputed tomography), and histological analysis were performed following euthanasia.

RESULTS

Retention of 125I-labeled-BV-265/CM was greater than BV-265/PCM, followed by BMP-2/ACS and BMP-2/buffer. The CM, 0.43 mg/cm3 BMP-2/ACS:MM, and 0.05 mg/cm3 BV-265/CM failed to generate PLFs. The 0.15-mg/cm3 BV-265/CM or 0.075-mg/cm3 BV-265/PCM combinations were partially effective. The 0.25-mg/cm3 BV-265/CM and 0.15 and 0.3-mg/cm3 BV-265/PCM combinations generated successful 2-level PLFs at 12 and 24 weeks.

CONCLUSIONS

BV-265/CM or PCM can induce fusion in a challenging nonhuman primate noninstrumented PLF model at substantially lower concentrations than BMP-2/ACS:MM.

CLINICAL RELEVANCE

BV-265/CM and PCM represent potential alternatives to induce PLF in humans at substantially lower concentrations than BMP-2/ACS:MM.

Bisphosphonate-Conjugated Photo-Crosslinking Polyanionic Hyaluronic Acid Microbeads for Controlled BMP2 Delivery and Enhanced Bone Formation Efficacy

In this study, we designed bisphosphonate-conjugated polyanionic hyaluronic acid (HA) microbeads (MBs) for the controlled delivery of bone morphogenetic protein 2 (BMP2). MBs were prepared via the photo-crosslinking of bisphosphonate (alendronate)-conjugated methacrylated HA (Alen-MHA). The polyanionic Alen-MHA MBs actively absorbed cationic BMP2 up to 91.0% of the loading efficacy and displayed a sustained release of BMP2 for 10 days. BMP2/Alen-MHA MBs induced osteogenic-related genes in cellular experiments and showed the highly increased bone formation efficacy in thigh muscle injection and rat spinal fusion animal models. Thus, BMP2/Alen-MHA MBs provide a promising opportunity to improve the delivery efficiency of BMP2.

Synergistic effect of electromagnetic fields and nanomagnetic particles on osteogenesis through calcium channels and p-ERK signaling

Electromagnetic fields (EMFs) are widely used in a number of cell therapies and bone disorder treatments, and nanomagnetic particles (NMPs) also promote cell activity. In this study, we investigated the synergistic effects of EMFs and NMPs on the osteogenesis of the human Saos-2 osteoblast cell line and in a rat calvarial defect model. The Saos-2 cells and critical-size calvarial defects of the rats were exposed to EMF (1 mT, 45 Hz, 8 h/day) with or without Fe₃ O₄ NMPs. Biocompatibility was evaluated with MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) and LDH (lactate dehydrogenase) assays. This analysis showed that NMP and EMF did not induce cell toxicity. Quantitative reverse-transcription polymerase chain reaction indicated that the osteogenesis-related markers were highly expressed in the NMP-incorporated Saos-2 cells after exposure to EMF. Also, the expression of gene-encoding proteins involved in calcium channels was activated and the calcium concentration of the NMP-incorporated + EMF-exposed group was increased compared with the control group. In particular, in the NMP-incorporated + EMF-exposed group, all osteogenic proteins were more abundantly expressed than in the control group. This indicated that the NMP incorporation + EMF exposure induced a signaling pathway through activation of p-ERK and calcium channels. Also, in vivo evaluation revealed that rat calvarial defects treated with EMFs and NMPs had good regeneration results with new bone formation and increased mineral density after 6 weeks. Altogether, these results suggest that NMP treatment or EMF exposure of Saos-2 cells can increase osteogenic activity and NMP incorporation following EMF exposure which is synergistically efficient for osteogenesis.

Bone Morphogenetic Proteins, Carriers, and Animal Models in the Development of Novel Bone Regenerative Therapies

Bone morphogenetic proteins (BMPs) possess a unique ability to induce new bone formation. Numerous preclinical studies have been conducted to develop novel, BMP-based osteoinductive devices for the management of segmental bone defects and posterolateral spinal fusion (PLF). In these studies, BMPs were combined with a broad range of carriers (natural and synthetic polymers, inorganic materials, and their combinations) and tested in various models in mice, rats, rabbits, dogs, sheep, and non-human primates. In this review, we summarized bone regeneration strategies and animal models used for the initial, intermediate, and advanced evaluation of promising therapeutical solutions for new bone formation and repair. Moreover, in this review, we discuss basic aspects to be considered when planning animal experiments, including anatomical characteristics of the species used, appropriate BMP dosing, duration of the observation period, and sample size.

Role of rhBMP-7, Fibronectin, And Type I Collagen in Dental Implant Osseointegration Process: An Initial Pilot Study on Minipig Animals

Background: The biological factors involved in dental implant osseointegration need to be investigated to improve implant success. Methods: Twenty-four implants were inserted into the tibias of six minipigs. Bone samples were obtained at 7, 14, and 56 days. Biomolecular analyses evaluated mRNA of BMP-4, -7, Transforming Growth Factor-β2, Interleukin-1β, and Osteocalcin in sites treated with rhBMP-7, Type 1 Collagen, or Fibronectin (FN). Inflammation and osteogenesis were evaluated by histological analyses. Results: At 7 and 14 days, BMP-4 and BMP-7 increased in the sites prepared with rhBMP-7 and FN. BMP-7 remained greater at 56 days in rhBMP-7 and FN sites. BPM-4 at 7 and 14 days increased in Type 1 Collagen sites; BMP-7 increased from day 14. FN increased the TGF-β2 at all experimental times, whilst the rhBMP-7 only did so up to 7 days. IL-1β increased only in collagen-treated sites from 14 days. Osteocalcin was high in FN-treated sites. Neutrophilic granulocytes characterized the inflammatory infiltrate at 7 days, and mononuclear cells at 14 and 56 days. Conclusions: This initial pilot study, in a novel way, evidenced that Type 1 Collagen induced inflammation and did not stimulate bone production; conversely FN or rhBMP-7 showed neo-osteogenetic and anti-inflammatory properties when directly added into implant bone site.

