Long-term delivery enhances in vivo osteogenic efficacy of bone morphogenetic protein-2 compared to short-term delivery

In vivo osteogenic response to different ratios of BMP-2 and VEGF released from a biodegradable porous system

Bone regeneration and vascularization with porous PLGA scaffolds loaded with VEGF (0.35 and 1.75 μg) and BMP-2 (3.5 and 17.5 μg), incorporated in PLGA microspheres, or the combination of either dose of BMP-2 with the low dose of VEGF were investigated in an intramedullary femur defect in rabbits. The system was designed to control growth factor (GF) release and maintain the GFs localized within the defect. An incomplete release was observed in vitro whereas in vivo VEGF and BMP-2 were totally delivered during 3 and 4 weeks, respectively. A weak synergistic effect of the dual delivery of VEGF and BMP-2 (high dose) was found by 4 weeks. However, the absence of an apparent synergistic long-term effect (12 weeks) of the combination over BMP-2 alone suggests that more work has to be done to optimize VEGF dose, sequential presentation, and the ratio of the two GFs to obtain a beneficial bone repair response.

Bone marrow-derived heparan sulfate potentiates the osteogenic activity of bone morphogenetic protein-2 (BMP-2)

Lowering the efficacious dose of bone morphogenetic protein-2 (BMP-2) for the repair of critical-sized bone defects is highly desirable, as supra-physiological amounts of BMP-2 have an increased risk of side effects and a greater economic burden for the healthcare system. To address this need, we explored the use of heparan sulfate (HS), a structural analog of heparin, to enhance BMP-2 activity. We demonstrate that HS isolated from a bone marrow stromal cell line (HS-5) and heparin each enhances BMP-2-induced osteogenesis in C2C12 myoblasts through increased ALP activity and osteocalcin mRNA expression. Commercially available HS variants from porcine kidney and bovine lung do not generate effects as great as HS5. Heparin and HS5 influence BMP-2 activity by (i) prolonging BMP-2 half-life, (ii) reducing interactions between BMP-2 with its antagonist noggin, and (iii) modulating BMP2 distribution on the cell surface. Importantly, long-term supplementation of HS5 but not heparin greatly enhances BMP-2-induced bone formation in vitro and in vivo. These results show that bone marrow-derived HS effectively supports bone formation, and suggest its applicability in bone repair by selectively facilitating the delivery and bioavailability of BMP-2.

Mineral coatings modulate β-TCP stability and enable growth factor binding and release

β-Tricalcium phosphate (β-TCP) is an attractive ceramic for bone tissue repair because of its similar composition to bone mineral and its osteoconductivity. However, compared with other ceramics β-TCP has a rapid and uncontrolled rate of degradation. In the current study β-TCP granules were mineral coated with the aim of influencing the dissolution rate of β-TCP, and also to use the coating as a carrier for controlled release of the growth factors recombinant human vascular endothelial growth factor (rhVEGF), modular VEGF peptide (mVEGF), and modular bone morphogenetic protein 2 peptide (mBMP2). The biomineral coatings were formed by heterogeneous nucleation in aqueous solution using simulated body fluid solutions with varying concentrations of bicarbonate (HCO(3)). Our results demonstrate that we could coat β-TCP granules with mineral layers possessing different dissolution properties. The presence of a biomineral coating delays the dissolution rate of the β-TCP granules. As the carbonate (CO(3)(2-)) content in the coating was increased the dissolution rate of the coated β-TCP also increased, but remained slower than the dissolution of uncoated β-TCP. In addition, we showed sustained release of multiple growth factors, with release kinetics that were controllable by varying the identity of the growth factor or the CO(3)(2-) content in the mineral coating. Released rhVEGF induced human umbilical vein endothelial cell (HUVEC) proliferation, and mVEGF enhanced migration of mouse embryonic endothelial cells in a scratch wound healing assay, indicating that each released growth factor was biologically active.

Rapid Heterotrophic Ossification with Cryopreserved Poly(ethylene glycol-) Microencapsulated BMP2-Expressing MSCs

Autologous bone grafting is the most effective treatment for long-bone nonunions, but it poses considerable risks to donors, necessitating the development of alternative therapeutics. Poly(ethylene glycol) (PEG) microencapsulation and BMP2 transgene delivery are being developed together to induce rapid bone formation. However, methods to make these treatments available for clinical applications are presently lacking. In this study we used mesenchymal stem cells (MSCs) due to their ease of harvest, replication potential, and immunomodulatory capabilities. MSCs were from sheep and pig due to their appeal as large animal models for bone nonunion. We demonstrated that cryopreservation of these microencapsulated MSCs did not affect their cell viability, adenoviral BMP2 production, or ability to initiate bone formation. Additionally, microspheres showed no appreciable damage from cryopreservation when examined with light and electron microscopy. These results validate the use of cryopreservation in preserving the viability and functionality of PEG-encapsulated BMP2-transduced MSCs.

Improved spinal fusion efficacy by long-term delivery of bone morphogenetic protein-2 in a rabbit model

BACKGROUND AND PURPOSE

Various new delivery systems for recombinant human bone morphogenetic protein-2 (rhBMP-2) have been introduced to improve its efficacy in osteogenesis. Of these, we have previously developed heparin-conjugated PLGA nanospheres (HCPN) as a long-term delivery system for BMP-2. In vitro studies have shown that the BMP-2 long-term delivery system enhances the level of bone formation. However, the long-term effects of BMP-2 on spinal fusion have not been assessed. Therefore, we now tested the hypothesis that the long-term delivery of BMP-2 using HCPN improves spinal fusion compared to short-term delivery in a rabbit fusion model.

