Bone morphogenetic protein encapsulated with a biodegradable and biocompatible polymer

A review on carrier systems for bone morphogenetic protein-2

Bone morphogenetic protein-2 (BMP-2) has unique bone regeneration property. The powerful osteoinductive nature makes it considered as second line of therapy in nonunion bone defect. A large number of carriers and delivery systems made up of different materials have been investigated for controlled and sustained release of BMP-2. The delivery systems are in the form of hydrogel, microsphere, nanoparticles, and fibers. The carriers used for the delivery are made up of metals, ceramics, polymers, and composites. Implantation of these protein-loaded carrier leads to cell adhesion, degradation which eventually releases the drug/protein at site specific. But, problems like ectopic growth, lesser protein delivery, inactivation of the protein are reported in the available carrier systems. Therefore, it is need of an hour to modify the available carrier systems as well as explore other biomaterials with desired properties. In this review, all the reported carrier systems made of metals, ceramics, polymers, composites are evaluated in terms of their processing conditions, loading capacity and release pattern of BMP-2. Along with these biomaterials, the attempts of protein modification by adding some functional group to BMP-2 or extracting functional peptides from the protein to achieve the desired effect, is also evaluated. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 904-925, 2017.

Studies of bone morphogenetic protein-based surgical repair

Over the past several decades, recombinant human bone morphogenetic proteins (rhBMPs) have been the most extensively studied and widely used osteoinductive agents for clinical bone repair. Since rhBMP-2 and rhBMP-7 were cleared by the U.S. Food and Drug Administration for certain clinical uses, millions of patients worldwide have been treated with rhBMPs for various musculoskeletal disorders. Current clinical applications include treatment of long bone fracture non-unions, spinal surgeries, and oral maxillofacial surgeries. Considering the growing number of recent publications related to clincal research of rhBMPs, there exists enormous promise for these proteins to be used in bone regenerative medicine. The authors take this opportunity to review the rhBMP literature paying specific attention to the current applications of rhBMPs in bone repair and spine surgery. The prospective future of rhBMPs delivered in combination with tissue engineered scaffolds is also reviewed.

Induction of bone formation in biphasic calcium phosphate scaffolds by bone morphogenetic protein-2 and primary osteoblasts

Bone tissue engineering strategies mainly depend on porous scaffold materials. In this study, novel biphasic calcium phosphate (BCP) matrices were generated by 3D-printing. High porosity was achieved by starch consolidation. This study aimed to characterise the porous BCP-scaffold properties and interactions of osteogenic cells and growth factors under in vivo conditions. Five differently treated constructs were implanted subcutaneously in syngeneic rats: plain BCP constructs (group A), constructs pre-treated with BMP-2 (group B; 1.6 µg BMP-2 per scaffold), seeded with primary osteoblasts (OB) (group C), seeded with OB and BMP-2 (group D) and constructs seeded with OB and pre-cultivated in a flow bioreactor for 6 weeks (group E). After 2, 4 and 6 weeks, specimens were explanted and subjected to histological and molecular biological analyses. Explanted scaffolds were invaded by fibrovascular tissue without significant foreign body reactions. Morphometric analysis demonstrated significantly increased bone formation in samples from group D (OB + BMP-2) compared to all other groups. Samples from groups B-E displayed significant mRNA expression of bone-specific genes after 6 weeks. Pre-cultivation in the flow bioreactor (group E) induced bone formation comparable with group B. In this study, differences in bone distribution between samples with BMP-2 or osteoblasts could be observed. In conclusion, combination of osteoblasts and BMP-2 synergistically enhanced bone formation in novel ceramic scaffolds. These results provide the basis for further experiments in orthotopic defect models with a focus on future applications in orthopaedic and reconstructive surgery.

Biodegradable inflatable balloon for reducing radiation adverse effects in prostate cancer

Carcinoma of the prostate is one of the most abundant killers for men in the western world, and it is frequently treated via Radiation therapy. Unfortunately, radiotherapy side effects include rectal irritation and bleeding, erectile dysfunction and urinary frequency. Because radiation intensity decays rapidly as a function of distance, displacing irradiated prostate away from normal tissues would reduce damage and therefore side effects. The objective of this study is to develop an inflatable balloon that is implanted via a minimal invasive procedure. The balloon is made of a biodegradable polymer called poly(lactide-co-epsilon-caprolactone). The implant is inserted rolled throughout the perineum; inflated in situ with a physiological saline; sealed and placed between the rectum wall, and the prostate gland. Balloon's mechanical and chemical properties were extensively characterized both in vitro and in vivo. The balloon's preparation ensures no bonding across surfaces as these may endanger the implant mechanical stability. Moreover, the coating method does not alter the polymer's molecular weight and therefore preserve its mechanical properties. Balloon's sterilization was carried out using ethylene oxide which, as our results show and in comparison with gamma-irradiation, doesn't damage the mechanical stability of the implant. The proper functionality of the insertion-mounting device as well as the balloon capability to retain its inflated form during patients' radiation session was demonstrated both in vitro and in vivo.

Pharmacokinetics and bone formation by BMP-2 entrapped in polyethylenimine-coated albumin nanoparticles

The osteoinductive growth factor, bone morphogenetic protein-2 (BMP-2), is capable of inducing de novo bone formation after implantation. A nanoparticulate (NP) system was developed for BMP-2 delivery based on NPs fabricated from bovine serum albumin (BSA) and stabilized by polyethylenimine (PEI) coating. In this study, the pharmacokinetics and osteoinductivity of BMP-2 delivered with different BSA NP formulations were determined by subcutaneous implantation in rats. A 7-day pharmacokinetics study showed that PEI coating on NPs effectively reduced the initial burst release of BMP-2 and prolonged the BMP-2 retention at implantation site. However, the uncoated BMP-2 NPs (BMP-2 loading of 1.44% w/w) were able to induce a robust ectopic bone formation, while no bone formation was found by the BMP-2 NPs coated with PEI. The toxicity of the PEI used for NP coating was determined to be the reason for lack of osteoinduction. Increasing BMP-2 loading (up to 5.76% w/w) was then employed to formulate NPs with lower PEI content; the higher BMP-2 loading was found to better promote induction of de novo bone. Our findings indicated that PEI coating on BSA NPs was effective for controlling BMP-2 release from NPs, but the toxicity of cationic PEI was a concern for the osteoinductive activity, which should be alleviated by further optimization of NP formulations.