3D Bioprinting of Human Tissues: Biofabrication, Bioinks, and Bioreactors

The field of tissue engineering has progressed tremendously over the past few decades in its ability to fabricate functional tissue substitutes for regenerative medicine and pharmaceutical research. Conventional scaffold-based approaches are limited in their capacity to produce constructs with the functionality and complexity of native tissue. Three-dimensional (3D) bioprinting offers exciting prospects for scaffolds fabrication, as it allows precise placement of cells, biochemical factors, and biomaterials in a layer-by-layer process. Compared with traditional scaffold fabrication approaches, 3D bioprinting is better to mimic the complex microstructures of biological tissues and accurately control the distribution of cells. Here, we describe recent technological advances in bio-fabrication focusing on 3D bioprinting processes for tissue engineering from data processing to bioprinting, mainly inkjet, laser, and extrusion-based technique. We then review the associated bioink formulation for 3D bioprinting of human tissues, including biomaterials, cells, and growth factors selection. The key bioink properties for successful bioprinting of human tissue were summarized. After bioprinting, the cells are generally devoid of any exposure to fluid mechanical cues, such as fluid shear stress, tension, and compression, which are crucial for tissue development and function in health and disease. The bioreactor can serve as a simulator to aid in the development of engineering human tissues from in vitro maturation of 3D cell-laden scaffolds. We then describe some of the most common bioreactors found in the engineering of several functional tissues, such as bone, cartilage, and cardiovascular applications. In the end, we conclude with a brief insight into present limitations and future developments on the application of 3D bioprinting and bioreactor systems for engineering human tissue.

BMP-6 carrying metal organic framework-embedded in bioresorbable electrospun fibers for enhanced bone regeneration

Biomolecule carrier structures have attracted substantial interest owing to their potential utilizations in the field of bone tissue engineering. In this study, MOF-embedded electrospun fiber scaffold for the controlled release of BMP-6 was developed for the first time, to enrich bone regeneration efficacy. The scaffolds were achieved by first, one-pot rapid crystallization of BMP-6 encapsulated ZIF-8 nanocrystals-as a novel carrier for growth factor molecules- and then electrospinning of the blending solution composed of poly (ε-caprolactone) and BMP-6 encapsulated ZIF-8 nanocrystals. BMP-6 molecule encapsulation efficiency for ZIF-8 nanocrystals was calculated as 98%. The in-vitro studies showed that, the bioactivity of BMP-6 was preserved and the release lasted up to 30 days. The release kinetics fitted the Korsmeyer-Peppas model exhibiting a pseudo-Fickian behavior. The in-vitro osteogenesis studies revealed the superior effect of sustained release of BMP-6 towards osteogenic differentiation of MC3T3-E1 pre-osteoblasts. In-vivo studies also revealed that the sustained slow release of BMP-6 was responsible for the generation of well-mineralized, new bone formation in a rat cranial defect. Our results proved that; MOF-carriers embedded in electrospun scaffolds can be used as an effective platform for bone regeneration in bone tissue engineering applications. The proposed approach can easily be adapted for various growth factor molecules for different tissue engineering applications.

Autologous bone graft substitute containing rhBMP6 within autologous blood coagulum and synthetic ceramics of different particle size determines the quantity and structural pattern of bone formed in a rat subcutaneous assay

Bone morphogenetic proteins (BMPs) are potent osteoinductive agents for bone tissue engineering. In order to define optimal properties of a novel autologous bone graft substitute (ABGS) containing rhBMP6 within the autologous blood coagulum (ABC) and ceramic particles as a compression resistant matrix (CRM), we explored the influence of their amount, chemical composition and particle size on the quantity and quality of bone formation in the rat subcutaneous assay. Tested ceramic particles included tricalcium phosphate (TCP), hydroxyapatite (HA) and biphasic calcium phosphate ceramic (BCP), containing TCP and HA in 80/20 ratio of different particle sizes (small 74-420 μm, medium 500-1700 μm and large 1000-4000 μm). RhBMP6 was either mixed with ABC or lyophilized on CRM prior to use with ABC. The experiments were terminated on day 21 and implants were analysed by microCT, histology and histomorphometry. Addition of CRM to ABGS containing rhBMP6 in ABC significantly increased the amount of newly formed bone and the optimal CRM/ABC ratio was found to be around 100 mg/500 μL. MicroCT analyses revealed that all tested ABGS formulations induced an extensive new bone formation and there were no differences between the two methods of rhBMP6 application as determined by the bone volume. However, the particle size played a significant role in the quantity and quality of newly formed bone. ABGS containing small particles induced new bone forming a dense trabecular network, cortical bone at the rim, bone and bone marrow in apposition to and in between ceramic particles. ABGS containing medium and large particles also resulted in new bone on the surface of particles as well as inside the pores. Histomorphometric analysis revealed that the ceramics particle size correlated with the quality of trabecular pattern of newly formed bone, bone/bone marrow ratio as observed in apposition and between particles, and the ratio between the cortical and trabecular bone. By employing rat subcutaneous implant assay, we showed for the first time that the size of synthetic ceramics particles affected the osteogenesis as defined by both the quantity and quality of ectopic bone.

