Ectopic induction of cartilage and bone by water-soluble proteins from bovine bone using a polyanhydride delivery vehicle

Galactosylation of rat natural scaffold for MSC differentiation into hepatocyte-like cells: A comparative analysis of 2D vs. 3D cell culture techniques

The liver plays a crucial role in drug detoxification, and the main source of liver transplants is brain-dead patients. However, the demand for transplants exceeds the available supply, leading to controversies in selecting suitable candidates for acute liver diseases. This research aimed to differentiate mesenchymal stem cells (MSCs) into hepatocyte-like cells using galactosylated rat natural scaffolds and comparing 2-D and 3-D cell culture methods. The study involved isolating and culturing Wharton's jelly cells from the umbilical cord, examining surface markers and adipogenic differentiation potential of MSCs, and culturing mesenchymal cells on galactosylated scaffolds. The growth and proliferation of stem cells on the scaffolds were evaluated using the MTT test, and urea synthesis was measured in different culture environments. Changes in gene expression were analyzed using real-time PCR. Flow cytometry results confirmed the presence of specific surface antigens on MSCs, indicating their identity, while the absence of a specific antigen indicated their differentiation into adipocytes. The MTT test revealed higher cell attachment to galactosylated scaffolds compared to the control groups. Urea secretion was observed in differentiated cells, with the highest levels in cells cultured on galactosylated scaffolds. Gene expression analysis showed differential expression patterns for OCT-4, HNF1, ALB, AFP, and CYP genes under different conditions. The findings indicated that hepatocyte-like cells derived from 3D cultures on galactosylated scaffolds exhibited superior characteristics compared to cells in other culture conditions. These cells demonstrated enhanced proliferation, stability, and urea secretion ability. The study also supported the differentiation potential of MSCs derived from Wharton's jelly umbilical cord into liver-like cells.

Biodegradable Polyphosphazenes for Regenerative Engineering

Regenerative engineering is a field that seeks to regenerate complex tissues and biological systems, rather than simply restore and repair individual tissues or organs. Since the first introduction of regenerative engineering in 2012, numerous research has been devoted to the development of this field. Biodegradable polymers such as polyphosphazenes in particular have drawn significant interest as regenerative engineering materials for their synthetic flexibility in designing into materials with a wide range of mechanical properties, degradation rates, and chemical functionality. These polyphosphazenes can go through complete hydrolytic degradation and provide harmlessly and pH neutral buffering degradation products such as phosphates and ammonia, which is crucial for reducing inflammation in vivo. Here, we discuss the current accomplishments of polyphosphazene, different methods for synthesizing them, and their applications in tissue regeneration such as bones, nerves, and elastic tissues.

The Role of Nanomaterials and Biological Agents on Rotator Cuff Regeneration

The rotator cuff is a musculotendon unit responsible for movement in the shoulder. Rotator cuff tears represent a significant number of musculoskeletal injuries in the adult population. In addition, there is a high incidence of retear rates due to various complications within the complex anatomical structure and the lack of proper healing. Current clinical strategies for rotator cuff augmentation include surgical intervention with autograft tissue grafts and beneficial impacts have been shown, but challenges still exist because of limited supply. For decades, nanomaterials have been engineered for the repair of various tissue and organ systems. This review article provides a thorough summary of the role nanomaterials, stem cells and biological agents have played in rotator cuff repair to date and offers input on next generation approaches for regenerating this tissue.

Regenerative Engineering Animal Models for Knee Osteoarthritis

Abstract

Osteoarthritis (OA) of the knee is the most common synovial joint disorder worldwide, with a growing incidence due to increasing rates of obesity and an aging population. A significant amount of research is currently being conducted to further our understanding of the pathophysiology of knee osteoarthritis to design less invasive and more effective treatment options once conservative management has failed. Regenerative engineering techniques have shown promising preclinical results in treating OA due to their innovative approaches and have emerged as a popular area of study. To investigate these therapeutics, animal models of OA have been used in preclinical trials. There are various mechanisms by which OA can be induced in the knee/stifle of animals that are classified by the etiology of the OA that they are designed to recapitulate. Thus, it is essential to utilize the correct animal model in studies that are investigating regenerative engineering techniques for proper translation of efficacy into clinical trials. This review discusses the various animal models of OA that may be used in preclinical regenerative engineering trials and the corresponding classification system.

Lay Summary

Osteoarthritis (OA) of the knee is the most common synovial joint disease worldwide, with high rates of occurrence due to an increase in obesity and an aging population. A great deal of research is currently underway to further our understanding of the causes of osteoarthritis, to design more effective treatments. The emergence of regenerative engineering has provided physicians and investigators with unique opportunities to join ideas in tackling human diseases such as OA. Once the concept is proven to work, the initial procedure for the evaluation of a treatment solution begins with an animal model. Thus, it is essential to utilize a suitable animal model that reflects the particular ailment in regenerative engineering studies for proper translation to human patients as each model has associated advantages and disadvantages. There are various ways by which OA can occur in the knee joint, which are classified according to the particular cause of the OA. This review discusses the various animal models of OA that may be used in preclinical regenerative engineering investigations and the corresponding classification system.