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

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

Biodegradable gelatin microparticles as delivery systems for the controlled release of bone morphogenetic protein-2

This work evaluated gelatin microparticles and biodegradable composite scaffolds for the controlled release of bone morphogenetic protein-2 (BMP-2) in vitro and in vivo. Gelatin crosslinking (10 and 40mM glutaraldehyde), BMP-2 dose (6 and 60ng BMP-2 per mg dry microparticles), buffer type (phosphate buffered saline (PBS) and collagenase-containing PBS), and gelatin type (acidic and basic) were investigated for their effects on BMP-2 release. Release profiles were also observed using poly(lactic-co-glycolic acid) (PLGA) microparticles with varying molecular weights (8300 and 57,500). In vitro and in vivo studies were conducted using radiolabeled BMP-2; the chloramine-T method was preferred over Bolton-Hunter reagent for radioiodination with this system. BMP-2 release from PLGA microparticles resulted in a moderate burst release followed by minimal cumulative release, while BMP-2 release from gelatin microparticles exhibited minimal burst release followed by linear release kinetics in vitro. Growth factor dose had a small effect on its normalized release kinetics probably because of an equilibrium between gelatin-bound and unbound BMP-2. Differences in release from acidic and basic gelatin microparticles may result from the different pretreatment conditions used for gelatin synthesis. The in vitro release kinetics for both gelatin microparticles alone and within composite scaffolds were dependent largely on the extent of gelatin crosslinking; varying buffer type served to confirm that controlled release relies on enzymatic degradation of the gelatin for controlled release. Finally, in vivo studies with composite scaffolds exhibited minimal burst and linear release up to 28 days. In summary, dose effects on BMP-2 release were found to be minimal while varying gelatin type and release medium can alter release kinetics. These results demonstrate that a systematic control of BMP-2 delivery from gelatin microparticles can be achieved by altering the extent of basic gelatin crosslinking.

RhBMP-2 microspheres-loaded chitosan/collagen scaffold enhanced osseointegration: an experiment in dog

The purpose of this study is to develop a novel recombinant human bone morphogenetic protein-2 (rhBMP-2) sustained release scaffold for dental implant osseointegration, and to evaluate the effect of this scaffold on promoting bone formation. RhBMP-2 was encapsulated in the poly-D,L-lactide-co-glycolide (PLGA) biodegradable microspheres, which were subsequently dispersed in a chitosan/collagen composite scaffold. This rhBMP-2 microspheres-loaded scaffold (S-MB) was compared with a chitosan/collagen scaffold without microspheres that directly encapsulated rhBMP-2 (S-B) in vitro and in vivo. The microstructure of the new scaffold was examined with scanning electron microscopy. The release profile of rhBMP-2 in vitro was measured at interval periods. The effect of rhBMP-2 encapsulated scaffolds on enhancing bone formation through implantation in dogs' mandibles was identified by histological examination of the regenerated bone after 4 weeks of implantation. Due to PLGA microspheres being loaded, the S-MB exhibited lower values at porosity and swelling rate, as well as a higher effective release dose than that of the S-B. Bone density, bone-implant contact, and bone-fill values measured from dog experiments demonstrated that the S-MB induced bone regeneration more quickly and was timely substituted by new bone. It was concluded that this sustained carrier scaffold based on microspheres was more effective to induce implant osseointegration.

Inhalable large porous microspheres of low molecular weight heparin: in vitro and in vivo evaluation

This study tests the feasibility of large porous particles as long-acting carriers for pulmonary delivery of low molecular weight heparin (LMWH). Microspheres were prepared with a biodegradable polymer, poly(lactic-co-glycolic acid) (PLGA), by a double-emulsion-solvent-evaporation technique. The drug entrapment efficiencies of the microspheres were increased by modifying them with three different additivespolyethyleneimine (PEI), Span 60 and stearylamine. The resulting microspheres were evaluated for morphology, size, zeta potential, density, in vitro drug-release properties, cytotoxicity, and for pulmonary absorption in vivo. Scanning electron microscopic examination suggests that the porosity of the particles increased with the increase in aqueous volume fraction. The amount of aqueous volume fraction and the type of core-modifying agent added to the aqueous interior had varying degrees of effect on the size, density and aerodynamic diameter of the particles. When PEI was incorporated in the internal aqueous phase, the entrapment efficiency was increased from 16.22+/-1.32% to 54.82+/-2.79%. The amount of drug released in the initial burst phase and the release-rate constant for the core-modified microspheres were greater than those for the plain microspheres. After pulmonary administration, the half-life of the drug from the PEI- and stearylamine-modified microspheres was increased by 5- to 6-fold compared to the drug entrapped in plain microspheres. The viability of Calu-3 cells was not adversely affected when incubated with the microspheres. Overall, the data presented here suggest that the newly developed porous microspheres of LMWH have the potential to be used in a form deliverable by dry-powder inhaler as an alternative to multiple parenteral administrations of LMWH.

Bone tissue engineering with novel rhBMP2-PLLA composite scaffolds

The aims of the present study were to fabricate a novel porous polylactic acid (PLLA) composite scaffold and evaluate the capacity of the scaffold in carrying recombinant bone morphogenetic protein 2 (rhBMP2) for engineering bone formation. The structures of the PLLA scaffolds were evaluated by SEM and the controlled release of rhBMP2 from the composite scaffolds was assayed by ELISA. Bone induction by the scaffolds loaded with or without rhBMP2 was performed in the calf muscle of twenty Wistar rats for 3, 7, 10, 14, and 28 days. Tissue specimens were examined by Masson's trichrome and von Kossa stainings, and immunohistochemistry of bone proteins. Our results indicated that a moderate foreign body reaction was found in control scaffolds, which lasted for 4 weeks. The addition of rhBMP2 to this novel scaffold dramatically alleviated the adverse responses to PLLA. Enhanced deposition of collagen matrix and endochondral formation were observed in rhBMP2-PLLA scaffolds at 7-10 days, compatible with an early release of rhBMP2 in the composite scaffolds. Bone sialoprotein and osteopontin were demonstrated simultaneously. Von Kossa staining was observed in the test group at 10-14 days. In conclusion, the PLLA scaffolds exhibited the capability of carrying rhBMP2 for inducing bone formation within 2 weeks. These results suggest that rhBMP2-PLLA scaffold may be applicable in tissue engineering.

Tailored release of TGF-β1 from porous scaffolds for cartilage tissue engineering

In view of cartilage tissue engineering, the possibility to prepare porous scaffolds releasing transforming growth factor-beta(1) (TGF-beta(1)) in a well controlled fashion was investigated by means of an emulsion-coating method. Poly(ether-ester) multiblock copolymers were used to prepare emulsions containing TGF-beta(1) which were subsequently applied onto prefabricated scaffolds. This approach resulted in defined porous structures (66%) with interconnected porosity, suitable to allow tissue ingrowth. The scaffolds were effectively associated with TGF-beta(1) and allowed to tailor precisely the release of the growth factor from 12 days to more than 50 days by varying the copolymer composition of the coating. An incomplete release was measured by ELISA, possibly linked to the rapid concentration decrease of the protein in solution. The released growth factor retained its biological activity as was assessed by a cell proliferation assay and by the ability of the released protein to induce chondrogenic differentiation of bone marrow-derived mesenchymal stem cells. However, exact bioactivity quantification was rendered difficult by the protein concentration decrease during storage. Therefore, this study confirms the interest of poly(ether-ester) multiblock copolymers for controlled release of growth factors, and indicates that emulsion-coated scaffolds are promising candidates for cartilage tissue engineering applications requiring precise TGF-beta(1) release rates.