Rational design of hydrogels to enhance osteogenic potential

Bone tissue engineering (BTE) encompasses the field of biomaterials, cells, and bioactive molecules to successfully guide the growth and repair of bone tissue. Current BTE strategies rely on delivering osteogenic molecules or cells via scaffolding materials. However, growth factor- and stem cell-based treatments have several limitations, such as source restriction, low stability, difficulties in predicting long-term efficacy, and high costs, among others. These issues have promoted the development of material-based therapy with properties of accessibility, high stability, tunable efficacy, and low-cost production. Hydrogels are widely used in BTE applications because of their unique hydrophilic nature and tunable physicochemical properties to mimic the native bone environment. However, current hydrogel materials are not ideal candidates due to minimal osteogenic capability on their own. Therefore, recent studies of BTE hydrogels attempt to counterbalance these issues by modifying their biophysical properties. In this article, we review recent progress in the design of hydrogels to instruct osteogenic potential, and present strategies developed to precisely control its bone healing properties.

A modular programmed biphasic dual-delivery system on 3D ceramic scaffolds for osteogenesis in vitro and in vivo

Single-factor delivery is the most common characteristic of bone tissue engineering techniques. However, bone regeneration is a complex process requiring multiple factors and specialized release mechanisms. Therefore, the development of a dual-delivery system allowing for programmed release kinetics would be highly desirable. Improvement of the molarity and versatility of the delivery system has rarely been studied. Herein, we report the development of a novel, modular programmed biphasic dual-release system (SCB), carrying a BMP2 and an engineered collagen I-derived recognition motif (Stath-DGEA), with a self-remodification feature on hydroxyapatite (HA)-based materials. The SCB system was loaded onto an additive manufactured (AM) scaffold in order to evaluate its bifactor osteogenic potential and its biphasic release behavior. Further, the biomechanical properties of the scaffold were studied by using the fluid-structure interaction (FSI) method. Section fluorescent labeling revealed that the HA scaffold has a relatively higher density and efficiency. Additionally, the results of the release and inhibition experiment suggested that the SCB system could facilitate the sustained release of therapeutic levels of two factors during the initial stage of implantation, thereby exhibiting a rapid high-dose release pattern at a specific time point during the second stage. The FSI prediction model indicated that the scaffold provides an excellent biomimetic mechanical and fluid dynamic microenvironment to promote osteogenesis. Our results indicated that incorporation of BMP2 with Stath-DGEA in the biphasic SCB system could have a synergetic effect in promoting the adhesion, proliferation, and differentiation of bone marrow mesenchymal stem cells (BMSCs) in vitro, under staged stimulations. Further, in vivo studies in both ectopic and orthotopic rat models showed that the SCB system loaded onto an AM scaffold could enhance osteointegration and osteoinduction throughout the osteogenic process. Thus, the novel synthetic SCB system described herein used on an AM scaffold provides a biomimetic extracellular environment that enhances bone regeneration and is a promising multifunctional, dual-release platform.

Evaluation of synthetic ceramics as compression resistant matrix to promote osteogenesis of autologous blood coagulum containing recombinant human bone morphogenetic protein 6 in rabbit posterolateral lumbar fusion model

Posterolateral lumbar fusion (PLF) is a commonly performed surgical procedure for the treatment of pathological conditions of the lumbosacral spine. In the present study, we evaluated an autologous bone graft substitute (ABGS) containing rhBMP6 in autologous blood coagulum (ABC) and synthetic ceramics used as compression resistant matrix (CRM) in the rabbit PLF model. In the pilot PLF rabbit experiment, we tested four different CRMs (BCP 500-1700 μm, BCP 1700-2500 μm and two different TCP in the form of slabs) which were selected based on achieving uniform ABC distribution. Next, ABGS implants composed of 2.5 mL ABC with 0.5 g ceramic particles (TCP or BCP (TCP/HA 80/20) of particle size 500-1700 μm) and 125 μg rhBMP6 (added to blood or lyophilized on ceramics) were placed bilaterally between transverse processes of the lumbar vertebrae (L5-L6) following exposition and decortication in 12 New Zealand White Rabbits observed for 7 weeks following surgery. Spinal fusion outcome was analysed by μCT, palpatory segmental mobility testing and selected specimens were either tested biomechanically (three-point bending test) and/or processed histologically. The total fusion success rate was 90.9% by both μCT analyses and by palpatory segmental mobility testing. The volume of newly formed bone between experimental groups with TCP or BCP ceramics and the different method of rhBMP6 application was comparable. The newly formed bone and ceramic particles integrated with the transverse processes on histological sections resulting in superior biomechanical properties. The results were retrospectively found superior to allograft devitalized mineralized bone as a CRM as reported previously in rabbit PLF. Overall, this novel ABGS containing rhBMP6, ABC and the specific 500-1700 μm synthetic ceramic particles supported new bone formation for the first time and successfully promoted posterolateral lumbar fusion in rabbits.