Effect of steroidal saponins-loaded nano-bioglass/phosphatidylserine/collagen bone substitute on bone healing

The objective of this study was to investigate the therapeutic potential of nano-bioglass/phosphatidylserine/collagen (nBG/PS/COL) scaffolds loaded with steroidal saponins as an inducer factor for skeletal defects. The drugs-encapsulated bone substitute was prepared by loading steroidal saponins-collagen microsphere suspension in nano-bioglass and phosphatidylserine (PS) composite. The scaffolds possess an interconnected porous structure with a porosity of about 82.3%. The pore size ranges from several micrometers up to about 400 μm. The drug release assays showed the long-term sustained release of steroidal saponins from the scaffolds with effective and safe bioactivity. Moreover,

Formulation, Delivery and Stability of Bone Morphogenetic Proteins for Effective Bone Regeneration

Bone morphogenetic proteins (BMPs) are responsible for bone formation during embryogenesis and bone regeneration and remodeling. The osteoinductive action of BMPs, especially BMP-2 and BMP-7, has led to their use in a range of insurmountable treatments where intervention is required for effective bone regeneration. Introduction of BMP products to the market, however, was not without reports of multiple complications and side effects. Aiming for optimization of the therapeutic efficacy and safety, efforts have been focused on improving the delivery of BMPs to lower the administered dose, localize the protein, and prolong its retention time at the site of action. A major challenge with these efforts is that the protein stability should be maintained. With this review we attempt to shed light on how the stability of BMPs can be affected in the formulation and delivery processes. We first provide a short overview of the current standing of the complications experienced with BMP products. We then discuss the different delivery parameters studied in association with BMPs, and their influence on the efficacy and safety of BMP treatments. In particular, the literature addressing the stability of BMPs and their possible interactions with components of the delivery system as well as their sensitivity to conditions of the formulation process is reviewed. In summary, recent developments in the fields of bioengineering and biopharmaceuticals suggest that a good understanding of the relationship between the formulation/delivery conditions and the stability of growth factors such as BMPs is a prerequisite for a safe and effective treatment.

Bone substitutes in orthopaedic surgery: from basic science to clinical practice

Bone substitutes are being increasingly used in surgery as over two millions bone grafting procedures are performed worldwide per year. Autografts still represent the gold standard for bone substitution, though the morbidity and the inherent limited availability are the main limitations. Allografts, i.e. banked bone, are osteoconductive and weakly osteoinductive, though there are still concerns about the residual infective risks, costs and donor availability issues. As an alternative, xenograft substitutes are cheap, but their use provided contrasting results, so far. Ceramic-based synthetic bone substitutes are alternatively based on hydroxyapatite (HA) and tricalcium phosphates, and are widely used in the clinical practice. Indeed, despite being completely resorbable and weaker than cortical bone, they have exhaustively proved to be effective. Biomimetic HAs are the evolution of traditional HA and contains ions (carbonates, Si, Sr, Fl, Mg) that mimic natural HA (biomimetic HA). Injectable cements represent another evolution, enabling mininvasive techniques. Bone morphogenetic proteins (namely BMP2 and 7) are the only bone inducing growth factors approved for human use in spine surgery and for the treatment of tibial nonunion. Demineralized bone matrix and platelet rich plasma did not prove to be effective and their use as bone substitutes remains controversial. Experimental cell-based approaches are considered the best suitable emerging strategies in several regenerative medicine application, including bone regeneration. In some cases, cells have been used as bioactive vehicles delivering osteoinductive genes locally to achieve bone regeneration. In particular, mesenchymal stem cells have been widely exploited for this purpose, being multipotent cells capable of efficient osteogenic potential. Here we intend to review and update the alternative available techniques used for bone fusion, along with some hints on the advancements achieved through the experimental research in this field.

Macroscale delivery systems for molecular and cellular payloads

Macroscale drug delivery (MDD) devices are engineered to exert spatiotemporal control over the presentation of a wide range of bioactive agents, including small molecules, proteins and cells. In contrast to systemically delivered drugs, MDD systems act as a depot of drug localized to the treatment site, which can increase drug effectiveness while reducing side effects and confer protection to labile drugs. In this Review, we highlight the key advantages of MDD systems, describe their mechanisms of spatiotemporal control and provide guidelines for the selection of carrier materials. We also discuss the combination of MDD technologies with classic medical devices to create multifunctional MDD devices that improve integration with host tissue, and the use of MDD technology in tissue-engineering strategies to direct cell behaviour. As our ever-expanding knowledge of human biology and disease provides new therapeutic targets that require precise control over their application, the importance of MDD devices in medicine is expected to increase.

Optimally porous and biomechanically compatible scaffolds for large-area bone regeneration

Large-area or critical-sized bone defects pose a serious challenge in orthopedic surgery, as all current treatment options present with shortcomings. Bone tissue engineering offers a more promising alternative treatment strategy. However, this approach requires mechanically stable scaffolds that support homogenous bone formation throughout the scaffold thickness. Despite advances in scaffold fabrication, current scaffold-based techniques are unable to support uniform, three-dimensional bone regeneration, and are limited to only the scaffold surface in vitro and in vivo. This is mainly because of inadequate scaffold pore sizes (<200 μm) and accessible pore volume, and the associated limited oxygen diffusion and vascular invasion. In this study, we have adopted a method combining microsphere-sintering and porogen-leaching techniques to fabricate scaffolds with an increased accessible pore volume. Of the scaffolds developed, moderately porous poly(85 lactide-co-15 glycolide) (PLGA) microsphere scaffolds were selected as most advantageous, since they retain mechanical strength in the range of human cancellous bone and display a significantly higher accessible pore volume, which is attributed to an increased percentage of larger pores (i.e., size range 200-600 μm). Unlike control scaffolds with a limited pore size and an accessible pore volume, moderately porous scaffolds displayed increased oxygen diffusion, pre-osteoblast cell infiltration, proliferation, and survival throughout the entire scaffold. Furthermore, moderately porous PLGA microsphere scaffolds displayed enhanced and homogenous mineralization in vitro. Since these newly designed moderately porous scaffolds are weight bearing, are fully osteoconductive, and have the ability to support vascularization, they may serve as effective scaffolds for large-area bone defect repair/regeneration. In addition, this study demonstrates the ability to modulate scaffold porosity and, in turn, to develop oxygen tension-controlled matrices that are effective for large-area bone regeneration.

