Engineering a clinically-useful matrix for cell therapy

The influence of collagen and hyaluronan matrices on the delivery and bioactivity of bone morphogenetic protein-2 and ectopic bone formation

Bone morphogenetic protein-2 (BMP-2) is known to enhance fracture healing when delivered via a bovine collagen sponge. However, collagen rapidly releases BMP-2 with a high burst phase that is followed by a low sustained phase. As a result, supra-physiological doses of BMP-2 are often required to successfully treat bone defects. High BMP-2 dosing can introduce serious side effects that include edema, bone overgrowth, cyst-like bone formation and significant inflammation. As the release behavior of BMP-2 carriers significantly affects the efficacy of fracture healing, we sought to compare the influence of two BMP-2 delivery matrices with contrasting release profiles on BMP-2 bioactivity and ectopic bone formation. We compared a thiol-modified hyaluronan (Glycosil™) hydrogel that exhibits a low burst followed by a sustained release of BMP-2 to a collagen sponge for the delivery of three different doses of BMP-2, the bioactivities of released BMP-2 and ectopic bone formation. Analysis of bone formation by micro-computed tomography revealed that low burst followed by sustained release of BMP-2 from a hyaluronan hydrogel induced up to 456% more bone compared to a BMP-2 dose-matched collagen sponge that has a high burst and sustained release. This study demonstrates that BMP-2 released with a low burst followed by a sustained release of BMP-2 is more desirable for bone formation. This highlights the therapeutic potential of hydrogels, particularly hyaluronan-based, for the delivery of BMP-2 for the treatment of bone defects and may help abrogate the adverse clinical effects associated with high dose growth factor use.

Systematic engineering of uniform, highly efficient, targeted and shielded viral-mimetic nanoparticles

In the past decades, numerous types of nanomedicines have been developed for the efficient and safe delivery of nucleic acid-based drugs for cancer therapy. Given that the destination sites for nucleic acid-based drugs are inside cancer cells, delivery systems need to be both targeted and shielded in order to overcome the extracellular and intracellular barriers. One of the major obstacles that has hindered the translation of nanotechnology-based gene-delivery systems into the clinic has been the complexity of the design and assembly processes, resulting in non-uniform nanocarriers with unpredictable surface properties and efficiencies. Consequently, no product has reached the clinic yet. In order to address this shortcoming, a multifunctional targeted biopolymer is genetically engineered in one step, eliminating the need for multiple chemical conjugations. Then, by systematic modulation of the ratios of the targeted recombinant vector to PEGylated peptides of different sizes, a library of targeted-shielded viral-mimetic nanoparticles (VMNs) with diverse surface properties are assembled. Through the use of physicochemical and biological assays, targeted-shielded VMNs with remarkably high transfection efficiencies (>95%) are screened. In addition, the batch-to-batch variability of the assembled targeted-shielded VMNs in terms of uniformity and efficiency is examined and, in both cases, the coefficient of variation is calculated to be below 20%, indicating a highly reproducible and uniform system. These results provide design parameters for engineering uniform, targeted-shielded VMNs with very high cell transfection rates that exhibit the important characteristics for in vivo translation. These design parameters and principles could be used to tailor-make and assemble targeted-shielded VMNs that could deliver any nucleic acid payload to any mammalian cell type.

Engineering the regenerative microenvironment with biomaterials

Modern synthetic biomaterials are being designed to integrate bioactive ligands within hydrogel scaffolds for cells to respond and assimilate within the matrix. These advanced biomaterials are only beginning to be used to simulate the complex spatio-temporal control of the natural healing microenvironment. With increasing understanding of the role of growth factors and cytokines and their interactions with components of the extracellular matrix, novel biomaterials are being developed that more closely mimic the natural healing environments of tissues, resulting in increased efficacy in applications of tissue repair and regeneration. Herein, the important aspects of the healing microenvironment, and how these features can be incorporated within innovative hydrogel scaffolds, are presented.

