Biological activity of rhBMP-2 released from PLGA microspheres

Bone Scaffold Materials in Periodontal and Tooth-supporting Tissue Regeneration: A Review

BACKGROUND AND OBJECTIVES

Periodontium is an important tooth-supporting tissue composed of both hard (alveolar bone and cementum) and soft (gingival and periodontal ligament) sections. Due to the multi-tissue architecture of periodontium, reconstruction of each part can be influenced by others. This review focuses on the bone section of the periodontium and presents the materials used in tissue engineering scaffolds for its reconstruction.

MATERIALS AND METHODS

The following databases (2015 to 2021) were electronically searched: ProQuest, EMBASE, SciFinder, MRS Online Proceedings Library, Medline, and Compendex. The search was limited to English-language publications and in vivo studies.

RESULTS

Eighty-three articles were found in primary searching. After applying the inclusion criteria, seventeen articles were incorporated into this study.

CONCLUSION

In complex periodontal defects, various types of scaffolds, including multilayered ones, have been used for the functional reconstruction of different parts of periodontium. While there are some multilayered scaffolds designed to regenerate alveolar bone/periodontal ligament/cementum tissues of periodontium in a hierarchically organized construct, no scaffold could so far consider all four tissues involved in a complete periodontal defect. The progress and material considerations in the regeneration of the bony part of periodontium are presented in this work to help investigators develop tissue engineering scaffolds suitable for complete periodontal regeneration.

Dual drug delivery platforms for bone tissue engineering

The dual delivery platforms used in bone tissue engineering provide supplementary bioactive compounds that include distinct medicines and growth factors thereby aiding enhanced bone regeneration. The delivery of these compounds can be adjusted for a short or prolonged time based on the requirement by altering various parameters of the carrier platform. The platforms thus used are fabricated to mimic the niche of the bone microenvironment, either in the form of porous 3D structures, microspheres, or films. Thus, this review article focuses on the concept of dual drug delivery platform and its importance, classification of various platforms for dual drug delivery specific to bone tissue engineering, and finally highlights the foresight into the future direction of these techniques for better clinical applications.

Nanotechnology Assisted Targeted Drug Delivery for Bone Disorders: Potentials and Clinical Perspectives

Nanotechnology and its allied modalities have brought revolution in tissue engineering and bone healing. The research on translating the findings of the basic and preclinical research into clinical practice is ongoing. Advances in the synthesis and design of nanomaterials along with advances in genomics and proteomics, and tissue engineering have opened a bright future for bone healing and orthopedic technology. Studies have shown promising outcomes in the design and fabrication of porous implant substrates that can be exploited as bone defect augmentation and drug-carrier devices. However, there are dozens of applications in orthopedic traumatology and bone healing for nanometer-sized entities, structures, surfaces, and devices with characteristic lengths ranging from tens 10s of nanometers to a few micrometers. Nanotechnology has made promising advances in the synthesis of scaffolds, delivery mechanisms, controlled modification of surface topography and composition, and biomicroelectromechanical systems. This study reviews the basic and translational sciences and clinical implications of the nanotechnology in tissue engineering and bone diseases. Recent advances in NPs assisted osteogenic agents, nanocomposites, and scaffolds for bone disorders are discussed.

The sialylation profile of IgG determines the efficiency of antibody directed osteogenic differentiation of iMSCs by modulating local immune responses and osteoclastogenesis

Antibody-mediated osseous regeneration (AMOR) has been proved as a promising strategy for osteogenic differentiation of induced pluripotent stem cells derived MSCs (iMSCs). The key characteristic of antibody that determines the AMOR potential is largely unknown. The glycosylation profile of immunoglobulin G (IgG) represents a key checkpoint that determines its effector functions. Herein, we modified the sialylation profile of BMP2 antibodies to investigate the effects of glycosylation on antibody-mediated osteogenic differentiation of iMSCs. We found that over-sialylated BMP2 antibodies stimulated the highest amount of new bone while those non- or low-sialylated led to bone porosity and collapse. The immune response aroused by BMP2 immune complexes (BMP2-ICs) was intensified by desialylation, which contributed to an environment that favored osteoclastogenesis while inhibited osteoblastogenesis. In vitro study further demonstrated that the osteogenic potential of BMP2-ICs was not significantly affected by the degree of sialylation. On the other hand, BMP2-ICs could stimulate osteoclastogenesis by binding FcγRs on preosteoclasts directly, which was significantly intensified by desialylation and attenuated by over-sialylation. Bone defects implanted with alginate microbeads loaded with iMSCs and over-sialylated antibodies showed more bone formation than those sites with non- or low sialylated antibodies. Taken together, our study demonstrated that sialylation profile is one of the traits that decide the AMOR potential of BMP2 antibodies. Enhancement of sialylation may be a promising strategy to optimize antibody for iMSCs application in bone tissue engineering. STATEMENT OF SIGNIFICANCE: Antibody-mediated osseous regeneration (AMOR) is a promising strategy for bone tissue engineering that takes advantage of the specific reactivity of antibodies to sequester endogenous BMP2 and present it to osteoprogenitor cells. We previously demonstrated that BMP2 immune complex can drive iPSCs derived MSCs to osteogenic lineage. In this study, we analyze the effects of glycosylation profile on antibody directed osteogenic differentiation of iMSCs because glycosylation profile represents a key checkpoint that determines the effector functions of antibodies, and it is susceptible to variations in different clones. The results showed that sialylation profile is one of the traits that decides the AMOR potential of BMP2 antibody, and the enhancement of sialylation maybe a promising strategy to optimize antibodies for AMOR.

Sintered porous Ti6Al4V scaffolds incorporated with recombinant human bone morphogenetic protein-2 microspheres and thermosensitive hydrogels can enhance bone regeneration

A well-controlled powder sintering technique was used to fabricate porous Ti6Al4V scaffold. The thermosensitive chitosan thioglycolic acid (CS-TA) hydrogel was used as a carrier to inject recombinant human bone morphogenetic protein-2 (rhBMP-2) microspheres into pores of the Ti6Al4V scaffold at 37 °C, and then the porous Ti6Al4V/rhBMP-2 loaded hydrogel composite was obtained. The bare Ti6Al4V scaffold was used as the control. The characteristics and mechanical properties of the scaffold, rheological properties of the hydrogels and the rhBMP-2 loaded hydrogel, the release of the rhBMP-2 loaded hydrogel, and the biological properties of the two types of samples were evaluated by in vitro and in vivo tests. Results indicated that the sintered porous Ti6Al4V had high porosity, large pore size with good mechanical properties. The hydrogel and rhBMP-2 loaded hydrogel showed thermosensity. The rhBMP-2 loaded hydrogel showed a stable and extended release profile without too high burst release of rhBMP-2. Both groups showed good biocompatibility and osteogenic ability. However, according to the results of cell tests and implantation, the group with rhBMP-2 loaded hydrogel had significantly higher cell proliferation rate, faster bone growth speed, and more bone ingrowth at every time point. Therefore, the sintered porous Ti6Al4V scaffolds incorporated with rhBMP-2 microspheres and CS-TA hydrogel was effective in enhancing the bone regeneration, and prospects a good candidate for application in orthopedics.