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

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

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

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

Bioactive scaffolds for bone and ligament tissue

Bone and ligament injuries present the greatest challenges in connective tissue regeneration. The design of materials for these applications lies at the forefront of material science and is the epitome of its current ambition. Indeed, its goal is to design and fabricate reproducible, bioactive and bioresorbable 3D scaffolds with tailored properties that are able to maintain their structure and integrity for predictable times, even under load-bearing conditions. Unfortunately, the mechanical properties of today's available porous scaffolds fall short of those exhibited by complex human tissues, such as bone and ligament. The manipulation of structural parameters in the design of scaffolds and their bioactivation, through the incorporation of soluble and insoluble signals capable of promoting cell activities, are discussed as possible strategies to improve the formation of new tissues both in vitro and in vivo. This review focuses on the different approaches adopted to develop bioactive composite systems for use as temporary scaffolds for bone and anterior ligament regeneration.

Degradable hydrogel scaffolds for in vivo delivery of single and dual growth factors in cartilage repair

OBJECTIVE

As our population ages, treatment for joint pain associated with articular cartilage damage is becoming a prevalent challenge. Accordingly, this work investigates local delivery of two regulatory proteins - transforming growth factor-beta1 (TGF-beta1) and insulin-like growth factor-1 (IGF-1) - to cartilage defects from degradable scaffolds as a potential strategy for improving cartilage repair.

METHOD

The effects of TGF-beta1 and/or IGF-1 delivery on osteochondral repair in adult rabbits were examined through histomorphometric analysis of 11 markers of osteochondral repair.

RESULTS

Complete scaffold degradation occurred allowing for assessment of the healing response at 12 weeks post-surgery. When compared to untreated defects, higher scores were observed with IGF-1-treated defects for the six markers of neo-surface repair: neo-surface morphology, cartilage thickness, surface regularity, chondrocyte clustering, and the chondrocyte/glycosaminoglycan content of the neo-surface and the cartilage surrounding the defect. Surprisingly, the benefits of IGF-1 delivery were not maintained when this growth factor (GF) was co-delivered with TGF-beta1, despite numerous in vitro reports of the combinatory actions of these GFs.

CONCLUSIONS

While localized delivery of IGF-1 may be a promising repair strategy, further in vivo assessment is necessary, since fibrous tissue was commonly observed in the neo-surface of all treatment groups. More importantly, this study highlights the need to rigorously examine GF interactions in the wound healing environment and demonstrates that in vitro observations do not directly translate to the in vivo setting.

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

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

Tissue engineering of human cartilage and osteochondral composites using recirculation bioreactors

Chondrocytes isolated from human foetal epiphyseal cartilage were seeded dynamically into polyglycolic acid (PGA) scaffolds and cultured in recirculation column bioreactors to produce tissue-engineered cartilage. Several culture techniques with the potential to provide endogenous growth factors and other conditions beneficial for de novo cartilage synthesis were investigated. Osteochondral composite constructs were generated by seeding separate PGA scaffolds with either foetal chondrocytes or foetal osteoblasts then suturing the scaffolds together before bioreactor cultivation. This type of co-culture system provided direct contact between the tissue-engineered cartilage and developing tissue-engineered bone and yielded significant improvements in cartilage quality. In the cartilage section of the composites, the concentrations of glycosaminoglycan (GAG) and total collagen were increased by 55% and 2.5-fold, respectively, compared with control cartilage cultures, while levels of collagen type II were similar to those in the controls. The osteochondral composites were harvested from the bioreactors as single units with good integration between the cartilage and bone tissues. Only the cartilage layer contained GAG while only the bone layer was mineralised. In other experiments, co-culture of tissue-engineered cartilage with pieces of ex-vivo cartilage or ex-vivo bone did not improve the quality of the cartilage relative to control cultures. Addition of 10(-6) M diacerein to the culture medium also had no effect on the properties of engineered cartilage. This work demonstrates the beneficial effects of generating cartilage tissues in contact with developing bone. It also demonstrates the feasibility of producing composite osteochondral constructs for clinical application using recirculation column bioreactors.

