Cross-linked hyaluronan-coated stents in the prevention of airway stenosis

Advances in modified hyaluronic acid-based hydrogels for skin wound healing

Hyaluronic acid (HA) is a natural linear anionic polysaccharide with many unique characteristics such as excellent biocompatibility and biodegradability, native biofunctionality, hydrophilicity, and non-immunoreactivity. HA plays crucial roles in numerous biological processes, including the inflammatory response, cell adhesion, migration, proliferation, differentiation, angiogenesis, and tissue regeneration. All these properties and biological functions of HA make it an appealing material for the synthesis of biomedical hydrogels for skin wound healing. Since HA is not able to be gelate alone, it must be processed and functionalized through chemical modifications and crosslinking to generate versatile HA-based hydrogels. In recent years, different physical and chemical crosslinking strategies for HA-based hydrogels have been developed and designed, such as radical polymerization, Schiff-base crosslinking, enzymatic crosslinking, and dynamic covalent crosslinking, and they have broad and promising applications in skin wound healing and tissue engineering. In this review, we focus on chemical modification and crosslinking strategies for HA-based hydrogels, aiming to provide an overview of the latest advances in the development of HA-based hydrogels for skin wound healing. We summarize and propose feasible measures for the application of HA-based hydrogels for skin treatment, and discuss future application trends, which may ultimately promote HA-based hydrogels as a promising biomaterial for clinical applications.

The Rise and Fall of Hyaluronan in Respiratory Diseases

In normal airways, hyaluronan (HA) matrices are primarily located within the airway submucosa, pulmonary vasculature walls, and, to a lesser extent, the alveoli. Following pulmonary injury, elevated levels of HA matrices accumulate in these regions, and in respiratory secretions, correlating with the extent of injury. Animal models have provided important insight into the role of HA in the onset of pulmonary injury and repair, generally indicating that the induction of HA synthesis is an early event typically preceding fibrosis. The HA that accumulates in inflamed airways is of a high molecular weight (>1600 kDa) but can be broken down into smaller fragments (<150 kDa) by inflammatory and disease-related mechanisms that have profound effects on HA pathobiology. During inflammation in the airways, HA is often covalently modified with heavy chains from inter-alpha-inhibitor via the enzyme tumor-necrosis-factor-stimulated-gene-6 (TSG-6) and this modification promotes the interaction of leukocytes with HA matrices at sites of inflammation. The clearance of HA and its return to normal levels is essential for the proper resolution of inflammation. These data portray HA matrices as an important component of normal airway physiology and illustrate its integral roles during tissue injury and repair among a variety of respiratory diseases.

Ophthalmic Uses of a Thiol-Modified Hyaluronan-Based Hydrogel

Significance: Hyaluronic acid (HA, or hyaluronan) is a ubiquitous naturally occurring polysaccharide that plays a role in virtually all tissues in vertebrate organisms. HA-based hydrogels have wound-healing properties, support cell delivery, and can deliver drugs locally. Recent Advances: A few HA hydrogels can be customized for composition, physical form, and biomechanical properties. No clinically approved HA hydrogel allows for in vivo crosslinking on administration, has a tunable gelation time to meet wound-healing needs, or enables drug delivery. Recently, a thiolated carboxymethyl HA (CMHA-S) was developed to produce crosslinked hydrogels, sponges, and thin films. CMHA-S can be crosslinked with a thiol-reactive crosslinker or by oxidative disulfide bond formation to form hydrogels. By controlled crosslinking, the shape and form of this material can be manipulated. These hydrogels can be subsequently lyophilized to form sponges or air-dried to form thin films. CMHA-S films, liquids, and gels have been shown to be effective in vivo for treating various injuries and wounds in the eye in veterinary use, and are in clinical development for human use. Critical Issues: Better clinical therapies are needed to treat ophthalmic injuries. Corneal wounds can be treated using this HA-based crosslinked hydrogel. CMHA-S biomaterials can help heal ocular surface defects, can be formed into a film to deliver drugs for local ocular drug delivery, and could deliver autologous limbal stem cells to treat extreme ocular surface damage associated with limbal stem cell deficiencies. Future Directions: This CMHA-S hydrogel increases the options that could be available for improved ocular wound care, healing, and regenerative medicine.

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.