Encapsulated stem cells for cancer therapy

Stem cells have inherent tumor‑trophic migratory properties and can serve as vehicles for delivering effective, targeted therapy to isolated tumors and metastatic disease, making them promising anti‑cancer agents. Encapsulation of therapeutically engineered stem cells in hydrogels has been utilized to provide a physical barrier to protect the cells from hostile extrinsic factors and significantly improve the therapeutic efficacy of transplanted stem cells in different models of cancer. This review aims to discuss the potential of different stem cell types for cancer therapy, various engineered stem cell based therapies for cancer, stem cell encapsulation process and provide an in depth overview of current applications of therapeutic stem cell encapsulation in the highly malignant brain tumor, glioblastoma multiforme (GBM), as well as the prospects for their clinical translation.

The translational imperative: making cell therapy simple and effective

The current practice of cell therapy, in which multipotent or terminally differentiated cells are injected into tissues or intravenously, is inefficient. Few therapeutic cells are retained at the site of administration and engraftment is low. An injectable and biologically appropriate vehicle for delivery, retention, growth and differentiation of therapeutic cells is needed to improve the efficacy of cell therapy. We focus on a hyaluronan-based semi-synthetic extracellular matrix (sECM), HyStem®, which is a manufacturable, approvable and affordable clinical product. The composition of this sECM can be customized for use with mesenchymal stem cells as well as cells derived from embryonic or induced pluripotent sources. In addition, it can support therapeutic uses of progenitor and mature cell populations obtained from skin, fat, liver, heart, muscle, bone, cartilage, nerves and other tissues. This overview presents four pre-clinical uses of HyStem® for cell therapy to repair injured vocal folds, improve post-myocardial infarct heart function, regenerate damaged liver tissue and restore brain function following ischemic stroke. Finally, we address the real-world limitations - manufacture, regulation, market acceptance and financing - surrounding cell therapy and the development of clinical combination products.

Drug targeting to tumors: principles, pitfalls and (pre-) clinical progress

Many different systems and strategies have been evaluated for drug targeting to tumors over the years. Routinely used systems include liposomes, polymers, micelles, nanoparticles and antibodies, and examples of strategies are passive drug targeting, active drug targeting to cancer cells, active drug targeting to endothelial cells and triggered drug delivery. Significant progress has been made in this area of research both at the preclinical and at the clinical level, and a number of (primarily passively tumor-targeted) nanomedicine formulations have been approved for clinical use. Significant progress has also been made with regard to better understanding the (patho-) physiological principles of drug targeting to tumors. This has led to the identification of several important pitfalls in tumor-targeted drug delivery, including I) overinterpretation of the EPR effect; II) poor tumor and tissue penetration of nanomedicines; III) misunderstanding of the potential usefulness of active drug targeting; IV) irrational formulation design, based on materials which are too complex and not broadly applicable; V) insufficient incorporation of nanomedicine formulations in clinically relevant combination regimens; VI) negligence of the notion that the highest medical need relates to metastasis, and not to solid tumor treatment; VII) insufficient integration of non-invasive imaging techniques and theranostics, which could be used to personalize nanomedicine-based therapeutic interventions; and VIII) lack of (efficacy analyses in) proper animal models, which are physiologically more relevant and more predictive for the clinical situation. These insights strongly suggest that besides making ever more nanomedicine formulations, future efforts should also address some of the conceptual drawbacks of drug targeting to tumors, and that strategies should be developed to overcome these shortcomings.

Modified hyaluronan hydrogels support the maintenance of mouse embryonic stem cells and human induced pluripotent stem cells

These studies provide evidence for the ability of a commercially available, defined, hyaluronan-gelatin hydrogel, HyStem-C™, to maintain both mouse embryonic stem cells (mESCs) and human induced pluripotent stem cells (hiPSCs) in culture while retaining their growth and pluripotent characteristics. Growth curve and doubling time analysis show that mESCs and hiPSCs grow at similar rates on HyStem-C™ hydrogels and mouse embryonic fibroblasts and Matrigel™, respectively. Immunocytochemistry, flow cytometry, gene expression and karyotyping reveal that both human and murine pluripotent cells retain a high level of pluripotency on the hydrogels after multiple passages. The addition of fibronectin to HyStem-C™ enabled the attachment of hiPSCs in a xeno-free, fully defined medium.