Delivery of bone morphogenetic proteins for orthopedic tissue regeneration

Carriers for bone morphogenetic proteins (BMPs) are used to increase retention of these factors at orthopedic treatment sites for a sufficient period of time to allow regenerative tissue forming cells to migrate to the area of injury and to proliferate and differentiate. Carriers can also serve as a matrix for cell infiltration while maintaining the volume in which repair tissue can form. Carriers have to be biocompatible and are often required to be bioresorbable. Carriers also have to be easily, and cost-effectively, manufactured for large-scale production, conveniently sterilized and have appropriate storage requirements and stability. All of these processes have to be approvable by regulatory agencies. The four major categories of BMP carrier materials include natural polymers, inorganic materials, synthetic polymers, composites of these materials. Autograft or allograft carriers have also used. Carrier configurations range from simple depot delivery systems to more complex systems mimicking the extracellular matrix structure and function. Bone regenerative carriers include depot delivery systems for fracture repair, three-dimensional polymer or ceramic composites for segmental repairs and spine fusion and metal or metal/ceramic composites for augmenting implant integration. Tendon/ligament regenerative carriers range from depot delivery systems to three-dimensional carriers that are either randomly oriented or linearly oriented to improve regenerative tissue alignment. Cartilage regenerative systems generally require three-dimensional matrices and often incorporate cells in addition to factors to augment the repair. Alternative BMP delivery systems include viral vectors, genetically altered cells, conjugated factors and small molecules.

In vitro and in vivo evaluation of differentially demineralized cancellous bone scaffolds combined with human bone marrow stromal cells for tissue engineering

Mineralized and partially or fully demineralized biomaterials derived from bovine bone matrix were evaluated for their ability to support human bone marrow stromal cell (BMSC) osteogenic differentiation in vitro and bone-forming capacity in vivo in order to assess their potential use in clinical tissue-engineering strategies. BMSCs were either seeded on bone-derived scaffolds and cocultured in direct cell-to-scaffold contact, allowing for the exposure of soluble and insoluble matrix-incorporated factors, or cocultured with the scaffold preparations in a transwell system, exposing them to soluble matrix-incorporated factors alone. Osteoblast-related markers, alkaline phosphatase (ALP) activity and bone sialoprotein (BSP) and osteopontin (OP) mRNA expression were evaluated in BMSCs following 14 days of cocultivation in both systems. The data demonstrate that BMSCs from some donors express significantly higher levels of all osteoblast-related markers following cocultivation in direct cell-to-scaffold contact with mineralized scaffolds in comparison to fully demineralized preparations, while BMSCs from other donors display no significant differences in response to various scaffold preparations. In contrast, BMSCs cocultured independently with soluble matrix-incorporated factors derived from each scaffold preparation displayed significantly lower levels of ALP activity and BSP mRNA expression in comparison to untreated controls, while no significant differences were observed in marker levels between cells cocultured similarly with different biomaterial preparations. In addition, BMSCs were seeded directly on mineralized and partially or fully demineralized biomaterials and implanted in subcutaneous sites of athymic mice for 8 weeks to evaluate their in vivo bone-forming capacity. The ex vivo incorporation of BMSCs into all bone-derived scaffold preparations substantially increased the mean extent and frequency of samples containing de novo bone formation over similar nonseeded controls, as determined by histological and histomorphometrical analysis. No statistically significant differences were observed in the extent or frequency of bone formation between various scaffold preparations seeded with BMSCs from different donors. These results demonstrate that the in vivo osteoinductivity of bone-derived scaffolds can be modulated by ex vivo incorporated BMSCs and the extent of scaffold demineralization plays a significant role in influencing in vitro osteogenic differentiation of BMSCs depending on the coculture system and BMSC donor.

Role of polyanhydrides as localized drug carriers

Many drugs that are administered in an unmodified form by conventional systemic routes fail to reach target organs in an effective concentration, or are not effective over a length of time due to a facile metabolism. Various types of targeting delivery systems and devices have been tried over a long period of time to overcome these problems. Targeted delivery or localized drug delivery offers an advantage of reduced body burden and systemic toxicity of the drugs, especially useful for highly toxic drugs like anticancer agents. Local drug delivery via polymer is a simple approach and hypothesized to avoid the above stated problems. Polyanhydrides are a unique class of polymer for drug delivery because some of them demonstrate a near zero order drug release and relatively rapid biodegradation in vivo. Further, the release rate of polyanhydride fabricated device can be altered over a thousand fold by simple changes in the polymer backbone. Hence, these are one of the best-suited polymers for drug delivery, with biodegradability and biocompatibility. The review focuses on the advantages of polyanhydride carriers in localized drug delivery along with their degradability behavior, toxicological profile and role in various disease conditions.

Development of a porous poly(L-lactic acid)/hydroxyapatite/collagen scaffold as a BMP delivery system and its use in healing canine segmental bone defect

A hydroxyapatite/collagen (HAC) composite was produced to mimic the natural extracellular matrix of bone, with the collagen serving as a template for apatite formation. A three-dimensional highly porous scaffold was developed by mixing HAC with poly(L-lactic acid) (PLA) using a thermally induced phase separation technique. Naturally derived bovine bone morphogenetic protein (bBMP) was incorporated into the porous HAC-PLA scaffolds, and the composite then was implanted in diaphyseal defects (2 cm in radius) of adult beagle dogs. Controls were implanted with scaffolds without BMP. The dogs were sacrificed at 6 months, at which time biocompatibility, biodegradability, and osteoinduction were evaluated by histologic and radiologic examination and by bone mineral density (BMD) measurements. All defects healed after treatment with BMP combined with HAC-PLA, and BMD at the site of the defect was higher than the BMD of the intact radius. Fibrous union developed in the control group animals. Histologic observation indicated that the presence of BMP not only promoted osteogenesis but that it also accelerated degradation of the biomaterials. Optimized design parameters of a three-dimensional porous biomaterial would give full scope to the role of BMP as an osteoinductive growth factor.

