Osteogenic effect of local, long versus short term BMP-2 delivery from a novel SPU-PLGA-βTCP concentric system in a critical size defect in rats

Sumecton reinforced gelatin-based scaffolds for cell-free bone regeneration

Bone tissue engineering has risen to tackle the challenges of the current clinical need concerning bone fractures that is already considered a healthcare system problem. Scaffold systems for the repair of this tissue have yielded different combinations including biomaterials with nanotechnology or biological agents. Herein, three-dimensional porous hydrogels were engineered based on gelatin as a natural biomaterial and reinforced with synthetic saponite nanoclays. Scaffolds were biocompatible and shown to enhance the inherent properties of pristine ones, in particular, proved to withstand pressures similar to load-bearing tissues. Studies with murine mesenchymal stem cells found that scaffolds had the potential to proliferate and promote cell differentiation. In vivo experiments were conducted to gain insight about the ability of these cell-free scaffolds to regenerate bone, as well as to determine the role that these nanoparticles in the scaffold could play as a drug delivery system. SDF-1 loaded scaffolds showed the highest percentage of bone formation, which was corroborated by osteogenic markers and new blood vessels. Albeit a first attempt in the field of synthetic nanosilicates, these results suggest that the designed constructs may serve as delivery platforms for biomimetic agents to mend bony defects, circumventing high doses of therapeutics and cell-loading systems.

Engineered dual affinity protein fragments to bind collagen and capture growth factors

Collagen type I lacks affinity for growth factors (GFs) and yet it is clinically used to deliver bone morphogenic protein 2 (BMP-2), a potent osteogenic growth factor. To mitigate this lack of affinity, supra-physiological concentrations of BMP-2 are loaded in collagen sponges leading to uncontrolled BMP-2 leakage out of the material. This has led to important adverse side effects such as carcinogenesis. Here, we design recombinant dual affinity protein fragments, produced in E. Coli, which contain two regions, one that spontaneously binds to collagen and a second one that binds BMP-2. By adding the fragment to collagen sponges, BMP-2 is sequestered enabling solid phase presentation of BMP-2. We demonstrate osteogenesis in vivo with ultra-low doses of BMP-2. Our protein technology enhances the biological activity of collagen without using complex chemistries or changing the manufacturing of the base material and so opens a pathway to clinical translation.

Sustained delivery of a heterodimer bone morphogenetic protein-2/7 via a collagen hydroxyapatite scaffold accelerates and improves critical femoral defect healing

Despite the glimmer of hope provided by the discovery and commercialization of bone morphogenetic protein-2 (BMP-2) as a bone graft substitute, side effects related to the use of supraphysiological doses have hindered its clinical usage. In this study, we compared the osteoinductive potential of BMP-2 homodimer with a heterodimer of BMP-2/7, both delivered via a collagen-hydroxyapatite (CHA) scaffold delivery system, with the aim to reduce the overall therapeutic BMP doses and the associated side-effects. We first show that the incorporation of hydroxyapatite in collagen-based BMP delivery systems is pivotal for achieving efficient BMP sequestration and controlled release. Using an ectopic implantation model, we then showed that the CHA+BMP-2/7 was more osteoinductive than CHA+BMP-2. Further evaluation of the molecular mechanisms responsible for this increased osteoinductivity at an early stage in the regeneration process indicated that the CHA+BMP-2/7 enhanced progenitor cell homing at the implantation site, upregulated the key transcriptomic determinants of bone formation, and increased the production of bone extracellular matrix components. Using fluorescently labelled BMP-2/7 and BMP-2, we demonstrated that the CHA scaffold provided a long-term delivery of both molecules for at least 20 days. Finally, using a rat femoral defect model, we showed that an ultra-low dose (0.5 µg) of BMP-2/7 accelerated fracture healing and performed at a level comparable to 20-times higher BMP-2 dose. Our results indicate that the sustained delivery of BMP-2/7 via a CHA scaffold could bring us a step closer in the quest for the use of physiological growth factor doses in fracture healing. STATEMENT OF SIGNIFICANCE: • Incorporation of hydroxyapatite (HA) in a collagen scaffold dramatically improves bone morphogenic protein (BMP) sequestration via biophysical interactions with BMP, thereby providing more controlled BMP release compared with pristine collagen. • We then investigate the molecular mechanisms responsible for increased osteoinductive potential of a heterodimer BMP-2/7 with is clinically used counterpart, the BMP-2 homodimer. • The superior osteoinductive properties of BMP-2/7 are a consequence of its direct positive effect on progenitor cell homing at the implantation site, which consequently leads to upregulation of cartilage and bone related genes and biochemical markers. • An ultra-low dose of BMP-2/7 delivered via a collagen-HA (CHA) scaffold leads to accelerated healing of a critical femoral defect in rats while a 20-times higher BMP-2 dose was required to achieve comparable results.