Modular extracellular matrices: solutions for the puzzle

The common technique of growing cells in two-dimensions (2-D) is gradually being replaced by culturing cells on matrices with more appropriate composition and stiffness, or by encapsulation of cells in three-dimensions (3-D). The universal acceptance of the new 3-D paradigm has been constrained by the absence of a commercially available, biocompatible material that offers ease of use, experimental flexibility, and a seamless transition from in vitro to in vivo applications. The challenge-the puzzle that needs a solution-is to replicate the complexity of the native extracellular matrix (ECM) environment with the minimum number of components necessary to allow cells to rebuild and replicate a given tissue. For use in drug discovery, toxicology, cell banking, and ultimately in reparative medicine, the ideal matrix would therefore need to be highly reproducible, manufacturable, approvable, and affordable. Herein we describe the development of a set of modular components that can be assembled into biomimetic materials that meet these requirements. These semi-synthetic ECMs, or sECMs, are based on hyaluronan derivatives that form covalently crosslinked, biodegradable hydrogels suitable for 3-D culture of primary and stem cells in vitro, and for tissue formation in vivo. The sECMs can be engineered to provide appropriate biological cues needed to recapitulate the complexity of a given ECM environment. Specific applications for different sECM compositions include stem cell expansion with control of differentiation, scar-free wound healing, growth factor delivery, cell delivery for osteochondral defect and liver repair, and development of vascularized tumor xenografts for personalized chemotherapy.

Prevention of peritendinous adhesions using a hyaluronan-derived hydrogel film following partial-thickness flexor tendon injury

Peritendinous adhesions are an important complication of flexor tendon injury. Three hyaluronan (HA)-derived biomaterials were evaluated for the reduction of peritendinous adhesions following partial-thickness tendon injury in rabbits. Rabbits (n = 24) were divided into three groups (n = 8 per group), which were used for gross evaluation, histologic assessment, or biomechanical testing. The fourth and third toes from both hindpaws of each rabbit were randomly assigned to one of four treatments: (i) untreated control, (ii) Seprafilm, (iii) Carbylan-SX in situ crosslinked hydrogel, and (iv) preformed Carbylan-SX film. Rabbits were sacrificed at 3 weeks postsurgery and evaluated anatomically, histologically, and mechanically. All materials used reduced adhesions relative to untreated controls for all three evaluations. Both the gross anatomic and histologic results revealed that Carbylan-SX film was statistically superior to Seprafilm and Carbylan-SX gel in preventing tendon adhesion formation. In biomechanical tests, the Carbylan-SX film-treated hindpaws required the least force to pull the tendon from the sheath. This force was statistically indistinguishable from that required to extrude an unoperated tendon (n = 8). Carbylan-SX gel was less effective than Carbylan-SX film but superior to Seprafilm for all evaluations. A crosslinked HA-derived film promoted healing of a flexor tendon injury without the formation of fibrosis at 3 weeks postoperatively.

Engineering a clinically-useful matrix for cell therapy

The design criteria for matrices for encapsulation of cells for cell therapy include chemical, biological, engineering, marketing, regulatory, and financial constraints. What is required is a biocompatible material for culture of cells in three-dimensions (3-D) that offers ease of use, experimental flexibility to alter composition and compliance, and a composition that would permit a seamless transition from in vitro to in vivo use. The challenge is to replicate the complexity of the native extracellular matrix (ECM) environment with the minimum number of components necessary to allow cells to rebuild a given tissue. Our approach is to deconstruct the ECM to a few modular components that can be reassembled into biomimetic materials that meet these criteria. These semi-synthetic ECMs (sECMs) employ thiol-modified derivatives of hyaluronic acid (HA) that can form covalently crosslinked, biodegradable hydrogels. These sECMs are "living" biopolymers, meaning that they can be crosslinked in the presence of cells or tissues to enable cell therapy and tissue engineering. Moreover, the sECMs allow inclusion of the appropriate biological cues needed to simulate the complexity of the ECM of a given tissue. Taken together, the sECM technology offers a manufacturable, highly reproducible, flexible, FDA-approvable, and affordable vehicle for cell expansion and differentiation in 3-D.

Synthesis of Hyaluronan Haloacetates and Biology of Novel Cross-Linker-Free Synthetic Extracellular Matrix Hydrogels

Hyaluronan (HA) derivatives containing thiol-reactive electrophilic esters were prepared to react with thiol-modified macromolecules to give cross-linker-free hydrogels. Specifically, HA was converted to two haloacetate derivatives, HA bromoacetate (HABA) and HA iodoacetate (HAIA). In cytotoxicity assays, these reactive macromolecules predictably induced cell death in a dose-dependent manner. Cross-linker-free synthetic extracellular matrix (sECM) hydrogels were prepared by thiol alkylation using HAIA and HABA as polyvalent electrophiles and thiol-modified HA (CMHA-S) with or without thiol-modified gelatin (Gtn-DTPH) as polyvalent nucleophiles. When primary human fibroblasts were seeded on the surface of the sECMs containing only the electrophilic HA haloacetate and nucleophilic CMHA-S components, no significant cytoadherence was observed. Cell attachment and viability was 17% (HABA) to 30% (HAIA) lower on HA haloacetate cross-linked hydrogels than on CMHA-S that had been oxidatively cross-linked via disulfide-bonds. In contrast, sECMs that included Gtn-DTPH allowed fibroblasts to attach, spread, and proliferate. Taken together, the HA haloacetates are attractive candidates for producing cross-linker-free sECM biomaterials that can function either as anti-adhesive barriers or as cytoadhesive sECMs for cell culture in pseudo-3-D.

