The majority of ocular conditions are treated by topical application of drugs as solutions or suspensions. A significant disadvantage of these systems is rapid and extensive pre-corneal drug loss resulting in poor corneal bioavailability of the drug. Consequently, relatively high dosage and frequent applications are required to maintain therapeutic levels at the cornea. This in turn presents a phenomenon known as pulse-drug entry. Initially this gives rise to toxic drug concentrations, followed by levels below therapeutic effectiveness. Improvement of pre-corneal retention is a key objective for the development of a more effective ocular delivery system.
An obvious starting point is the use of contact lenses for controlled ocular drug delivery. This is currently an under-exploited route but one that provides potential solutions. Typical compounds of ophthalmic interest range from new drugs such as pirenzepine, which has attracted clinical interest for the control and management of myopia progression, to the well-established family of non steroidal anti-inflammatory and anti allergenic drugs which have a range of ophthalmic applications. This spectrum of ophthalmic drugs has widely differing structures, sizes, solubility profiles and pH sensitivities.
Our work involves the fundamental understanding of the ways in which these different characteristics influence incorporation and release behaviour. It targets release into both the aqueous layer of the tear film and the lipid layer, which has a much lower turnover rate and provides an additional reservoir for drug delivery, particularly for more hydrophobic drugs.