Initial clinical experience with matrix detachable coils for the treatment of intracranial aneurysms

OBJECT

The Matrix detachable coil is a new bioactive, bioabsorbable coil used in the endovascular embolization of intracranial aneurysms. It has a platinum core covered with a bioactive, bioabsorbable polymer (polyglycolic acid/lactide). The authors report on their initial midterm clinical experience with the first-generation Matrix detachable coil.

METHODS

One hundred twelve patients harboring 118 aneurysms were treated using Matrix coils. Forty-nine aneurysms (41.5%) were associated with acute subarachnoid hemorrhage (SAH). Twenty-four lesions (49%) were harbored by patients with Hunt and Hess Grade I, 11 (23.4%) by patients with Grade II, eight (16.3%) by those with Grade III, and six (12.2%) by those with Grade IV. Four aneurysms (3.4%) were harbored by patients who had presented with nonacute SAH. Sixty-five aneurysms (55%) were unruptured. Fifty-seven lesions (48.3%) were small with a small neck, 29 (24.6%) were small with a wide neck, 30 (25.4%) were large, and two (1.7%) were giant. All patients were followed up to obtain angiography and clinical outcome data. Technical complications occurred in six patients: two thromboembolic complications and four aneurysm perforations. Of these six patients, the status of two deteriorated because of aneurysm perforation and another two because of thrombus formation (morbidity 3.6%). There were five deaths--one due to rerupture after embolization. Angiography follow-up studies of 87 aneurysms were obtained. Seventy aneurysms demonstrated progressive occlusion or a stable neck (80.5%), and 17 had some degree of recanalization (19.5%). The aneurysms originally diagnosed as a neck remnant showed a 15% rate of recanalization.

CONCLUSIONS

Matrix coils can be delivered into aneurysms with technical complications similar to those encountered using GDCs. Midterm anatomical outcomes to date have shown moderate improvement in the recanalization rate when compared with those realized using the GDC system. Because of the increased friction associated with the first-generation Matrix coil, the packing density in most aneurysms was less than that achieved with GDCs. Prolonged angiography follow-up evaluations are needed to document long-term efficacy.

Dual release of proteins from porous polymeric scaffolds

To create porous scaffolds releasing in a controlled and independent fashion two different proteins, a novel approach based on protein-loaded polymeric coatings was evaluated. In this process, two water-in-oil emulsions are forced successively through a prefabricated scaffold to create coatings, containing each a different protein and having different release characteristics. In a first step, a simplified three-layered system was designed with model proteins (myoglobin and lysozyme). Poly(ether-ester) multiblock copolymers were chosen as polymer matrix, to allow the diffusion of proteins through the coatings. The model system showed the independent release of the two proteins. The myoglobin release was tailored from a burst to a linear release still on-going after 60 days, while the lysozyme release rate was kept constant. Macro-porous scaffolds, with a porosity of 59 vol.%, showed the same ability to control the release rate of the model proteins independently. The relation between the coatings properties and their release characteristics were investigated with the use of a mathematical diffusion model based on Fick's second law. It confirmed that the multiple coated scaffolds are biphasic system, where each coating controls the release of the protein that it contains. This approach could be of value for tissue engineering applications.

Delivery systems for small molecule drugs, proteins, and DNA: the neuroscience/biomaterial interface

Manipulation of cellular processes in vivo by the delivery of drugs, proteins or DNA is of paramount importance to neuroscience research. Methods for the presentation of these molecules vary widely, including direct injection (either systemic or stereotactic), osmotic pump-mediated chronic delivery, or even implantation of cells engineered to indefinitely secrete a factor of interest. Biomaterial-based delivery systems represent an alternative to more traditional approaches, with the possibility of increased efficacy. Drug-releasing biomaterials, either as injectable microspheres or as three-dimensional implants, can deliver a molecule of interest (including small molecule drugs, biologically active proteins, or DNA) over a more prolonged period of time than by standard bolus injection, avoiding the need for repeated administration. Furthermore, sustained-release systems can maintain therapeutic concentrations at a target site, thus reducing the chance for toxicity. This review summarizes applications of polymer-based delivery of small molecule drugs, proteins, and DNA specifically relevant to neuroscience research. We detail the fabrication procedures for the polymeric systems and their utility in various experimental models. The biomaterial field offers unique experimental tools with downstream clinical application for the study and treatment of neurologic disease.

Controlled release of hyaluronan oligomers from biodegradable polymeric microparticle carriers

In the present study, biodegradable microparticles of blends of poly(DL-lactic-co-glycolic acid) (PLGA) and poly(ethylene glycol) (PEG) were explored as a potential carrier for the controlled release of polysaccharide oligomers. To this end, hyaluronan (HY) oligomers of varying molecular weights were incorporated into PLGA/PEG microparticles. Using a two-level fractional factorial experimental design, four microparticle formulation parameters, the amount of PEG included in the microparticles, the initial HY loading of the microparticles, the molecular weight of HY, and the molecular weight of PLGA, were studied for their influence on the incorporation and in vitro release of HY over the period of 28 days. The entrapment efficiencies were found to range between 10+/-1% and 24+/-2% depending on the initial loading and the molecular weight of the HY oligomer used in the fabrication of the microparticles. The HY was released in a multiphasic fashion including an initial burst release, followed by two separate periods of linear release. The normalized cumulative mass released during the burst release ranged from 25.1+/-9.2% to 93.0+/-0.7% and was found to be significantly influenced by the initial HY loading, the HY molecular weight, and the PLGA molecular weight. The initial period of linear release lasted from day 1 to day 14 and displayed normalized cumulative rates of release from 0.1+/-0.0%/day to 1.4+/-0.2%/day. During this period, PEG content of the microparticles and HY molecular weight exerted the greatest influence on the rate of release. Finally, the second period of linear release lasted through the final time-point at day 28. Here, the normalized cumulative rate of release values ranged from 0.2+/-0.1%/day to 3.6+/-0.7%/day and were dependent on all formulation parameters studied. These results demonstrate the potential of PLGA/PEG blend microparticles for the controlled release of HY oligomers.