Nanoclay-reinforced HA/alginate scaffolds as cell carriers and SDF-1 delivery-platforms for bone tissue engineering

Bone tissue engineering has come on the scene to overcome the difficulties of the current treatment strategies. By combining biomaterials, active agents and growth factors, cells and nanomaterials, tissue engineering makes it possible to create new structures that enhance bone regeneration. Herein, hyaluronic acid and alginate were used to create biologically active hydrogels, and montmorillonite nanoclay was used to reinforce and stabilize them. The developed scaffolds were found to be biocompatible and osteogenic with mMSCs in vitro, especially those reinforced with the nanoclay, and allowed mineralization even in the absence of differentiation media. Moreover, an in vivo investigation was performed to establish the potential of the hydrogels to mend bone and act as cell-carriers and delivery platforms for SDF-1. Scaffolds embedded with SDF-1 exhibited the highest percentages of bone regeneration as well as of angiogenesis, which confirms the suitability of the scaffolds for bone. Although there are a number of obstacles to triumph over, these bioengineered structures showed potential as future bone regeneration treatments.

Effective Osteogenic Priming of Mesenchymal Stem Cells through LNA-ASOs-Mediated Sfrp1 Gene Silencing

Mesenchymal stem cell (MSC) transplantation has emerged as a promising approach for bone regeneration. Importantly, the beneficial effects of MSCs can be improved by modulating the expression levels of specific genes to stimulate MSC osteogenic differentiation. We have previously shown that Smurf1 silencing by using Locked Nucleic Acid-Antisense Oligonucleotides, in combination with a scaffold that sustainably releases low doses of BMP-2, was able to increase the osteogenic potential of MSCs in the presence of BMP-2 doses significantly smaller than those currently used in the clinic. This would potentially allow an important reduction in this protein in MSs-based treatments, and thus of the side effects linked to its administration. We have further improved this system by specifically targeting the Wnt pathway modulator Sfrp1. This approach not only increases MSC bone regeneration efficiency, but is also able to induce osteogenic differentiation in osteoporotic human MSCs, bypassing the need for BMP-2 induction, underscoring the regenerative potential of this system. Achieving successful osteogenesis with the sole use of LNA-ASOs, without the need of administering pro-osteogenic factors such as BMP-2, would not only reduce the cost of treatments, but would also open the possibility of targeting these LNA-ASOs specifically to MSCs in the bone marrow, allowing us to treat systemic bone loss such as that associated with osteoporosis.

Injectable Scaffold for Bone Marrow Stem Cells and Bone Morphogenetic Protein-2 to Repair Cartilage

OBJECTIVE

The limits of the microfracture (MFX) treatment in terms of lesion size and long-term tissue functionality makes it necessary to investigate different alternatives to repair focal cartilage lesions. The present study aims at evaluating the efficacy of a minimally invasive approach against the conventional MFX to repair a chondral defect in rabbits. An injectable scaffold of BMP-2 pre-encapsulated in PLGA microspheres dispersed in a Pluronic F-127 solution is proposed as support of cells and controlled delivery system for the growth factor.

DESIGN

MFX was compared versus the injectable system seeded with mesenchymal stem cells (MSCs), both without BMP-2 and under controlled release of BMP-2 at 2 different doses (3 and 12 µg/scaffold). The different treatments were evaluated on a 4-mm diameter chondral defect model using 9 experimental groups of 4 rabbits (8 knees) each, throughout 24 weeks.

RESULTS

Histologically, all the treated groups, except MFX treated, responded significantly better than the control group (nontreated defect). Although no significant differences were found between the treated groups, only BMP(12), MSC-BMP(12), and MFX-BMP(3) groups showed nonsignificant differences when compared with the normal cartilage.

CONCLUSIONS

The hydrogel system proposed to control the release rate of the BMP-2 was safe, easily injectable, and also provided good support for cells. Treatments with MSCs or BMP-2 repaired efficiently the chondral lesion created in rabbits, being less invasive than MFX treatment.