METHODS

24 adult New Zealand White rabbits underwent posterolateral fusion (6 animals in 4 groups). The autograft group received an autologous iliac chip bone graft as a positive control. The BMP-2-PN group received rhBMP-2 (20 μg per implant) and PLGA nanospheres (PN) suspended in fibrin gel, and served as a short-term release group. The HCPN group received HCPN suspended in fibrin gel without BMP-2 as a negative control. The BMP-2-HCPN group received rhBMP-2 (20 μg per implant)-bound HCPN suspended in fibrin gel and served as a long-term release group. All animals were killed 12 weeks after surgery. Manual palpation, axial tensile tests, radiography, and histological evaluations were then performed.

RESULTS

The spinal fusion rate and Young's modulus of the fusion mass were better in the BMP-2 long-term delivery group than in the short-term delivery group at an equivalent dose. However, the outcome of the long-term delivery was inferior to that of the autograft group.

INTERPRETATION

The HCPN system showed potential as an effective carrier that might improve the osteogenic efficacy of BMP-2 for spinal fusion.

Functional restoration of critically sized segmental defects with bone morphogenetic protein-2 and heparin treatment

BACKGROUND

Bone defects and fracture nonunions remain a substantial challenge for clinicians. Grafting procedures are limited by insufficient volume and donor site morbidity. As an alternative, biomaterial scaffolds functionalized through incorporation of growth factors such as bone morphogenetic proteins (BMPs) have been developed and appear to regenerate the structure and function of damaged or degenerated skeletal tissue.

OBJECTIVES/PURPOSES

Our objectives were therefore to determine whether: (1) the addition of heparin alone to collagen scaffolds sufficed to promote bone formation in vivo; (2) collagen-heparin scaffold improved BMP-mediated bone regeneration; and (3) precomplexed heparin and BMP-2 delivered on collagen scaffold could restore long bone biomechanical strength.

METHODS

We created bilateral surgical defects in the femora of 20 rats and filled the defects with PCL scaffolds with one of five treatments: collagen matrix (n = 5), collagen/heparin matrix (n = 7), collagen matrix + BMP-2 (n = 9), collagen/heparin matrix + BMP-2 (n = 9), or collagen matrix + BMP-2/heparin complex (n = 9). Bone formation was observed with radiographs and micro-CT analysis and biomechanical testing was used to assess strength.

RESULTS

The addition of heparin alone to collagen did not promote bone ingrowth and the addition of heparin to collagen did not improve BMP-mediated bone regeneration. Delivery of precomplexed BMP-2 and heparin in a collagen matrix resulted in new bone formation with mechanical properties similar to those of intact bone.

CLINICAL RELEVANCE

Our findings suggest delivery of precomplexed BMP-2 and heparin may be an advantageous strategy for treatment of clinically challenging bone defects.

Biodegradable composite scaffolds incorporating an intramedullary rod and delivering bone morphogenetic protein-2 for stabilization and bone regeneration in segmental long bone defects

In this study, a two-part bone tissue engineering scaffold was investigated. The scaffold consists of a solid poly(propylene fumarate) (PPF) intramedullary rod for mechanical support surrounded by a porous PPF sleeve for osseointegration and delivery of poly(dl-lactic-co-glycolic acid) (PLGA) microspheres with adsorbed recombinant human bone morphogenetic protein-2 (rhBMP-2). Scaffolds were implanted into critical size rat segmental femoral defects with internal fixation for 12 weeks. Bone formation was assessed throughout the study via radiography, and following euthanasia, via microcomputed tomography and histology. Mechanical stabilization was evaluated further via torsional testing. Experimental implant groups included the PPF rod alone and the rod with a porous PPF sleeve containing PLGA microspheres with 0, 2 or 8 μg of rhBMP-2 adsorbed onto their surface. Results showed that presence of the scaffold increased mechanical stabilization of the defect, as evidenced by the increased torsional stiffness of the femurs by the presence of a rod compared to the empty defect. Although the presence of a rod decreased bone formation, the presence of a sleeve combined with a low or high dose of rhBMP-2 increased the torsional stiffness to 2.06 ± 0.63 and 1.68 ± 0.56 N·mm, respectively, from 0.56 ± 0.24 N·mm for the rod alone. The results indicate that, while scaffolds may provide structural support to regenerating tissues and increase their mechanical properties, the presence of scaffolds within defects may hinder overall bone formation if they interfere with cellular processes.

The effect of immobilization of heparin and bone morphogenic protein-2 to bovine bone substitute on osteoblast-like cell's function

PURPOSE

This study was performed to investigate the ability of recombinant human-bone morphogenic protein-2 immobilized on a heparin-grafted bone substrate to enhance the osteoblastic functions.

MATERIALS AND METHODS

The Bio-Oss®, not coated with any material, was used as a control group. In rhBMP-2-Bio-Oss® group, rhBMP-2 was coated with Bio-Oss® using only deep and dry methods (50 ng/mL, 24 h). In heparinized rhBMP-2-Bio-Oss® group, dopamine was anchored to the surface of Bio-Oss®, and coated with heparin. rhBMP-2 was immobilized onto the heparinized- Bio-Oss® surface. The release kinetics of the rhBMP-2-Bio-Oss® and heparinized rhBMP-2-Bio-Oss® were analyzed using an enzyme-linked immunosorbent assay. The biological activities of the MG63 cells on the three groups were investigated via cytotoxicity assay, cell proliferation assay, alkaline phosphatase (ALP) measurement, and calcium deposition determination. Statistical comparisons were carried out by one-way ANOVA test. Differences were considered statistically significant at ^*P<.05 and ^**P<.001.