In vitro bone formation using muscle-derived cells: a new paradigm for bone tissue engineering using polymer-bone morphogenetic protein matrices

Over 800,000 bone grafting procedures are performed in the United States annually, creating a demand for viable alternatives to autogenous bone, the grafting standard in osseous repair. The objective of this study was to examine the efficacy of a BMP-polymer matrix in inducing the expression of the osteoblastic phenotype and in vitro bone formation by muscle-derived cells. Specifically, we evaluated the ability of bone morphogenetic protein-7 (BMP-7), delivered from a poly(lactide-co-glycolide) (PLAGA) matrix, to induce the differentiation of cells derived from rabbit skeletal muscle into osteoblast-like cells and subsequently form mineralized tissue. Results confirmed that muscle-derived cells attached and proliferated on the PLAGA substrates. BMP-7 released from PLAGA induced the muscle-derived cells to increase bone marker expression and form mineralized cultures. These results demonstrate the efficacy of a BMP-polymer matrix in inducing the expression of the osteoblastic phenotype by muscle-derived cells and present a new paradigm for bone tissue engineering.

Bone tissue engineering: recent advances and promising therapeutic agents

Bone regeneration can be accomplished with growth factors, cells and delivery systems. This review is a summary of these components that may be used for tissue regeneration. Support for the potential therapeutic applications of transcription factors in bone tissue engineering will also be discussed.

Delivery of osteoinductive growth factors from degradable PEG hydrogels influences osteoblast differentiation and mineralization

Degradable poly(ethylene glycol) (PEG) hydrogels with varying mass loss profiles were investigated to assess their applicability as delivery vehicles for osteoinductive growth factors in bone tissue engineering. Protein release is readily controlled by changes in both the structure (i.e., macromer concentration) and chemistry (i.e., number of degradable units) of the starting macromer. In vitro studies indicate an increase in total protein levels, alkaline phosphatase, and mineralization by osteoblasts cultured in the presence of osteoinductive growth factors compared to cells cultured with standard media. When growth factors are delivered from a 25 wt% hydrogel, a significant increase in both alkaline phosphatase and mineralization was seen after 3 weeks of culture over growth factor delivery in a bolus fashion. Additionally, gene expression levels of both osteocalcin and type I collagen were higher at early timepoints when growth factors were released from hydrogels. These results indicate that growth factors remain active after photoencapsulation, that the sustained delivery of growth factors alters various markers of osteoblastic differentiation, and that these networks could be useful as delivery vehicles for growth factors in bone tissue engineering. Finally, ectopic bone formation was present in subcutaneous rat tissue after implantation of hydrogel networks loaded with osteoinductive growth factors.

Bone morphogenetic protein delivery systems

STUDY DESIGN

A review was conducted.

OBJECTIVES

To review the rationale for the use of carrier systems to deliver bone morphogenetic proteins to sites of orthopedic repair, and to discuss commonly used carriers.

SUMMARY OF BACKGROUND DATA

Carriers for bone morphogenetic protein in spine fusion are used to increase the retention of these osteogenic factors at the treatment site, and to serve as an osteoconductive matrix for bone forming cells while maintaining a space or volume in which bone formation can occur.

METHODS

The literature is reviewed and discussed.

RESULTS

Although bone morphogenetic proteins can induce bone formation when delivered in formulation buffer in small animal models, carriers often are used in larger animal models and human clinical trials to maintain the concentration of osteogenic factors at the treatment site for a sufficient period to allow bone-forming cells to migrate to the area of injury and to proliferate and differentiate. For spine fusion, carriers also are required to serve as an osteoconductive matrix for bone-forming cells while maintaining a space or volume in which bone formation can occur. Four major categories of carrier materials are used for osteogenic factor delivery: inorganic materials, synthetic polymers, natural polymers, and composites of the first three materials. In addition, allograft bone has been used to deliver osteogenic factors to the site of orthopedic repairs. The efficacy of osteogenic carrier combinations often is site specific and species specific. The requirement for supraphysiologic concentrations of osteogenic factors may be related to the ability of the delivery system to increase the retention time at the treatment site and overcome tight regulation of these factors by their inhibitors. Dose escalation in large animal models also may be related to a decrease in the number of responding cells and a slower rate of bone formation. New delivery systems being evaluated include depot delivery systems, viral vector systems, conjugated osteogenic factor delivery systems, and oral small molecule targets.

CONCLUSIONS

Delivery systems play an important role in the use of osteogenic factors to augment spine fusions and other orthopedic repairs.

Delivering on the promise of bone morphogenetic proteins

The advent of bone growth factors has been widely anticipated since their successful production using recombinant DNA technology. Bone morphogenetic proteins (BMPs) are an important class of bone growth factors and will be the focus of this article. In the near future these therapeutics might revolutionize how clinicians treat such diverse orthopedic applications as the healing of broken bones, increasing bone density lost through aging, and strengthening the spine. These potent proteins require application directly at the site of repair via a delivery system. The choice of delivery system has a profound effect on the clinical outcome. In the past decade, researchers have focused on developing efficient delivery systems and advancing these factors from the bench to the clinic.