The Bone Regeneration Capacity of BMP-2 + MMP-10 Loaded Scaffolds Depends on the Tissue Status

Biomaterials-mediated bone formation in osteoporosis (OP) is challenging as it requires tissue growth promotion and adequate mineralization. Based on our previous findings, the development of scaffolds combining bone morphogenetic protein 2 (BMP-2) and matrix metalloproteinase 10 (MMP-10) shows promise for OP management. To test our hypothesis, scaffolds containing BMP-2 + MMP-10 at variable ratios or BMP-2 + Alendronate (ALD) were prepared. Systems were characterized and tested in vitro on healthy and OP mesenchymal stem cells and in vivo bone formation was studied on healthy and OP animals. Therapeutic molecules were efficiently encapsulated into PLGA microspheres and embedded into chitosan foams. The use of PLGA (poly(lactic-co-glycolic acid)) microspheres as therapeutic molecule reservoirs allowed them to achieve an in vitro and in vivo controlled release. A beneficial effect on the alkaline phosphatase activity of non-OP cells was observed for both combinations when compared with BMP-2 alone. This effect was not detected on OP cells where all treatments promoted a similar increase in ALP activity compared with control. The in vivo results indicated a positive effect of the BMP-2 + MMP-10 combination at both of the doses tested on tissue repair for OP mice while it had the opposite effect on non-OP animals. This fact can be explained by the scaffold’s slow-release rate and degradation that could be beneficial for delayed bone regeneration conditions but had the reverse effect on healthy animals. Therefore, the development of adequate scaffolds for bone regeneration requires consideration of the tissue catabolic/anabolic balance to obtain biomaterials with degradation/release behaviors suited for the existing tissue status.

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

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

A facile one-stage treatment of critical bone defects using a calcium sulfate/hydroxyapatite biomaterial providing spatiotemporal delivery of bone morphogenic protein-2 and zoledronic acid

Bone morphogenic proteins (BMPs) are the only true osteoinductive molecules. Despite being tremendously potent, their clinical use has been limited for reasons including supraphysiological doses, suboptimal delivery systems, and the pro-osteoclast effect of BMPs. Efforts to achieve spatially controlled bone formation using BMPs are being made. We demonstrate that a carrier consisting of a powder of calcium sulfate/hydroxyapatite (CaS/HA) mixed with bone active molecules provides an efficient drug delivery platform for critical femoral defect healing in rats. The bone-active molecules were composed of osteoinductive rhBMP-2 and the bisphosphonate, and zoledronic acid (ZA) was chosen to overcome BMP-2-induced bone resorption. It was demonstrated that delivery of rhBMP-2 was necessary for critical defect healing and restoration of mechanical properties, but codelivery of BMP-2 and ZA led to denser and stronger fracture calluses. Together, the CaS/HA biomaterial with rhBMP-2 and/or ZA can potentially be used as an off-the-shelf alternative to autograft bone.

Oxytocin-loaded sustained-release hydrogel graft provides accelerated bone formation: An experimental rat study

Restoration of the lost bone volume is one of the most deliberate issues in dentistry. Sustained-release microspherical oxytocin hormone in a poloxamer hydrogel scaffold combined with a mixture of β-tricalcium phosphate and hydroxyapatite (CP) may serve as a suitable bone graft. The aim of this study was to design and test a novel thermosensitive hydrogel graft incorporating oxytocin-loaded poly(d, l-lactide-co-glycolide) (PLGA) sustained-release microspheres and CP. Thermosensitive poloxamer hydrogel containing CP (HCP graft) was prepared as a base and combined with hollow microspheres (HCPM) and oxytocin-loaded microspheres (HCPOM). Eighty Wistar rats were used for testing the grafts and a control group in 8-mm-diameter critical-sized calvarial defects (CSD); (n = 20). Bone healing at the 4th and 8th weeks was evaluated by histological, histomorphometric, and radiological (micro-computed tomography [µCT]) analyses. The results were analyzed by two-way analysis of variance (P < .05). Oxytocin-loaded PLGA microspheres prepared by the solvent displacement method yielded a high encapsulation efficiency of 89.5% and a slow drug release. Incorporation of the microspheres into the hydrogel graft slowed the release rate down and the release completed within 32 days. HCPOM revealed the highest new bone formation (26.45% ± 6.65% and 30.76% ± 4.37% at the 4th and 8th weeks, respectively; P < .0001) while HCPM and HCP groups revealed a bone formation of around 10% (P > .05). µCT findings of HCPOM group showed the highest mean bone mineral density values (42.21 ± 5.14 and 46.94 ± 3.30 g/cm³ for the 4th and 8th weeks, respectively; P < .0027). The proposed oxytocin-loaded sustained-release PLGA microspheres containing thermosensitive hydrogel graft (HCPOM) provide an accelerated bone regeneration in the rat calvaria.