Therapeutic antibody directed osteogenic differentiation of induced pluripotent stem cell derived MSCs

Induced pluripotent stem cells (iPSCs) are regarded as a new cell source for regenerative medicine. Recent advances in tissue engineering have brought to light the therapeutic application of induced pluripotent stem cells (iPSCs) in bone defect repair. However, a safe and efficient way to differentiate iPSCs into osteogenic lineage remains to be a major challenge. Here we describe an approach using anti-BMP2 antibodies (Abs) to mediate osteogenic differentiation of iPSC-derived mesenchymal stromal cells (iMSCs). We first proved that 3G7 (an anti-BMP2 Ab) not only bound to BMP2, but also allowed the bound BMP2 to engage the BMP2 receptors on iMSCs. Subcutaneous implantation sites loaded with iMSCs + 3G7 group showed significant bone formation and vascularization in mice while those sites with exogenous BMP2 exhibited dystrophic calcification and significantly lower vascularization. Our in vitro study demonstrated that the anti-BMP2 Ab/BMP2 immune complex were capable of dictating the acquisition of osteogenic phenotype of iMSCs and subsequent mineralization. The study provided the first evidence of antibody-mediated differentiation of iMSCs and osseous regeneration in vivo. This novel strategy takes full advantage of the endogenous bioactive molecules for osseous regeneration and its potential therapeutic application is promising.

STATEMENT OF SIGNIFICANCE

Induced pluripotent stem cells (iPSCs) and its derived cells hold significant promise for the treatment of bone defects. In present study, we carried out the concept of antibody-mediated bone regeneration into the iPSC research for the first time. We demonstrated that anti-BMP2 Ab/BMP2 immune complex was capable of promoting osteogenic differentiation of iPSC-derived MSCs (iMSCs), likely through the classical BMP2/Smad1/Runx2 pathway. Subcutaneous co-delivery of iMSCs and anti-BMP2 Abs resulted in significant bone formation and vascularization. These findings suggested antibody mediated osteogenic differentiation may be a favorable approach for iPSC-based bone tissue engineering.

Effect of different sustained bone morphogenetic protein-2 release kinetics on bone formation in poly(propylene fumarate) scaffolds

To investigate the effect of sustained bone morphogenetic protein-2 (BMP-2) release kinetics on bone formation in poly(propylene fumarate) (PPF) scaffolds, different poly(lactic-co-glycolic acid) (PLGA) microspheres were used as delivery vehicles. All PPF scaffolds had the same 75% porous structure, while the degradation rate of the embedded PLGA microspheres was changed to tailor BMP-2 release by varying the lactic-to-glycolic acid (L:G) ratio in the copolymer. Four PLGA microsphere formulations with 50/50, 65/35, 75/25, and 85/15 L:G ratios and varying in vivo degradation rates were fabricated. The in vitro and in vivo BMP-2 release kinetics were determined by analyzing radiolabeled ¹²⁵ I-BMP-2. Biological activity of released BMP-2 was tested using a W20-17 cell culture model in vitro and a subcutaneous rat model in vivo. Corresponding outcome parameters were defined as capacity to increase the in vitro AP activity in weekly consecutive cell cultures over 14 weeks and the in vivo bone formation after 7 and 14 weeks. The PLGA/PPF composites showed similar biological activity and BMP-2 release profiles in vitro. In vivo, PPF/PLGA 85:15 composite released significantly less BMP-2 per time point in the first weeks. Micro-CT and histological analysis after 7 and 14 weeks of implantation showed bone formation, which significantly increased over time for all composites. No significant differences were seen between the composites. Overall, the results of this study show that small differences in BMP-2 sustained release had no significant effect on BMP-2 osteogenic efficacy in PPF/PLGA composites. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 477-487, 2018.

Biomechanical analysis of engineered bone with anti-BMP2 antibody immobilized on different scaffolds

Recently we have demonstrated the ability of monoclonal antibodies (mAb) specific for bone morphogenetic protein (BMP)-2 immobilized on different scaffolds to mediate bone formation, a process referred to as Antibody Mediated Osseous Regeneration (AMOR). One of the key properties of regenerated bone is its biomechanical strength, in particular in load-bearing areas. This study sought to test the hypothesis that the biomechanical strength of regenerated bone depends of the mode of regeneration, as well as the scaffold used. Four different scaffolds, namely titanium granules (Ti), alginate hydrogel, anorganic bovine bone mineral (ABBM), and absorbable collagen sponge (ACS) were functionalized with anti-BMP-2 or isotype control mAb and implanted into rat critical-size calvarial defects. The morphology, density and strength of the regenerated bone were evaluated after 8 weeks. Results demonstrated that scaffolds functionalized with anti-BMP-2 mAb exhibited varying degrees of bone volume and density. Ti and ABBM achieved the highest bone volume, density, and strength of bone. When anti-BMP-2 mAb was immobilized on Ti or ABBM, the strength of the regenerated bone were 80% and 77% of native bone respectively, compared with 60% of native bone in sites implanted with rh-BMP-2. Control interventions with isotype mAb did not promote considerable bone regeneration and exhibited significantly lower mechanical properties. SEM analysis showed specimens immobilized with anti-BMP-2 mAb formed new bone with organized structure bridging the crack areas. Altogether, the present data demonstrated that the morphological and mechanical properties of bone bioengineered through AMOR could approximate that of native bone, when appropriate scaffolds are used. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 104B: 1465-1473, 2016.

Effects of the orientation of anti-BMP2 monoclonal antibody immobilized on scaffold in antibody-mediated osseous regeneration

Recently, we have shown that anti-BMP2 monoclonal antibodies (mAbs) can trap endogenous osteogenic BMP ligands, which can in turn mediate osteodifferentiation of progenitor cells. The effectiveness of this strategy requires the availability of the anti-BMP-2 monoclonal antibodies antigen-binding sites for anti-BMP-2 monoclonal antibodies to bind to the scaffold through a domain that will leave its antigen-binding region exposed and available for binding to an osteogenic ligand. We examined whether antibodies bound to a scaffold by passive adsorption versus through Protein G as a linker will exhibit differences in mediating bone formation. In vitro anti-BMP-2 monoclonal antibodies was immobilized on absorbable collagen sponge (ACS) with Protein G as a linker to bind the antibody through its Fc region and implanted into rat calvarial defects. The biomechanical strength of bone regenerated by absorbable collagen sponge/Protein G/anti-BMP-2 monoclonal antibodies immune complex was compared to ACS/anti-BMP-2 monoclonal antibodies or ACS/Protein G/isotype mAb control group. Results demonstrated higher binding of anti-BMP-2 monoclonal antibodies/BMPs to C2C12 cells, when the mAb was initially attached to recombinant Protein G or Protein G-coupled microbeads. After eight weeks, micro-CT and histomorphometric analyses revealed increased bone formation within defects implanted with absorbable collagen sponge/Protein G/anti-BMP-2 monoclonal antibodies compared with defects implanted with absorbable collagen sponge/anti-BMP-2 monoclonal antibodies (p < 0.05). Confocal laser scanning microscopy (CLSM) confirmed increased BMP-2, -4, and -7 detection in sites implanted with absorbable collagen sponge/Protein G/anti-BMP-2 monoclonal antibodies in vivo. Biomechanical analysis revealed the regenerated bone in sites with Protein G/anti-BMP-2 monoclonal antibodies had higher mechanical strength in comparison to anti-BMP-2 monoclonal antibodies. The negative control group, Protein G/isotype mAb, did not promote bone regeneration and exhibited significantly lower mechanical properties (p < 0.05). Altogether, our results demonstrated that application of Protein G as a linker to adsorb anti-BMP-2 monoclonal antibodies onto the scaffold was accompanied by increased in vitro binding of the anti-BMP-2 mAb/BMP immune complex to BMP-receptor positive cell, as well as increased volume and strength of de novo bone formation in vivo.