Poly(lactide-co-glycolide)/hydroxyapatite delivery of BMP-2-producing cells: a regional gene therapy approach to bone regeneration

Currently, functional treatment of fracture non-unions and bone loss remains a significant challenge in the field of orthopaedic surgery. Tissue engineering of bone has emerged as a new treatment alternative in bone repair and regeneration. Our approach is to combine a polymeric matrix with a cellular vehicle for delivery of bone morphogenetic protein-2 (BMP-2), constructed through retroviral gene transfer. The objective of this study is to develop an osteoinductive, tissue-engineered bone replacement system by culturing BMP-2-producing cells on an osteoconductive, biodegradable, polymeric-ceramic matrix. The hypothesis is that retroviral gene transfer can be used effectively in combination with a biodegradable matrix to promote bone formation. First, we examined the in vitro attachment and growth of transfected BMP-producing cells on a PLAGA-HA scaffold. Second, the bioactivity of the produced BMP in vitro was evaluated using a mouse model. It was found that the polymer-ceramic scaffold supported BMP-2 production, allowing the attachment and growth of retroviral transfected, BMP-2-producing cells. In vivo, the scaffold successfully functioned as a delivery vehicle for bioactive BMP-2, as it induced heterotopic bone formation in a SCID mouse model.

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.

A biodegradable polymer scaffold for delivery of osteotropic factors

Despite discoveries and developments in osteotropic factors, therapies exploiting these macromolecules have been limited due to a lack of suitable delivery vehicles and three dimensional (3D) scaffolds that promote bone regeneration. To address this limitation, an emulsion freeze-drying process was developed to fabricate biodegradable scaffolds with controlled microarchitecture, and the ability to incorporate and deliver bioactive macromolecules for bone regeneration. The effect of median pore size and protein loading on protein release kinetics was investigated using scaffolds with different protein loading and median pore sizes ranging from 7 to 70 microm. Graphs of protein release from scaffolds showed an initial burst followed by a slower sustained release. Release kinetics were characterized using an unsteady-state, diffusion-controlled model with an effective diffusivity that took tortuosity (tau) and partition coefficient for protein adsorption (Kp) onto the scaffold walls into account. Tortuosity and partition coefficient significantly reduced the protein diffusivity by a factor of 41 +/- 43 and 105 +/- 51 for 60 and 30-microm median pore-sized scaffolds, respectively. The activity of the protein released from these scaffolds was demonstrated by delivering rhBMP 2 and [A-4] (an amelogenin derived polypeptide) proteins from the scaffold and regenerating bone in a rat ectopic bone induction assay [Whang et al. J Biomed Mater Res 1998;42:491-9, Veis et al. J Bone Mineral Res, Submitted].

Heterotopic ossification of degenerating rat skeletal muscle induced by adenovirus-mediated transfer of bone morphogenetic protein-2 gene

In vivo gene transfer is a recently developed device for efficient delivery of a therapeutic recombinant protein. We formulated the hypothesis that a high level of expression of bone morphogenetic protein 2 (BMP-2) could be a future therapeutic modality in terms of inducing substantial bone formation in vivo. First, to test this hypothesis, adenoviruses carrying BMP-2 gene were directly injected into the soleus muscle of adult rat. The BMP-2 gene was successfully overexpressed in the target muscle by adenovirus-mediated transfer, whereas bone formation in and around the muscle failed to occur in this case. Second, to recruit putative osteoprogenitor cells, we then induced ischemic degeneration of the target muscle by orthotopically grafting it simultaneously with the gene transfer. The combination of BMP-2 gene transfer and orthotopic muscle grafting resulted in successful ossification of almost the whole grafted muscle, whereas neither muscle grafting alone nor the combination of muscle grafting and adenovirus-mediated transfer of reporter gene LacZ induced any bone formation in the muscle. The ossification process was evident by positive von Kossa staining of the histological sections and roentgenographical radio-opacity of the region. It was also found that the BMP-2 transgene overexpressed in grafted muscles inhibited muscle regeneration, which should otherwise follow the muscle degeneration. We further demonstrated an up-regulation of BMP receptor type IA in grafted muscles, suggesting its involvement in the bone-formation process. In conclusion, overexpression of BMP-2 gene induced massive heterotopic ossification in skeletal muscles under graft-induced ischemic degeneration, which possibly up-regulates osteoprogenitor cells in situ.

Biocompatibility and effect on osteogenesis of poly(ortho ester) compared to poly(DL-lactic acid)

Implantation of demineralized bone induces new bone formation by the action of contained growth factors, of which bone morphogenetic proteins are of prime importance. A biodegradable polymer may be used as a carrier for demineralized bone particles or recombinant bone growth factors to prevent displacement of the implant, preserve its volume and shape, and assure sustained release of the incorporated active components. A polymer for this use should be biocompatible and completely absorbed without interfering with the osteogenesis. We investigated the host-tissue response and effect on demineralized bone-induced bone formation by two biodegradable polymers, a poly(ortho ester) and an amorphous low-molecular poly(DL-lactic acid). Both polymers had a plastic consistency, could easily be molded, and adhered well to the demineralized bone particles. Demineralized bone particles were implanted alone and in combination with each of the polymers in the abdominal muscles of 45 male Wistar rats. Four weeks after the operation the implants were recovered and subjected to (85)Sr uptake analysis to quantify bone formation and histologic examination. The poly(ortho ester) provoked little inflammation; it was largely absorbed by 4 weeks, and no qualitative or quantitative effect on bone formation was found. The poly(DL-lactic acid) provoked a chronic inflammation with multinuclear giant cells, macrophages with engulfed material, and proliferating fibroblasts; part of the material was still present, and the bone formation was inhibited.