Newly designed Protein Transduction Domain (PTD)-mediated BMP-7 is a potential therapeutic for peritoneal fibrosis

While the bone morphogenetic protein‐7 (BMP‐7) is a well‐known therapeutic growth factor reverting many fibrotic diseases, including peritoneal fibrosis by peritoneal dialysis (PD), soluble growth factors are largely limited in clinical applications owing to their short half‐life in clinical settings. Recently, we developed a novel drug delivery model using protein transduction domains (PTD) overcoming limitation of soluble recombinant proteins, including bone morphogenetic protein‐7 (BMP‐7). This study aims at evaluating the therapeutic effects of PTD‐BMP‐7 consisted of PTD and full‐length BMP‐7 on epithelial‐mesenchymal transition (EMT)‐related fibrosis. Human peritoneal mesothelial cells (HPMCs) were then treated with TGF‐β1 or TGF‐β1 + PTD‐BMP‐7. Peritoneal dialysis (PD) catheters were inserted into Sprague‐Dawley rats, and these rats were infused intra‐peritoneally with saline, peritoneal dialysis fluid (PDF) or PDF + PTD‐BMP‐7. In vitro, TGF‐β1 treatment significantly increased fibronectin, type I collagen, α‐SMA and Snail expression, while reducing E‐cadherin expression in HPMCs (P < .001). PTD‐BMP‐7 treatment ameliorated TGF‐β1‐induced fibronectin, type I collagen, α‐SMA and Snail expression, and restored E‐cadherin expression in HPMCs (P < .001). In vivo, the expressions of EMT‐related molecules and the thickness of the sub‐mesothelial layer were significantly increased in the peritoneum of rats treated with PDF, and these changes were significantly abrogated by the intra‐peritoneal administration of PTD‐BMP‐7. PTD‐BMP‐7 treatment significantly inhibited the progression of established PD fibrosis. These findings suggest that PTD‐BMP‐7, as a prodrug of BMP‐7, can be an effective therapeutic agent for peritoneal fibrosis in PD patients.

Synthetic ceramic macroporous blocks as a scaffold in ectopic bone formation induced by recombinant human bone morphogenetic protein 6 within autologous blood coagulum in rats

PURPOSE

We have recently developed an autologous bone graft substitute (ABGS) containing recombinant human bone morphogenetic protein 6 (rhBMP6) in autologous blood coagulum (ABC) that induces new bone formation in vivo. In order to improve biomechanical properties of the implant, compression resistant matrix (CRM) consisting of synthetic ceramics in the form of macroporous cylinders was added to the ABGS and we evaluated the biomechanical properties and the quantity and quality of bone formation following subcutaneous implantation in rats.

METHODS

ABGS implants containing rhBMP6 in ABC with cylindrical ceramic blocks were implanted subcutaneously (n = 6 per time point) in the axillary region of Sprague-Dawley rats and removed at specified time points (7, 14, 21, 35, and 50 days). The quantity and quality of newly formed bone were analyzed by microCT, histology, and histomorphometric analyses. Biomechanical properties of ABGS formulations were determined by employing the cut test.

RESULTS

MicroCT analyses revealed that ABGS implants induced formation of new bone within ceramic blocks. Histological analysis revealed that on day seven following implantation, the endochondral ossification occupied the peripheral part of implants. On days 14 and 21, newly formed bone was present both around the ceramic block and through the pores inside the block. On both days 35 and 50, cortical bone encircled the ceramic block while inside the block, bone covered the ceramic surface surrounding the pores. Within the osseous circles, there were few trabeculae and bone marrow containing adipocytes. ABGS containing cylindrical ceramic blocks were more rigid and had significantly increased stiffness compared with implants containing ceramic particles as CRM.

CONCLUSION

We demonstrated that macroporous ceramic blocks in a form of cylinders are promising CRMs with good handling and enhanced biomechanical properties, supporting bone formation with ABGS containing rhBMP6 within autologous blood coagulum. Hence, ABGS containing ceramic blocks should be tested in preclinical models including diaphyseal segmental defects and non-unions in larger animals.

Compression-Resistant Polymer/Ceramic Composite Scaffolds Augmented with rhBMP-2 Promote New Bone Formation in a Nonhuman Primate Mandibular Ridge Augmentation Model

PURPOSE

This study was designed to test the hypothesis that compression-resistant (CR) scaffolds augmented with recombinant human bone morphogenetic protein-2 (rhBMP-2) at clinically relevant doses in a nonhuman primate lateral ridge augmentation model enhances bone formation in a dose-responsive manner without additional protective membranes.

MATERIALS AND METHODS

Defects (15 mm long × 8 mm wide × 5 mm deep) were created bilaterally in the mandibles of nine hamadryas baboons. The defect sites were implanted with poly(ester urethane) (PEUR)/ceramic CR scaffolds augmented with 0 mg/mL rhBMP-2 (CR control), 0.75 mg/mL rhBMP-2 (CR-L), or 1.5 mg/mL rhBMP-2 (CR-H). The primary outcome of ridge width and secondary outcomes of new bone formation, cellular infiltration, and integration with host bone were evaluated using histology, histomorphometry, and microcomputed tomography (micro-CT) at 16 weeks following implantation.

RESULTS

New bone formation in the mandible was observed in a dose-responsive manner. CR-H promoted significantly greater new bone formation compared with the CR control group. In all groups, ridge width was maintained without an additional protective membrane.

CONCLUSION

CR scaffolds augmented with a clinically relevant dose of rhBMP-2 (1.5 mg/mL) promoted significant new bone formation. These results suggest that a CR PEUR/ceramic composite scaffold without a protective membrane may be a potential new rhBMP-2 carrier for clinical use.