Effect of phosphophoryn on rhBMP-2-induced bone formation

In the present study, dentin phosphophoryn (DPP) derived from fresh bovine dentin was evaluated as a co-factor for recombinant human bone morphogenetic protein-2 (rhBMP-2) in rhBMP-2-induced bone formation in rats. A 5 microg amount of Escherichia coli-derived rhBMP-2 variant was combined with DPP cross-linked to type I collagen (2.4 microg DPP/360 microg collagen), acting as carrier. Next, rhBMP-2/DPP/collagen composites were implanted by onlay-grafting beneath the cranial periosteum in 4-week-old Wistar rats. Rats were sacrificed at 2 and 3 weeks after implantation. Throughout the experimental period, rhBMP-2/DPP/collagen composite induced more bone formation than the rhBMP-2/collagen composite. Moreover, the degradation rate of rhBMP-2/DPP/collagen composite in rat was faster than that of rhBMP-2/collagen composite. Neither DPP/collagen composite nor collagen alone conducted bone formation even at 3 weeks postimplantation. These results indicate that the bone-inducing activity of rhBMP-2 is enhanced by DPP as a co-factor of rhBMP-2 in vivo.

rhBMP-2 release from injectable poly(DL-lactic-co-glycolic acid)/calcium-phosphate cement composites

BACKGROUND

In bone tissue engineering, poly(DL-lactic-co-glycolic acid) (PLGA) microparticles are frequently used as a delivery vehicle for bioactive molecules. Calcium phosphate cement is an injectable, osteoconductive, and degradable bone cement that sets in situ. The objective of this study was to create an injectable composite based on calcium phosphate cement embedded with PLGA microparticles for sustained delivery of recombinant human bone morphogenetic protein-2 (rhBMP-2).

METHODS

(125) I-labeled rhBMP-2 was incorporated in PLGA microparticles. PLGA microparticle/calcium-phosphate cement composites were prepared in a ratio of 30:70 by weight. Material properties were evaluated by scanning electron microscopy, microcomputed tomography, and mechanical testing. Release kinetics of rhBMP-2 from PLGA/calcium-phosphate cement disks and PLGA microparticles alone were determined in vitro in two buffer solutions (pH 7.4 and pH 4.0) for up to twenty-eight days.

RESULTS

The entrapment yield of rhBMP-2 in PLGA microparticles was a mean (and standard deviation) of 79% +/- 8%. Analysis showed spherical PLGA microparticles (average size, 17.2 +/-1.3 micro m) distributed homogeneously throughout the nanoporous disks. The average compressive strength was significantly lower (p < 0.001) for PLGA and calcium-phosphate cement composite scaffolds than for calcium-phosphate cement scaffolds alone (6.4 +/- 0.6 MPa compared with 38.6 +/- 2.6 MPa, respectively). Average rhBMP-2 loading was 5.0 +/- 0.4 micro g per 75-mm (3) disk. Release of rhBMP-2 was limited for all formulations. At pH 7.4, 3.1% +/- 0.1% of the rhBMP-2 was released from the PLGA/calcium-phosphate cement disks and 18.0% +/- 1.9% was released from the PLGA microparticles alone after twenty-eight days. At pH 4.0, PLGA/calcium-phosphate cement disks revealed more release of rhBMP-2 than did PLGA microparticles alone (14.5% +/- 6.3% compared with 5.4% +/- 0.7%) by day 28.

CONCLUSIONS

These results indicate that preparation of a PLGA/calcium-phosphate cement composite for the delivery of rhBMP-2 is feasible and that the release of rhBMP-2 is dependent on the composite composition and nanostructure as well as the pH of the release medium.

Matrix and bioabsorbable polymeric coils accelerate healing of intracranial aneurysms: long-term experimental study

BACKGROUND AND PURPOSE

Acceleration of intra-aneurysmal clot organization and fibrosis may be a solution to preventing aneurysm recanalization after endovascular treatment. The purpose of this study was to evaluate the short-term efficacy and long-term safety of the new Matrix coil system.

METHODS

Matrix coils consist of thin platinum coils covered with a bioabsorbable, polymeric material (polyglycolic acid/lactide). Fifty-two experimental aneurysms were created in 26 swine. All of the aneurysms were tightly packed with Matrix or Guglielmi detachable coils (GDC). Comparative angiographic and histopathologic data were analyzed at 2 weeks (n=14), 3 months (n=6), and 6 months (n=6) after embolization.

RESULTS

Three aneurysms treated with GDC ruptured despite tight packing. No recanalization or rupturing was observed in the aneurysms embolized with Matrix coils. At 14 days after embolization, the aneurysms treated with Matrix coils exhibited a more extensive area of organized thrombus when compared with the aneurysms treated with GDC (87% versus 75%, P=0.008, n=11). At 3 months, both Matrix and GDC-treated aneurysms demonstrated complete clot organization. Neck tissue thickness was higher in Matrix-treated aneurysms at 14 days and 3 months, but not at 6 months. No untoward parent artery stenosis was observed in aneurysms treated with Matrix during follow-up. The angiographic cross-sectional area of the Matrix-treated aneurysms was smaller than those treated with GDC at the 3 months.

CONCLUSIONS

Matrix accelerated aneurysm fibrosis and neointima formation without parent artery stenosis. The Matrix system might prevent aneurysmal recanalization after endovascular treatment of cerebral aneurysms.

Human periodontal ligament cell responses to recombinant human bone morphogenetic protein-2 with and without bone allografts

BACKGROUND

Recombinant human bone morphogenetic protein-2 (rhBMP-2) has been found to promote the osteoblastic differentiation of human periodontal ligament cells. Its effect depends on the delivery system used. In this study we examined the effect of rhBMP-2 on the proliferation and osteoblastic differentiation of human periodontal ligament cells cultured alone or with 3 different bone allografts.

METHODS

The rhBMP-2 effect on cell proliferation and osteoblastic differentiation was examined by measuring [3H] thymidine incorporation and ALPase activity, respectively, on human periodontal ligament (hPDL) cells. Two human demineralized freeze-dried allografts of cortical (DFDBAco) and cancellous (DFBDAca) bone origin and 1 non-demineralized freeze-dried allograft (FDBA) of cancellous bone origin, derived from different tissue banks, were used to evaluate the rhBMP-2 effect on cell osteoblastic differentiation. The measurements were taken on various days.

RESULTS

rhBMP-2 decreased hPDL cell proliferation. rhBMP-2 acted on the third day of the process of cell differentiation, had a specific time of action, achieved its peak effect on the fourth and fifth days, and then did not provoke any further effects. The 3 bone allografts were efficiently combined with rhBMP-2. The combination of rhBMP-2 and DFDBAco showed the effect with the longest duration. rhBMP-2, on day 4, made the inactive bone allograft more active while, on the other days, its effect was dependent on the allograft alone.

CONCLUSIONS

rhBMP-2 promotes the osteoblastic differentiation of human periodontal ligament cells and decreases cell proliferation. In this study rhBMP-2 in the presence of the bone allografts tested resulted in hPDL cell differentiation.