Postoperative Pericardial Adhesion Prevention Using Carbylan-SX in a Rabbit Model

INTRODUCTION

The presence of dense adhesions within the pericardial space complicates reoperative cardiac surgery. Prior attempts to reduce adhesion formation after primary cardiac surgery using medications or biomaterials have had variable success. Carbylan-SX (Carbylan Biosurgery Inc., Palo Alto, CA) is a hyaluronan-based biomaterial, which has been shown to be effective at reducing adhesions in a nonthoracic rat model. This study evaluates whether Carbylan-SX can effectively reduce postoperative adhesions within the pericardial cavity.

METHODS

Thirty-eight New Zealand white rabbits underwent a left lateral thoracotomy. A pericardiotomy was made and epicardial adhesions were induced on the anterior surface of the heart using a Dremel device (Racine, WI). The rabbits were divided into four groups: controls with abrasions only receiving no treatment (n=10), Carbylan-SX films (n=10), Carbylan-SX aerosolized hydrogel (n=10), and Seprafilm (n=8). The pericardial sac and chest were subsequently closed. Rabbits were sacrificed at a mean of 15 days. For histological analysis, each heart was divided into 12 separate 1 mm sections. Computer imaging software was used to measure the adhesion thickness and the mean of 12 random measurements for each animal was recorded and statistical analysis performed.

RESULTS

Histological analysis revealed all treatment groups to be significantly better than the control (2159 mum thickness, P<0.0001) at preventing adhesions. The Carbylan-SX film and Carbylan-SX aerosolized hydrogel both proved to be better at preventing adhesions than Seprafilm (Genzyme Corp., Cambridge, MA) with an average adhesion thickness of 454 and 577 microm, respectively, compared with 1319 microm for Seprafilm (P<0.0001 and P<0.0005, respectively). The Carbylan-SX film and Carbylan-SX aerosolized hydrogel were equally effective at preventing adhesion formation.

CONCLUSION

Carbylan-SX film and Carbylan-SX aerosolized crosslinkable hydrogel are equally effective methods of reducing postoperative pericardial adhesions within the pericardial cavity. Both the Carbylan-SX film and aerosolized hydrogel showed a significantly greater reduction in adhesions than Seprafilm. Clinical application of Carbylan-SX could have significant therapeutic implications in the future.

Reduced postoperative intra-abdominal adhesions using Carbylan-SX, a semisynthetic glycosaminoglycan hydrogel

OBJECTIVE

To compare the efficacy of crosslinked Carbylan-SX (Carbylan BioSurgery, Inc., Palo Alto, CA) hydrogel films and sprayable gels as physical barriers in reducing postoperative intra-abdominal adhesions in the rat cecum-abdominal wall and rat uterine horn models.

DESIGN

Pre-formed crosslinked Carbylan-SX films and sprayable in situ crosslinkable Carbylan-SX gels were evaluated in rat cecum-abdominal wall and rat uterine horn models and compared with commercially available and clinically used Seprafilm.

SETTING

University animal research facility.

ANIMALS

Female Wistar rats.

INTERVENTION(S)

Abrasions were made with the foot-pedal-operated Flex-shaft (Dremel, Racine, WI) on both the cecum and abdominal wall (each area 10 mm in diameter) in female rats as one model and on both uterine horns (3 x 10 mm) in female rats as the other model. In each of the two adhesion models, four groups were assigned with eight rats in each group: (1) untreated control, (2) treated with Seprafilm (Genzyme Corporation, Cambridge, MA), (3) treated with preformed Carbylan-SX hydrogel films, and (4) treated with sprayable Carbylan-SX gel.

MAIN OUTCOME MEASURE(S)

Extent and severity of postoperative adhesions between the cecum and the abdominal wall in rat cecum-abdominal wall model and between the uterine horns in rat uterine horn model.

RESULT(S)

The Carbylan-SX film and the Carbylan-SX sprayable gel led to fewer adhesions than Seprafilm in both rat adhesion models. Interestingly, a single physical form was not optimal for both models: the Carbylan film was more efficacious in the rat uterine horn model, whereas Carbylan gel gave the best results in the rat cecum-abdominal wall model.

CONCLUSION(S)

Both Carbylan-SX film and gel were efficacious in reducing postoperative intra-abdominal adhesion formation in rat cecum-abdominal wall and uterine horn models.