Overcoming barriers confronting application of protein therapeutics in bone fracture healing

Bone fracture is a major contributor to debilitation and death among patients with bone diseases. Thus, osteogenic protein therapeutics and their delivery to bone have been extensively researched as strategies to accelerate fracture healing. To prevent morbidity and mortality of fractures, which occur frequently in the aging population, there is a critical need for development of first-line therapeutics. Bone morphogenic protein-2 (BMP-2) has been at the forefront of bone regeneration research for its potent osteoinduction, despite safety concerns and biophysiological obstacles of delivery to bone. However, continued pursuit of osteoinductive proteins as a therapeutic option is largely aided by drug delivery systems, playing an imperative role in enhancing safety and efficacy. In this work, we highlighted several types of drug delivery platforms and their biomaterials, to evaluate the suitability in overcoming challenges of therapeutic protein delivery for bone regeneration. To showcase the clinical considerations for each type of platform, we have assessed the most common route of administration strategies for bone regeneration, classifying the platforms as implantable or injectable. Additionally, we have analyzed the commonly utilized models and methodology for safety and efficacy evaluation of these osteogenic protein-loaded systems, to present clinical opinions for future directions of research in this field. It is hoped that this review will promote research and development of clinically translatable osteogenic protein therapeutics, while targeting first-line treatment status for achieving desired outcomes of fracture healing. Graphical abstract.

A BMP/activin A chimera is superior to native BMPs and induces bone repair in nonhuman primates when delivered in a composite matrix

Bone morphogenetic protein (BMP)/carriers approved for orthopedic procedures achieve efficacy superior or equivalent to autograft bone. However, required supraphysiological BMP concentrations have been associated with potential local and systemic adverse events. Suboptimal BMP/receptor binding and rapid BMP release from approved carriers may contribute to these outcomes. To address these issues and improve efficacy, we engineered chimeras with increased receptor binding by substituting BMP-6 and activin A receptor binding domains into BMP-2 and optimized a carrier for chimera retention and tissue ingrowth. BV-265, a BMP-2/BMP-6/activin A chimera, demonstrated increased binding affinity to BMP receptors, including activin-like kinase-2 (ALK2) critical for bone formation in people. BV-265 increased BMP intracellular signaling, osteogenic activity, and expression of bone-related genes in murine and human cells to a greater extent than BMP-2 and was not inhibited by BMP antagonist noggin or gremlin. BV-265 induced larger ectopic bone nodules in rats compared to BMP-2 and was superior to BMP-2, BMP-2/6, and other chimeras in nonhuman primate bone repair models. A composite matrix (CM) containing calcium-deficient hydroxyapatite granules suspended in a macroporous, fenestrated, polymer mesh-reinforced recombinant human type I collagen matrix demonstrated improved BV-265 retention, minimal inflammation, and enhanced handling. BV-265/CM was efficacious in nonhuman primate bone repair models at concentrations ranging from ¹/₁₀ to ¹/₃₀ of the BMP-2/absorbable collagen sponge (ACS) concentration approved for clinical use. Initial toxicology studies were negative. These results support evaluations of BV-265/CM as an alternative to BMP-2/ACS in clinical trials for orthopedic conditions requiring augmented healing.

Effect of Single Injection of Recombinant Human Bone Morphogenetic Protein-2-Loaded Artificial Collagen-Like Peptide in a Mouse Segmental Bone Transport Model

This study aimed to investigate whether a single injection of recombinant human bone morphogenetic protein-2-loaded artificial collagen-like peptide gel (rhBMP-2/ACG) accelerates consolidation at the bone defect site and bone union at the docking site in a mouse segmental bone transport (SBT) model. A critical sized bone defect (2 mm) was created in the femur of mice and subsequently reconstructed using SBT with an external fixator. Mice were divided into four treatment groups: Group CONT (immobile control), Group 0.2 (bone segments moved 0.2 mm/day for 10 days), Group 1.0 (bone segments moved 1.0 mm/day for 2 days), and Group 1.0/BMP-2 (rhBMP-2/ACG injected into the bone defect and segments moved 1.0 mm/day for 2 days). Consolidation at the bone defect site and bone union at the docking site was evaluated radiologically and histologically across eight weeks. Bone volume and bone mineral content were significantly higher in Group 0.2 than in Group 1.0. Group 0.2 showed evidence of rebuilding of the medullary canal eight weeks after surgery at the bone defect site. However, in Group 1.0, maturation of regenerative bone at the bone defect site was poor, with the central area between the proximal and distal bone composed mainly of masses of fibrous and adipose tissue. Group 1.0/BMP-2 had higher bone volume and bone mineral content compared to Group 1.0, and all mice achieved bone union at the bone defect and docking sites. Single injection of rhBMP-2/ACG combined with SBT may be effective for enhancing bone healing in large bone defects.