A novel method to obtain protein release from porous polymer scaffolds: emulsion coating

To obtain the controlled release of proteins from macro-porous polymeric scaffolds, a novel emulsion-coating method has been developed. In this process, a water-in-oil emulsion, from an aqueous protein solution and a polymer solution, is forced through a prefabricated scaffold by applying a vacuum. After solvent evaporation, a polymer film, containing the protein, is then deposited on the porous scaffold surface. This paper reports the effect of processing parameters on the emulsion coating characteristics, scaffold structure, and protein release and stability. Poly(ether-ester) multiblock copolymers were chosen as the polymer matrix for both scaffolds and coating. Macro-porous scaffolds, with a porosity of 77 vol% and pores of approximately 500 microm were prepared by compression moulding/salt leaching. A micro-porous, homogeneous protein-loaded coating could be obtained on the scaffold surface. Due to the coating, the scaffold porosity was decreased, whereas the pore interconnection was increased. A model protein (lysozyme) could effectively be released in a controlled fashion from the scaffolds. Complete lysozyme release could be achieved within 3 days up to more than 2 months by adjusting the coated emulsion parameters. In addition, the coating process did not reduce the enzymatic activity. This new method appears to be promising for tissue engineering applications.

Controlled release of an osteogenic peptide from injectable biodegradable polymeric composites

Poly(D,L-lactic-co-glycolic acid)/poly(ethylene glycol) (PLGA/PEG) blend microparticles loaded with the osteogenic peptide TP508 were added to a mixture of poly(propylene fumarate) (PPF), poly(propylene fumarate)-diacrylate (PPF-DA), and sodium chloride (NaCl) for the fabrication of PPF composite scaffolds that could allow for tissue ingrowth as well as for the controlled release of TP508 when implanted in an orthopedic defect site. In this study, PPF composites were fabricated and the in vitro release kinetics of TP508 were determined. TP508 loading within the PLGA/PEG microparticles, PEG content within the PLGA/PEG microparticles, the microparticle content of the PPF composite polymer component, and the leachable porogen initial mass percent of the PPF composites were varied according to a fractional factorial design and the effect of each variable on the release kinetics was determined for up to 28 days. Each composite formulation released TP508 with a unique release profile. The initial release (release through day 1) of the PLGA/PEG microparticles was reduced upon inclusion in the PPF composite formulations. Day 1 normalized cumulative mass release from PPF composites ranged from 0.14+/-0.01 to 0.41+/-0.01, whereas the release from PLGA/PEG microparticles ranged from 0.31+/-0.02 to 0.58+/-0.01. After 28 days, PPF composites released 53+/-4% to 86+/-2% of the entrapped peptide resulting in cumulative mass releases ranging from 0.14+/-0.01 microg TP508/mm(3) scaffold to 2.46+/-0.05 microg TP508/mm(3) scaffold. The results presented here demonstrate that PPF composites can be used for the controlled release of TP508 and that alterations in the composite's composition can lead to modulation of the TP508 release kinetics. These composites can be used to explore the effects varied release kinetics and dosages on the formation of bone in vivo.

Experimental studies on bone induction using low-molecular-weight poly (DL-lactide-co-glycolide) as a carrier for recombinant human bone morphogenetic protein-2

An appropriate carrier acting as a slow delivery vehicle for the BMPs is required for maximal clinical effectiveness of these bone-inductive proteins. The purpose of this study was to evaluate a low-molecular-weight PLGA copolymer as a synthetic, biodegradable carrier for rhBMP-2 implantation in vivo. Two, 10, or 50 microg of recombinant human BMP-2 were mixed with 10 mg of a poly (DL-lactide-co-glycolide) (PLGA) 50:50 copolymer and implanted into the calf muscles of Wistar rats. Soft X-ray analysis and histologic examination indicated that new bone formation occurred at all rhBMP-2-implanted sites within 3 weeks after implantation. Correlation of rhBMP-2 concentration with the amount of bone induction was confirmed by specific alkaline phosphatase activity and calcium content assay. In vitro analysis indicated that 78.5% of the PLGA copolymer was degraded to smaller molecular weight material after 14 days in PBS solution. It is suggested that rhBMP-2 was released in an active form at the implant site during the degradation of the copolymer, resulting in the induction of new bone formation. Thus this low-molecular-weight PLGA copolymer material represents a promising delivery vehicle for BMPs, and possibly other growth factors, around dental and orthopedic implants.

Bone grafting alternatives in spinal surgery

BACKGROUND CONTEXT

Bone grafting is used to augment bone healing and provide stability after spinal surgery. Autologous bone graft is limited in quantity and unfortunately associated with increased surgical time and donor-site morbidity. Alternatives to bone grafting in spinal surgery include the use of allografts, osteoinductive growth factors such as bone morphogenetic proteins and various synthetic osteoconductive carriers.

PURPOSE

Recent research has provided insight into methods that may modulate the bone healing process at the cellular level in addition to reversing the effects of symptomatic disc degeneration, which is a potentially disabling condition, managed frequently with various fusion procedures. With many adjuncts and alternatives available for use in spinal surgery, a concise review of the current bone grafting alternatives in spinal surgery is necessary.

STUDY DESIGN/SETTING

A systematic review of the contemporary English literature on bone grafting in spinal surgery, including abstract information presented at national meetings.

METHODS

Bone grafting alternatives were reviewed as to their efficacy in extending or replacing autologous bone graft sources in spinal applications.

RESULTS

Alternatives to autologous bone graft include allograft bone, demineralized bone matrix, recombinant growth factors and synthetic implants. Each of these alternatives could possibly be combined with autologous bone marrow or various growth factors. Although none of the presently available substitutes provides all three of the fundamental properties of autograft bone (osteogenicity, osteoconductivity and osteoinductivity), there are a number of situations in which they have proven clinically useful.

CONCLUSIONS

Alternatives to autogenous bone grafting find their greatest appeal when autograft bone is limited in supply or when acceptable rates of fusion may be achieved with these substitutes (or extenders) despite the absence of one or more of the properties of autologous bone graft. In these clinical situations, the morbidity of autograft harvest is reasonably avoided. Future research may discover that combinations of materials may cumulatively result in the expression of osteogenesis, osteoinductivity and osteoconductivity found in autogenous sources.

Pharmaceutical and immunological evaluation of a single-shot hepatitis B vaccine formulated with PLGA microspheres

A single-shot Hepatitis B vaccine formulation using poly(d,l)-lactide-co-glycolide acid (PLGA) microspheres as a delivery system was examined using a variety of biophysical and biochemical techniques as well as immunological evaluation in C3H mice. PLGA microsphere encapsulation of the Hepatitis B surface antigen (HBsAg), a lipoprotein particle, resulted in good recoveries of protein mass, protein particle conformational integrity, and in vitro antigenicity. Some partial delipidation of the HBsAg, however, was observed. The loading and encapsulation efficiency of HBsAg into the PLGA microspheres were measured along with the morphology and size distribution of the vaccine-loaded PLGA microspheres. The in vitro release kinetics of HBsAg from the PLGA microspheres was evaluated and found to be affected by experimental conditions such as stirring rate. HBsAg showed enhanced storage stability at 37 degrees C in the slightly acidic pH range reported to be found inside PLGA microspheres; thus, the antigen is relatively stable under conditions of temperature and pH that may mimic in vivo conditions. The immunogenicity of the microsphere formulations of HBsAg was compared with conventional aluminum adjuvant formulated HBsAg vaccine in C3H mice. Comparisons were made between aluminum formulations (one and two injections), PLGA microsphere formulations (single injection), and a mixture of aluminum and PLGA microsphere formulations (single injection). The nine-month serum antibody titers indicate that a single injection of a mixture of aluminum and PLGA-formulated HBsAg results in equal or better immune responses than two injections of aluminum-formulated HBsAg vaccine. Based on these in vitro and in vivo studies, it is concluded that HBsAg can be successfully encapsulated and recovered from the PLGA microspheres and a mixture of aluminum-adjuvanted and PLGA-formulated HBsAg can auto-boost an immune response in manner comparable to multiple injections of an aluminum-formulated vaccine.