Polypeptide growth factors: targeted delivery systems

Growth factors are becoming extremely valuable tools in our attempts to understand the mechanisms that modulate cellular activities. Their targeting to appropriate cells and maintaining adequate pharmacological levels becomes essential, particularly in view of the different effects that these compounds have on various cells and the dose dependence of their response. Within this context, this review focuses primarily on the delivery of growth factors involved in the processes of wound healing and tissue repair.

Enhanced healing of segmental tibial defects in sheep by a composite bone substitute composed of tricalcium phosphate cylinder, bone morphogenetic protein, and type IV collagen

Diaphyseal segmental defects in the tibia of 18 sheep were used to evaluate the healing potential of a composite bone substitute device (CBS) composed of a tricalcium phosphate cylinder (TCP), naturally occurring sheep bone morphogenetic protein (sBMP), and type IV collagen. A total of 100 mg of sBMP and 20 mg of type IV collagen in the high-dose group (CBSH), and 13 mg of sBMP and 2.5 mg of type IV collagen in the low-dose group (CBSL) were adsorbed to TCP cylinders, respectively. TCP cylinders impregnated with type IV collagen alone (TCPC) were used as control. A significantly larger area and more highly integrated intensity of newly formed external callus between CBSH and CBSL or TCPC group were quantified by computerized image analyzer at both 3 and 6 weeks. A torsion test showed that the maximal torque capacity, maximal angular deformation, and bone stiffness of healed osteotomized tibia with implants recovered 117-125% in CBSH, 72-109% in CBSL, and 63-80% in TCPC, compared with the corresponding contralateral tibia at 16 weeks. A healing superiority of the segmental bone defects replaced by the implants was demonstrated in the CBSH group. Thus, the composite bone substitute device defined in this study was shown to possess osteoinductivity, osteoconductivity, and mechanical strength.

Bone morphogenetic protein encapsulated with a biodegradable and biocompatible polymer

To develop a controlled release system for bone morphogenetic protein (BMP), poly(DL-lactide-co-glycolide) (PLGA) capsules containing BMP were prepared by an interfacial precipitation method using a water-in-oil-in-water emulsion. The surface morphology, particle size distribution, and hydrolytic degradation rate of the PLGA capsules were examined. The encapsulation yield and release rate of BMP in vitro were measured using fluorescein isothiocyanate-labeled BMP. The amount of BMP released from PLGA capsules increased between days 3 and 5. In addition, the effectiveness of BMP encapsulated in PLGA to induce bone formation in vivo was also examined by subcutaneous implantation in rats. Complete digestion of the capsules and new bone formation including bone marrow were identified by histologic examination of harvested tissues at 3 weeks after implantation. These results demonstrated that encapsulation of BMP with PLGA could be a promising method to induce bone in clinics.

Alternative materials for fracture fixation

Biodegradable polymers are gaining acceptance as alternative materials to metallic devices in fracture fixation applications. Self-reinforced polyglycolide and polylactide fixation devices have been used effectively in managing several thousand clinical cases involving small-fragment fractures. An eight percent incidence of non-specific inflammatory reaction and insufficient mechanical strength to deal with load-bearing situations are the two issues preventing the widespread acceptance of these biodegradable fixation devices. This brief review examines the potential of other biodegradable polymers, discusses the biodegradable composite design, and considers future research directions that might realize the full potential of biodegradable polymeric prostheses.

Histological changes of implanted collagen material during bone induction

Studies were made on the fate of implanted material during bone induction. Mixtures of 1 mg of crude bone morphogenetic protein (BMP), or bovine serum albumin as a control, and 1.5 mg of bovine collagen, were pressed into discs and implanted under the fascia of the rectus abdominus muscle of rats. The tissues with implants were fixed 7, 10, and 14 days later and examined histologically. On day 7 after implantation, the implant was surrounded and invaded by alkaline phosphatase-positive cells. New bone and cartilage were seen at the periphery of the implant. In the regions of calcified cartilage and bone, these osteogenic matrices were intermixed with the implant. The mineral deposits were seen by electron microscopy not only on the osteogenic matrices but also on the implanted collagen. On day 14, the bone had spread to the center of the implant. No osteogenesis or chondrogenesis was seen in control implants. It was concluded that the calcification occurred on the implanted collagen during bone induction, and that it was related to successive bone formation and remodeling.

Resorbable synthetic polymers as replacements for bone graft

The potential of resorbable synthetic polymers derived from the poly(alpha-hydroxy acids), poly(lactide) and poly(glycolide), to fulfill a role as bone graft substitutes is reviewed. The various elements of the relationship between the degradation behaviour of resorbable implants and polymer synthesis and chain structure, implant morphology, processing and dimensions have been defined. The production of resorbable polymeric implants has been extensively documented so as to provide a wide basis for selection of an appropriate manufacturing technique. The key requirement of implant dimensional stability over the early stages of bone healing is emphasised so as to provide a stable surface on which osteoblasts and/or their precursor cells may migrate and secrete bone matrix. Minimisation of the content of slow resorbing polymers such as poly(L-lactide) is recommended, consistent with retention of an adequate implant degradation characteristic. The review concludes with a summary of alternative resorbable polymers such as the polyphosphazines which are interesting candidate materials for bone repair and reconstruction.