Hyaluronic acid-based hydrogels functionalized with heparin that support controlled release of bioactive BMP-2

Bone morphogenetic protein-2 (BMP-2) is a potent osteoinductive factor, yet its clinical use is limited by a short biological half-life, rapid local clearance and propensity for side effects. Heparin (HP), a highly sulfated glycosaminoglycan (GAG) that avidly binds BMP-2, has inherent biological properties that may circumvent these limitations. Here, we compared hyaluronan-based hydrogels formulated to include heparin (Heprasil™) with similar gels without heparin (Glycosil™) for their ability to deliver bioactive BMP-2 in vitro and in vivo. The osteogenic activity of BMP-2 released from the hydrogels was evaluated by monitoring alkaline phosphatase (ALP) activity and SMAD 1/5/8 phosphorylation in mesenchymal precursor cells. The osteoinductive ability of these hydrogels was determined in a rat ectopic bone model by 2D radiography, 3D μ-CT and histological analyses at 8 weeks post-implantation. Both hydrogels sustain the release of BMP-2. Importantly, the inclusion of a small amount of heparin (0.3% w/w) attenuated release of BMP-2 and sustained its osteogenic activity for up to 28 days. In contrast, hydrogels lacking heparin released more BMP-2 initially but were unable to maintain BMP-2 activity at later time points. Ectopic bone-forming assays using transplanted hydrogels emphasized the therapeutic importance of the initial burst of BMP-2 rather than its long-term osteogenic activity. Thus, tuning the burst release phase of BMP-2 from hydrogels may be advantageous for optimal bone formation.

Therapeutic potential of gel-based injectables for vocal fold regeneration

Vocal folds are anatomically and biomechanically unique, thus complicating the design and implementation of tissue engineering strategies for repair and regeneration. Integration of an enhanced understanding of tissue biomechanics, wound healing dynamics and innovative gel-based therapeutics has generated enthusiasm for the notion that an efficacious treatment for vocal fold scarring could be clinically attainable within several years. Fibroblast phenotype and gene expression are mediated by the three-dimensional mechanical and chemical microenvironment at an injury site. Thus, therapeutic approaches need to coordinate spatial and temporal aspects of the wound healing response in an injured vocal tissue to achieve an optimal clinical outcome. Successful gel-based injectables for vocal fold scarring will require a keen understanding of how the native inflammatory response sets into motion the later extracellular matrix remodeling, which in turn will determine the ultimate biomechanical properties of the tissue. We present an overview of the challenges associated with this translation as well as the proposed gel-based injectable solutions.

Thiolated carboxymethyl-hyaluronic-Acid-based biomaterials enhance wound healing in rats, dogs, and horses

The progression of wound healing is a complicated but well-known process involving many factors, yet there are few products on the market that enhance and accelerate wound healing. This is particularly problematic in veterinary medicine where multiple species must be treated and large animals heal slower, oftentimes with complicating factors such as the development of exuberant granulation tissue. In this study a crosslinked-hyaluronic-acid (HA-) based biomaterial was used to treat wounds on multiple species: rats, dogs, and horses. The base molecule, thiolated carboxymethyl HA, was first found to increase keratinocyte proliferation in vitro. Crosslinked gels and films were then both found to enhance the rate of wound healing in rats and resulted in thicker epidermis than untreated controls. Crosslinked films were used to treat wounds on forelimbs of dogs and horses. Although wounds healed slower compared to rats, the films again enhanced wound healing compared to untreated controls, both in terms of wound closure and quality of tissue. This study indicates that these crosslinked HA-based biomaterials enhance wound healing across multiple species and therefore may prove particularly useful in veterinary medicine. Reduced wound closure times and better quality of healed tissue would decrease risk of infection and pain associated with open wounds.

