Emerging Trends in New Care Regimens for Silicone Hydrogel CLs Materials

Lyndon Jones, PhD FCOptom DipCLP DipOrth FAAO (DipCL) FIACLE Lyndon Jones is currently Professor at the School of Optometry, and the Departments of Physics, Biology, Chemistry & Chemical Engineering (cross-appointed), and Associate Director of the Centre for Contact Lens Research at the University of Waterloo in Ontario, Canada. He graduated in Optometry from the University of Wales, UK in 1985 and gained his PhD from the Biomaterials Research Unit at Aston University, UK in 1998. He is a Fellow and Diplomate of the American Academy of Optometry, has authored over 200 papers and conference abstracts, one text-book and given over 200 invited lectures at conferences worldwide.

The recent release of silicone hydrogel lenses aimed primarily at the daily wear market, along with the increase in popularity of silicone hydrogel daily wear [1] is driving the need for care regimens that perform optimally with silicone hydrogel materials. The fundamental differences in surface properties and the preservative uptake and release behaviour between conventional and silicone hydrogel lenses has resulted in the contact lens industry looking for novel formulations that adequately clean deposits from silicone hydrogels, suitably wet and “condition” the lens surface, and do not contain agents that are preferentially adsorbed and subsequently released. Moreover, the issue of incompatibility between lens care solutions and silicone hydrogels has become a matter of concern to both practitioner and patient alike because of the recent reports of solution toxicity.

Many patients who are fitted into a silicone hydrogel lens will, quite naturally, continue with their previous contact lens solution. However, recent studies show that certain care regimens can result in abnormally high levels of relatively asymptomatic corneal staining with silicone hydrogels [2-8], even though patients have successfully used the same solution with conventional lens materials. Recent evidence suggests that this long-term staining may result in low grade inflammation, eventually resulting in increased numbers of inflammatory events and reduced comfort [9].

Over the past 5-6 years, there has been marked growth in the use of “no-rub” care regimens. This idea primarily came about because it was well recognised that patients were generally non-compliant with instructions relating to the rub-and-rinse step, with many merely removing the lens from their eye and dropping it into their care regimen overnight. These no-rub products were developed to work optimally on the large amount of loosely bound, non-denatured protein that is found on many conventional lens materials [10, 11]. However, silicone hydrogels deposit small amounts of heavily denatured proteins and increased amounts of lipid, compared to conventional polyHEMA-based materials [12, 13] and there is growing evidence to suggest that patients using silicone hydrogels should consider rubbing their lenses with their care regimen if they are to maintain optimal performance [14].

This article looks briefly at three examples of new solution formulations that have recently become available. These solutions contain novel active ingredients, and incorporate a range of surfactant and wetting agents to improve efficacy of the care regimens and to enhance wettability. Because these solutions are relatively new, very little data has been published on their clinical performance. The aim of this article therefore is to highlight the unique aspects of each solution which may help optimise the performance of daily wear silicone hydrogel materials.

Regard

Regard is unique because it is the only unpreserved care regimen available that requires no post-disinfection neutralisation prior to lens insertion (Table 1). It is primarily based on the chlorite ion, which interacts with acidic microbial components (such as bacterial cell membranes) and transforms temporarily into chlorine dioxide, a potent disinfectant often used to disinfect municipal drinking water [15, 16]. However, chlorine dioxide is notoriously unstable and in Regard is stabilised by trace amounts of hydrogen peroxide. This small amount of peroxide also enhances the antimicrobial action of the chlorite ion, by enhancing its ability to penetrate cells [17]. Out of the bottle and exposed to light, sodium chlorite breaks down into sodium chloride (salt) and oxygen. In the ocular environment hydrogen peroxide is broken down by enzymes (catalase and superoxide desmutase) within the tears to water and oxygen.

Previous animal and human studies have shown that both sodium chlorite and chlorine dioxide are safe when exposed to tissues [18-20]. According to the manufacturers Regard can be instilled directly into the eye because of the very low concentration of hydrogen peroxide present (0.01% or 100 parts per million), which is clearly different to that seen with traditional 3% (or 30,000 parts per million) hydrogen peroxide systems. Regard also contains a wetting agent (0.15% hydroxypropylmethylcellulose; HPMC) and a surfactant cleaner (Pluronic F68).

The lack of preservative in Regard should avoid the onset of toxicity staining seen with certain combinations of preserved products and silicone hydrogel lenses. Preliminary studies [21] indicate Regard is highly effective against bacteria [22] and Acanthamoeba [23] and is safe when used in the eye [24].
         
ReNu with MoistureLoc

ReNu with MoistureLoc (Table 1) [25-27] is unique in that it employs a preservative/disinfectant, alexidine, which is new to the ophthalmic field. This agent is a cationic bisbiguanide that has a long history of use in oral mouthwashes [28-32] and is highly effective against a variety of bacteria [32-35]. In addition, the incorporation of MoistureLoc, a novel wetting and cleaning “complex” which consists of a synergistic combination of polyquaternium 10 and poloxamer 407, acts to enhance the wettability of silicone hydrogel lenses. Polyquaternium 10 is a positively charged polymer with a cellulosic backbone that helps to attract water to the lens surface [36]. Poloxamer 407 is a non-ionic surfactant that also helps retain moisture on the lens surface, removes debris and helps to retain proteins in their native state [26, 36].

Available data shows ReNu with MoistureLoc to be extremely effective against Acanthamoeba trophozoites and cysts [37]. The product was designed to perform optimally when used with PureVision lenses [36, 38], and recent preliminary data show that this product performs well when used with a range of silicone hydrogel lens types [39, 40]. No associated corneal staining has been observed with the silicone hydrogels tested and enhanced comfort – particularly at the end of the day - compared with OptiFree Express has been reported [39, 40].

OptiFree Replenish

OptiFree RepleniSH is the most recent solution designed specifically for silicone hydrogel lenses (Table 1). It uses the same preservative contained in OptiFree Express, polyquad (poyquaternium-1), as well as Aldox, an antifungal agent. The unique aspect of RepleniSH is that has been formulated to optimize the performance of silicone hydrogels through the incorporation of a novel wetting and cleaning agent – “TearGlyde”, which is designed to retain lens surface moisture and enhance wettability. This agent is comprised of two surface-active agents - Tetronic 1304 and C9-ED3A (nonanoyl ethylenediaminetriacetic acid), which form a network on the surface of the lens material, enhancing wettability and reducing deposition of proteins, which should result in improved in-eye comfort.

Preliminary data to be presented at the ARVO meeting in May 2006 suggests that this solution provides minimal levels of corneal staining, good cleaning efficacy and good wettability on FDA group IV lenses [41]. Time will tell regarding its performance with silicone hydrogel lenses.

Summary

It is estimated that some 25% of multipurpose contact lens solutions are generic, private-label brands, which are almost exclusively older, obsolete formulations [42, 43]. It is imperative that contact lens practitioners educate patients about the benefits of using new, complex, dedicated care products such as those described in this article, if the proven physiological benefits of silicone hydrogels are to be further enhanced. 

All patients should be carefully questioned about the type of solution they are using and how they are using it. All patients – whether they appear to be symptomatic or not - should be encouraged to try newly developed products that have been optimised for use with silicone hydrogels. By adopting a proactive approach to lens care usage then hopefully patients will see an increase in comfort and improved satisfaction and contact lens dropout rates will be reduced.

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