Alginate-hydrogel versus alginate-solid system. Efficacy in bone regeneration in osteoporosis

In the present study, two different PLGA-Alginate scaffolds, a hydrogel (HY) and a solid sponge (SS), were developed for β-estradiol and BMP-2 sustained delivery for bone regeneration in osteoporosis. β-Estradiol and BMP-2 were encapsulated in PLGA and PLGA-Alginate microspheres respectively. Scaffolds were characterized in vitro in terms of porosity, water uptake, release rate and HY rheological properties. BMP-2 release profiles were also analysed in vivo. The bone regeneration induced by both HY and SS was evaluated using a critical-sized bone defect in an osteoporotic (OP) rat model. Compared to HY, SS presented 30% higher porosity, more than double water absorption capacity and almost negligible mass loss compared to the 40% of HY. Both systems were flexible and fit well the defect shape, however, HY has the advantage of being injectable. Despite both delivery systems had similar composition and release profile, bone repair was around 30% higher with SS than with HY, possibly due to its longer residence time at the defect site. The incorporation of mesenchymal stem cells obtained from OP rats did not result in any improvement or synergistic effect on bone repair.

PLGA-BMP-2 and PLA-17β-Estradiol Microspheres Reinforcing a Composite Hydrogel for Bone Regeneration in Osteoporosis

The controlled release of active substances-bone morphogenetic protein 2 (BMP-2) and 17β-estradiol-is one of the main aspects to be taken into account to successfully regenerate a tissue defect. In this study, BMP-2- and 17β-estradiol-loaded microspheres were combined in a sandwich-like system formed by a hydrogel core composed of chitosan (CHT) collagen, 2-hidroxipropil γ-ciclodextrin (HP-γ-CD), nanoparticles of hydroxyapatite (nano-HAP), and an electrospun mesh shell prepared with two external electrospinning films for the regeneration of a critical bone defect in osteoporotic rats. Microspheres were made with poly-lactide-co-glycolide (PLGA) to encapsulate BMP-2, whereas the different formulations of 17β-estradiol were prepared with poly-lactic acid (PLA) and PLGA. The in vitro and in vivo BMP-2 delivered from the system fitted a biphasic profile. Although the in vivo burst effect was higher than in vitro the second phases (lasted up to 6 weeks) were parallel, the release rate ranged between 55 and 70 ng/day. The in vitro release kinetics of the 17β-estradiol dissolved in the polymeric matrix of the microspheres depended on the partition coefficient. The 17β-estradiol was slowly released from the core system using an aqueous release medium (Deff = 5.58·10-16 ± 9.81·10-17m2s-1) and very fast in MeOH-water (50:50). The hydrogel core system was injectable, and approximately 83% of the loaded dose is uniformly discharged through a 20G needle. The system placed in the defect was easily adapted to the defect shape and after 12 weeks approximately 50% of the defect was refilled by new tissue. None differences were observed between the osteoporotic and non-osteoporotic groups. Despite the role of 17β-estradiol on the bone remodeling process, the obtained results in this study suggest that the observed regeneration was only due to the controlled rate released of BMP-2 from the PLGA microspheres.

Smurf1 Silencing Using a LNA-ASOs/Lipid Nanoparticle System to Promote Bone Regeneration

Despite the great advance of bone tissue engineering in the last few years, repair of bone defects remains a major problem. Low cell engraftment and dose‐dependent side effects linked to the concomitant administration of bone morphogenetic proteins (BMPs) are the main problems currently hindering the clinical use of mesenchymal stem cell (MSC)‐based therapies in this field. We have managed to bypass these drawbacks by combining the silencing the Smurf1 ubiquitin ligase in MSCs with the use of a scaffold that sustainably releases low doses of BMP‐2. In this system, Smurf1 silencing is achieved by using GapmeRs, a clinically safe method that avoids the use of viral vectors, facilitating its translation to the clinic. Here, we show that a single transient transfection with a small quantity of a Smurf1‐specific GapmeR is able to induce a significant level of silencing of the target gene, enough to prime MSCs for osteogenic differentiation. Smurf1 silencing highly increases MSCs responsiveness to BMP‐2, allowing a dramatic reduction of the dose needed to achieve the desired therapeutic effect. The combination of these primed cells with alginate scaffolds designed to sustainably and locally release low doses of BMP‐2 to the defect microenvironment is able to induce the formation of a mature bone matrix both in an osteoporotic rat calvaria system and in a mouse ectopic model. Importantly, this approach also enhances osteogenic differentiation in MSCs from osteoporotic patients, characterized by a reduced bone‐forming potential, even at low BMP doses, underscoring the regenerative potential of this system. stem cells translational medicine2019;8:1306&1317

The BMP‐Smad signaling cascade is an effective therapeutic target to promote bone formation. Silencing of Smurf1, a known BMP signaling inhibitor, increases the responsiveness of Mesenchymal stem cells to BMP, allowing a dramatic reduction of the doses used in the clinic to promote bone formation and therefore, avoiding secondary effects associated to the use of these factors.

