Published online last week in Contact Lens and Anterior Eye, the article reviews studies that have been conducted using polymeric hydrogels as soft contact lens-based ophthalmic drug delivery systems.

According to the authors, who are based in China and the UK, contact lenses "hold interest as therapeutic devices for delivery of ophthalmic drugs" and "we believe that the ability of soft contact lenses to provide an alternative ophthalmic drugs delivery system warrants further investigation".

The majority of conventional eye treatments consist of eye drops, but these are largely ineffective at delivering the drugs as only about five per cent of the drug is actually absorbed into the cornea where it is needed. In addition, residence time of the drug in the eye is short; about two minutes.

"A number of researchers have proposed using contact lenses for ophthalmic drug delivery. It has been shown that in the presence of a lens, ophthalmic drugs have a much longer residence time in the post-lens tear film, compared with two minutes in the case of topical application as eye drops," the report said.

The article reviews six different types of contact lens-based ophthalmic drug delivery systems.

One of the methods investigated was the pre-soaking of contact lenses with the drug.

"Results showed that the ability of conventional soft contact lenses to be a drug reservoir strongly depends on the water content and thickness of the lens, the molecular weight of the drug, the concentration of the drug loading solution, the solubility of the drugs in the gel matrix and the time the lens remains in it."

However, this delivery system saw that the lenses could only deliver drugs for a few hours.

The authors found incorporating the drug solution or drug plate in a hollow cavity in the lens looked promising in in vivo albino rabbit studies, but the oxygen and carbon dioxide permeability was lower than that recommended for safe daily wearing of contact lenses.

The review also looked at modifying the surface of the lenses to bind drug-containing liposomes, dispersing micro-emulsion drug drops into soft contact lenses, binding ionised drugs to ion ligands in the lens which would release the drugs in the eye through ion exchange, and molecularly imprinting the lenses.

In August, at the Drug Discovery and Development of Innovative Therapeutics (DDT) conference in Boston, Pfizer head of worldwide strategic alliances Dr BJ Bormann said the US drug giant was keen to look at advancing into ophthalmology, though she said it would be dependent on drug delivery issues.