Bone Regeneration Using Bone Morphogenetic Proteins and Various Biomaterial Carriers

Trauma and disease frequently result in fractures or critical sized bone defects and their management at times necessitates bone grafting. The process of bone healing or regeneration involves intricate network of molecules including bone morphogenetic proteins (BMPs). BMPs belong to a larger superfamily of proteins and are very promising and intensively studied for in the enhancement of bone healing. More than 20 types of BMPs have been identified but only a subset of BMPs can induce de novo bone formation. Many research groups have shown that BMPs can induce differentiation of mesenchymal stem cells and stem cells into osteogenic cells which are capable of producing bone. This review introduces BMPs and discusses current advances in preclinical and clinical application of utilizing various biomaterial carriers for local delivery of BMPs to enhance bone regeneration.

Immobilization of murine anti-BMP-2 monoclonal antibody on various biomaterials for bone tissue engineering

Biomaterials are widely used as scaffolds for tissue engineering. We have developed a strategy for bone tissue engineering that entails application of immobilized anti-BMP-2 monoclonal antibodies (mAbs) to capture endogenous BMPs in vivo and promote antibody-mediated osseous regeneration (AMOR). The purpose of the current study was to compare the efficacy of immobilization of a specific murine anti-BMP-2 mAb on three different types of biomaterials and to evaluate their suitability as scaffolds for AMOR. Anti-BMP-2 mAb or isotype control mAb was immobilized on titanium (Ti) microbeads, alginate hydrogel, and ACS. The treated biomaterials were surgically implanted in rat critical-sized calvarial defects. After 8 weeks, de novo bone formation was assessed using micro-CT and histomorphometric analyses. Results showed de novo bone regeneration with all three scaffolds with immobilized anti-BMP-2 mAb, but not isotype control mAb. Ti microbeads showed the highest volume of bone regeneration, followed by ACS. Alginate showed the lowest volume of bone. Localization of BMP-2, -4, and -7 antigens was detected on all 3 scaffolds with immobilized anti-BMP-2 mAb implanted in calvarial defects. Altogether, these data suggested a potential mechanism for bone regeneration through entrapment of endogenous BMP-2, -4, and -7 proteins leading to bone formation using different types of scaffolds via AMOR.

Effect of different sintering methods on bioactivity and release of proteins from PLGA microspheres

Macromolecule release from poly(d,l-lactide-co-glycolide) (PLGA) microspheres has been well-characterized, and is a popular approach for delivering bioactive signals from tissue-engineered scaffolds. However, the effect of some processing solvents, sterilization, and mineral incorporation (when used in concert) on long-term release and bioactivity has seldom been addressed. Understanding these effects is of significant importance for microsphere-based scaffolds, given that these scaffolds are becoming increasingly more popular, yet growth factor activity following sintering and/or sterilization is heretofore unknown. The current study evaluated the 6-week release of transforming growth factor (TGF)-β3 and bone morphogenetic protein (BMP)-2 from PLGA and PLGA/hydroxyapatite (HAp) microspheres following exposure to ethanol (EtOH), dense phase carbon dioxide (CO2), or ethylene oxide (EtO). EtO was chosen based on its common use in scaffold sterilization, whereas EtOH and CO2 were chosen given their importance in sintering microspheres together to create scaffolds. Release supernatants were then used in an accelerated cell stimulation study with human bone marrow stromal cells (hBMSCs) with monitoring of gene expression for major chondrogenic and osteogenic markers. Results indicated that in microspheres without HAp, EtOH exposure led to the greatest amount of delivery, while those treated with CO2 delivered the least growth factor. In contrast, formulations with HAp released almost half as much protein, regardless of EtOH or CO2 exposure. Notably, EtO exposure was not found to significantly affect the amount of protein released. Cell stimulation studies demonstrated that eluted protein samples performed similarly to positive controls in PLGA-only formulations, and ambiguously in PLGA/HAp composites. In conclusion, the use of EtOH, subcritical CO2, and EtO in microsphere-based scaffolds may have only slight adverse effects, and possibly even desirable effects in some cases, on protein availability and bioactivity.

Functionalization of scaffolds with chimeric anti-BMP-2 monoclonal antibodies for osseous regeneration

Recent studies have demonstrated the ability of murine anti-BMP-2 monoclonal antibodies (mAb) immobilized on an absorbable collagen sponge (ACS) to mediate de novo bone formation, a process termed antibody-mediated osseous regeneration (AMOR). The objectives of this study were to assess the efficacy of a newly generated chimeric anti-BMP-2 mAb in mediating AMOR, as well as to evaluate the suitability of different biomaterials as scaffolds to participate in AMOR. Chimeric anti-BMP-2 mAb was immobilized on 4 biomaterials, namely, titanium microbeads (Ti), alginate hydrogel, macroporous biphasic calcium phosphate (MBCP) and ACS, followed by surgical implantation into rat critical-size calvarial defects. Animals were sacrificed after 8 weeks and the degree of bone fill was assessed using micro-CT and histomorphometry. Results demonstrated local persistence of chimeric anti-BMP-2 mAb up to 8 weeks, as well as significant de novo bone regeneration in sites implanted with chimeric anti-BMP-2 antibody immobilized on each of the 4 scaffolds. Ti and MBCP showed the highest volume of bone regeneration, presumably due to their resistance to compression. Alginate and ACS also mediated de novo bone formation, though significant volumetric shrinkage was noted. In vitro assays demonstrated cross-reactivity of chimeric anti-BMP-2 mAb with BMP-4 and BMP-7. Immune complex of anti-BMP-2 mAb with BMP-2 induced osteogenic differentiation of C2C12 cells in vitro, involving expression of RUNX2 and phosphorylation of Smad1. The present data demonstrated the ability of chimeric anti-BMP-2 mAb to functionalize different biomaterial with varying characteristics to mediate osteogenesis.

