Contact Lens Design & Materials
Update on Drug Delivery Lenses
BY RONALD K. WATANABE, OD, FAAO
Soft contact lenses for drug delivery have been under study for years. The concept of slow, sustained release of medication is attractive, and it theoretically would be a more effective method of delivering therapeutic doses compared to the conventional eye drop. Drops can drain quickly or may not even make it into the eye at all. This can be problematic for patients suffering both acute ocular complications and chronic ocular diseases.
Commercially available soft lenses have been investigated as drug delivery devices, but pose challenges to the predictable delivery of medications. Lens water content, thickness, and surface properties as well as the drug's molecular weight and concentration affect the amount of drug absorbed into the lens and delivered to the eye. In most cases, the lens releases almost all its medication in a short burst that lasts at most a few hours. For most therapeutic applications, this is not much better than eye drops, making their clinical usefulness questionable.
Various forms of modified lenses have been developed recently.
Liposomes (small vesicles made of lipid bilayers) containing drug molecules have been used to coat the surface of soft lenses and have been dispersed in the matrix of soft lenses as micro-emulsion drops. Both can increase the drug molecules' release time, though the entrapped liposomes may have a longer release duration of up to seven or eight days (See “Dry Eye Dx and Tx of this month for more about liposomes.)
Molecularly imprinted contact lenses are formed with cavities that have an affinity for a specific drug molecule. They are able to hold up to three times the amount of drug as non-imprinted lenses and then slowly release the drug over several days.
Cyclodextrin, a sugar-based compound, has been incorporated into HEMA materials to bind drug molecules and increase their concentrations within the lens matrix. Drug molecules held this way have been shown to release slowly for up to several weeks, though the duration depends on the size of the drug molecule.
Biomimetic hydrogels are formed by polymerizing functional monomers that have similar structures to those that bind drugs in nature. An example is a functional group that resembles an H1 receptor and can bind anti-histamine molecules. In this way, a drug can be more readily absorbed within the lens and slowly released over the course of about a week.
Surfactant aggregates may also help bind drug molecules within the lens matrix. One system sustained drug release over the course of one month (Ciolino, 2009; Xinming, 2008).
PLGA and vitamin E are two of the latest developments in this area. Researchers in Boston have developed a drug-polymer film, poly(lactic-coglycolic) or PLGA, able to hold a large quantity of drug and release it in a controlled manner over a long period of time. The PLGA was coated with HEMA and formed into a contact lens. In laboratory experiments, the lens was able to sustain the release of ciprofloxacin at therapeutic levels at a constant rate for at least 30 days (Ciolino, 2009).
Researchers in Florida have developed a vitamin E-laden lens that has been shown to increase the drug release time of timolol and dexamethasone by up to 100 times. Purportedly, the vitamin E aggregates block the transport of the drug molecules, making it more difficult for them to be released from the lens (Peng, 2010).
More to Come
Lens drug delivery is still in the experimental stages, though great advances keep coming. Let's hope we see a true, clinically available contact lens drug delivery system in the foreseeable future. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #177.
Dr. Watanabe is an associate professor of optometry at the New England College of Optometry. He is a Diplomate in the American Academy of Optometry's Section on Cornea and Contact Lenses and Refractive Technologies and is in private practice in Andover, Mass. You can reach him at firstname.lastname@example.org.