Synthesis and characterization of osteoinductive visible light-activated adhesive composites with antimicrobial properties

Orthopedic surgical procedures based on the use of conventional biological graft tissues are often associated with serious post-operative complications such as immune rejection, bacterial infection, and poor osseointegration. Bioresorbable bone graft substitutes have emerged as attractive alternatives to conventional strategies because they can mimic the composition and mechanical properties of the native bone. Among these, bioactive glasses (BGs) hold great potential to be used as biomaterials for bone tissue engineering owing to their biomimetic composition and high biocompatibility and osteoinductivity. Here, we report the development of a novel composite biomaterial for bone tissue engineering based on the incorporation of a modified strontium- and lithium-doped 58S BG (i.e., BG-5/5) into gelatin methacryloyl (GelMA) hydrogels. We characterized the physicochemical properties of the BG formulation via different analytical techniques. Composite hydrogels were then prepared by directly adding BG-5/5 to the GelMA hydrogel precursor, followed by photocrosslinking of the polymeric network via visible light. We characterized the physical, mechanical, and adhesive properties of GelMA/BG-5/5 composites, as well as their in vitro cytocompatibility and osteoinductivity. In addition, we evaluated the antimicrobial properties of these composites in vitro, using a strain of methicillin-resistant Staphylococcus Aureus. GelMA/BG-5/5 composites combined the functional characteristics of the inorganic BG component, with the biocompatibility, biodegradability, and biomimetic composition of the hydrogel network. This novel biomaterial could be used for developing osteoinductive scaffolds or implant surface coatings with intrinsic antimicrobial properties and higher therapeutic efficacy.

Bone Morphogenic Protein 2-Loaded Porous Silicon Carriers for Osteoinductive Implants

Bone morphogenetic proteins (BMPs) are probably the most important growth factors in bone formation and healing. However, the utilization of BMPs in clinical applications is mainly limited due to the protein poor solubility at physiological pH, rapid clearance and relatively short biological half-life. Herein, we develop degradable porous silicon (PSi)-based carriers for sustained delivery of BMP-2. Two different loading approaches are examined, physical adsorption and covalent conjugation, and their effect on the protein loading and release rate is thoroughly studied. The entrapment of the protein within the PSi nanostructures preserved its bioactivity for inducing osteogenic differentiation of rabbit bone marrow mesenchymal stems cells (BM-MSCs). BM-MSCs cultured with the BMP-2 loaded PSi carriers exhibit a relatively high alkaline phosphatase (ALP) activity. We also demonstrate that exposure of MSCs to empty PSi (no protein) carriers generates some extent of differentiation due to the ability of the carrier's degradation products to induce osteoblast differentiation. Finally, we demonstrate the integration of these promising BMP-2 carriers within a 3D-printed patient-specific implant, constructed of poly(caprolactone) (PCL), as a potential bone graft for critical size bone defects.

Anisotropic Cryostructured Collagen Scaffolds for Efficient Delivery of RhBMP-2 and Enhanced Bone Regeneration

In the treatment of bone non-unions, an alternative to bone autografts is the use of bone morphogenetic proteins (BMPs), e.g., BMP–2, BMP–7, with powerful osteoinductive and osteogenic properties. In clinical settings, these osteogenic factors are applied using absorbable collagen sponges for local controlled delivery. Major side effects of this strategy are derived from the supraphysiological doses of BMPs needed, which may induce ectopic bone formation, chronic inflammation, and excessive bone resorption. In order to increase the efficiency of the delivered BMPs, we designed cryostructured collagen scaffolds functionalized with hydroxyapatite, mimicking the structure of cortical bone (aligned porosity, anisotropic) or trabecular bone (random distributed porosity, isotropic). We hypothesize that an anisotropic structure would enhance the osteoconductive properties of the scaffolds by increasing the regenerative performance of the provided rhBMP–2. In vitro, both scaffolds presented similar mechanical properties, rhBMP–2 retention and delivery capacity, as well as scaffold degradation time. In vivo, anisotropic scaffolds demonstrated better bone regeneration capabilities in a rat femoral critical-size defect model by increasing the defect bridging. In conclusion, anisotropic cryostructured collagen scaffolds improve bone regeneration by increasing the efficiency of rhBMP–2 mediated bone healing.

Porous PolyHIPE microspheres for protein delivery from an injectable bone graft

Delivery of osteoinductive factors such as bone morphogenetic protein 2 (BMP-2) has emerged as a prominent strategy to improve regeneration in bone grafting procedures. However, it remains challenging to identify a carrier that provides the requisite loading efficiency and release kinetics without compromising the mechanical properties of the bone graft. Previously, we reported on porous, polymerized high internal phase emulsion (polyHIPE) microspheres fabricated using controlled fluidics. Uniquely, this solvent-free method provides advantages over current microsphere fabrication strategies including in-line loading of growth factors to improve loading efficiency. In the current study, we utilized this platform to fabricate protein-loaded microspheres and investigated the effect of particle size (∼400 vs ∼800 μm) and pore size (∼15 vs 30 μm) on release profiles. Although there was no significant effect of these variables on the substantial burst release profile of the microspheres, the incorporation of the protein-loaded microspheres within the injectable polyHIPE resulted in a sustained release of protein from the bulk scaffold over a two-week period with minimal burst release. Bioactivity retention of encapsulated BMP-2 was confirmed first using a genetically-modified osteoblast reporter cell line. A functional assay with human mesenchymal stem cells established that the BMP-2 release from microspheres induced osteogenic differentiation. Finally, microsphere incorporation had minimal effect on the cure and compressive properties of an injectable polyHIPE bone graft. Overall, this work demonstrates that these microsphere-polyHIPE composites have strong potential to enhance bone regeneration through controlled release of BMP-2 and other growth factors. STATEMENT OF SIGNIFICANCE: This manuscript describes a method for solvent-free fabrication of porous microspheres from high internal phase emulsions using a controlled fluids setup. The principles of emulsion templating and fluid dynamics provide exceptional control of particle size and pore architecture. In addition to the advantage of solvent-free fabrication, this method provides in-line loading of protein directly into the pores of the microspheres with high loading efficiencies. The incorporation of the protein-loaded microspheres within an injectable polyHIPE scaffold resulted in a sustained release of protein over a two-week period with minimal burst release. Retention of BMP-2 bioactivity and incorporation of microspheres with minimal effect on scaffold compressive properties highlights the potential of these new bone grafts.