Biomodulation of an implant for enhanced bone-implant anchorage

Aseptic loosening of implants is the major cause for revision surgery. By modulating the bone-implant interface, early bone-implant anchorage could be improved. Implant surface manipulation by the addition of osteopromotive molecules locally and systemically to promote implant integration has been described with limited success. This study describes a novel approach by making the implant capable of biologically modulating its surroundings. It was hypothesized that the early implant fixation would improve by filling the interior of the implant with a carrier providing spatio-temporal release of bone active drugs with known osteogenic effect. The implant consisted of a threaded polyether ether ketone (PEEK) hollow chamber with holes at the bottom. The implant was filled with a calcium sulphate (CaS)/hydroxyapatite (HA) carrier, delivering two bone active molecules; zoledronic acid (ZA) and bone morphogenic protein-2 (BMP-2). At first, a rat abdominal muscle pouch model indicated a sustained in-vivo release of both ¹²⁵I-rhBMP-2 (57%) and ¹⁴C-ZA (22%) from the CaS/HA carrier over a period of 4-weeks. The biomodulated implant was then inserted in the proximal tibia in rats with the following experimental groups: G1) Empty implant, G2) Implant + CaS/HA, G3) Implant + CaS/HA + ZA and G4) Implant + CaS/HA + ZA + rhBMP-2. Significantly higher bone volume (BV) was seen around the implant in groups G3 (3.3 ± 0.7 mm³) and G4 (3.1 ± 0.7 mm³) compared to the control (1.3 ± 0.4 mm³) using micro-computed tomography and qualitative histology. Group G3, also exhibited significantly higher pull-out force and absorbed energy when compared to the control group G1. These findings indicate that a low dose of ZA alone, released in a controlled manner from within a fenestrated implant is enough to improve implant anchorage without the need of adding rhBMP-2. This simple method of using a fenestrated implant containing a ceramic carrier releasing bone active molecules improved bone anchorage and could clinically reduce prosthetic failure. STATEMENT OF SIGNIFICANCE: Aseptic loosening remains as a major cause for implant revisions and early reaction of surrounding bone to the prosthesis is important for longevity. A novel approach to enhance early bone-implant anchorage is presented. The implant is filled with a carrier providing controlled release of bone active molecules. In an animal model, a calcium sulphate (CaS)/hydroxyapatite (HA) carrier was used to provide a spatio-temporal release of bone morphogenic protein-2 (BMP-2) and zoledronic acid (ZA). Significantly better bone-implant integration was achieved using ZA alone, thereby eliminating the need for adding BMP-2. The developed method of implant biomodulation holds potential to prevent implant loosening and is an alternative to prosthetic coatings or systemic drug treatment. Importantly, all constituents are approved for clinical use.

Effect of Biomaterial Electrical Charge on Bone Morphogenetic Protein-2-Induced In Vivo Bone Formation

IMPACT STATEMENT

Biomaterials can play a dual role in bone regeneration: they enable local sustained delivery of growth factors, such as bone morphogenetic protein-2 (BMP-2), while they provide structural support as scaffold. By better imitating the properties of native bone tissue, scaffolds may be both osteoconductive and osteoinductive. The latter can be achieved by modifying the electrical charge of the surface. The present work uses tunable oligo[(polyethylene glycol) fumarate] hydrogel and demonstrates that negative charge enhances BMP-2-induced bone formation compared with neutral or positive charge. Altogether, this indicates that tissue-specific surface charge modifications of biomaterials hold great promise in the field of tissue regeneration.

Controlled release of BMP-2 from titanium with electrodeposition modification enhancing critical size bone formation

In this study, a porous Ti-alloy based implant with an interconnected channel structure (MAO-CaP-BMP2) is fabricated using a method combining 3D printing, microarc oxidation (MAO) treatment, and co-precipitation of Ca,P layer with BMP-2 technique. The macroporous structure with pore size of 600 μm made by 3D printing not only enhances the ingrowth of cells but also allows the formation of blood vessels inside the implant. As a result, the new bond formation is promoted. In addition, the microporous dioxide layer formed on the implant surface by MAO provides the sites for co-precipitation of Ca,P layer with BMP-2. The microstructure allows the prolonged release of BMP-2. Our results show that a sustained release of BMP-2 over 35 days is achieved for MAO-CaP-BMP2 group longer than Ti without MAO modification group and without Ca,P electrochemical deposition group. The slow release of BMP-2 at the bone/implant interface for a long period of time leads to enhancement of the osseointegration between the implant and surrounding bones. This result indicates that MAO-CaP-BMP2 is a good candidate of growth factor carrier. Successful regeneration of bone requires the concomitant processes of osteogenesis and neovascularization. MAO-CaP-BMP2 modified Ti-alloy implant is both osteoinductive and osteoconductive which can create better osteogenesis and angiogenesis. As a result, it can enhance bone formation.

