Scientists, manufacturers and clinicians are always looking for ways to minimize the occurrence of infection in contact lens wearers. One initiative that receives attention from time to time is the use of antimicrobial surfaces on contact lenses and lens cases to reduce the risk of microbial keratitis.
The concept of using an antimicrobial surface such as a heavy metal coating is not new; antimicrobial coatings have been employed in orthopedic implants, urinary catheters and wound dressings for decades.1 For contact lens wearers. preventing bacterial adhesion and its resulting colonization would minimize biofilm deposition—likely preventing (or at least minimizing) the chance for infection and reducing inflammation.1,2
Researchers are constantly examining antimicrobial agents suitable for contact lenses and lens cases as viable protective mechanisms against potential risk for microbial keratitis secondary to contact lens wear. Strategies include using cationic metals such as silver and peptides, selenium, furanones, chitosan, quorum-sensing inhibitors and various biocidal and non-cidal agents such as polyquats.1,2 Also, several animal studies support biocompatibility and safety of lenses coated with antimicrobial peptides.3
A New Contender
Melimine, a cationic peptide, has broad-spectrum activity and is part of the innate immune system of all multicellular organisms. It also has the ability to inhibit microbial growth.3 Melimine’s proven effectiveness against bacteria and non-bacterial pathogens has caught researchers’ attention. This coating does not alter the physical dimensions of the lens and is even heat stable.3 Might it be the best option so far?
The goal is to develop lenses with an ideal surface coating or an infused material: non-toxic with significant antimicrobial activity. In addition, it should have minimal impact on normal ocular flora or the innate immune system responsible for warding off and responding to possible infection.1,3 Researchers hope lenses with melimine will inhibit microbial adhesion and consequently reduce adverse events—including microbial keratitis.
Keep in mind that our bodies have robust anti-infective defenses that naturally provide surveillance through neutrophils and macrophages.4 Small peptides called defensins have naturally occurring anti-infective properties. Additionally, lactoferrin is found in mucous membrane secretions along with other tear film components and may actually prevent adherence of bacteria to the lens surface and block biofilm development.1,4
Literature has yet to uncover what happens to the elaborate innate defense system when a lens with surface coating is introduced to the ocular surface, and blocking any of the inflammatory mechanisms may not serve a desirable purpose. Only clinical trials can fully assess whether antimicrobial lenses (and cases) induce resistance. According to Mark Willcox, PhD, of the University of New South Wales, laboratory tests have reported that bacteria have trouble becoming resistant to antimicrobials such as silver, selenium and melimine.
Unfortunately, several impediments to the development of effective antimicrobial lens surfaces exist. First, the cost of production poses limitations, especially with single-use lenses. In addition, there have been concerns about unintended effects on the whole tear film ecosystem when blocking or killing off normal flora. Surprisingly, the biggest hurdle may be the regulatory approval process. According to Dr. Willcox, you may actually need to prove reduction in microbial keratitis, and that likely poses some study design problems. Lastly, there is the question of the proposed agent’s compatibility with current accoutrement.1
Fortunately, microbial keratitis with subsequent vision loss is rare. But wouldn’t it be nice to make it exceedingly rare? The notion of producing lenses with antimicrobial surfaces or polymers (with similar qualities) has been investigated for decades with little progress, but there’s motivation to reduce the rate of infection in lens wear. Will melimine or another agent be the answer in the near future?
1. Gabriel M, Wesibarth RE. Developing antimicrobial surfaces for silicone hydrogels. Silicone Hydrogels. www.siliconehydrogels.org/editorials/09-dec.asp. Accessed January 30, 2018.