Silk–hyaluronan-based composite hydrogels: A novel, securable vehicle for drug delivery

A new, biocompatible hyaluronic acid (HA)–silk hydrogel composite was fabricated and tested for use as a securable drug delivery vehicle. The composite consisted of a hydrogel formed by cross-linking thiol–modified HA with poly(ethylene glycol)-diacrylate, within which was embedded a reinforcing mat composed of electrospun silk fibroin protein. Both HA and silk are biocompatible, selectively degradable biomaterials with independently controllable material properties. Mechanical characterization showed the composite tensile strength as fabricated to be 4.43 ± 2.87 kPa, two orders of magnitude above estimated tensions found around potential target organs. In the presence of hyaluronidase (HAse) in vitro, the rate of gel degradation increased with enzyme concentration although the reinforcing silk mesh was not digested. Composite gels demonstrated the ability to store and sustainably deliver therapeutic agents. Time constants for in vitro release of selected representative antibacterial and anti-inflammatory drugs varied from 46.7 min for cortisone to 418 min for hydrocortisone. This biocomposite showed promising mechanical characteristics for direct fastening to tissue and organs, as well as controllable degradation properties suitable for storage and release of therapeutically relevant drugs.

Regulation of hepatic stem/progenitor phenotype by microenvironment stiffness in hydrogel models of the human liver stem cell niche

Human livers have maturational lineages of cells within liver acini, beginning periportally in stem cell niches, the canals of Hering, and ending in polyploid hepatocytes pericentrally and cholangiocytes in bile ducts. Hepatic stem cells (hHpSCs) in vivo are partnered with mesenchymal precursors to endothelia (angioblasts) and stellate cells, and reside in regulated microenvironments, stem cell niches, containing hyaluronans (HA). The in vivo hHpSC niche is modeled in vitro by growing hHpSC in two-dimensional (2D) cultures on plastic. We investigated effects of 3D microenvironments, mimicking the liver's stem cell niche, on these hHpSCs by embedding them in HA-based hydrogels prepared with Kubota's Medium (KM), a serum-free medium tailored for endodermal stem/progenitors. The KM-HA hydrogels mimicked the niches, matched diffusivity of culture medium, exhibited shear thinning and perfect elasticity under mechanical loading, and had predictable stiffness depending on their chemistry. KM-HA hydrogels, which supported cell attachment, survival and expansion of hHpSC colonies, induced transition of hHpSC colonies towards stable heterogeneous populations of hepatic progenitors depending on KM-HA hydrogel stiffness, as shown by both their gene and protein expression profile. These acquired phenotypes did not show morphological evidence of fibrotic responses. In conclusion, this study shows that the mechanical properties of the microenvironment can regulate differentiation in endodermal stem cell populations.

Integrin-mediated adhesion and proliferation of human MSCs elicited by a hydroxyproline-lacking, collagen-like peptide

In this study, we evaluated the competence of a rationally designed collagen-like peptide (CLP-Cys) sequence - containing the minimal essential Glycine-Glutamic acid-Arginine (GER) triplet but lacking the hydroxyproline residue - for supporting human mesenchymal stem cell (hMSC) adhesion, spreading and proliferation. Cellular responses to the CLP-Cys sequence were analyzed by conjugating the peptide to two different substrates - a hard, planar glass surface and a soft hyaluronic acid (HA) particle-based hydrogel. Integrin-mediated cell spreading and adhesion were observed for hMSCs cultivated on the CLP-Cys functionalized surfaces, whereas on control surfaces lacking the peptide motif, cells either did not adhere or maintained a round morphology. On the glass surface, CLP-Cys-mediated spreading led to the formation of extended and well developed stress fibers composed of F-actin bundles and focal adhesion complexes while on the soft gel surface, less cytoskeletal reorganization organization was observed. The hMSCs proliferated significantly on the surfaces presenting CLP-Cys, compared to the control surfaces lacking CLP-Cys. Competitive binding assay employing soluble CLP-Cys revealed a dose-dependent inhibition of hMSC adhesion to the CLP-Cys-presenting surfaces. Blocking the α(2)β(1) receptor on hMSC also resulted in a reduction of cell adhesion on both types of CLP-Cys surfaces, confirming the affinity of CLP-Cys to α(2)β(1) receptors. These results established the competence of the hydroxyproline-free CLP-Cys for eliciting integrin-mediated cellular responses including adhesion, spreading and proliferation. Thus, CLP-Cys-modified HA hydrogels are attractive candidates as bioactive scaffolds for tissue engineering applications.