RESULTS

The heparinized rhBMP-2-Bio-Oss® showed more sustained release compared to the rhBMP-2-Bio-Oss® over an extended time. In the measurement of the ALP activity, the heparinized group showed a significantly higher ALP activity when compared with the non-heparinized groups (P<.05). The MG63 cells cultivated in the group with rhBMP-2 showed increased calcium deposition, and the MG63 cells from the heparinized group increased more than those that were cultivated in the non-heparinized groups.

CONCLUSION

Heparin increased the rhBMP-2 release amount and made sustained release possible, and heparinized Bio-Oss® with rhBMP-2 successfully improved the osteoblastic functions.

Apatite-coated collagen scaffold for bone morphogenetic protein-2 delivery

Bone morphogenetic proteins (BMPs) are the most potent osteoinductive growth factors. BMP-2 is clinically used for spine fusion and bone fracture healing. Commercially available BMP-2 uses a type I collagen scaffold as a carrier, but it only releases BMP-2 for a short period of time, which may release the bone formation efficacy. In the present study, we hypothesize that apatite coating of a collagen scaffold increases the release period as well as the osteogenic efficacy of BMP-2. Apatite coating was achieved by incubating collagen scaffolds in simulated body fluids (SBFs). Apatite coating on collagen scaffolds was confirmed by X-ray diffraction, electron spectroscopy for chemical analysis, attenuated total reflectance-Fourier transform infrared spectroscopy, and scanning electron microscopy. The rate and period of BMP-2 release from apatite-coated collagen scaffolds varied depending on the concentration of SBFs used. The 5× and 10× SBF apatite-coated collagen scaffolds released 91.8%±11.5% and 82.2%±13.1% of their loaded BMP-2 over 13 days in vitro, respectively, whereas noncoated collagen scaffold released 98.3%±2.2% over the initial one day. BMP-2 released from apatite-coated collagen scaffold significantly increased the alkaline phosphatase activity of cultured osteoblasts, compared with BMP-2 released from noncoated collagen scaffold. Computed tomography and histomorphometry showed that BMP-2 delivery using apatite-coated collagen scaffolds resulted in 2.5-fold higher bone formation volume and 4.0-fold higher bone formation area than BMP-2 delivery using noncoated collagen scaffolds. This study shows that simple apatite coating of a collagen scaffold results in a BMP-2 carrier that renders long-term release of BMP-2 and dramatically enhances osteogenic efficacy.

Tunable dual growth factor delivery from polyelectrolyte multilayer films

A promising strategy to accelerate joint implant integration and reduce recovery time and failure rates is to deliver a combination of certain growth factors to the integration site. There is a need to control the quantity of growth factors delivered at different times during the healing process to maximize efficacy. Polyelectrolyte multilayer (PEM) films, built using the layer-by-layer (LbL) technique, are attractive for releasing controlled amounts of potent growth factors over a sustained period. Here, we present PEM films that sequester physiological amounts of osteogenic rhBMP-2 (recombinant human bone morphogenetic protein-2) and angiogenic rhVEGF₁₆₅ (recombinant human vascular endothelial growth factor) in different ratios in a degradable [poly(β-amino ester)/polyanion/growth factor/polyanion] LbL tetralayer repeat architecture where the biologic load scaled linearly with the number of tetralayers. No burst release of either growth factor was observed as the films degraded. The release of rhBMP-2 was sustained over a period of 2 weeks, while rhVEGF₁₆₅ eluted from the film over the first 8 days. Both growth factors retained their efficacy, as quantified with relevant in vitro assays. rhBMP-2 initiated a dose dependent differentiation cascade in MC3T3-E1S4 pre-osteoblasts while rhVEGF₁₆₅ upregulated HUVEC proliferation, and accelerated closure of a scratch in HUVEC cell cultures in a dose dependent manner. In vivo, the mineral density of ectopic bone formed de novo by rhBMP-2/rhVEGF₁₆₅ PEM films was approximately 33% higher than when only rhBMP-2 was introduced, with a higher trabecular thickness, which would indicate a decrease in the risk of osteoporotic fracture. Bone formed throughout the scaffold when both growth factors were released, which suggests more complete remodeling due to an increased local vascular network. This study demonstrates a promising approach to delivering precise doses of multiple growth factors for a variety of implant applications where control over spatial and temporal release profile of the biologic is desired.

Future of local bone regeneration - Protein versus gene therapy

The most promising attempts to achieve bone regeneration artificially are based on the application of mediators such as bone morphogenetic proteins (BMPs) directly to the deficient tissue site. BMPs, as promoters of the regenerative process, have the ability to induce de novo bone formation in various tissues, and many animal models have demonstrated their high potential for ectopic and orthotopic bone formation. However, the biological activity of the soluble factors that promote bone formation in vivo is limited by diffusion and degradation, leading to a short half-life. Local delivery remains a problem in clinical applications. Several materials, including hydroxyapatite, tricalcium phosphate, demineralised bone matrices, poly-lactic acid homo- and heterodimers, and collagen have been tested as carriers and delivery systems for these factors in a sustained and appropriate manner. Unfortunately these delivery vehicles often have limitations in terms of biodegradability, inflammatory and immunological rejection, disease transmission, and most importantly, an inability to provide a sustained, continuous release of these factors at the region of interest. In coping with these problems, new approaches have been established: genes encoding these growth factor proteins can be delivered to the target cells. In this way the transfected cells serve as local "bioreactors", as they express the exogenous genes and secrete the synthesised proteins into their vicinity. The purpose of this review is to present the different methods of gene versus growth factor delivery in tissue engineering. Our review focuses on these promising and innovative methods that are defined as regional gene therapy and provide an alternative to the direct application of growth factors. Various advantages and disadvantages of non-viral and viral vectors are discussed. This review identifies potential candidate genes and target cells, and in vivo as well as ex vivo approaches for cell transduction and transfection. In explaining the biological basis, this paper also refers to current experimental and clinical applications.