Cellular responses of bioabsorbable polymeric material and Guglielmi detachable coil in experimental aneurysms

BACKGROUND AND PURPOSE

Acceleration of healing mechanisms is a promising approach to improve current limitations of endovascular aneurysm therapy with the use of platinum coils. We evaluated a new endovascular therapeutic, bioabsorbable polymeric material (BPM), which may promote cellular reaction in the aneurysms.

METHODS

Four different concentrations of lactide/glycolic acid copolymer [poly(D-L-lactic-co-glycolic acid)] (PLGA), 85/15, 75/25, 65/35, and 50/50, were used as BPMs. Sixteen experimental aneurysms were created in 8 swine. Eight-millimeter-long spiral-shaped BPMs were surgically implanted in the aneurysms without tight packing (n=3 for each BPM). Guglielmi detachable coils (GDCs) were used as control (n=4). The animals were killed 14 days after embolization, and angiographic, histological, and immunohistochemical analyses were performed.

RESULTS

Despite loose packing of aneurysms with BPMs, faster BPMs such as 50/50 or 65/35 PLGA demonstrated more mature collagen formation and fibrosis in the sac and neck of the aneurysm. One aneurysm treated with 65/35 PLGA, 1 treated with 75/25 PLGA, and all 3 treated with 85/15 PLGA showed a neck remnant on angiography. There was a linear relationship between collagen levels and polymer degradation properties (r=-0.9513).

CONCLUSIONS

This preliminary animal study indicates that acceleration of aneurysm healing with the use of BPM is feasible. This concept can be applied to decrease and perhaps prevent aneurysmal recanalization after endovascular treatment of cerebral aneurysms.

Slow and continuous application of human recombinant bone morphogenetic protein via biodegradable poly(lactide-co-glycolide) foamspheres

Bone morphogenetic proteins (BMPs) are multifunctional cytokines that were originally identified as molecules that induce bone and cartilage formation in vivo. In order to increase the efficacy of this potent protein for application in medicine, a carrier system is needed to retain the BMP at the preferred site. Here we present and characterize a slow-release carrier system for pure human recombinant (rh)BMP. The large porous microspheres, called 'foamspheres', are biodegradable, because they consist of poly(lactide-co-glycolide) acids and release loaded rhBMP slowly and continuously. In vivo studies in rodents revealed that rhBMP-loaded foamspheres increased the thickness of the calvarial bone of rats by 222%. When the same amount of rhBMP was applied via a gelatine-based hydrogel, the increase in bone height was only 66%. Thus, the carrier system for rhBMP is an important factor for the efficacy of BMPs.

Poly(L-lactide)acid/alginate composite membranes for guided tissue regeneration

The barrier membranes for guided tissue regeneration (GTR) to treat bone defects have to satisfy the criteria of biocompatibility, cell-occlusiveness, space-making, tissue integration and clinical manageability. In this study a system constituted of a poly(L-lactide) acid (PLLA) asymmetric membrane combined with an alginate film was prepared. The PLLA membrane functions to both support the alginate film and separate the soft tissue; the alginate film is intended to act as potential vehicle for the growth factors to promote osteogenesis. The structural, morphological, and mechanical properties of the bilamellar membrane and its stability in culture medium were evaluated. Moreover, the feasibility of using the alginate membranes as controlled-release delivery vehicles of TGF-beta was monitored. Finally, the bacterial adhesion and permeability of Streptococcus mutans, selected for the high adhesive affinity, were monitored. The results showed that the surfaces of the alginate side, to be used in contact with the bone defect, were rougher than PLLA ones. When in contact with complete culture medium, the PLLA-alginate membrane retained its mechanical and structural properties for more than 100 days. Then, the degradation processes occurred but the membrane continued to be stable and manageable for 6 months. Growth factors such as TGF-beta can be incorporated into alginate membranes functioning as drug delivery vehicle, and retain the biological activity when tested in an in vitro model system. The obtained membrane acted as a barrier to the passage of S. mutans bacteria and showed to promote a lower bacterial adhesion with respect to commercial GTR membranes.

Bioabsorbable polymeric material coils for embolization of intracranial aneurysms: a preliminary experimental study

OBJECT

A new embolic agent, bioabsorbable polymeric material (BPM), was incorporated into Guglielmi detachable coils (GDCs) to improve long-term anatomical results in the endovascular treatment of intracranial aneurysms. The authors investigated whether BPM-mounted GDCs (BPM/GDCs) accelerated the histopathological transformation of unorganized blood clot into fibrous connective tissue in experimental aneurysms created in swine.

METHODS

Twenty-four experimental aneurysms were created in 12 swine. In each animal, one aneurysm was embolized using BPM/GDCs and the other aneurysm was embolized using standard GDCs. Comparative angiographic and histopathological data were analyzed at 2 weeks and 3 months postembolization. At 14 days postembolization, angiograms revealed evidence of neck neointima in six of eight aneurysms treated with BPM/GDCs compared with zero of eight aneurysms treated with standard GDCs (p < 0.05). At 3 months postembolization, angiograms demonstrated that four of four aneurysms treated with BPM/GDC were smaller and had neck neointima compared with zero of four aneurysms treated with standard GDCs (p = 0.05). At 14 days, histological analysis of aneurysm healing favored BPM/GDC treatment (all p < 0.05): the grade of cellular reaction around the coils was 3 +/- 0.9 (mean +/- standard deviation) for aneurysms treated using BPM/GDCs compared with 1.6 +/- 0.7 for aneurysms treated using GDCs alone; the percentage of unorganized thrombus was 16 +/- 12% compared with 37 +/- 15%, and the neck neointima thickness was 0.65 +/- 0.26 mm compared with 0.24 +/- 0.21 mm, respectively. At 3 months postembolization, only neck neointima thickness was significantly different (p < 0.05): 0.73 +/- 0.37 mm in aneurysms filled with BPM/GDCs compared with 0.16 +/- 0.14 mm in aneurysms filled with standard GDCs.