Co-encapsulation of anti-BMP2 monoclonal antibody and mesenchymal stem cells in alginate microspheres for bone tissue engineering

Recently, it has been shown that tethered anti-BMP2 monoclonal antibodies (mAbs) can trap BMP ligands and thus provide BMP inductive signals for osteo-differentiation of progenitor cells. The objectives of this study were to: (1) develop a co-delivery system based on murine anti-BMP2 mAb-loaded alginate microspheres encapsulating human bone marrow mesenchymal stem cells (hBMMSCs); and (2) investigate osteogenic differentiation of encapsulated stem cells in alginate microspheres in vitro and in vivo. Alginate microspheres of 1 ± 0.1 mm diameter were fabricated with 2 × 10(6) hBMMSCs per mL of alginate. Critical-size calvarial defects (5 mm diameter) were created in immune-compromised mice and alginate microspheres preloaded with anti-BMP mAb encapsulating hBMMSCs were transplanted into defect sites. Alginate microspheres pre-loaded with isotype-matched non-specific antibody were used as the negative control. After 8 weeks, micro CT and histologic analyses were used to analyze bone formation. In vitro analysis demonstrated that anti-BMP2 mAbs tethered BMP2 ligands that can activate the BMP receptors on hBMMSCs. The co-delivery system described herein, significantly enhanced hBMMSC-mediated osteogenesis, as confirmed by the presence of BMP signal pathway-activated osteoblast determinants Runx2 and ALP. Our results highlight the importance of engineering the microenvironment for stem cells, and particularly the value of presenting inductive signals for osteo-differentiation of hBMMSCs by tethering BMP ligands using mAbs. This strategy of engineering the microenvironment with captured BMP signals is a promising modality for repair and regeneration of craniofacial, axial and appendicular bone defects.

A differential effect of bone morphogenetic protein-2 and vascular endothelial growth factor release timing on osteogenesis at ectopic and orthotopic sites in a large-animal model

In bone tissue engineering, growth factors are widely used. Bone morphogenetic proteins (BMPs) and vascular endothelial growth factor (VEGF) are the most well-known regulators of osteogenesis and angiogenesis. We investigated whether the timing of dual release of VEGF and BMP-2 influences the amount of bone formation in a large-animal model. Poly(lactic-co-glycolic acid) (PLGA) microparticles (MPs) were loaded with BMP-2 or VEGF to create sustained-release profiles, and rapidly degrading gelatin was loaded with either growth factor for fast-release profiles. To study in vivo osteogenicity, the two delivery vehicles were combined with biphasic calcium phosphate (BCP) scaffolds and implanted in 10 Beagle dogs for 9 weeks, at both ectopic (paraspinal muscles) and orthotopic sites (critical-size ulnar defect). The 9 ectopic groups contained combined or single BMP/VEGF dosage, in sustained- or fast-release profiles. In the ulnae of 8 dogs, fast VEGF and sustained BMP-2 were applied to one leg, and the other received the opposite release profiles. The two remaining dogs received bilateral control scaffolds. Bone growth dynamics was analyzed by fluorochrome injection at weeks 3, 5, and 7. Postoperative and posteuthanization X-rays of the ulnar implants were taken. After 9 weeks of implantation, bone quantity and bone growth dynamics were studied by histology, histomorphometry, and fluorescence microscopy. The release of the growth factors resulted in both enhanced orthotopic and ectopic bone formation. Bone formation started before 3 weeks and continued beyond 7 weeks. The ectopic BMP-2 fast groups showed significantly more bone compared to sustained release, independent of the VEGF profile. The ulna implants revealed no significant differences in the amount of bone formed. This study shows that timing of BMP-2 release largely determines speed and amount of ectopic bone formation independent of VEGF release. Furthermore, at the orthotopic site, no significant effect on bone formation was found from a timed release of growth factors, implicating that timed-release effects are location dependent.

Biomaterial delivery of morphogens to mimic the natural healing cascade in bone

Complications in treatment of large bone defects using bone grafting still remain. Our understanding of the endogenous bone regeneration cascade has inspired the exploration of a wide variety of growth factors (GFs) in an effort to mimic the natural signaling that controls bone healing. Biomaterial-based delivery of single exogenous GFs has shown therapeutic efficacy, and this likely relates to its ability to recruit and promote replication of cells involved in tissue development and the healing process. However, as the natural bone healing cascade involves the action of multiple factors, each acting in a specific spatiotemporal pattern, strategies aiming to mimic the critical aspects of this process will likely benefit from the usage of multiple therapeutic agents. This article reviews the current status of approaches to deliver single GFs, as well as ongoing efforts to develop sophisticated delivery platforms to deliver multiple lineage-directing morphogens (multiple GFs) during bone healing.

Tunable hydrogel-microsphere composites that modulate local inflammation and collagen bulking

Injectable biomaterials alone may alter local tissue responses, including inflammatory cascades and matrix production (e.g. stimulatory dermal fillers are used as volumizing agents that induce collagen production). To expand upon the available material compositions and timing of presentation, a tunable hyaluronic acid (HA) and poly(lactide-co-glycolide) (PLGA) microsphere composite system was formulated and assessed in subcutaneous and cardiac tissues. HA functionalized with hydroxyethyl methacrylate (HeMA) was used as a precursor to injectable and degradable hydrogels that carry PLGA microspheres (~50 μm diameter) to tissues, where the HA hydrogel degradation (~20 or 70 days) and quantity of PLGA microspheres (0-300 mgml(-1)) are readily varied. When implanted subcutaneously, faster hydrogel degradation and more microspheres (e.g. 75 mgml(-1)) generally induced more rapid tissue and cellular interactions and a greater macrophage response. In cardiac applications, tissue bulking may be useful to alter stress profiles and to stabilize the tissue after infarction, limiting left ventricular (LV) remodeling. When fast degrading HeMA-HA hydrogels containing 75 mgml(-1) microspheres were injected into infarcted tissue in sheep, LV dilation was limited and the thickness of the myocardial wall and the presence of vessels in the apical infarct region were increased ~35 and ~60%, respectively, compared to empty hydrogels. Both groups decreased volume changes and infarct areas at 8 weeks, compared to untreated controls. This work illustrates the importance of material design in expanding the application of tissue bulking composites to a range of biomedical applications.

A sulfate polysaccharide/TNF-related apoptosis-inducing ligand (TRAIL) complex for the long-term delivery of TRAIL in poly(lactic-co-glycolic acid) (PLGA) microspheres

OBJECTIVES

The aim was to develop a long-term delivery system for Apo2 ligand/tumour necrosis factor-related apoptosis-inducing ligand (TRAIL) without chemical modification (such as pegylation).

METHODS

A nanocomplex system between the positively charged TRAIL and the negatively charged chondroitin sulfate (CS) (CS/TRAIL) was designed and applied in poly(lactide-co-glycolide) (PLGA) microspheres (MSs).

KEY FINDINGS

A nanocomplex of approximately 200 nm was easily formed in a weight ratio of 2 TRAIL to CS (TC2) at pH 5.0. The cytotoxicity of CS/TRAIL against HeLa cells was similar to that of native TRAIL. The complex also had higher loading efficiency (above 95%) in PLGA MSs prepared by the multi-emulsion method than that of native TRAIL. The release behaviour of TRAIL from the PLGA MSs was monitored. Although the release of TRAIL from native TRAIL-loaded PLGA MSs (TMS0) was almost complete after 3 days, TC2-loaded PLGA MSs (TMS2) showed sustained TRAIL release without an initial burst for 10 days. The released TRAIL from TMS2 led to cytotoxicity accompanied by massive apoptosis of cancer cells. TMS2 significantly inhibited tumour growth in an in-vivo xenograft model in mice, without any loss of body weight after treatment.