Tissue engineering approaches for bone repair: concepts and evidence

Over the last decades, the medical world has advanced dramatically in the understanding of fracture repair. The three components needed for fracture healing are osteoconduction, osteoinduction and osteogenesis. With newly designed scaffolds, ex vivo produced growth factors and isolated stem cells, most of the challenges of critical size bone defects have been resolved in vitro, and in some cases in animal models as well. However, there are still challenges needed to be overcome before these technologies can be fully converted from the bench to the bedside. These technological and biological advancements need to be converted to mass production of affordable products that can be used in every part of the world. Vascularity, full substation of scaffolds by native bone, and bio-safety are the three most critical steps to be challenged before reaching the clinical setting.

Osteogenic activities of MC3T3-E1 cells on heparin-immobilized poly(caprolactone) membranes

The aim of this study was to investigate the effects of heparin on the activity of osteoblast-like cells seeded on poly(caprolactone) (PCL) membranes. The membranes were prepared by solvent-casting technique in ~150 µm thickness. Then they were treated with 1,6-hexanediamine solution and functionalized with covalently bound heparin. The morphology, proliferation, and differentiation of MC3T3-E1 preosteoblasts on these membranes were investigated in vitro. The heparin functionalized PCL membranes, compared to non-functionalized membranes, significantly stimulated osteoblast proliferation. The Scanning electron microscope images confirmed the stimulative effect of covalently bound heparin on the osteoblast-like cell proliferation. The alkaline phosphatase and osteocalcin levels for cells proliferated on heparin containing PCL membranes were higher than that of nonfunctionalized membranes.

Hyaline cartilage regeneration by combined therapy of microfracture and long-term bone morphogenetic protein-2 delivery

Microfracture of cartilage induces migration of bone-marrow-derived mesenchymal stem cells. However, this treatment often results in fibrocartilage regeneration. Growth factors such as bone morphogenetic protein (BMP)-2 induce the differentiation of bone-marrow-derived mesenchymal stem cells into chondrocytes, which can be used for hyaline cartilage regeneration. Here, we tested the hypothesis that long-term delivery of BMP-2 to cartilage defects subjected to microfracture results in regeneration of high-quality hyaline-like cartilage, as opposed to short-term delivery of BMP-2 or no BMP-2 delivery. Heparin-conjugated fibrin (HCF) and normal fibrin were used as carriers for the long- and short-term delivery of BMP-2, respectively. Rabbit articular cartilage defects were treated with microfracture combined with one of the following: no treatment, fibrin, short-term delivery of BMP-2, HCF, or long-term delivery of BMP-2. Eight weeks after treatment, histological analysis revealed that the long-term delivery of BMP-2 group (microfracture + HCF + BMP-2) showed the most staining with alcian blue. A biochemical assay, real-time polymerase chain reaction assay and Western blot analysis all revealed that the long-term delivery of BMP-2 group had the highest glucosaminoglycan content as well as the highest expression level of collagen type II. Taken together, the long-term delivery of BMP-2 to cartilage defects subjected to microfracture resulted in regeneration of hyaline-like cartilage, as opposed to short-term delivery or no BMP-2 delivery. Therefore, this method could be more convenient for hyaline cartilage regeneration than autologous chondrocyte implantation due to its less invasive nature and lack of cell implantation.

The efficacy of bone morphogenetic protein-2 depends on its mode of delivery

Bone morphogenetic protein-2 (BMP-2) induces bone regeneration in a dose-dependent manner, with higher doses of BMP-2 inducing greater bone formation. Previously, we showed that long-term delivery of BMP-2 provides better ectopic bone formation than short-term delivery of an equivalent dose. In the present study, we investigated the efficacy of orthotopic bone formation over a range of BMP-2 doses, using different delivery modes. Heparin-conjugated poly(lactic-co-glycolic acid) nanospheres suspended in fibrin gel were used as a long-term delivery system, and fibrin gel was used as a short-term delivery system. Different doses of BMP-2 were delivered to mouse calvarial defects using either long-term or short-term delivery systems. Eight weeks after treatment, bone regeneration was evaluated by histomorphometry. For both delivery systems, bone regeneration increased as the BMP-2 dose increased up to 1 µg and did not increase beyond this dose. Importantly, at BMP-2 doses higher than 1 µg, long-term delivery resulted in much greater bone formation than short-term delivery. This study shows that long-term delivery of BMP-2 is more effective at enhancing orthotopic bone formation than short-term delivery over a range of doses.

Experimental construction of BMP2 and VEGF gene modified tissue engineering bone in vitro

The purpose of this study was to investigate the feasibility and advantages of constructing a novel tissue engineering bone, using β-tricalcium phosphate (β-TCP) and rat bone marrow mesenchymal stem cells (MSCs), modified with human bone morphogenetic protein 2 gene (hBMP2) and human vascular endothelial growth factor 165 gene (hVEGF165), through lentiviral transfection. Both genes were successfully co-expressed in the co-transfection group for up to eight weeks confirmed by enzyme-linked immunosorbent assay (ELISA). After seeding MSCs onto the scaffolds, scanning electron microscopy (SEM) observation showed that MSCs grew and proliferated well in co-transfection group at 7 and 14 days. There was no significant difference among all the groups in hoechst DNA assay for cell proliferation for 14 days after cell seeding (P > 0.05), but the highest alkaline phosphatase (ALP) activity was observed in the co-transfection group at 14 days after cell seeding (p < 0.01). These results demonstrated that it was advantageous to construct tissue engineering bone using β-TCP combined with MSCs lentivirally co-transfected with BMP2 and VEGF165, providing an innovative way for treating bone defects.