In Vitro and In Vivo Correlation of Bone Morphogenetic Protein-2 Release Profiles from Complex Delivery Vehicles

Local sustained delivery of bioactive molecules from biomaterials is a promising strategy to enhance bone regeneration. To optimize delivery vehicles for bone formation, the design characteristics are tailored with consequential effect on bone morphogenetic protein-2 (BMP-2) release and bone regeneration. Complying with the 3R principles (Replacement, Reduction, and Refinement), the growth factor release is often investigated in vitro using several buffers to mimic the in vivo physiological environment. However, this remains an unmet need. Therefore, this study investigates the in vitro-in vivo correlation (IVIVC) of BMP-2 release from complex delivery vehicles in several commonly used in vitro buffers: cell culture model, phosphate buffered saline, and a strong desorption buffer. The results from this study showed that the release environment affected the BMP-2 release profiles, creating distinct relationships between release versus time and differences in extent of release. According to the guidance set by the U.S. Food and Drug Administration (FDA), IVIVC resulted in level A internal predictability for individual composites. Since the IVIVC was influenced by the BMP-2 loading method and composite surface chemistry, the external predictive value of the IVIVCs was limited. These results show that the IVIVCs can be used for predicting the release of an individual composite. However, the models cannot be used for predicting in vivo release for different composite formulations since they lack external predictability. Potential confounding effects of drug type, delivery vehicle formulations, and application site should be added to the equation to develop one single IVIVC applicable for complex delivery vehicles. Altogether, these results imply that more sophisticated in vitro systems should be used in bone regeneration to accurately discriminate and predict in vivo BMP-2 release from different complex delivery vehicles.

Combined sustained release of BMP2 and MMP10 accelerates bone formation and mineralization of calvaria critical size defect in mice

The effect of dual delivery of bone morphogenetic protein-2 (BMP-2) and matrix metalloproteinase 10 (MMP10) on bone regeneration was investigated in a murine model of calvarial critical-size defect, hypothesizing that it would result in an enhanced bone formation. Critical-size calvarial defects (4 mm diameter) were created in mice and PLGA microspheres preloaded with either BMP-2, MMP10 or a microsphere combination of both were transplanted into defect sites at different doses. Empty microspheres were used as the negative control. Encapsulation efficiency was assessed and in vivo release kinetics of BMP-2 and MMP10 were examined over 14 days. Histological analyses were used to analyze bone formation after four and eight weeks. Combination with MMP10 (30 ng) significantly enhanced BMP-2 (600 ng)-mediated osteogenesis, as confirmed by the increase in percentage of bone fill (p < .05) at four weeks. Moreover, it also increased mineral apposition rate (p < .05), measured by double labeling with tetracycline and calceine. MMP10 accelerates bone repair by enhancing BMP-2-promoted bone healing and improving the mineralization rate. In conclusion combination of MMP10 and BMP-2 may become a promising strategy for repair and regeneration of bone defects.

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

IMPACT STATEMENT

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

Calcium Sulphate/Hydroxyapatite Carrier for Bone Formation in the Femoral Neck of Osteoporotic Rats

This study investigated bone regeneration in the femoral neck canal of osteoporotic rats using a novel animal model. A calcium sulphate (CS)/hydroxyapatite (HA) carrier was used to deliver a bisphosphonate, zoledronic acid (ZA), locally, with or without added recombinant human bone morphogenic protein-2 (rhBMP-2). Twenty-eight-week-old ovariectomized Sprague–Dawley rats were used. A 1 mm diameter and 8 mm long defect was created in the femoral neck by drilling from the lateral cortex in the axis of the femoral neck, leaving the surrounding cortex intact. Three treatment groups and one control group were used: (1) CS/HA alone, (2) CS/HA + ZA (10 μg) (3) CS/HA + ZA (10 μg) + rhBMP-2 (4 μg), and (4) empty defect (control). The bone formation was assessed at 4 weeks post surgery using in vivo micro computed tomography (micro-CT). At 8 weeks post surgery, the animals were sacrificed, and both defect and contralateral femurs were subjected to micro-CT, mechanical testing, and histology. Micro-CT results showed that the combination of CS/HA with ZA or ZA + rhBMP-2 increased the bone formation in the defect when compared to the other groups and to the contralateral hips. Evidence of new dense bone formation in CS/HA + ZA and CS/HA + ZA + rhBMP-2 groups was seen histologically. Mechanical testing results showed no differences in the load to fracture between the treatments in either of the treated or contralateral legs. The CS/HA biomaterial can be used as a carrier for ZA and rhBMP-2 to regenerate bone in the femoral neck canal of osteoporotic rats.

