Close encounter: Delivering medication to the surface of the eye

The eye's anatomy doesn't make it easy for foreign objects to penetrate it, for better and for worse.

The epithelium and stroma are physical barriers. Tears and eyelashes naturally wash out unwanted objects. Conversely, those same protective mechanisms can dilute medications—eye drops, for instance—meant to treat ocular conditions. For decades, researchers have attempted to find more efficient and effective ways of delivering and releasing drugs into the eyes. A new study underscores the ongoing research being done to find a solution.

Published online in the February 2018 issue of Drug Delivery and Translational Research, a new study reports on testing soluble ocular inserts developed to deliver controlled amounts of moxifloxacin to the eye for treatment of such bacterial infections as corneal keratitis and bacterial endophthalmitis. The tiny drug-releasing inserts, made from bio-adhesive polymeric materials, were tested on fresh and frozen rabbit corneas.

"Inserts allow the reduction of systemic absorption and in some cases, increase patient compliance, as there is reduced frequency of administration and lower incidence of visual and systemic side effects," say the study's researchers in the pharmacy department at the CEU Cardenal Herrera University in Valencia, Spain.

The result was promising, the study's authors say.

"The ophthalmic insert we have developed allows a larger quantity of moxifloxacin to permeate through the cornea than existing commercial formulations of the drug," the researchers say. "Ocular delivery of moxifloxacin with this insert could be a new approach for the treatment of eye diseases."

Seeking effective and efficient treatment

Previous studies have noted that as little as 5% of a dose of eye drops may reach its desired target—the anterior tissues of the eye.

Researchers hope to do better, says Thomas Quinn, O.D. Dr. Quinn, who practices in Athens, Ohio, is past chair of the AOA's Contact Lens and Cornea Section (CLCS). Researchers are studying the potential to deliver drugs via contact lens, and the tiny polymer packets. Researchers also are looking at nano-drops to treat nearsightedness and farsightedness.

"When drops are applied to the ocular surface, much of the medication is washed away or neutralized by components in the tears," Dr. Quinn says. "In cases of infection in the eye (endophthalmitis), it can be difficult to deliver adequate levels of medication through the globe without resorting to injection.

"Efforts have been explored for years employing contact lenses as vehicles to deliver medication to the ocular surface," he adds. "The greatest challenge has been in controlling the rate of release of medication from the contact lens. This new study suggests technology is on the horizon that may help in each of these scenarios. With recent strides being made, perhaps we will soon have yet another tool for enhancing the treatment of ocular disease."

Edward Bennett, O.D., current chair of the AOA's CLCS, agrees.

"This study confirms that we are coming closer to our goal of having a much more prolonged and effective method of providing ocular medications than the use of drops," says Dr. Bennett, University of Missouri-St. Louis College of Optometry assistant dean.

"This complements the ongoing research with diffusion of ophthalmic medications—notably for management of conditions such as dry eye and glaucoma, which a diffusion process is more effective for the drug to stay in contact with the eye for long periods of time and may, in fact, aid patient compliance. Regardless of the method, the future for ophthalmic drug delivery is very exciting."