Enhancement of human peripheral blood mononuclear cell transplantation-mediated bone formation

Recent studies have demonstrated the existence of osteoblast progenitor cells in circulating blood. Here we show that local delivery of bone morphogenetic protein-2 (BMP-2) to cell transplantation sites induces in situ osteogenic differentiation of transplanted human peripheral blood mononuclear cells (PBMNCs) and enhances in vivo bone formation mediated by PBMNC transplantation. Human PBMNCs were seeded on scaffolds with or without BMP-2 and implanted subcutaneously into athymic mice. Nonseeded scaffolds with BMP-2 were also implanted. Eight weeks later, radiographic and histological analyses showed that the PBMNC + BMP-2 group had undergone much more extensive bone formation than either the PBMNC group or BMP-2 group. Only the PBMNC + BMP-2 group expressed human Cbfa1, osteonectin, and osteocalcin, suggesting in situ osteogenic differentiation of and bone formation by transplanted human PBMNCs, while the other groups did not express these genes. This study provides a method to enhance human PBMNC transplantation-mediated bone formation.

Evaluation of an injectable silk fibroin enhanced calcium phosphate cement loaded with human recombinant bone morphogenetic protein-2 in ovine lumbar interbody fusion

The objective of this study was to investigate the efficacy of an injectable calcium phosphate cement/silk fibroin/human recombinant bone morphogenetic protein-2 composite (CPC/SF/rhBMP-2) in an ovine interbody fusion model. Twenty-four mature sheep underwent anterior lumbar interbody fusion at the levels of L1/2, L3/4, and L5/6 with random implantation of CPC/SF, CPC/rhBMP-2, CPC/SF/rhBMP-2, or autogenous iliac bone. After the sheep were sacrificed, the fusion segments were evaluated by manual palpation, CT scan, undestructive biomechanical testing, undecalcified histology, and histomorphology. The fusion rates of CPC/SF/rhBMP-2 were 55.56% and 77.78% at 6 and 12 months, respectively. The fusion was superior to all the biomaterial grafts in stiffness, and reached the same stiffness as the autograft at 12 months. The new bone formation was less than autograft at 6 months, but similar with that at 12 months. However, the ceramic residue volume of CPC/SF/rhBMP-2 was significantly decreased compared with CPC/SF and CPC/rhBMP-2 at both times. The results indicated that CPC/SF/rhBMP-2 composite had excellent osteoconduction and osteoinduction, and balanced degradation and osteogenesis.

Effect of encapsulation or grafting on release kinetics of recombinant human bone morphogenetic protein-2 from self-assembled poly(lactide-co-glycolide ethylene oxide fumarate) nanoparticles

The objective of this work was to compare the release characteristics of Recombinant human bone morphogenetic protein-2 (rhBMP-2) encapsulated in thermally self-assembled poly(lactide ethylene oxide fumarate) (PLEOF) nanoparticles (NPs) with rhBMP-2 grafted to succinimide-terminated poly(lactide fumarate) (PLAF-NHS) or poly(lactide-co-glycolide fumarate) (PLGF-NHS) NPs. The amphiphilic PLEOF NPs had average size of 110 +/- 50 nm. The hydrophobic PLAF-NHS and PLGF-NHS NPs had average size of 242 +/- 67 and 195 +/- 42 nm, respectively. PLEOF NPs had rhBMP-2 encapsulation efficiency ranging from 65 to 93%. Grafting efficiency of rhBMP-2 to PLAF-NHS and PLGF-NHS NPs was 97% +/- 1% and 98% +/- 1%, respectively. PLEOF NPs displayed a relatively high-release rate of rhBMP-2 in the first week, which rapidly dropped to zero after 10 days. PLEOF NPs grafted with 10 and 20 microg/mL rhBMP-2 released 67 and 80% of the protein in the active conformation after degradation. PLGF-NHS NPs displayed sustained release of rhBMP-2 in the first 2 weeks but dropped to almost zero rate (< 3 ng/day) after 20 days. PLAF-NHS NPs showed the longest period of sustained release of active rhBMP-2 at two rates: a high rate of 25-35 ng/mL in the first 2 weeks followed by a low rate of 5-10 ng/mL from 2 to 6 weeks. Nearly, 25 and 50% of the rhBMP-2 released from PLGF-NHS and PLAF-NHS NPs, respectively, were enzymatically active after degradation of the NPs. PLEOF NPs provided a fast release of rhBMP-2 for 1 week, whereas PLAF-NHS NPs provided a slow release for up to 6 weeks.

