Nowadays, most ophthalmic drug formulations are applied as eyedrops. Although this administration form is easy to use and well accepted by patients, it requires frequent applications, leads to significant drug losses (> 95%) and may cause undesirable side effects, due to the rapid drug absorption into the bloodstream.
The development of more efficient drug delivery systems, that enhance the ocular bioavailability of the drugs, has been pursued by several researchers in the last years and is arising great interest among clinicians and industry. Among the several studied possibilities, soft contact lenses (SCLs) have deserved special attention due to their high degree of comfort, biocompatibility and prolonged contact with the eye. Drug soaked SCLs demonstrated to be more efficient than eyedrops, but still lead to short release times and are not commercially available. In order to enhance their drug loading capacity and to achieve a controlled drug release, various methods have been explored. These methods may be extended to other ophthalmic lenses - the intraocular lenses (IOLs) - due to the similarity of the constituent materials and functionality.
SCLs and IOLs materials (hydrogels) can be loaded with drugs by soaking. Although this is a simple and inexpensive method, which leads to a larger drug uptake by the ocular tissues than topical application of drops, it is ineffective in several cases, since drug may be quickly released, generally with an initial burst. To enhance the drug loading capability and achieve a controlled drug release, different strategies have been attempted.
Among them, there is one which is particularly welcomed both by clinicians and industry: the modification or coating of the hydrogel surfaces to create drug diffusion barriers which prevent the rapid diffusion and loss of the drug. This method has the advantage that it can be applied to the surfaces of ophthalmic lenses already available in the market and whose properties are extensively studied.