Founder's award to Antonios G. Mikos, Ph.D., 2011 Society for Biomaterials annual meeting and exposition, Orlando, Florida, April 13-16, 2011: Bones to biomaterials and back again--20 years of taking cues from nature to engineer synthetic polymer scaffolds

For biomaterials scientists focusing on tissue engineering applications, the gold standard material is healthy, autologous tissue. Ideal material properties and construct design parameters are thus both obvious and often times unachievable; additional considerations such as construct delivery and the underlying pathology necessitating new tissue yield additional design challenges with solutions that are not evident in nature. For the past nearly two decades, our laboratory and collaborators have aimed to develop both new biomaterials and a better understanding of the complex interplay between material and host tissue to facilitate bone and cartilage regeneration. Various approaches have ranged from mimicking native tissue material properties and architecture to developing systems for bioactive molecule delivery as soluble factors or bound directly to the biomaterial substrate. Such technologies have allowed others and us to design synthetic biomaterials incorporating increasing levels of complexity found in native tissues with promising advances made toward translational success. Recent work focuses on translation of these technologies in specific clinical situations through the use of adjunctive biomaterials designed to address existing pathologies or guide host-material integration.

The effect of mesenchymal stromal cell-hyaluronic acid hydrogel constructs on immunophenotype of macrophages

During the past several years, multipotent mesenchymal stromal cells (MSCs) have rapidly moved from in vitro and animal studies into clinical trials as a therapeutic modality potentially applicable to a wide range of disorders. It has been proposed that ex vivo culture-expanded MSCs exert their tissue regeneration potential through their immunomodulatory and anti-inflammatory properties, and paracrine effects more than their ability to differentiate into multiple tissue lineages. Since extracellular matrix (ECM) deposition and tissue support is also one of many physiological roles of MSCs, there is increasing interest in their potential use for tissue engineering, particularly in combination with ECM-based scaffolds such as hyaluronic acid (HA). We investigated the effect of MSCs on immunophenotype of macrophages in the presence of an HA-hydrogel scaffold using a unique 3D coculture system. MSCs were encapsulated in the hydrogel and peripheral blood CD14+ monocyte-derived macrophages plated in direct contact with the MSC-gel construct. To determine the immunophenotype of macrophages, we looked at the expression of cell surface markers CD14, CD16, CD206, and human leukocyte antigen (HLA)-DR by flow cytometry. MSCs and macrophages cultured on the HA-hydrogel remained viable and were able to be recovered from the construct. There was a significant difference in the immunophenotype observed between monocyte-derived macrophages cultured on the HA scaffold compared to tissue culture polystyrene. Macrophages cultured on gels with MSCs expressed lower CD16 and HLA-DR with higher expression of CD206, indicating the least inflammatory profile overall, compatible with the immunophenotype of alternatively activated macrophages. Development of macrophages, with this immunophenotype, upon interaction with the MSC-hydrogel constructs may play a potentially significant role in tissue repair when using a cellular-biomaterial therapeutic approach.