The effect of immobilization of heparin and bone morphogenic protein-2 (BMP-2) to titanium surfaces on inflammation and osteoblast function

The aim of this study was to investigate biologic function of bone morphorgenic protein-2 (rhBMP-2) immobilized on the heparin-grafted Ti surface. Ti surfaces were first modified by 3-aminopropyltriethoxysilane (ATPES), followed by grafting of heparin. BMP-2 was then immobilized on the heparin-grafted Ti surfaces. Pristine Ti and functionalized Ti surfaces were characterized by X-ray photoelectron spectroscopy (XPS), measurement of water contact angles, and protein adsorption. The biological activity of MG-63 cells on pristine and functionalized Ti surfaces was investigated by cell proliferation assays, measurement of alkaline phosphate (ALP) activity, and determination of calcium deposition. Anti-inflammatory effects were assessed by RT-PCR to measure the transcript levels of IL-6 and TNF-α. XPS revealed that heparin and BMP-2 were successfully grafted and immobilized on the Ti surfaces, respectively. In addition, Ti surfaces with BMP-2 immobilized were more hydrophilic than pristine Ti. Furthermore, BMP-2 immobilized Ti promoted significantly higher ALP activity and calcium deposition by MG-63 cells than pristine Ti. The inflammatory response was also decreased when cells were grown on heparin-grafted, BMP-2-immobilized Ti surfaces. The results of this study suggest that by grafting heparin and immobilizing BMP-2 on Ti surfaces, inflammation can be inhibited and osteoblast function promoted.

Fibrin as a delivery system for therapeutic drugs and biomolecules

Fibrin is a natural biopolymer involved in the coagulation cascade. It acts as a reservoir for growth factors, cells, and enzymes during wound healing and provides a scaffold for the synthesis of extracellular matrix. Thus, the use of fibrin has expanded in recent years from traditional use as a sealant for surgical applications, to a tissue engineering scaffold capable of providing nature's cues for tissue regeneration. This paper reviews the advantageous biological aspects of fibrin, the history of the scaffold material, and its present role in the delivery of drugs, growth factors, cells, and gene vectors. Examples are given of studies where the structure and form of the scaffold have been manipulated for optimal release of the therapeutic agent, optimal cellular activity, and investigation into stem cell differentiation. It is evident from the body of literature presented that the benefits of fibrin are being exploited for a vast range of tissue engineering applications and that fibrin remains a key scaffold material for the delivery of drugs and biomolecules.

Heparin-conjugated fibrin as an injectable system for sustained delivery of bone morphogenetic protein-2

Long-term release of bone morphogenetic protein-2 (BMP-2) can promote bone regeneration. We developed an injectable system for long-term delivery of BMP-2 by covalently conjugating heparin to fibrinogen. The heparin-conjugated fibrinogen formed an injectable, heparin-conjugated fibrin (HCF) gel when mixed with thrombin. HCF released 89.4 +/- 3.8% of the loaded BMP-2 for 13 days, whereas normal fibrin released 83.7 +/- 7.6% for the initial 3 days. BMP-2 released from HCF significantly increased alkaline phosphatase activity of cultured osteoblasts, whereas BMP-2 released from normal fibrin did not do so, indicating that BMP-2 released from HCF is bioactive and suggesting that long-term delivery of BMP-2 is advantageous over short-term delivery for bone regeneration. HCF, BMP-2-loaded HCF, and BMP-2-loaded normal fibrin containing free heparin were contained in polyester cylindrical tubes and implanted into the hind limb muscle pockets of rats for 8 weeks. Soft X-ray radiography, computed tomography, histomorphometry, calcium assay, and western blot analysis showed that BMP-2-loaded HCF yielded the most extensive bone formation among the groups. Since HCF can deliver BMP-2 over a long term, is an injectable system, and is made of clinically benign materials, this system would have advantages for clinical applications to regenerate bone.

Enhanced initial bone regeneration with inorganic polyphosphate-adsorbed hydroxyapatite

Inorganic polyphosphate (poly(P)) can promote binding between fibroblast growth factors and their receptors and enhance osteoblastic cell differentiation and calcification. This study evaluated the possibilities for poly(P) adsorbed onto interconnected porous calcium hydroxyapatite (IP-CHA) as a new bone regeneration material. Prepared 1%, 5%, 25% and 50% poly(P)/IP-CHA composites showed the elution peak of poly(P) between 15 and 20 min, respectively, with the highest value from 50% poly(P)/IP-CHA in vitro. Histologically, at 1 week of placement into the femur of rabbits, granulation tissue had penetrated into the pores in all composites and IP-CHA as a control. In contrast, at 2 weeks of placement, newly formed lamellar bone was found in all groups, although a higher amount of bone regeneration was obviously formed in the 25% and 50% poly(P)/IP-CHA with a significantly higher value of bone regeneration ratio of 50% poly(P)/IP-CHA. These results indicate that 25% and 50% poly(P)/IP-CHA composites may enhance initial bone regeneration.

Sequential BMP-2/BMP-7 delivery from polyester nanocapsules

The aim of this study was to develop a nanosized, controlled growth factor release system to incorporate into tissue engineering scaffolds and thus activate the cells seeded in the scaffold. Nanocapsules of poly(lactic acid-co-glycolic acid) (PLGA) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) were loaded with the bone morphogenetic proteins BMP-2 and BMP-7, respectively, and with bovine serum albumin (BSA), the model protein. BSA-loading efficiency and release kinetics were used to determine the most appropriate nanocapsule pair to achieve the delivery of growth factors in a sequential manner, as occurs in natural processes. BSA-encapsulation efficiency was highest when the polymer concentration used in the preparation of PLGA and PHBV nanocapsules was 10% (w/v) (84.75% and 16.72%, respectively). Release of BSA was faster from PLGA than it was from PHBV. Based on the encapsulation efficiency and release data, 10% PLGA and 10% PHBV nanocapsules were chosen to provide the early BMP-2 and later BMP-7 release, respectively. Simultaneous, sequential delivery and individual release of the BMPs were studied for 7, 14, and 21 days, using rat bone marrow mesenchymal stem cells. Individual BMP-2 release suppressed cell proliferation while providing higher alkaline phosphatase activity with respect to BMP-7. The sequential delivery of BMP-2 and BMP-7 provided slightly lower proliferation than did simultaneous delivery, but the highest alkaline phosphatase activity of all indicated a synergistic effect on the osteogenic differentiation of mesenchymal stem cells caused by the use of the two growth factors in a sequential fashion.