CONCLUSIONS

From the results, we concluded that TC-loaded PLGA MSs have the potential for long-term delivery of TRAIL without side effects.

Sustained release of PTH(1-34) from PLGA microspheres suppresses osteoarthritis progression in rats

We previously reported that PTH(1-34) inhibits the terminal differentiation of articular chondrocytes and, in turn, suppresses the progression of osteoarthritis (OA). However, this treatment requires an injection of PTH(1-34) once every 3 days over the treatment period. In this study, we studied the effect of sustained administration of PTH(1-34) in a papain-induced OA rat model. We developed an effective controlled-release system for prolonging the treatment duration of an intra-articular injection for OA treatment in rats. The effects of released PTH(1-34) from PLGA(65:35)-encapsulated PTH(1-34) microspheres (PTH/PLGA) on papain-induced OA in rat knees were studied. Microsphere morphology was observed in vitro by scanning electron microscopy, and microsphere size was determined with a particle size analyzer. The PTH(1-34) encapsulation efficiency and release profile, as well as the toxicity of PTH/PLGA, were examined. The bioactivity of released PTH(1-34) was tested by examining cAMP levels in MC3T3E1 cells. In vivo, we evaluated the changes of localized GAG, Col II, and Col X in the articular cartilage of rat knees. Our results demonstrated that the surface of the PLGA microspheres was smooth, and the size of the microspheres was in the range of 51-127 μm. PTH/PLGA microspheres sustainably released PTH(1-34) for 19 days with a concentration range of 0.01-100 nM that covered the expected concentration of 10nM at 37°C. The cAMP levels of MC3T3E1 cells were elevated in the response to released PTH(1-34) from PTH/PLGA microspheres, indicating that the released PTH(1-34) is bioactive. Most importantly, intra-articular treatment with either PTH(1-34) (0.1-100 nM) 3 days/injection or PTH/PLGA microspheres (15 days/injection) for 5 weeks revealed the similar effect on suppressing papain-induced OA changes (decreasing GAG and Col II and increasing Col X) in rat knee cartilage. The effect of PTH/PLGA microspheres on suppressing OA progression was similar to that of a once-every-three-day injection of PTH(1-34), indicating that both the sustained and intermittent action of PTH(1-34) effectively suppress OA progression. The developed PLGA microspheres with sustained release and long-term effect may be potent carriers for PTH(1-34) used to treat early OA.

Antibody-mediated osseous regeneration: a novel strategy for bioengineering bone by immobilized anti-bone morphogenetic protein-2 antibodies

Bone regeneration often requires harvesting of autologous bone with significant potential morbidity and cost. Recombinant human bone morphogenetic protein (rhBMP)-2 has been approved by the U.S. Food and Drug Administration for specific regenerative indications. However, administration of exogenous growth factors has many drawbacks. The objective of the present proof-of-concept study was to determine whether immobilized anti-BMP-2 antibodies (Abs) could capture endogenous BMP-2 in local sites to mediate osteogenesis, a strategy we refer to as antibody-mediated osseous regeneration (AMOR). We have generated a murine anti-BMP-2 monoclonal antibody library, which was tested along with commercially available Abs in vitro and in vivo for their ability to mediate AMOR. In vitro studies demonstrated that only some anti-BMP-2 Abs tested formed immune complexes with BMP-2, which can bind to BMP cellular receptor, whereas other BMP-2/anti-BMP-2 complexes failed to bind. To investigate whether anti-BMP-2 Abs were able to mediate AMOR in vivo, anti-BMP-2 Abs were immobilized on absorbable collagen sponge (ACS) and surgically placed in rat calvarial defects. Microcomputed tomography analysis of live animals at 2, 4, and 6 weeks demonstrated that some anti-BMP-2 Abs immobilized on ACS mediated significant bone regeneration, whereas other clones did not mediate any bone regeneration. In situ BMP-2 and osteocalcin expression was investigated by immunohistochemistry. Results demonstrated higher BMP-2 and osteocalcin expression in sites with increased bone regeneration. Results provide first evidence for the ability of anti-BMP2 Abs to form an immune complex with endogenous BMP-2 and mediate bone regeneration in vivo, suggesting a promising therapeutic method for tissue engineering.

Enhanced bone morphogenetic protein-2-induced ectopic and orthotopic bone formation by intermittent parathyroid hormone (1-34) administration

Bone morphogenetic proteins (BMPs) play a central role in local bone regeneration strategies, whereas the anabolic features of parathyroid hormone (PTH) are particularly appealing for the systemic treatment of generalized bone loss. The aim of the current study was to investigate whether local BMP-2-induced bone regeneration could be enhanced by systemic administration of PTH (1-34). Empty or BMP-2-loaded poly(lactic-co glycolic acid)/poly(propylene fumarate)/gelatin composites were implanted subcutaneously and in femoral defects in rats (n = 9). For the orthotopic site, empty defects were also tested. Each of the conditions was investigated in combination with daily administered subcutaneous PTH (1-34) injections in the neck. After 8 weeks of implantation, bone mineral density (BMD) and bone volume were analyzed using microcomputed tomography and histology. Ectopic bone formation and almost complete healing of the femoral defect were only seen in rats that received BMP-2-loaded composites. Additional treatment of the rats with PTH (1-34) resulted in significantly (p < 0.05) enhanced BMD and bone volume in the BMP-2 composites at both implantation sites. Despite its effect on BMD in the humerus and vertebra, PTH (1-34) treatment had no significant effect on BMD and bone volume in the empty femoral defects and the ectopically or orthotopically implanted empty composites. Histological analysis showed that the newly formed bone had a normal woven and trabecular appearance. Overall, this study suggests that intermittent administration of a low PTH dose alone has limited potential to enhance local bone regeneration in a critical-sized defect in rats. However, when combined with local BMP-2-releasing scaffolds, PTH administration significantly enhanced osteogenesis in both ectopic and orthotopic sites.

Use of a bioactive scaffold for the repair of bone defects in a novel reproducible vertebral body defect model

Bone defects in vertebral bodies (VB) usually occur after the reduction of fractures or are caused by bone disease. Besides the treatment of original disease, repair of the bone defect can restore the structure of VB and improve stabilization of the spine to protect the spinal cord nerves. To aid studies of the efficacy of bioengineering techniques for repair of VB, we developed a rat model with a critical size bone defect in VB. Air-motivated burrs were used to create two sizes of bone defect (2 x 3 x 1.5 mm; 2 x 3 x 3 mm) in the anterior part of VB in 6-month-old Fischer 344 rats. Quantitative CT analyses and histological assays demonstrated that neither defects self-repaired by 8 weeks post surgery. Moreover, the tendency of bone formation was monitored in the same animal by serial CT image evaluations, allowing us to demonstrate that there was significant bone growth during the 4- to 6-week period after the creation of the bone defect. We then implanted sintered poly(lactic-co-glycolic acid) (PLGA) microsphere scaffolds loaded with Matrigel with or without recombinant human bone morphogenetic protein 2 (rhBMP2; 2.0 microg rhBMP2/10 microL Matrigel/scaffold) into the bone defect (2 x 3 x 3 mm) in the VB. Bone formation was detected by quantitative analyses of serial CT images, which demonstrated bone growth in rats that received the rhBMP2 implant, in both surrounding areas and inside area of the scaffold. In addition to a rapid increase within 2 weeks of the operation, another significant bone formation period was found between 4 and 8 weeks after the implantation. By contrast, the control group that received the implant without rhBMP2 did not show similar bone formation tendencies. The results of CT analyses were confirmed by histological studies. This study suggests that a critical size bone defect of the anterior VB can be developed in a rat model. Characterization of this model demonstrated that 4 to 6 weeks after creation of the defect was a significant bone growth period for VB bone repair in rats. This animal model has further utility for the study of different biomaterials for VB bone repair. Implantation of a bioactive PLGA scaffold carrying rhBMP2 allowed more successful repair of the VB defect. Although further characterization studies are needed, the bioactive PLGA scaffold developed in this study will likely adapt easily to other in vivo osteogenesis applications.