Bone morphogenetic protein-2 release profile modulates bone formation in phosphorylated hydrogel

The optimal release profile of locally delivered bone morphogenetic protein-2 (BMP-2) for safe and effective clinical application is unknown. In this work, the effect of differential BMP-2 release on bone formation was investigated using a novel biomaterial oligo[(polyethylene glycol) fumarate] bis[2-(methacryloyloxy) ethyl] phosphate hydrogel (OPF-BP) containing poly(lactic-co-glycolic acid) microspheres. Three composite implants with the same biomaterial chemistry and structure but different BMP-loading methods were created: BMP-2 encapsulated in microspheres (OPF-BP-Msp), BMP-2 encapsulated in microspheres and adsorbed on the phosphorylated hydrogel (OPF-BP-Cmb), and BMP-2 adsorbed on the phosphorylated hydrogel (OPF-BP-Ads). These composites were compared with the clinically used BMP-2 carrier, Infuse® absorbable collagen sponge (ACS). Differential release profiles of bioactive BMP-2 were achieved by these composites. In a rat subcutaneous implantation model, OPF-BP-Ads and ACS generated a large BMP-2 burst release (>75%), whereas a more sustained release was seen for OPF-BP-Msp and OPF-BP-Cmb (~25% and 50% burst, respectively). OPF-BP-Ads generated significantly more bone than did all other composites, and the bone formation was 12-fold higher than that of the clinically used ACS. Overall, this study clearly shows that BMP-2 burst release generates more subcutaneous bone than do sustained release in OPF-BP-microsphere composites. Furthermore, composites should not only function as a delivery vehicle but also provide a proper framework to achieve appropriate bone formation.

Bone regeneration in osteoporosis by delivery BMP-2 and PRGF from tetronic-alginate composite thermogel

As the life expectancy of the world population increases, osteoporotic (OP) fracture risk increase. Therefore in the present study a novel injectable thermo-responsive hydrogel loaded with microspheres of 17β-estradiol, microspheres of bone morphogenetic protein-2 (BMP-2) and plasma rich in growth factors (PRGF) was applied locally to regenerate a calvaria critical bone defect in OP female rats. Three systems were characterized: Tetronic® 1307 (T-1307) reinforced with alginate (T-A), T-A with PRGF and T-A-PRGF with microspheres. The addition of the microspheres increased the viscosity but the temperature for the maximum viscosity did not change (22-24 °C). The drugs were released during 6 weeks in one fast phase (three days) followed by a long slow phase. In vivo evaluation was made in non-OP and OP rats treated with T-A, T-A with microspheres of 17β-estradiol (T-A-βE), T-A-βE prepared with PRGF (T-A-PRGF-βE), T-A-βE with microspheres of BMP-2 (T-A-βE-BMP-2) and the combination of the three (T-A-PRGF-βE-BMP). After 12 weeks, histological and histomorphometric analyzes showed a synergic effect due to the addition of BMP-2 to the T-A-βE formulation. The PRGF did not increased the bone repair. The new bone filling the OP defect was less mineralized than in the non-OP groups.

Nanomedicine for safe healing of bone trauma: Opportunities and challenges

Historically, high-energy extremity injuries resulting in significant soft-tissue trauma and bone loss were often deemed unsalvageable and treated with primary amputation. With improved soft-tissue coverage and nerve repair techniques, these injuries now present new challenges in limb-salvage surgery. High-energy extremity trauma is pre-disposed to delayed or unpredictable bony healing and high rates of infection, depending on the integrity of the soft-tissue envelope. Furthermore, orthopedic trauma surgeons are often faced with the challenge of stabilizing and repairing large bony defects while promoting an optimal environment to prevent infection and aid bony healing. During the last decade, nanomedicine has demonstrated substantial potential in addressing the two major issues intrinsic to orthopedic traumas (i.e., high infection risk and low bony reconstruction) through combatting bacterial infection and accelerating/increasing the effectiveness of the bone-healing process. This review presents an overview and discusses recent challenges and opportunities to address major orthopedic trauma through nanomedical approaches.

Structure-Performance Relationships of Temperature-Responsive PLGA-PEG-PLGA Gels for Sustained Release of Bone Morphogenetic Protein-2

PLGA (poly(lactic-co-glycolic) acid)-PEG (polyethylene glycol)-PLGA synthesis conditions have an impact on the physicochemical features of the copolymer and its usefulness as biomaterial. This study reports on an analysis of the composition and structural properties of PLGA-PEG-PLGA copolymers applying a variety of analytical techniques. Viscoelastic properties and particularly the temperature-responsive behavior of PLGA-PEG-PLGA showed a marked dependence on copolymer structural features. Physicochemical and biological properties, such as bioadhesion, biocompatibility and cell viability, of the raw copolymers and their gels were also evaluated. The most promising copolymer was chosen to formulate the osteoinductive protein bone morphogenetic protein-2 (¹²⁵I-BMP-2), and the ability of its gels to sustain the release both in vitro and in vivo was monitored in situ using a gamma counter. In vitro diffusion studies were carried out using a bioinspired set-up that included a biorelevant receptor medium. In vivo release tests after implantation in a critical-size calvarial defect model showed an important burst, but then the release fitted well to the square-root kinetics. Importantly, the release rate constants recorded in vitro and in vivo matched each other suggesting close in vitro-in vivo correlation. Overall, the information gathered opens new perspectives in the biomedical application of these temperature-sensitive materials.