Controlled delivery systems for proteins using polyanhydride microspheres

A method to provide near-constant sustained release of high molecular weight, water-soluble proteins from polyanhydride microspheres is described. The polyanhydrides used were poly(fatty acid dimer) (PFAD), poly(sebacic acid) (PSA), and their copolymers [P(FAD-SA)]. P(FAD-SA) microspheres containing proteins of different molecular sizes--lysozyme, trypsin, heparinase, ovalbumin, albumin, and immunoglobulin--were prepared by a solvent evaporation method using a double emulsion. The microspheres containing proteins were spherical, with diameters of 50-125 microns, and encapsulated more than 80% of the protein, irrespective of the protein used. Enzymatic activity studies showed that encapsulation of enzymes inside polyanhydride microspheres can protect them from activity loss. When not placed inside polyanhydride microspheres, trypsin lost 80% of its activity in solution at 37 degrees C at pH 7.4 in 12 hr, whereas inside the polyanhydride microspheres the activity loss was less than 10% under these conditions. About 47% of the enzymatic activity of heparinase encapsulated in the microspheres was lost at 37 degrees C in 24 hr, while in solution it lost over 90% of its activity. The protein-loaded microspheres displayed near-zero-order erosion kinetics over 5 days as judged by the release of sebacic acid (SA) from the microspheres. The microspheres degraded to form SA and FAD monomers. All proteins were released at a near-constant rate without any large initial burst, irrespective of polymer molecular weight and protein loading. The period of protein release was longer than that of SA and continued protein release was observed even after the microsphere matrix had completely degraded. Differential scanning calorimetric studies demonstrated an interaction between protein and the FAD monomers produced with microsphere degradation. It is likely that the protein interaction with FAD monomers permits formation of water-insoluble protein aggregates which slowly dissolve and diffuse out of the matrix, leading to delayed protein release. For trypsin-loaded microspheres, trypsin lost 40% of its activity during microsphere preparation. Activity studies demonstrated that the sonication process was primarily responsible for activity loss. A reduction in the period of ultrasound exposure decreased the loss of protein activity to around 20%.

The functional equivalence of demineralized bone and tooth matrices in ectopic bone induction

The objective of this study was to determine whether demineralized rat incisor matrices were a more potent inducer of ectopic endochondral bone formation than demineralized diaphyseal bone matrices derived from the same donors. Twenty-five-milligram disks of demineralized bone or tooth matrix obtained from adolescent Long-Evans rats were implanted in a standardized ectopic site. Biochemical and histometric measurements of bone formation revealed that the two matrices were functionally equivalent inducers of endochondral bone formation. The induced pellicle of bone reached a maturation point 18 days after implantation. Dentin matrix implants generated a significantly greater amount of mineralized tissue than did bone matrix implants. This difference could be explained on the basis of remineralization of the dentin particles to a greater degree than the bone matrix particles. Initial observations suggesting a more robust osteoinductive activity in demineralized incisor matrix can be attributed to the decreasing activity of bone matrix from older donors when compared to younger donors. The extent of osteoinduction by the two substrata was equivalent when the matrices were matched for age.

Bone-derived growth factor release from poly(alpha-hydroxy acid) implants in vitro

Matrix proteins were extracted from bovine cortical bone and polymer implant discs (13 mm x 2 mm composed of 50:50 poly DL-lactide-co-glycolide; mol. wt. approximately 9000) prepared by compression moulding granules with lyophilized bone matrix extracts (BMX) 10.1 (w/w). BMX-containing polymers were cultured for 5 wk in either serum-free Dulbecco's modification of Eagle's medium (DMEM) or phosphate buffer, and growth factor activity released into the media assayed by its ability to stimulate the proliferation of murine fibroblast BALB/c/3T3 cells. Approximately 60-75% of the biological activity was released during the first week of culture; however, less than half of the growth factor units originally incorporated into the implants retained biological activity. Scanning electron microscopy revealed the development of significant internal porosity by week 2; the size of the channels, pores and surface openings suggested they were of the right order for bone ingrowth. These preliminary findings suggest that poly(alpha-hydroxy acid) polymers containing bone-derived growth factors could have potential for stimulating osseous regeneration in vivo.

Bone induction by composite of bioerodible polyorthoester and demineralized bone matrix in rats

A composite of a local, sustained, drug-release system, Alzamer bioerodible polyorthoester, and demineralized bone-matrix (DBM) particles implanted in the abdominal muscle of 89 Wistar rats induced cartilage and bone formation at the same rate as DBM when evaluated histologically and by 85Sr uptake. The composite implant was technically easier to use than DBM alone.

Biocompatible controlled release polymers for delivery of polypeptides and growth factors

The development of biocompatible, controlled release systems for macromolecules has provided the opportunity for researchers and clinicians to target and deliver, on site, biologically active factors. This advance has also facilitated the purification and characterization of a number of important biomolecules. These systems include controlled release delivery systems which release proteins through porous polymer matrices, degradable polymeric delivery systems, and modulated polymer release systems. These areas of research will be reviewed with regards to their design, release kinetics, and biocompatibilities. The utilization of these systems to release such biologically important polypeptides as growth factors (e.g., fibroblast growth factor, epidermal growth factor, transforming growth factor-B) as well as a number of important inhibitory factors (e.g., nitrosoureas, angiogenesis inhibitors) in both in vivo and in vitro studies will be discussed.