Nanotechnology and bone healing

Nanotechnology and its attendant techniques have yet to make a significant impact on the science of bone healing. However, the potential benefits are immediately obvious with the result that hundreds of researchers and firms are performing the basic research needed to mature this nascent, yet soon to be fruitful niche. Together with genomics and proteomics, and combined with tissue engineering, this is the new face of orthopaedic technology. The concepts that orthopaedic surgeons recognize are fabrication processes that have resulted in porous implant substrates as bone defect augmentation and medication-carrier devices. However, there are dozens of applications in orthopaedic traumatology and bone healing for nanometer-sized entities, structures, surfaces, and devices with characteristic lengths ranging from 10s of nanometers to a few micrometers. Examples include scaffolds, delivery mechanisms, controlled modification of surface topography and composition, and biomicroelectromechanical systems. We review the basic science, clinical implications, and early applications of the nanotechnology revolution and emphasize the rich possibilities that exist at the crossover region between micro- and nanotechnology for developing new treatments for bone healing.

Delivery systems for bone growth factors — the new players in skeletal regeneration

Given the challenge of an increasing elderly population, the ability to repair and regenerate traumatised or lost tissue is a major clinical and socio-economic need. Pivotal in this process will be the ability to deliver appropriate growth factors in the repair cascade in a temporal and tightly regulated sequence using appropriately designed matrices and release technologies within a tissue engineering strategy. This review outlines the current concepts and challenges in growth factor delivery for skeletal regeneration and the potential of novel delivery matrices and biotechnologies to influence the healthcare of an increasing ageing population.

Effect of autologous bone marrow stromal cell seeding and bone morphogenetic protein-2 delivery on ectopic bone formation in a microsphere/poly(propylene fumarate) composite

A biodegradable microsphere/scaffold composite based on the synthetic polymer poly(propylene fumarate) (PPF) holds promise as a scaffold for cell growth and sustained delivery vehicle for growth factors for bone regeneration. The objective of the current work was to investigate the in vitro release and in vivo bone forming capacity of this microsphere/scaffold composite containing bone morphogenetic protein-2 (BMP-2) in combination with autologous bone marrow stromal cells (BMSCs) in a goat ectopic implantation model. Three composites consisting of 0, 0.08, or 8 microg BMP-2 per mg of poly(lactic-co-glycolic acid) microspheres, embedded in a porous PPF scaffold, were combined with either plasma (no cells) or culture-expanded BMSCs. PPF scaffolds impregnated with a BMP-2 solution and combined with BMSCs as well as empty PPF scaffolds were also tested. The eight different composites were implanted subcutaneously in the dorsal thoracolumbar area of goats. Incorporation of BMP-2-loaded microspheres in the PPF scaffold resulted in a more sustained in vitro release with a lower burst phase, as compared to BMP-2-impregnated scaffolds. Histological analysis after 9 weeks of implantation showed bone formation in the pores of 11/16 composites containing 8 microg/mg BMP-2-loaded microspheres with no significant difference between composites with or without BMSCs (6/8 and 5/8, respectively). Bone formation was also observed in 1/8 of the BMP-2-impregnated scaffolds. No bone formation was observed in the other conditions. Overall, this study shows the feasibility of bone induction by BMP-2 release from microspheres/scaffold composites.

Targeted delivery with peptidomimetic conjugated self-assembled nanoparticles

Peptides produce specific nanostructures, making them useful for targeting in biological systems but they have low bioavailability, potential immunogenicity and poor metabolic stability. Peptidomimetic self-assembled NPs can possess biological recognition motifs as well as providing desired engineering properties. Inorganic NPs, coated with self-assembled macromers for stability and anti-fouling, and conjugated with target-specific ligands, are advancing imaging from the anatomy-based level to the molecular level. Ligand conjugated NPs are attractive for cell-selective tumor drug delivery, since this process has high transport capacity as well as ligand dependent cell specificity. Peptidomimetic NPs can provide stronger interaction with surface receptors on tumor cells, resulting in higher uptake and reduced drug resistance. Self-assembled NPs conjugated with peptidomimetic antigens are ideal for sustained presentation of vaccine antigens to dendritic cells and subsequent activation of T cell mediated adaptive immune response. Self-assembled NPs are a viable alternative to encapsulation for sustained delivery of proteins in tissue engineering. Cell penetrating peptides conjugated to NPs are used as intracellular delivery vectors for gene expression and as transfection agents for plasmid delivery. In this work, synthesis, characterization, properties, immunogenicity, and medical applications of peptidomimetic NPs in imaging, tumor delivery, vaccination, tissue engineering, and intracellular delivery are reviewed.

Non-invasive monitoring of BMP-2 retention and bone formation in composites for bone tissue engineering using SPECT/CT and scintillation probes

Non-invasive imaging can provide essential information for the optimization of new drug delivery-based bone regeneration strategies to repair damaged or impaired bone tissue. This study investigates the applicability of nuclear medicine and radiological techniques to monitor growth factor retention profiles and subsequent effects on bone formation. Recombinant human bone morphogenetic protein-2 (BMP-2, 6.5 microg/scaffold) was incorporated into a sustained release vehicle consisting of poly(lactic-co-glycolic acid) microspheres embedded in a poly(propylene fumarate) scaffold surrounded by a gelatin hydrogel and implanted subcutaneously and in 5-mm segmental femoral defects in 9 rats for a period of 56 days. To determine the pharmacokinetic profile, BMP-2 was radiolabeled with (125)I and the local retention of (125)I-BMP-2 was measured by single photon emission computed tomography (SPECT), scintillation probes and ex vivo scintillation analysis. Bone formation was monitored by micro-computed tomography (microCT). The scaffolds released BMP-2 in a sustained fashion over the 56-day implantation period. A good correlation between the SPECT and scintillation probe measurements was found and there were no significant differences between the non-invasive and ex-vivo counting method after 8 weeks of follow up. SPECT analysis of the total body and thyroid counts showed a limited accumulation of (125)I within the body. Ectopic bone formation was induced in the scaffolds and the femur defects healed completely. In vivo microCT imaging detected the first signs of bone formation at days 14 and 28 for the orthotopic and ectopic implants, respectively, and provided a detailed profile of the bone formation rate. Overall, this study clearly demonstrates the benefit of applying non-invasive techniques in drug delivery-based bone regeneration strategies by providing detailed and reliable profiles of the growth factor retention and bone formation at different implantation sites in a limited number of animals.