Bone morphogenic protein-2 (BMP-2) loaded nanoparticles mixed with human mesenchymal stem cell in fibrin hydrogel for bone tissue engineering

Recent tissue engineering efforts have been focused on the use of natural or synthetic matrices which combine the characteristics of biodegradable properties with those of protein delivery vehicles, allowing for implanted cell actions and enhanced tissue regeneration. The principal objective of this study was to assess the feasibility of ectopic bone formation in a three-dimensional fibrin construct mixed with bone morphogenic protein-2 (BMP-2) loaded in nano-carriers for the osteogenic differentiation of human mesenchymal stem cells (hMSCs). The results of our evaluation showed that the osteogenic differentiation of hMSCs embedded in the fibrin construct was affected significantly by the stimulation of growth factors loaded in nanoparticles. When the osteoinduction activity of hMSCs in fibrin construct was evaluated in an in vitro test followed by RT-PCR, real time-QPCR, Western blotting, histological and immunohistochemical examinations, significant homogeneous bone formation was observed histologically throughout the fibrin construct containing the growth factor (BMP-2) loaded into the nanoparticles. With the above detection techniques, the BMP-2-loaded nanoparticles encapsulated in fibrin constructs evidenced more potent effects of hMSCs on bone regeneration as compared to the control or BMP-2 loaded fibrin constructs without nanoparticles. In the current study, we conclude that fibrin constructs containing BMP-2 loaded nanoparticles will be a promising method by which bone regeneration can be enhanced.

Bone tissue engineering: a review in bone biomimetics and drug delivery strategies

Critical-sized defects in bone, whether induced by primary tumor resection, trauma, or selective surgery have in many cases presented insurmountable challenges to the current gold standard treatment for bone repair. The primary purpose of a tissue-engineered scaffold is to use engineering principles to incite and promote the natural healing process of bone which does not occur in critical-sized defects. A synthetic bone scaffold must be biocompatible, biodegradable to allow native tissue integration, and mimic the multidimensional hierarchical structure of native bone. In addition to being physically and chemically biomimetic, an ideal scaffold is capable of eluting bioactive molecules (e.g., BMPs, TGF-betas, etc., to accelerate extracellular matrix production and tissue integration) or drugs (e.g., antibiotics, cisplatin, etc., to prevent undesired biological response such as sepsis or cancer recurrence) in a temporally and spatially controlled manner. Various biomaterials including ceramics, metals, polymers, and composites have been investigated for their potential as bone scaffold materials. However, due to their tunable physiochemical properties, biocompatibility, and controllable biodegradability, polymers have emerged as the principal material in bone tissue engineering. This article briefly reviews the physiological and anatomical characteristics of native bone, describes key technologies in mimicking the physical and chemical environment of bone using synthetic materials, and provides an overview of local drug delivery as it pertains to bone tissue engineering is included.

Correlation of computed tomography with histology in the assessment of periprosthetic defect healing

Computed tomography (CT) may more accurately assess the healing of grafted osteolytic lesions around acetabular components compared with plain radiographs, although clinical validation is lacking. To determine whether clinical or micro-CT imaging could assess accurately the grafted lesion compared with histology, we therefore quantified bone healing and ingrowth to determine an effective rhBMP-2 dose and ratio to allograft bone when grafted adjacent to a cementless porous-coated component. We grafted surgically created acetabular defects in canines (n = 20) before uncemented total hip arthroplasty. At 6 weeks, embedded acetabula were imaged and the CT slice images matched to histology section images. The percentage of bone in the defect and growth into the porous surface was assessed quantitatively. Low-dose rhBMP-2 with allograft (1:5 ratio) resulted in a higher percentage of defect healing (43.8%) than rhBMP-2 alone (29.2%) and a higher percentage of bone ingrowth (15.7%) than allograft bone alone (1.1%) as measured by histology. Micro-CT measurements were similar to histologic measurements of defect healing, whereas clinical CT overestimated periprosthetic bone by 38%. Neither clinical CT nor micro-CT techniques are adequate for assessing ingrowth or the bone-implant interface with metal artifacts.

Ultrasound enhances recombinant human BMP-2 induced ectopic bone formation in a rat model

Two methods to improve bone repair include the use of recombinant human bone morphogenetic protein-2 (rhBMP-2) and low-intensity pulsed ultrasound (LIPUS). The present study was designed to determine if LIPUS enhances the effect of rhBMP-2-induced bone formation in a well characterized ectopic implant model. Absorbable collagen sponges loaded with 0-, 1-, 2.5- or 5-microg doses of rhBMP-2 were implanted subcutaneously in 11-week-old, male Long Evans rats, followed by daily 20-min LIPUS or sham LIPUS treatment beginning 1 d after surgery. Explanted sponges were assessed for bone volume, mineral density and mineral content by microcomputed tomography (microCT). At two weeks, LIPUS had no effect on rhBMP-2-induced bone formation, but at four weeks, LIPUS increased bone volume in the 1-microg rhBMP-2-treated implants 117.7-fold (0.02 +/- 0.04 mm(3)vs. 2.07(S.E.M.) +/- 1.67 mm(3);p = 0.028), and 2.3-fold in the 5-microg dose implants (5.96 +/- 3.68 mm(3)vs. 13.52 +/- 6.81 mm(3);p = 0.077) compared with sham LIPUS. Bone mineral density was not affected by LIPUS treatment. Total mineral content followed the same pattern as bone volume. Histologic staining for mineralized tissue was consistent with the microCT observations. The present study is the first to demonstrate that LIPUS enhances bone formation induced by rhBMP-2.