Bone Regeneration from PLGA Micro-Nanoparticles

Poly-lactic-co-glycolic acid (PLGA) is one of the most widely used synthetic polymers for development of delivery systems for drugs and therapeutic biomolecules and as component of tissue engineering applications. Its properties and versatility allow it to be a reference polymer in manufacturing of nano- and microparticles to encapsulate and deliver a wide variety of hydrophobic and hydrophilic molecules. It additionally facilitates and extends its use to encapsulate biomolecules such as proteins or nucleic acids that can be released in a controlled way. This review focuses on the use of nano/microparticles of PLGA as a delivery system of one of the most commonly used growth factors in bone tissue engineering, the bone morphogenetic protein 2 (BMP2). Thus, all the needed requirements to reach a controlled delivery of BMP2 using PLGA particles as a main component have been examined. The problems and solutions for the adequate development of this system with a great potential in cell differentiation and proliferation processes under a bone regenerative point of view are discussed.

Bone critical defect repair with poloxamine-cyclodextrin supramolecular gels

The aim of this study was to evaluate the osteoinductive capacity of a poloxamine (Tetronic(®) 908, T) and α-cyclodextrin (αCD) supramolecular gel (T-CD) as scaffold in a critical size defect in rat calvaria. The T-CD gel was evaluated solely and after being loaded with simvastatin (SV) and bone morphogenetic protein (BMP-2) separately and in combinations in order to reduce the doses of the active substances. Three doses of SV (7.5, 75, 750 μg) and two doses of BMP-2 (3 and 6 μg) were tested. The histology and histomorphometrical analysis showed improved bone repair with T-CD compared to T, probably due to better release control of both SV and BMP-2. In addition, as T-CD eroded more slowly than poloxamine alone, it remained longer in the defect site. Although synergism was not obtained with BMP-2 and SV, according to the observed regeneration of the defect, the dose of BMP-2 and SV can be reduced to 3 μg and 7.5 μg, respectively.

BMP-2, PDGF-BB, and bone marrow mesenchymal cells in a macroporous β-TCP scaffold for critical-size bone defect repair in rats

The aim of this work was to study the bone repair induced by bone morphogenetic protein-2 (BMP-2), rat mesenchymal stem cells (rMSCs), and platelet-derived growth factor (PDGF-BB) incorporated in a macroporous beta-tricalcium phosphate (β-TCP) system fabricated by robocasting, and to identify the most beneficial combination in a critical rat calvaria defect. BMP-2 was formulated in microspheres to provide a prolonged, local concentration, whereas PDGF-BB, which acts during the initial stage of defect repair, was incorporated in a thin layer of crosslinked alginate. Approximately 80% of PDGF-BB and 90% of BMP-2 were released into the defect during the first 2 d and 3 weeks, respectively. Histological analyses indicated a minor synergistic effect in the BMP-2-MSC groups. In contrast, significant antagonism was found with combined BMP-2 and PDGF-BB defect treatment. The high-grade repair induced by BMP-2 rules out any advantage from combining BMP-2 with PDGF-BB or MSCs, at least with this scaffold and defect model.

Nanotechnology in bone tissue engineering

Nanotechnology represents a major frontier with potential to significantly advance the field of bone tissue engineering. Current limitations in regenerative strategies include impaired cellular proliferation and differentiation, insufficient mechanical strength of scaffolds, and inadequate production of extrinsic factors necessary for efficient osteogenesis. Here we review several major areas of research in nanotechnology with potential implications in bone regeneration: 1) nanoparticle-based methods for delivery of bioactive molecules, growth factors, and genetic material, 2) nanoparticle-mediated cell labeling and targeting, and 3) nano-based scaffold construction and modification to enhance physicochemical interactions, biocompatibility, mechanical stability, and cellular attachment/survival. As these technologies continue to evolve, ultimate translation to the clinical environment may allow for improved therapeutic outcomes in patients with large bone deficits and osteodegenerative diseases.

FROM THE CLINICAL EDITOR

Traditionally, the reconstruction of bony defects has relied on the use of bone grafts. With advances in nanotechnology, there has been significant development of synthetic biomaterials. In this article, the authors provided a comprehensive review on current research in nanoparticle-based therapies for bone tissue engineering, which should be useful reading for clinicians as well as researchers in this field.