Nanostructured Biomaterials for Regeneration

Biomaterials play a pivotal role in regenerative medicine, which aims to regenerate and replace lost/dysfunctional tissues or organs. Biomaterials (scaffolds) serve as temporary 3D substrates to guide neo tissue formation and organization. It is often beneficial for a scaffolding material to mimic the characteristics of extracellular matrix (ECM) at the nanometer scale and to induce certain natural developmental or/and wound healing processes for tissue regeneration applications. This article reviews the fabrication and modification technologies for nanofibrous, nanocomposite, and nanostructured drug-delivering scaffolds. ECM-mimicking nanostructured biomaterials have been shown to actively regulate cellular responses including attachment, proliferation, differentiation and matrix deposition. Nano-scaled drug delivery systems can be successfully incorporated into a porous 3D scaffold to enhance the tissue regeneration capacity. In conclusion, nano-structured biomateials are a very exciting and rapidly expanding research area, and are providing new enabling technologies for regenerative medicine.

Growth factor gradients via microsphere delivery in biopolymer scaffolds for osteochondral tissue engineering

Temporally and spatially controlled delivery of growth factors in polymeric scaffolds is crucial for engineering composite tissue structures, such as osteochondral constructs. In the present study, microsphere-mediated growth factor delivery in polymer scaffolds and its impact on osteochondral differentiation of human bone marrow-derived mesenchymal stem cells (hMSCs) was evaluated. Two growth factors, bone morphogenetic protein 2 (rhBMP-2) and insulin-like growth factor I (rhIGF-I), were incorporated as a single concentration gradient or reverse gradient combining two factors in the scaffolds. To assess the gradient making system and the delivery efficiency of polylactic-co-glycolic acid (PLGA) and silk fibroin microspheres, initially an alginate gel was fabricated into a cylinder shape with microspheres incorporated as gradients. Compared to PLGA microspheres, silk microspheres were more efficient in delivering rhBMP-2, probably due to sustained release of the growth factor, while less efficient in delivering rhIGF-I, likely due to loading efficiency. The growth factor gradients formed were shallow, inducing non-gradient trends in hMSC osteochondral differentiation. Aqueous-derived silk porous scaffolds were used to incorporate silk microspheres using the same gradient process. Both growth factors formed deep and linear concentration gradients in the scaffold, as shown by enzyme-linked immunosorbent assay (ELISA). After seeding with hMSCs and culturing for 5 weeks in a medium containing osteogenic and chondrogenic components, hMSCs exhibited osteogenic and chondrogenic differentiation along the concentration gradients of rhBMP-2 in the single gradient of rhBMP-2 and reverse gradient of rhBMP-2/rhIGF-I, but not the rhIGF-I gradient system, confirming that silk microspheres were more efficient in delivering rhBMP-2 than rhIGF-I for hMSCs osteochondrogenesis. This novel silk microsphere/scaffold system offers a new option for the delivery of multiple growth factors with spatial control in a 3D culture environment for both understanding natural tissue growth process and in vitro engineering complex tissue constructs.

2007 AIChE Alpha Chi Sigma Award: From Material to Tissue: Biomaterial Development, Scaffold Fabrication, and Tissue Engineering

The need for techniques to facilitate the regeneration of failing or destroyed tissues remains great with the aging of the worldwide population and the continued incidence of trauma and diseases such as cancer. A 16-year history in biomaterial scaffold development and tissue engineering is examined, beginning with the synthesis of novel materials and fabrication of 3D porous scaffolds. Exploring cell-scaffold interactions and subsequently cellular delivery using biomaterial carriers, we have developed a variety of techniques for bone and cartilage engineering. In addition to delivering cells, we have utilized growth factors, DNA, and peptides to improve the in vitro and in vivo regeneration of tissues. This review covers important developments and discoveries within our laboratory, and the increasing breadth in the scope of our work within the expanding field of tissue engineering is presented.

Nanoparticle technology in bone tissue engineering

Nanotechnology has been increasingly utilized to enhance bone tissue engineering strategies. In particular, nanotechnology has been employed to overcome some of the current limitations associated with bone regeneration methods including insufficient mechanical strength of scaffold materials, ineffective cell growth and osteogenic differentiation at the defect site, as well as unstable and insufficient production of growth factors to stimulate bone cell growth. Among the tremendous technologies of nanoparticles in biological systems, we focus here on the three major nanoparticle research areas that have been developed to overcome these limitations and disadvantages: (a) the generation of nanoparticle-composite scaffolds to provide increased mechanical strength for bone graft, (b) the fabrication of nanofibrous scaffolds to support cell growth and differentiation through morphologically-favored architectures, and (c) the development of novel delivery and targeting systems of genetic material, especially those encoding osteogenic growth factors. These nanoparticle-based bone tissue engineering technologies possess a great potential to ensure the efficacy of clinical bone regeneration.

Poly(lactic-co-glycolic acid) nanospheres and microspheres for short- and long-term delivery of bioactive ciliary neurotrophic factor

Ciliary neurotrophic factor (CNTF) has been shown to be neuroprotective in the central nervous system (CNS). However, systemic administration and bolus injections have shown significant side effects and limited efficacy. Sustained, local delivery may lead to effective neuroprotection and avoid or limit adverse side effects, but sustained CNTF delivery has proven difficult to achieve and control. For controlled, sustained delivery, we investigated several processing variables in making poly(DL-lactic-co-glycolic acid) (PLGA) nano- and microspheres to optimize CNTF encapsulation and release. Nano- and microspheres were 314.9 +/- 24.9 nm and 11.69 +/- 8.16 microm in diameter, respectively. CNTF delivery from nanospheres was sustained over 14 days, and delivery from microspheres continued over more than 70 days. To assess protein bioactivity after encapsulation, neural stem cells (NSCs) were treated with CNTF released from nanospheres and compared to those treated with unencapsulated CNTF as a control. NSCs treated with CNTF expressed markers specific to mature cells, notably astrocytes; some increase in oligodendrocytic and neuronal marker expression was also observed. Significantly, cells treated with CNTF released by nanospheres exhibited a similar degree of differentiation when compared to those treated with control CNTF of equivalent concentration, showing that the process of protein encapsulation did not reduce its potency.

Non-invasive screening method for simultaneous evaluation of in vivo growth factor release profiles from multiple ectopic bone tissue engineering implants

The purpose of this study was to develop and validate a screening method based on scintillation probes for the simultaneous evaluation of in vivo growth factor release profiles of multiple implants in the same animal. First, we characterized the scintillation probes in a series of in vitro experiments to optimize the accuracy of the measurement setup. The scintillation probes were found to have a strong geometric dependence and experience saturation effects at high activities. In vitro simulation of 4 subcutaneous limb implants in a rat showed minimal interference of surrounding implants on local measurements at close to parallel positioning of the probes. These characteristics were taken into consideration for the design of the probe setup and in vivo experiment. The measurement setup was then validated in a rat subcutaneous implantation model using 4 different sustained release carriers loaded with (125)I-BMP-2 per animal. The implants were removed after 42 or 84 days of implantation, for comparison of the non-invasive method to ex vivo radioisotope counting. The non-invasive method demonstrated a good correlation with the ex vivo counting method at both time-points of all 4 carriers. Overall, this study showed that scintillation probes could be successfully used for paired measurement of 4 release profiles with minimal interference of the surrounding implants, and may find use as non-invasive screening tools for various drug delivery applications.