Release characteristics and osteogenic activity of recombinant human bone morphogenetic protein-2 grafted to novel self-assembled poly(lactide-co-glycolide fumarate) nanoparticles

Functionalized biodegradable nanoparticles (NPs) provide reactive groups and large surface area for grafting recombinant human bone morphogenetic protein-2 (rhBMP-2) to reduce protein diffusion and maintain sufficient concentration for recruitment and differentiation of osteoprogenitor cells. The objective of this work was to investigate release characteristics and osteogenic activity of rhBMP-2, grafted to biodegradable NPs based on succinimide-terminated poly(lactide fumarate) (PLAF-NHS) and poly(lactide-co-glycolide fumarate) (PLGF-NHS) macromers. The release of rhBMP-2 from the NPs, measured by enzyme-linked immunosorbent assay, was linear with time in the first two weeks, and 24.70+/-1.30% and 48.7+/-0.7% of the protein grafted to PLGF-NHS and PLAF-NHS NPs, respectively, was released in the enzymatically active conformation after complete degradation/erosion of the NPs. After 14 days of incubation with bone marrow stromal (BMS) cells, rhBMP-2 grafted to PLAF-NHS and PLGF-NHS NPs was as effective in inducing mineralization as the native rhBMP-2 that was directly added to the cell culture media. At any incubation time, rhBMP-2 grafted to PLAF had the highest expression of osteopontin (OP) and osteocalcin (OC), followed by rhBMP-2 grafted to PLGF and rhBMP-2 directly added to media. Higher OP and OC expression for BMP-gPLAF and BMP-gPLGF groups may be related to other factors in the cascade of osteogenesis, such as differentiation of BMS cells to the vasculogenic lineage and formation of a vascularized/mineralized matrix.

Delivery of recombinant bone morphogenetic proteins for bone regeneration and repair. Part A: Current challenges in BMP delivery

Recombinant human bone morphogenetic proteins (rhBMPs) have been extensively investigated for developing therapeutic strategies aimed at the restoration and treatment of orthopaedic as well as craniofacial conditions. In this first part of the review, we discuss the rationale for the necessary use of carrier systems to deliver rhBMP-2 and rhBMP-7 to sites of bone tissue regeneration and repair. General requirements for growth factor delivery systems emphasizing the distinction between localized and release-controlled delivery strategies are presented highlighting the current limitations in the development of an effective rhBMP delivery system applicable in clinical bone tissue engineering.

Nanoparticulate systems for growth factor delivery

The field of nanotechnology, which aims to control and utilize matter generally in 1-100 nm range, has been at the forefront of pharmaceutical development. Nanoparticulate delivery systems, with their potential to control drug release profiles, prolonging the presence of drugs in circulation, and to target drugs to a specific site, hold tremendous promise as delivery strategies for therapeutics. Growth factors are endogenous polypeptides that initiate intracellular signals to regulate cellular activities, such as proliferation, migration and differentiation. With improved understanding of their roles in physiopathology and expansion of their availability through recombinant technologies, growth factors are becoming leading therapeutic candidates for tissue engineering approaches. However, the outcome of growth factor therapeutics largely depends on the mode of their delivery due to their rapid degradation in vivo, and non-specific distribution after systemic administration. In order to overcome these impediments, nanoparticulate delivery systems are being harnessed for spatiotemporal controlled delivery of growth factors. This review presents recent advances and some disadvantages of various nanoparticulate systems designed for effective intact growth factor delivery. The therapeutic applications of growth factors delivered by such systems are reviewed, especially for bone, skin and nerve regeneration as well as angiogenesis. Finally, future challenges and directions in the field are presented in addition to the current limitations.

Sustained release of BMP-2 in a lipid-based microtube vehicle

Sustained release systems have been developed for the use of growth factors in tissue engineering applications. However, many of these systems continue to have limitations associated with low loading efficiencies and reduced biological activity after release. In this paper, we utilized a lipid-based microtube system for the sustained release of BMP-2. The lipid microtubes were fabricated using a self-assembly method, in order to avoid the use of harsh organic solvents that may damage the protein. BMP-2 was loaded into the microtubes by rehydrating dried microtubes in the protein solution. The loading efficiency and release kinetics of BMP-2 in the microtubes were measured using in vitro immunoassays. Loading efficiency was found to be dependent on microtube concentration. The potential for this system to deliver biologically active BMP-2 was assessed using the alkaline phosphatase assay and von Kossa staining on human mesenchymal stem cell cultures. The results demonstrate that the lipid microtube system is able to provide sustained delivery of biologically active BMP-2 and thereby induce osteogenic differentiation.

Current status of gene therapy for rheumatoid arthritis

Gene therapy offers great possibilities for treating rheumatoid arthritis (RA). Traditional surgical and pharmaceutical methods of treating RA have met with limited therapeutic success and have failed to produce a cure, but the past several years have seen extensive progress toward development of a gene therapy for arthritis. Numerous vectors and therapeutic genes have been investigated in animal models of arthritis, and the potential of gene therapy to treat or manage RA has been demonstrated in several clinical studies. Gene therapy offers the possibility of overcoming many of the limitations of current biologic therapies by providing long-term, high-level localized expression of therapeutic genes, potentially in as little as a single dose. In this review, we explore the advances in gene therapy for RA and summarize the recent preclinical and clinical data. In addition, we provide an overview of vectors and targets for RA gene therapy.