Hyaluronic acid-based clinical biomaterials derived for cell and molecule delivery in regenerative medicine

The development of injectable and biocompatible vehicles for delivery, retention, growth, and differentiation of stem cells is of paramount importance for regenerative medicine. For cell therapy and the development of clinical combination products, we created a hyaluronan (HA)-based synthetic extracellular matrix (sECM) that provides highly reproducible, manufacturable, approvable, and affordable biomaterials. The composition of the sECM can be customized for use with progenitor and mature cell populations obtained from skin, fat, liver, heart, muscle, bone, cartilage, nerves, and other tissues. This overview describes the design criteria for "living" HA derivatives, and the many uses of this in situ crosslinkable HA-based sECM hydrogel for three-dimensional (3-D) culture of cells in vitro and translational use in vivo. Recent advances allow rapid expansion and recovery of cells in 3-D, and the bioprinting of engineered tissue constructs. The uses of HA-derived sECMs for cell and molecule delivery in vivo will be reviewed, including applications in cancer biology and tumor imaging.

Hyaluronic acid hydrogels for biomedical applications

Hyaluronic acid (HA), an immunoneutral polysaccharide that is ubiquitous in the human body, is crucial for many cellular and tissue functions and has been in clinical use for over thirty years. When chemically modified, HA can be transformed into many physical forms-viscoelastic solutions, soft or stiff hydrogels, electrospun fibers, non-woven meshes, macroporous and fibrillar sponges, flexible sheets, and nanoparticulate fluids-for use in a range of preclinical and clinical settings. Many of these forms are derived from the chemical crosslinking of pendant reactive groups by addition/condensation chemistry or by radical polymerization. Clinical products for cell therapy and regenerative medicine require crosslinking chemistry that is compatible with the encapsulation of cells and injection into tissues. Moreover, an injectable clinical biomaterial must meet marketing, regulatory, and financial constraints to provide affordable products that can be approved, deployed to the clinic, and used by physicians. Many HA-derived hydrogels meet these criteria, and can deliver cells and therapeutic agents for tissue repair and regeneration. This progress report covers both basic concepts and recent advances in the development of HA-based hydrogels for biomedical applications.

Photocrosslinkable hyaluronan-gelatin hydrogels for two-step bioprinting

Bioprinting by the codeposition of cells and biomaterials is constrained by the availability of printable materials. Herein we describe a novel macromonomer, a new two-step photocrosslinking strategy, and the use of a simple rapid prototyping system to print a proof-of-concept tubular construct. First, we synthesized the methacrylated ethanolamide derivative of gelatin (GE-MA). Second, partial photochemical cocrosslinking of GE-MA with methacrylated hyaluronic acid (HA-MA) gave an extrudable gel-like fluid. Third, the new HA-MA:GE-MA hydrogels were biocompatible, supporting cell attachment and proliferation of HepG2 C3A, Int-407, and NIH 3T3 cells in vitro. Moreover, hydrogels injected subcutaneously in nude mice produced no inflammatory response. Fourth, using the Fab@Home printing system, we printed a tubular tissue construct. The partially crosslinked hydrogels were extruded from a syringe into a designed base layer, and irradiated again to create a firmer structure. The computer-driven protocol was iterated to complete a cellularized tubular construct with a cell-free core and a cell-free structural halo. Cells encapsulated within this printed construct were viable in culture, and gradually remodeled the synthetic extracellular matrix environment to a naturally secreted extracellular matrix. This two-step photocrosslinkable biomaterial addresses an unmet need for printable hydrogels useful in tissue engineering.

Modular elastic patches: mechanical and biological effects

A modular approach to engineering cross-linked elastic biomaterials is presented for fine-tuning of material mechanical and biological properties. The three components, soluble elastin, hyaluronic acid, and silk fibroin, contribute with different features to the overall properties of the final material system. The elastic biomaterial is chemically cross-linked via interaction between primary amine groups naturally present on the two proteins, silk and elastin, or chemically introduced on hyaluronan and N-succinimide functionalities of the cross-linker. The materials obtained by cross-linking the three components in different ratios have Young's moduli ranging from ∼ 100 to 230 kPa, strain to failure between ∼ 15-40% and ultimate tensile strengths of ∼ 30 kPa. The biological effects and enzymatic degradation rates of the different composites are also different based on material composition. These findings further underline the strength of modular, multicomponent systems in creating a range of biomaterials, targeted tissue engineering, and regenerative medicine applications, with application-tailored mechanical and biological properties.