Retention of in vitro and in vivo BMP-2 bioactivities in sustained delivery vehicles for bone tissue engineering

In this study, we investigated the in vitro and in vivo biological activities of bone morphogenetic protein 2 (BMP-2) released from four sustained delivery vehicles for bone regeneration. BMP-2 was incorporated into (1) a gelatin hydrogel, (2) poly(lactic-co-glycolic acid) (PLGA) microspheres embedded in a gelatin hydrogel, (3) microspheres embedded in a poly(propylene fumarate) (PPF) scaffold and (4) microspheres embedded in a PPF scaffold surrounded by a gelatin hydrogel. A fraction of the incorporated BMP-2 was radiolabeled with (125)I to determine its in vitro and in vivo release profiles. The release and bioactivity of BMP-2 were tested weekly over a period of 12 weeks in preosteoblast W20-17 cell line culture and in a rat subcutaneous implantation model. Outcome parameters for in vitro and in vivo bioactivities of the released BMP-2 were alkaline phosphatase (AP) induction and bone formation, respectively. The four implant types showed different in vitro release profiles over the 12-week period, which changed significantly upon implantation. The AP induction by BMP-2 released from gelatin implants showed a loss in bioactivity after 6 weeks in culture, while the BMP-2 released from the other implants continued to show bioactivity over the full 12-week period. Micro-CT and histological analysis of the delivery vehicles after 6 weeks of implantation showed significantly more bone in the microsphere/PPF scaffold composites (Implant 3, p<0.02). After 12 weeks, the amount of newly formed bone in the microsphere/PPF scaffolds remained significantly higher than that in the gelatin and microsphere/gelatin hydrogels (p<0.001), however, there was no statistical difference compared to the microsphere/PPF/gelatin composite. Overall, the results from this study show that BMP-2 could be incorporated into various bone tissue engineering composites for sustained release over a prolonged period of time with retention of bioactivity.

Oxysterols enhance osteoblast differentiation in vitro and bone healing in vivo

Oxysterols, naturally occurring cholesterol oxidation products, can induce osteoblast differentiation. Here, we investigated short-term 22(S)-hydroxycholesterol + 20(S)-hydroxycholesterol (SS) exposure on osteoblastic differentiation of marrow stromal cells. We further explored oxysterol ability to promote bone healing in vivo. Osteogenic differentiation was assessed by alkaline phosphatase (ALP) activity, osteocalcin (OCN) mRNA expression, mineralization, and Runx2 DNA binding activity. To explore the effects of osteogenic oxysterols in vivo, we utilized the critical-sized rat calvarial defect model. Poly(lactic-co-glycolic acid) (PLGA) scaffolds alone or coated with 140 ng (low dose) or 1400 ng (high dose) oxysterol cocktail were implanted into the defects. Rats were sacrificed at 6 weeks and examined by three-dimensional (3D) microcomputed tomography (microCT). Bone volume (BV), total volume (TV), and BV/TV ratio were measured. Culture exposure to SS for 10 min significantly increased ALP activity after 4 days, while 2 h exposure significantly increased mineralization after 14 days. Four-hour SS treatment increased OCN mRNA measured after 8 days and nuclear protein binding to an OSE2 site measured after 4 days. The calvarial defects showed slight bone healing in the control group. However, scaffolds adsorbed with low or high-dose oxysterol cocktail significantly enhanced bone formation. Histologic examination confirmed bone formation in the defect sites grafted with oxysterol-adsorbed scaffolds, compared to mostly fibrous tissue in control sites. Our results suggest that brief exposure to osteogenic oxysterols triggered events leading to osteoblastic cell differentiation and function in vitro and bone formation in vivo. These results identify oxysterols as potential agents in local and systemic enhancement of bone formation.

Matrices and scaffolds for delivery of bioactive molecules in bone and cartilage tissue engineering

Regeneration of bone and cartilage defects can be accelerated by localized delivery of appropriate growth factors incorporated within biodegradable carriers. The carrier essentially allows the impregnated growth factor to release at a desirable rate and concentration, and to linger at injury sites for a sufficient time to recruit progenitors and stimulate tissue healing processes. In addition, the carrier can be formulated to have particular structure to facilitate cellular infiltration and growth. In this review, we present a summary of growth factor delivery carrier systems for bone and cartilage tissue engineering. Firstly, we describe a list of growth factors implicated in repair and regeneration of bone and cartilage by addressing their biological effects at different stages of the healing process. General requirements for localized growth factor delivery carriers are then discussed. We also provide selective examples of material types (natural and synthetic polymers, inorganic materials, and their composites) and fabricated forms of the carrier (porous scaffolds, microparticles, and hydrogels), highlighting the dose-dependent efficacy, release kinetics, animal models, and restored tissue types. Extensive discussion on issues involving currently investigated carriers for bone and cartilage tissue engineering approaches may illustrate future paths toward the development of an ideal growth factor delivery system.

Computed tomography-based tissue-engineered scaffolds in craniomaxillofacial surgery

INTRODUCTION

Tissue engineering provides an alternative modality allowing for decreased morbidity of donor site grafting and decreased rejection of less compatible alloplastic tissues.

METHODS

Using image-based design and computer software, a precisely sized and shaped scaffold for osseous tissue regeneration can be created via selective laser sintering. Polycaprolactone has been used to create a condylar ramus unit (CRU) scaffold for application in temporomandibular joint reconstruction in a Yucatan minipig animal model. Following sacrifice, micro-computed tomography and histology was used to demonstrate the efficacy of this particular scaffold design.

RESULTS

A proof-of-concept surgery has demonstrated cartilaginous tissue regeneration along the articulating surface with exuberant osseous tissue formation. Bone volumes and tissue mineral density at both the 1 and 3 month time points demonstrated significant new bone growth interior and exterior to the scaffold.

CONCLUSION

Computationally designed scaffolds can support masticatory function in a large animal model as well as both osseous and cartilage regeneration. Our group is continuing to evaluate multiple implant designs in both young and mature Yucatan minipig animals.

Biodegradable polymeric scaffolds. Improvements in bone tissue engineering through controlled drug delivery

Recent advances in biology, medicine, and engineering have led to the discovery of new therapeutic agents and novel materials for the repair of large bone defects caused by trauma, congenital defects, or bone tumors. These repair strategies often utilize degradable polymeric scaffolds for the controlled localized delivery of bioactive molecules to stimulate bone ingrowth as the scaffold degrades. Polymer composition, hydrophobicity, crystallinity, and degradability will affect the rate of drug release from these scaffolds, as well as the rate of tissue ingrowth. Accordingly, this chapter examines the wide range of synthetic degradable polymers utilized for osteogenic drug delivery. Additionally, the therapeutic proteins involved in bone formation and in the stimulation of osteoblasts, osteoclasts, and progenitor cells are reviewed to direct attention to the many critical issues influencing effective scaffold design for bone repair.