CONCLUSIONS

In experimental aneurysms in swine, BPM/GDCs accelerated aneurysm fibrosis and intensified neck neointima formation without causing parent artery stenosis or thrombosis. The use of BPM/GDCs may improve long-term anatomical outcomes by decreasing aneurysm recanalization due to stronger in situ anchoring of coils by organized fibrous tissue. The retraction of this scar tissue may also decrease the size of aneurysms and clinical manifestations of mass effect observed in large or giant aneurysms.

Growth-factor-induced healing of partial-thickness defects in adult articular cartilage

OBJECTIVE

We have previously shown (Hunziker and Rosenberg, J Bone Joint Surg 1996;78A:721-33) that synovial cells can be induced to migrate into partial-thickness articular cartilage defects, therein to proliferate and subsequently to deposit a scar-like tissue. We now wished to ascertain whether these synovial cells could be stimulated to transform into chondrocytes, and thus to lay down cartilage tissue, by the timely introduction of a differentiation factor.

DESIGN

Partial-thickness defects were created in the knee-joint cartilage of adult miniature pigs. These were then filled with a fibrin matrix containing a free chemotactic/mitogenic factor and a liposome-encapsulated chondrogenic differentiation one. Tissue was analyzed (immuno)histochemically at 2, 6 and 12 months.

RESULTS

Defects became filled with cartilage-like tissue which registered positive for all major cartilage-matrix components; it remained compositionally stable throughout the entire follow-up period.

CONCLUSION

Although still requiring considerable refinement, our one-step, growth-factor-based treatment strategy has the basic potential to promote intrinsic healing of partial-thickness articular cartilage defects, thus obviating the need for transplanting cells or tissue.

Surface and bulk modifications to photocrosslinked polyanhydrides to control degradation behavior

A unique class of surface-eroding polyanhydrides was developed and explored for use in medical applications requiring high-strength biomaterials (e.g., orthopedics). In particular, dimethacrylated anhydride monomers were synthesized that photopolymerize quickly to render densely crosslinked polymer networks that degrade from the surface only by hydrolysis of labile anhydride linkages. Previous research on these materials has shown that the rate of hydrolysis of the degradable linkages is dependent on the hydrophobicity of the network composition. This article demonstrates the versatility in controlling the degradation process and resulting cellular response in these materials through the incorporation of new chemistries and the formation of polymer-polymer composite structures. Specifically, the rate of mass loss was controlled by the addition of hydrophobic linear polymers [e.g., poly(methyl methacrylate)] or monovinyl monomers based on hydrophobic natural components (e.g., cholesterol, steric acid). In addition, a newly established photografting method was used to modify the network surface chemistry with cholesterol- and stearic acid-based polymer grafts to control the degradation front and cellular interactions at the polymer-tissue interface. Finally, a porogen leaching method was used to form porous polyanhydride constructs, which can be subsequently filled with osteoblasts photoencapsulated in a hydrogel, as potential synthetic allograft materials for tissue engineering bone.

Restoration of segmental bone defects in rabbit radius by biodegradable capsules containing recombinant human bone morphogenetic protein-2

Recombinant human bone morphogenetic protein-2 (rhBMP-2) was encapsulated in biodegradable poly(DL-lactide-co-glycolide) (PLGA) capsules to regenerate bone by controlling the release rate of rhBMP-2. The rhBMP-2/PLGA capsules containing 12 microg of rhBMP-2 were implanted in seven 15-mm segmental defects of rabbits radii to examine the healing capacity of the rhBMP-2/PLGA capsules. For the control group, four segmental defects were left empty and two were implanted with ghost PLGA capsules. Healing of the defects was followed for 24 weeks and periodically evaluated by radiographs and histological examination. Mechanical testing was applied to three regenerated bone samples at 24 weeks postoperatively when the mature cortex was observed. Mechanical properties of regenerated bone were not significantly different from normal intact bone statistically. Histologically, the rhBMP-2/PLGA capsules disappeared completely during the process of bone regeneration. These results increased possibilities for clinical application of rhBMP-2/PLGA capsules.

Bone morphogenetic protein excipients: comparative observations on poloxamer

Clinicians await the availability of synthetic bioimplants that will replace the need for autogeneic bone grafts in bone reconstructive surgery. For more than a decade, researchers have evaluated delivery vehicles for the tissue morphogen bone morphogenetic protein. The object of this investigation was to measure induced bone development when bone morphogenetic protein was delivered by human tendon collagen, human demineralized bone matrix, hydroxyapatite, a composite of human tendon collagen and human demineralized bone matrix (tendon collagen + demineralized bone matrix), Poloxamer 407, and a composite of human demineralized bone matrix and Poloxamer 407. Sixty-three adult male Swiss Webster mice (Harlan Sprague-Dawley, Indianapolis, Ind.) received 126 implants. The animals were divided into seven groups of nine animals, depending on carrier (six carriers plus the positive control group) used. Each animal received a bone morphogenetic protein-enhanced carrier in one hindquarter muscle mass, with the contralateral leg being implanted with the carrier alone. Implants were evaluated by quantitative radiomorphometry validated by histologic methods. Radiographically, no significant differences were identified among any of the implants evaluated (p > 0.05). Histomorphometric analysis demonstrated that Poloxamer 407 was significantly (p < 0.05) better at delivering bone morphogenetic protein than the other carriers involved in this investigation. The new bone developed in a tubular or spherical shape. Interaction of endogenous and exogenous delivery systems seems to be essential for optimal transmission of bone morphogenetic protein. The importance of the excipient to deliver bone morphogenetic protein and develop a bone morphogenetic protein concentration gradient has been emphasized by other investigators and confirmed by our research on poloxamer. With further research on the physicochemical mechanisms of localization and transmission of bone morphogenetic protein, it may be possible to avoid hazardous operations with autogeneic bone.

New synthetic absorbable polymers as BMP carriers: plastic properties of poly-D,L-lactic acid-polyethylene glycol block copolymers

Bone morphogenetic proteins (BMPs) are biologically active molecules capable of eliciting new bone formation. In combination with biomaterials, these proteins can be used in a clinical setting as bone-graft substitutes to promote bone repair. To find new synthetic absorbable polymers with plastic nature that can be used as BMP-carrier materials, six types of poly-D,L-lactic acid-polyethylene glycol block copolymer (PLA-PEG) with various molecular weights of PLA and PEG were synthesized. These were PLA6, 500-PEG3,000 (P-1), PLA11,500-PEG3,000 (P-2), PLA17,500-PEG3,000 (P-3), PLA6,500-PEG1,000 (P-4), PLA15,000-PEG8,000 (P-5), and PLA8, 500-PEG1,000 (P-6). Fifty milligrams of these polymers was mixed with 0 microg (control) or 5, 10, or 20 microg of recombinant human BMP-2 (rhBMP-2). These pellets were implanted into the dorsal muscle pouches of 144 mice (six pellets consisting of the same polymer and dose of rhBMP-2 for a specific group). Three weeks after surgery, the pellets were harvested and examined by radiographic and histological methods. All P-1 pellets with 10 or 20 microg of rhBMP-2 showed bone formation with hematopoietic marrow and bony trabeculae, as did one third of those with 5 microg of rhBMP-2. The incidence of new bone formation with P-2 pellets or that of P-5 pellets was lower than that of P-1 pellets. No bone was formed in any other type of pellet. These results indicated that the PLA6, 500-PEG3,000 polymer with plastic properties was found to work well as a BMP carrier.

