Reader and Industry Forum

Medication Delivery Via Contact Lens

By Agustin Gonzalez, OD

Managing drug therapy and ensuring that patients adhere to their prescribed regimens are common challenges for all clinicians. Adherence, drug toxicity and systemic absorption are particularly concerning for eyecare practitioners when treating chronic eye diseases and managing postsurgical patients, who require multiple doses of multiple medications for prolonged periods.

Delivering medications to the eye has been a challenging task from a pharmaceutical point of view. The unique anatomical and physiological characteristics of the eye restrict molecules in a mechanical or physiological manner, and this, in turn, restricts drug delivery to the site of action. About 90 percent of current ophthalmic medications are delivered topically in the form of ointment, suspension or solution. These medications are typically used to target anterior segment disease. This article will explore the potential of using a contact lens as a vehicle for drug delivery.

Benefits

The use of contact lenses to deliver medication to the eye has been studied since at least the mid-1960s. Some advantages of this method of drug delivery include the following:

1. Improved penetration, because high concentrations of the drug remain in contact with the cornea for a longer time
2. Amount of drug delivered is five times higher than that released toward the external lacrimal fluid
3. Reduced drug concentration required for therapeutic effect
4. Improved ocular bioavailability
5. Minimized risk of systemic toxicity, because medication remains at the site of action
6. Simplified administration with potential for improved adherence
7. Vision correction during therapeutic management.

Contact lenses as vehicles for drug delivery would be particularly useful for chronic diseases that require long-term therapeutic management, such as glaucoma, age-related macular degeneration, dry eye, allergies and chronic inflammation.

Ideal Drug Delivery Lens

With a goal of slow, sustained, constant release of medication to the eye, a contact lens for drug delivery needs to meet several criteria. Ideally, the lens should be comfortable enough to be worn for an extended period during therapy. Comfort is also an important consideration when treating patients who have surface inflammation or dry eyes, as the instillation of drops, such as artificial tears, would disrupt medication dynamics. The lens would also need to incorporate antimicrobial and anti-inflammatory agents that would be released over time.

Another consideration would be the ability of the material to reservoir the medication. This has many variables, such as the water content and thickness of the lens, the molecular weight of the drug, the concentration of the drug-loading solution and the time required for therapeutic effectiveness.

Development Challenges

Several studies have looked at the ability of contact lenses to enhance the penetration of topically applied therapeutic agents. A conventional approach is to soak the lenses in the drug solution before applying them to the patient's eyes. This approach produces unsatisfactory results because the drug release rate drops quickly over time.

A process called chemical functionalization allows for coordination or physical bonds between the material and the drug molecules. This is accomplished mainly through the application of molecular molding or imprinting. The procedure uses ion exchange as a way to release medication to tissue. Chemical functionalization has been used in the delivery of some medications, such as naphazoline, but the ionization process can deteriorate some of the lens properties, such as light transmittance, because it promotes lipid and protein deposition.

Molecular imprinting is another pro cess whereby synthesis of the contact lens occurs in the presence of drug molecules. The limited number of functional monomers available for contact lens manufacturing and the reduced physical stability of the receptors are barriers to the application of this technique.

Encapsulation of drug in nanoparticles is also being studied, and this may be one of the most promising methods for delivering medications to tissue. With nanoparticle vesicle technology, the drug is encapsulated in solution inside the monomers that make up the contact lens. The nanoparticles are less than 50nm. In studies, the release profiles of lidocaine, which is considered a model hydrophobic drug, travel through the lens matrix and enter the post-lens tear film, and the thin tear film is trapped between the cornea and the lens.

Extended Release

With delivery via contact lens, drug molecules reside in the post-lens tear film longer than the 2 to 5 minutes that is typical with topically applied drops. The presumption is the longer residence time provides a higher drug flux through the cornea while reducing drug absorption into the blood stream through the conjunctiva or the nasolacrimal duct. Owing to a slow diffusion of drug molecules through the particles and the lens matrix, a contact lens can provide continuous drug release for extended periods. The longer residence time would presumably result in higher drug bioavailability through the tissue.

Researchers have looked at the drug release rates of commercially available soft contact lenses (silicone hydrogel and acrylate hydrogel) and the extended release of ciprofloxacin hydrochloride. They found the majority of the conventional soft lenses released their drug within the first 10 to 15 minutes. Of the silicone hydrogel materials, balafilcon A released the highest amount of drug and appears to be the most encouraging for drug delivery.

More recently, contact lenses were made by coating poly (-lac-tic-co-glycolic acid) (PLGA) films containing test compounds with poly (hydroxyethyl methacrylate) (poly-HEMA) to deliver high doses of medication for up to 100 days. The drug release rate can be controlled by altering the properties of the polymer film and the lens using an infusion process. There is much interest in the development of commercial lenses using materials approved by the Food and Drug Administration to deliver drugs for up to 30 days per single use.

Promising Application

Soft contact lenses are familiar devices, easy to use and relatively economical to manufacture, making them attractive potential carriers for controlled-release medications for ocular disease treatment. Although there are some hurdles to clear and more research is needed, recent studies suggest long-term delivery of medications via contact lenses has several advantages, including prolonging the residence time of drugs in the precorneal area, reducing systemic absorption and improving patients' adherence to dosage regimens. I believe we will soon see a soft contact lens designed specifically as a vehicle for ophthalmic drug delivery in an efficient and time-dependent manner for managing eye diseases. CLS

For references, please visit www.clspectrum.com/references.asp and click on document #193.

---

Dr. Gonzalez practices in Dallas, where he specializes in medical optometry with primary interests in cornea and lacrimal disorders. He also serves as adjunct faculty at Inter American University of Puerto Rico. He is a consultant to Bausch + Lomb.