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Eye-drop application of Hyaluronic acid

Hyaluronic acid is an innovative new visco-enhancer for use in topical eye care formulations. It is produced by fermenting the safe bacterial strain Bacillus subtilis – the world’s first hyaluronic acid (HA) that is 100% free of animal-derived raw materials and organic-solvent remnants. In addition to all the well-known benefits of traditional HA, Hyaluronic acid also features outstanding filterability and heat stability, which streamline the production process, reduce batch times, and increase profit margins.

 

Hyaluronic acid is a key comfort ingredient for topical ophthalmic formulations since it is a natural compound that is biocompatible, non-immunogenic, and biodegradable. In fact, it is one of the most hygroscopic molecules found in nature; hydrated hyaluronic acid can contain up to 1,000-fold more water than its own weight. These exceptional water retention properties result in enhanced hydration of the corneal surface. Moreover, applications of ophthalmic formulations containing HA reduce tear elimination and enhance precorneal tear film stability, which is a useful property against dry eye syndrome.

 

The non-Newtonian and shear-thinning properties Hyaluronic acid produce solutions with a high viscosity at low shear rate (when the eye is open) and a low viscosity at high shear rate (during blinking), facilitate even distribution of the solution and lubrication of the ocular surface. Moreover, the muco-adhesivity of Hyaluronic acid provides effective coating and long-lasting protection of the cornea as well as extended residence times on the ocular surface. Finally, when topically instilled on the eye, HA promotes physiological wound healing by stimulating corneal epithelial migration and proliferation of keratocytes as well as reducing the healing time of corneal epithelium.

 

Water-binding capacity

The capacity of hyaluronic acids of various chain lengths to bind water was assessed from the amount of non frozen water in Differential Scanning Calorimetry (DSC). Overall, the results of our studies show that all HA samples bind very high amounts of water: 4-5 g/g. Moreover, the amount of bound water is independent of the molecular weight and the origin of the hyaluronic acid. When topically applied to the eye, Hyaluronic acid enhances water retention on the corneal surface and increases corneal wettability thanks to its superior water retention properties.

 

Residence time
The corneal residence time of Hyaluronic acid at 0.1% (w/v) is similar to that of Streptococcus HA at the same concentration, according to gamma scintigraphic analysis.In topical ophthalmic solutions loaded with a drug, Hyaluronic acid can increase the contact time with the ocular surface, thereby improving the bioavailability of the drug.

 

Tolerance/biocompatibility
The following figure illustrates the percentage of corneal injury in terms of epithelial cell loss as measured by confocal laser scanning ophthalmoscopy. These measurements were made after applying HA-containing formulations to the eyes of three rabbits. The observed levels are lower than 25% and indicate that all tested formulations were well tolerated and acceptable for topical ophthalmic use. This in vivo study highlights the biocompatibility of Hyaluronic acid on the corneal surface. 

 

Filtration
The time to filter 50 ml of a HA solution through a 0.22 nm filter was recorded. The molecular weight does not affect the filtration time up to concentrations of 0.1% (w/v). However, at 0.2% concentration, it takes up to three times longer to filter a high molecular weight HA solution than a medium molecular weight HA solution such as Hyaluronic acid. The effect is even more pronounced when the concentration is increased to 0.3% (w/v): A Hyaluronic acid solution is filtered within less than 10 minutes while filtration of a high molecular weight HA solution requires nearly an hour. Overall, Hyaluronic acid is easy to filter and can significantly reduce the production time for formulations that are sterilized by filtration.

 

Viscosity
The rheological properties of eye drops and artificial tears largely depend on the concentration, the molecular weight, and the nature of the viscosity-enhancing agents. As the viscosity-enhancing agent of choice, Hyaluronic acid decreases the drainage rate of ophthalmic solutions. In eye drops designed for drug delivery, a highly viscous HA solution prolongs the contact time of the drug with the cornea, resulting in improved bioavailability of the drug. But there can also be disadvantages related to the use of thickened eye drops, including ocular discomfort, increased eye blink frequency, and possible interference with vision. Solutions containing Hyaluronic acid feature well-defined rheological properties while allowing for maximum comfort and efficacy.

 

Heat stability
Size-exclusion chromatography analyses have shown that Hyaluronic acid remains very stable during heat sterilization of ophthalmic solutions. In fact, the retained HA molecular weight is 82% after treatment at 121 oC (250 oF) for 16 minutes, while that of Streptococcus-based HA is 70% under the same conditions.
 
Thus, Hyaluronic acid-containing formulations can be heat sterilized under standard conditions without compromising the final attributes of the formulations.