Bone regeneration produced in rat femur defects by polymer capsules containing recombinant human bone morphogenetic protein-2

PURPOSE

This study examines the effect of polymer capsules containing recombinant human bone morphogenetic protein-2 (rhBMP-2) on bone regeneration in a large segmental bone defect in the rat.

MATERIALS AND METHODS

Poly(DL-lactide-co-glycolide) (PLGA) capsules containing 3 microg of rhBMP-2 were implanted in a 5-mm segmental femoral defect in rats, and the femurs were harvested at 4 and 8 weeks after implantation and observed by radiographically and microscopically.

RESULTS

At 8 weeks after implantation, union of bone was found radiographically and histologically in the femoral defects implanted with the rhBMP-2/PLGA capsules. In contrast, the control animals did not show bridging of the defect.

CONCLUSIONS

This study provides evidence that use of rhBMP-2/PLGA capsules is a promising delivery system to regenerate bone.

The role of recombinant human bone morphogenetic protein-2 in PLGA capsules at an extraskeletal site of the rat

Bone morphogenetic protein-2 (BMP-2) is a member of the transforming growth factor-beta (TGF-beta) superfamily and has strong bone-inductive activity in vivo. To examine the role of BMP-2 in an extraskeletal site of rat using a controlled release system of peptides, we encapsulated the recombinant human BMP-2 (rhBMP-2) with poly(DL-lactide-co-glycolide) (PLGA) and implanted the rhBMP-2/PLGA capsules in the subcutaneous area of rats. Upon histochemical examination, it was found that bone-inducing cells having alkaline phosphatase (ALP) activity appeared around the capsules by the suitably released rhBMP-2. In addition, the temporal histological examination showed that direct bone formation without cartilage occurred in the process of this ectopic bone induction. These data indicate that the role of rhBMP-2 in the extraskeletal site of rats is to induce the differentiation of mesenchymal cells into the osteoblasts.

Implant fixation by bone ingrowth

The term osseointegration referred originally to an intimate contact of bone tissue with the surface of a titanium implant; the term bone ingrowth refers to bone formation within an irregular (beads, wire mesh, casting voids, cut grooves) surface of an implant. The section dealing with the historical background describes the development of macroporous, microporous, and textured surfaces with an emphasis on the evolution of porous and textured metal surfaces. The principal requirements for osseointegration and bone ingrowth are systematically reviewed as follows: i) the physiology of osseointegration and bone ingrowth, including biomaterial biocompatibility with respect to cellular and matrix response at the interface; ii) the implant surface geometry characteristics; iii) implant micromotion and fixation modes; and iv) the implant-bone interface distances. Based on current methods of bone ingrowth assessment, this article comparatively reviews and discusses the results of experimental studies with the objective of determining local and systemic factors that enhance bone ingrowth fixation.

Ectopic bone formation induced by biodegradable hydrogels incorporating bone morphogenetic protein

Biodegradable hydrogels were prepared from gelatin by glutaraldehyde cross-linking for release matrix of recombinant human bone morphogenetic protein-2 (BMP-2). BMP-2 solution was impregnated into the dried hydrogels to prepare BMP-2-incorporating gelatin hydrogels. In the in vitro study, enhanced retention of BMP-2 was observed from the BMP-2-incorporating gelatin hydrogels after an initial burst of BMP-2 incorporated initially in the hydrogel. Following subcutaneous implantation of (125)I-labeled BMP-2-incorporating gelatin hydrogels in the back of mice, the radioactivity remaining in the hydrogels was measured to estimate the in vivo release profile of BMP-2. It was found that BMP-2 was retained in the hydrogels for longer than 30 days, whereas 99% of BMP-2 injected in the solution form was cleared from the injected site within one day, completely disappearing within 3 days. Ectopic bone formation studies demonstrated that BMP-2-incorporating gelatin hydrogels exhibited a more potent ability for bone induction than solution injection of BMP-2. This finding indicates that enhanced retention of BMP-2 is promotes its ability to induce ectopic bone formation.

Healing bone using recombinant human bone morphogenetic protein 2 and copolymer

Middiaphyseal 2.5-cm segmental defects in the right femurs of 12 sheep were stabilized with stainless steel plates and implanted with (1) 2 mg recombinant human bone morphogenetic protein 2 and poly[D,L-(lactide-co-glycolide)] bioerodible polymer with autologous blood (n = 7), (2) 4 mg recombinant human bone morphogenetic protein 2 and poly[D,L-(lactide-co-glycolide)] and blood (n = 3), or (3) poly[D,L-(lactide-co-glycolide)] and blood only (n = 2). Bone healing was evaluated for 1 year using clinical, radiographic, gross pathologic, and histologic techniques. Union occurred in three sheep in Group 1, two in Group 2, and none in Group 3. In the animals that healed, new bone first was visible radiographically between Weeks 2 and 6 after implantation; new bone mineral content equaled that of the intact femur not surgically treated by Week 16; recanalization of the medullary cavity approached completion at Week 52; and at necropsy the surgical treated femurs were rigidly healed, the poly[D,L-(lactide-co-glycolide)] was resorbed completely, and woven and lamellar bone bridged the defect site. In two Group 1 sheep euthanized at Weeks 2 and 6, polymer particles were permeated by occasional multinucleated giant cells. Some plasma cells, lymphocytes, and neutrophils were present locally. The poly[D,L-(lactide-co-glycolide)] tended to fragment during surgical implantation. Despite these observations, the recombinant human bone morphogenetic protein 2/poly[D,L-(lactide-co-glycolide)] implant was able to heal large segmental bone defects in this demanding model.

Biodegradation and biocompatibility of PLA and PLGA microspheres

A fundamental understanding of the in vivo biodegradation phenomenon as well as an appreciation of cellular and tissue responses which determine the biocompatibility of biodegradable PLA and PLGA microspheres are important components in the design and development of biodegradable microspheres containing bioactive agents for therapeutic application. This chapter is a critical review of biodegradation, biocompatibility and tissue/material interactions, and selected examples of PLA and PLGA microsphere controlled release systems. Emphasis is placed on polymer and microsphere characteristics which modulate the degradation behaviour and the foreign body reaction to the microspheres. Selected examples presented in the chapter include microspheres incorporating bone morphogenetic protein (BMP) and leuprorelin acetate as well as applications or interactions with the eye, central nervous system, and lymphoid tissue and their relevance to vaccine development. A subsection on nanoparticles and nanospheres is also included. The chapter emphasizes biodegradation and biocompatibility; bioactive agent release characteristics of various systems have not been included except where significant biodegradation and biocompatibility information have been provided.