The generation of 3-D tissue models based on hyaluronan hydrogel-coated microcarriers within a rotating wall vessel bioreactor

With the increasing necessity for functional tissue- and organ equivalents in the clinic, the optimization of techniques for the in vitro generation of organotypic structures that closely resemble the native tissue is of paramount importance. The engineering of a variety of highly differentiated tissues has been achieved using the rotating wall vessel (RWV) bioreactor technology, which is an optimized suspension culture allowing cells to grow in three-dimensions (3-D). However, certain cell types require the use of scaffolds, such as collagen-coated microcarrier beads, for optimal growth and differentiation in the RWV. Removal of the 3-D structures from the microcarriers involves enzymatic treatment, which disrupts the delicate 3-D architecture and makes it inapplicable for potential implantation. Therefore, we designed a microcarrier bead coated with a synthetic extracellular matrix (ECM) composed of a disulfide-crosslinked hyaluronan and gelatin hydrogel for 3-D tissue engineering, that allows for enzyme-free cell detachment under mild reductive conditions (i.e. by a thiol-disulfide exchange reaction). The ECM-coated beads (ECB) served as scaffold to culture human intestinal epithelial cells (Int-407) in the RWV, which formed viable multi-layered cell aggregates and expressed epithelial differentiation markers. The cell aggregates remained viable following dissociation from the microcarriers, and could be returned to the RWV bioreactor for further culturing into bead-free tissue assemblies. The developed ECBs thus offer the potential to generate scaffold-free 3-D tissue assemblies, which could further be explored for tissue replacement and remodeling.

Stimulation of in vivo angiogenesis by in situ crosslinked, dual growth factor-loaded, glycosaminoglycan hydrogels

As part of a study of elicited angiogenesis, hyaluronan (HA)-based hydrogels crosslinked by polyethylene glycol diacrylate (PEGDA) were loaded with combinations of the cytokine growth factors vascular endothelial growth factor (VEGF), angiopoietin-1 (Ang-1), keratinocyte growth factor (KGF) and platelet-derived growth factor (PDGF). GF release in vivo was controlled by covalent incorporation of thiol-modified heparin into thiolated HA hydrogels, which were injected into the ear pinnae of mice and allowed to crosslink in situ. GF release in vivo was controlled by covalent incorporation of thiol-modified heparin in the gels. The ears were harvested at 7 or 14 days post-implantation, and vascularization evaluated via a Neovascularization Index (NI). The study demonstrates that in situ gelling implants produced no gross inflammation, redness or swelling, and an improved tolerance compared to HA-based dry film implants. All treatments showed significantly more vascularization than either contralateral ears or ears receiving a sham surgery. The maximum response was observed after 14 days in the ears receiving 0.3% Hp, gelatin-containing gels loaded with VEGF + KGF (NI = 3.91). The study revealed injected growth factor-loaded HA-based hydrogels can successfully produce localized controllable vascularization, while minimizing tissue necrosis, polymorphonuclear leukocytes and inflammation. The ability to target and controllably release growth factors can prove a useful tool in specific diseased tissue/organ angiogenesis.

New Thermo-responsive Hydrogels Based on Poly (N-isopropylacrylamide)/ Hyaluronic Acid Semi-interpenetrated Polymer Networks: Swelling Properties and Drug Release Studies

New pH and temperature sensitive semi-interpenetrated polymer networks (semi-IPNs) were developed by combining cross-linked PNIPAAm with hyaluronic acid (HA). At pH 7.4, the PNIPAAm/HA semi-IPN hydrogels had significantly greater and faster swelling at 25°C and a more complete deswelling at 37°C than PNIPAAm hydrogels. The temperature-dependent reversibility behavior was analyzed for biomaterial applications. Gentamicin was incorporated as a model drug, and the release profile from the hydrogels was followed under physiological conditions. The presence of HA, even in small quantities, allowed a more complete release of the gentamicin from the hydrogel. The PNIPAAm/HA semi-IPN hydrogels respond to both changes in temperature and pH making it suitable as a delivery system for therapeutics.