Preparation of fibrin hydrogels to promote the recruitment of anti-inflammatory macrophages

Macrophages play an important role in regulating inflammation and tissue regeneration. In the present study, uniform fibrin hydrogel scaffolds were engineered in millimeters. These scaffolds induced anti-inflammatory macrophages to digest and infiltrate the scaffold. The culture conditions of the fibrin hydrogels decreased the secretion of tumor necrosis factor-α (TNF-α), a pro-inflammatory cytokine, and increased the secretion of interleukin-10 (IL-10), an anti-inflammatory cytokine, in mouse bone marrow-derived macrophages. Similar results were also observed in human monocyte-derived macrophages (HMDMs). In addition, most of cells that infiltrated the fibrin hydrogels were macrophages expressing CD163, CD204, and CD206, which are anti-inflammatory macrophages markers, both in mice and in human cells. Therefore, to induce increased macrophage infiltration, we attempted to combine fibrin hydrogels with SEW2871, a monocyte/macrophage recruitment agent that is known to be a sphingosine-1 phosphate receptor 1 agonist, solubilized in water by micelle formation with a cholesterol-grafted gelatin. However, the fibrin hydrogels alone retained the same monocyte migration activity as the hydrogels with SEW2871-incorporated micelles in the hydrogel-bearing mouse model. These findings indicate that fibrin hydrogels have a strong promoting effect on the recruitment of anti-inflammatory macrophages. Therefore, fibrin hydrogels may be an optimal biomaterial in the design of medicines for macrophage-induced regenerative therapies. STATEMENT OF SIGNIFICANCE: The immune response to tissue injury is important for determining the speed and the result of the regeneration. Alternatively activated macrophages (M2 macrophages) resolve inflammatory response and promote tissue repair by producing anti-inflammatory factors. Promoting the recruitment of macrophages is a hopeful strategy in the design of biomaterials for tissue regeneration. In the present study, we combined the fibrin hydrogel, which promotes anti-inflammatory polarization, with a macrophage recruitment agent. We revealed that the fibrin hydrogel significantly promoted anti-inflammatory polarization in mouse in vivo and human in vitro. Moreover, macrophages significantly infiltrated into the fibrin hydrogel regardless of the agent combination. Fibrin hydrogels may become a reliable biomaterial for tissue regeneration, and the present study is believed to provide information for many researchers.

A randomized controlled clinical trial evaluating efficacy and adverse events of different types of recombinant human bone morphogenetic protein-2 delivery systems for alveolar ridge preservation

OBJECTIVES

This 12-week clinical trial evaluated efficacy and adverse events for two recombinant human bone morphogenetic protein-2 (rhBMP-2) delivery systems in alveolar ridge preservation.

MATERIALS AND METHODS

Sixty-four patients had a single tooth that required replacement with an implant, surrounded by > 50% alveolar bone height. Two cohorts (n = 32 patients each) were randomized to receive a rhBMP-2-soaked absorbable collagen sponge (test group), or β-tricalcium phosphate and hydroxyapatite particles (control group) immersed in rhBMP-2, at the implant site. Bone height and width changes at 25%, 50%, and 75% of extraction socket level (ESL) were compared. Adverse events were assessed in the same period. In addition to the randomized controlled clinical trial, histological analysis of 21 patients (test group [n = 12], control group [n = 9]) was conducted, 4 months after alveolar ridge preservation. A non-inferiority test was used to analyze changes in alveolar bone height between groups (p = 0.05). A Wilcoxon rank-sum test was used to analyze changes in alveolar bone width and histomorphometric results between groups (p = 0.05).

RESULTS

All patients showed good healing without severe adverse events. The lower limit of the one-sided 97.5% confidence interval in the difference between the two groups was 0.0033 (non-inferiority margin: -0.185); thus, the test group showed non-inferiority to the control group. Wilcoxon rank-sum test analysis did not show statistically significant differences between groups with regard to changes in alveolar bone width and histomorphometric analysis.

CONCLUSIONS

The delivery systems showed similar efficacy for alveolar ridge preservation without severe adverse events.

Bone Morphogenetic Proteins (BMPs) and Bone Regeneration

Many research methods exist to elucidate the role of BMP-2 during bone regeneration. This chapter briefly reviews important animal models used in these studies and provides details on the rat femur defect model. This animal model is frequently utilized to measure the efficacy of osteogenic factors like BMP-2. Detailed information about delivery methods, dose range, and dose duration used in BMP-2-related studies are provided.

Osteogenic effectiveness of photo-immobilized bone morphogenetic protein-2 using different azidophenyl-natural polymer carriers in rat calvarial defect model

The osteogenetic potential of photo-immobilized azdiophenyl (Az)-natural polymers as a carrier of bone morphogenetic protein-2 (BMP-2) was assessed in 56 rats randomized to four groups. The control group comprised implanted collagen sheet with BMP-2. In the three experimental groups, the implant comprised collagen sheet with photo-immobilized BMP-2 on Az-gelatin (Az-Gel), Az-O-carboxymethyl chitosan (Az-OMC), or Az‑O‑carboxymethyl low molecular chitosan (Az-LMC). Micro-computed tomography analysis revealed more regenerated bone in Az-Gel at 8weeks. Immunohistochemical analysis at 4weeks revealed that the positively expressed cellular ratio in RUNX2-stained cells was significantly higher in Az-Gel and Az-OMC groups. At 8weeks, only the Az-Gel group showed higher positively expressed cellular ratio compared with the control group. These results demonstrate the superior osteogenetic potential of photo-immobilized BMP-2 using Az-Gel carrier in a rat calvarial defect model compared with control group. Photo-immobilization of BMP-2 using Az-gelatin could be a more effective carrier system of BMP-2 than a chitosan-based carrier system.

