Biomedical applications of HA and its derivatives

- Hyaluronic acid: a natural biopolymer with a broad range of biomedical and industrial applications

Hyaluronic acid is a major component of the vitreous body of the eye, and is a key macromolecule in ophthalmology. Because of its viscoelastic properties, HA is used in a number of key ophthalmologic surgeries. Preparations of HA protect delicate eye tissues and provide space during surgical manipulations. Its major use, however, is as a substitute or replacement for the vitreous fluid lost during procedures such as cataract surgery or lens implantation. The first product on the market was Healon derived from rooster combs, manufactured initially by Biotrics, Inc. (Arlington, MA) and later by Pharmacia, Sweden, now Pfizer (New York, NY). This product came on the market in 1979 and was soon followed by other products. This preparation was also used as a viscoelastic protector of the corneal endothelium during corneal transplantation. Currently, a number of preparations of varying molecular size HA chains are available, including an HA and chondroitin sulfate combination, termed Viscoat (Alcon Labs, Inc., Fort Worth, TX). Most recently, Maltese et al. (2006), based on the extensive study of the rheological properties of pure materials and their blends, concluded that a new binary combination of sodium hyaluronate and hydroxypropylmethyl cellulose named VISC26 fulfills most optimally the requirements for use as an ophthalmic surgery device.

The second major application of HA is in viscosupplementation in the joints affected by arthritis in a preparation marketed by Seikagaku (Tokyo, Japan). A normal/healthy joint allows nearly frictionless and pain-free movement. However, when damaged or affected by arthritis, joints become stiff and painful. Of the more than one hundred arthritic disorders, osteoarthritis and rheumatoid arthritis are the most common chronic conditions affecting mostly the elderly population. While osteoarthritis is a degenerative disease of the cartilage and bone resulting in pain and stiffness in the affected joint, rheumatoid arthritis is classified as a systemic inflammatory disease, in which pain of the joint(s) is often accompanied with degenerative changes in additional organs, such as lungs, heart, and blood vessels. It is estimated that over 10% of all people over the age of 55 are affected by osteoarthritis.

Although the etiology and pathogenesis of rheumatoid arthritis are as yet unknown, a progressive degradation of polymeric carbohydrates—mainly HA—in synovial fluid can be observed in the course of the disease. In acute phases, a high number of neutrophils is accumulated in the patient’s synovial fluid. These cells alter the oxidative homeostasis and their products, especially reactive oxygen species, can contribute to the destruction of joint structures. Due to chronic inflammation of the joint, the reactive oxygen species alter/destruct the joint structure to such an extent that it is no longer functional. The altered tissues are recognized as “foreign”, and subsequently autoimmune reactions promote the disease and make rheumatoid arthritis a systemic ailment affecting the entire body (Šoltés et al. 2006).

Although HA is ubiquitous in the body, it is most concentrated in developing and specialized tissues such as vocal folds, synovial fluid, umbilical cord, and cartilage. In these tissues, it influences several different functions including tissue viscosity, tissue flow, tissue osmosis, shock absorption, wound healing and space filling. These functions are especially important in vocal folds due to the constant trauma caused by the vibratory actions of phonation. The osmotic, viscoelastic, and space-filling properties of HA are important in voice because they directly affect the thickness and viscosity of the vocal fold (Butler et al. 2001; Chan et al. 2001). Viscoaugmentation of the vocal cord, the repair of injured or scarred vocal cords, and treatment of glottal insufficiency are additional uses of HA derivatives. However, a major drawback to using HA as a lamina propria bioimplant for the treatment of vocal fold disorders is that its residence time within vocal folds is short—its half-life in rabbit vocal folds is only 3–5 days. To overcome this obstacle, the molecular structure of HA should be modified in order to increase the residence time. Various strategies including chemical, enzymatic, and mechanical cross-linking were implemented to prolong HA residence in vocal folds (Ward et al. 2002). Hylan B slurries (a cross-linked HA) injected into vocal cords produce no inflammatory reactions, and the material continues to be present even after one year (Hertegard et al. 2002).

In hearing disorders therapy, films of HA esters, such as HYAFF manufactured by Fidia (Abano Terme, Italy), are used in ear and sinus surgery. These preparations promote wound healing of the tympanic membrane, facilitate re-epithelization, as well as prevent adhesion between layers of mucous tissues.

Preparations of slightly cross-linked HA are currently commonly used for augmentation, to fill facial wrinkles and depressed scars. Such HA gels are more effective in maintaining cosmetic corrections than collagen-based products (Narins et al. 2003). Restylane, produced by the Medicis Corp. (Scottsdale, AZ) is prominent among such HA-based injectable materials (Kanchwala et al. 2005). Unlike collagen-based fillers, HA is extremely elastic, providing the elasticity required by spaces in which it is injected, such as facial wrinkles and depressed scars, vocal cord augmentation, laryngeal and glottal reconstruction, or sphincter muscle support. The HA preparations are also longer lasting. In comparing the applicability of the two commercial products used for soft tissue augmentation—Restylane produced by bacterial fermentation and Hylaform (Hylan B) from rooster combs (Genzyme, Cambridge, MA), Manna et al. (1999) concluded that Hylaform demonstrated better rheological properties behaving as a strong hydrogel, whereas Restylane acted as a weak hydrogel. Moreover, the former product contained four times less protein than the bacterial product, which offered Hylaform a better safety profile.

Shu et al. (2004) described development of novel HA-based cross-linkable hydrogels that did not need surgical implantation, but were injectable and showed improved cytocompatibility with fibroblasts. This demonstrated potential use of such hydrogels for tissue regeneration.

A commercial benzyl ester derivative of HA (HYAFF 11, Fidia, Abano Terme, Italy) and laboratory cross-linked Hylan (Genzyme, Cambridge, MA) were shown to be excellent biomaterials for promotion of adherence of vascular endothelial cells and vascular tissue engineering (Turner et al. 2004; Amarnath et al. 2006).

High molecular size HA preparations, applied topically, promote healing of fresh skin wounds. They also promote the healing of venous leg ulcers and are useful in the management of chronic wounds (Edmonds and Foster 2006).

A new product, a combination of HA with dexpanthenol (Hylactive, Promedic, Seville, Spain) is used as moisturizing, anti-erythematous, and skin regenerative/protecting topical preparation. Due to its antioxidant properties, HA serves as an anti-inflammatory component in the wound dressing materials (Moseley et al. 2003).

The carboxylate groups of HA have been used to create a cross-linked hydrogel for DNA entrapment and also for drug delivery. HA can be either conjugated directly to drugs or used to prepare microcapsules for optimized drug delivery (Esposito et al. 2005). HA is also used to improve biocompatibility of chitosan microspheres used as drug delivery capsules (Vasiliu et al. 2005). HA microspheres are also used for the delivery of plasmid DNA and monoclonal antibodies in gene transfer and site-specific targeting (Yun et al. 2004).