Hyaluronan-CD44 Interactions in Cancer: Paradoxes and Possibilities

Hyaluronan is a prominent component of the micro-environment in most malignant tumors and can be prognostic for tumor progression. Extensive experimental evidence in animal models implicates hyaluronan interactions in tumor growth and metastasis, but it is also evident that a balance of synthesis and turnover by hyaluronidases is critical. CD44, a major hyaluronan receptor, is commonly but not uniformly associated with malignancy, and is frequently used as a marker for cancer stem cells in human carcinomas. Multivalent interactions of hyaluronan with CD44 collaborate in driving numerous tumor-promoting signaling pathways and transporter activities. It is widely accepted that hyaluronan-CD44 interactions are crucial in both malignancy and resistance to therapy, but major challenges for future research in the field are the mechanism of activation of hyaluronan-CD44 signaling in cancer cells, the relative importance of variant forms of CD44 and other hyaluronan receptors, e.g., Rhamm, in different tumor contexts, and the role of stromal versus tumor cell production and turnover of hyaluronan. Despite these caveats, it is clear that hyaluronan-CD44 interactions are an important target for translation into the clinic. Among the approaches that show promise are antibodies and vaccines to specific variants of CD44 that are uniquely expressed at critical stages of progression of a particular cancer, hyaluronidase-mediated reduction of barriers to drug access, and small hyaluronan oligosaccharides that attenuate constitutive hyaluronan-receptor signaling and enhance chemosensitivity. In addition, hyaluronan is being used to tag drugs and delivery vehicles for targeting of anticancer agents to CD44-expressing tumor cells. (Clin Cancer Res 2009;15(24):7462-8).

Evaluating dual activity LPA receptor pan-antagonist/autotaxin inhibitors as anti-cancer agents in vivo using engineered human tumors

Using an in situ cross-linkable hydrogel that mimics the extracellular matrix (ECM), cancer cells were encapsulated and injected in vivo following a "tumor engineering" strategy for orthotopic xenografts. Specifically, we created several three-dimensional (3D) human tumor xenografts and evaluated the tumor response to BrP-LPA, a novel dual function LPA antagonist/ATX inhibitor (LPAa/ATXi). First, we describe the model system and the optimization of semi-synthetic ECM (sECM) compositions and injection parameters for engineered xenografts. Second, we summarize a study to compare angiogenesis inhibition in vivo, comparing BrP-LPA to the kinase inhibitor sunitinib maleate (Sutent). Third, we compare treatment of engineered breast tumors with LPAa/ATXi alone with treatment with Taxol. Fourth, using a re-optimized sECM for non-small cell lung cancer cells, we created reproducibly sized subcutaneous lung tumors and evaluated their response to treatment with LPAa/ATXi. Fifth, we summarize the data on the use of LPAa/ATXi to treat a model for colon cancer metastasis to the liver. Taken together, these improved, more realistic xenografts show considerable utility for evaluating the potential of novel anti-metastatic, anti-proliferative, and anti-angiogenic compounds that modify signal transduction through the LPA signaling pathway.

Rheological properties of cross-linked hyaluronan-gelatin hydrogels for tissue engineering

Hydrogels that mimic the natural extracellular matrix (ECM) are used in three-dimensional cell culture, cell therapy, and tissue engineering. A semi-synthetic ECM based on cross-linked hyaluronana offers experimental control of both composition and gel stiffness. The mechanical properties of the ECM in part determine the ultimate cell phenotype. We now describe a rheological study of synthetic ECM hydrogels with storage shear moduli that span three orders of magnitude, from 11 to 3 500 Pa, a range important for engineering of soft tissues. The concentration of the chemically modified HA and the cross-linking density were the main determinants of gel stiffness. Increase in the ratio of thiol-modified gelatin reduced gel stiffness by diluting the effective concentration of the HA component.