Solutions that do not include an enzyme are formulated to contain other elements that help to remove protein and prevent it from binding to lens surfaces.
Contact lenses become coated with a protein-rich film soon after being placed on the eye. In the short term, this improves wettability and reduces friction, making lenses more comfortable. Over time, the protein (mostly lysozyme) can denature and firmly adhere to the lens surface. Bound protein can decrease comfort and vision, and in some cases precipitate a giant papillary conjunctivitis.
You could argue that the discovery that a proteolytic enzyme could extend the useable life of contact lenses saved the pre-disposable soft contact lens industry. (On the other hand, had this discovery not taken place, we may have gotten to disposables much sooner.) For years, the use of papain, pancreatin and subtilisin made soft contact lens wear significantly less problem-prone.
Multi-purpose soft contact lens solutions simplified (revolutionized?) soft contact lens care. The goal of a safe, effective single reagent system had merit, as well as enthusiastic consumer interest. One catch is that our traditional enzymes can cause significant stinging and ocular irritation when they get in the eye, so the patient still needed a multi-step approach, usually involving separate enzyme soaking, to remove bound protein from the eye. A liquid form of the enzyme helped, but patients still had to thoroughly rinse lenses prior to insertion to avoid irritation. Disposability may be the ultimate approach to avoiding lens deposit problems, but we know that many patients don"t dispose as regularly as we recommend, and they don"t always take their lenses out. We can expect more of the latter with FDA approval of 30-night wear.
"No separate enzyme required"
Practitioners and patients alike misinterpret the "no separate enzyme required" phrase many multi-purpose solutions include in their labeling as meaning "enzyme included." These products do not include an enzyme. Rather, they are formulated to include other elements that help to remove protein and prevent it from binding to lens surfaces.
Ionic lens materials, from which nearly 70% of soft lenses are made, have the greatest propensity to attract lysozyme. So much so that considerable effort has gone into developing solution elements that will prevent protein-binding on ionic lens materials, which carry a negative surface charge. Tear proteins are positively charged and are thus attracted to the lens surface. One approach to reducing protein deposits is for the solution to contain a positively-charged element that effectively competes with the protein for the lens surface. (This "ionic exchange" is similar to the way a home water softener removes calcium from water, by letting a sodium-saturated solution compete with and displace calcium from ionic-binding sites. The sodium does not bind nearly as tenaciously as the calcium, and is easily flushed away.) In addition to this ionic competition, other added elements will form ionic bonds with the protein in such a way that the protein is easily removed from the lens even by simply passive soaking.
An interesting twist on this chemistry is the addition of protein emulsifier to a rewetting drop. This is the concept behind Alcon"s Clerz Plus drops. Cleaning and soaking reduce the amount of protein on the lens, but never back to the pre-wear condition. The protein accumulation/reduction cycle repeats each day, and the amount of protein on a lens gradually increases. Regular use of Clerz Plus drops has been shown to reduce protein on Group IV lenses by about 20 percent.
The latest approach to limiting protein-binding to lens surfaces is the application of methods of plasma polymerization, a modification of the surface layer of the lens at the molecular level. The resulting impact on ionization of the lens surface affects both wettability and deposit resistance. Variations of this technique are employed to improve the surface characteristics of CIBA Vision"s Focus Night & Day and Bausch & Lomb"s PureVision silicone hydrogel lenses.
Enzymes not yet obsolete
In the not too distant future, enzyme products will likely take their place with salt tablets and other dated contact lens adjuncts. In the meantime, they still play an important role with our non-disposable lenses, including rigid lenses.
Dr. Bergenske (email@example.com) is on the faculty at Pacific University College of Optometry.