Tailoring the Interface of Biomaterials to Design Effective Scaffolds

Tissue engineering (TE) is a multidisciplinary science, which including principles from material science, biology and medicine aims to develop biological substitutes to restore damaged tissues and organs. A major challenge in TE is the choice of suitable biomaterial to fabricate a scaffold that mimics native extracellular matrix guiding resident stem cells to regenerate the functional tissue. Ideally, the biomaterial should be tailored in order that the final scaffold would be (i) biodegradable to be gradually replaced by regenerating new tissue, (ii) mechanically similar to the tissue to regenerate, (iii) porous to allow cell growth as nutrient, oxygen and waste transport and (iv) bioactive to promote cell adhesion and differentiation. With this perspective, this review discusses the options and challenges facing biomaterial selection when a scaffold has to be designed. We highlight the possibilities in the final mold the materials should assume and the most effective techniques for its fabrication depending on the target tissue, including the alternatives to ameliorate its bioactivity. Furthermore, particular attention has been given to the influence that all these aspects have on resident cells considering the frontiers of materiobiology. In addition, a focus on chitosan as a versatile biomaterial for TE scaffold fabrication has been done, highlighting its latest advances in the literature on bone, skin, cartilage and cornea TE.

The Biological Enhancement of Spinal Fusion for Spinal Degenerative Disease

In this era of aging societies, the number of elderly individuals who undergo spinal arthrodesis for various degenerative diseases is increasing. Poor bone quality and osteogenic ability in older patients, due to osteoporosis, often interfere with achieving bone fusion after spinal arthrodesis. Enhancement of bone fusion requires shifting bone homeostasis toward increased bone formation and reduced resorption. Several biological enhancement strategies of bone formation have been conducted in animal models of spinal arthrodesis and human clinical trials. Pharmacological agents for osteoporosis have also been shown to be effective in enhancing bone fusion. Cytokines, which activate bone formation, such as bone morphogenetic proteins, have already been clinically used to enhance bone fusion for spinal arthrodesis. Recently, stem cells have attracted considerable attention as a cell source of osteoblasts, promising effects in enhancing bone fusion. Drug delivery systems will also need to be further developed to assure the safe delivery of bone-enhancing agents to the site of spinal arthrodesis. Our aim in this review is to appraise the current state of knowledge and evidence regarding bone enhancement strategies for spinal fusion for degenerative spinal disorders, and to identify future directions for biological bone enhancement strategies, including pharmacological, cell and gene therapy approaches.

The Osteoinductive Effect of Controlled Bone Morphogenic Protein 2 Release Is Location Dependent

IMPACT STATEMENT

The main challenge in bone morphogenic protein 2 (BMP-2)-based application lies in finding strategies to prolong its biologic activity as it has a short biological half-life. The present study uses a phosphate-modified oligo[(polyethylene glycol) fumarate] hydrogel that can be tuned to achieve differential release profiles of biologically active BMP-2 release. We demonstrate that this platform outperforms Infuse^®, currently used in the clinic and that the osteoinductive effect of BMP-2 is location dependent. Altogether, this study stresses the importance of evaluating efficacy of bone tissue engineering strategies at an orthotopic location rather than subcutaneously. Even more so, it emphasizes the role of biomaterials as a scaffold to achieve proper bone tissue engineering.

Biodegradable Screws Containing Bone Morphogenetic Protein-2 in an Osteoporotic Rat Model

OBJECTIVE

The aim of this study was to evaluate the effect for biodegradable screws containing bone morphogenetic protein-2 (BMP-2) in an osteoporotic rat model.

METHODS

Twenty-four female Wistar rat (250-300 g, 12 weeks of age) were randomized into four groups. Three groups underwent bilateral ovariectomy (OVX). Biodegradable screws with or without BMP-2 were inserted in the proximal tibia in two implantation groups. The extracted proximal metaphysis of the tibiae were scanned by exo-vivo micro-computed tomography. Evaluated parameters included bone mineral density (BMD), trabecular bone volume (BV/TV), trabecular number, trabecular thickness, and trabecular separation (Tb.Sp). The tibia samples were pathologically evaluated by staining with by Hematoxylin and Eosin, and trichrome.

RESULTS

Trabecular formation near screw insertion site was evident only in rats receiving BMP-2 screws. BMD and BV/TV significantly differed between controls and the OVX and OVX with screw groups. However, there were no significant differences between control and OVX with screw BMP groups. Tb.Sp significantly differed between control and OVX and OVX with screw groups (p<0.05), and between the OVX and OVX with screw BMP group (p<0.05), with no statistically significant difference between control and OVX with screw BMP groups. Over the 12 weeks after surgery, bone lamellae in direct contact with the screw developed more extensive and thicker trabecular bone around the implant in the OVX with screw BMP group compared to the OVX with screw group.

CONCLUSION

Biodegradable screws containing BMP-2 improve nearby bone conditions and enhance ostoeintegration between the implant and the osteoporotic bone.