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Solving the Preservative Paradox
Better packaging for nonpreserved tears is the answer.
BY HERBERT L. GOULD M.D., F.A.C.S.

With the growing popularity of laser vision correction in recent years and resulting dry eye syndrome, along with strong demographic trends that point to an aging population, the need for beneficial tear substitutes has become critical.

The mounting evidence of toxicity and allergy from preservatives in these eyedrops has caused the industry to respond by formulating non-preserved tear substitutes in unit-dose packaging. However, this format has met with patient resistance both as awkward to use and costly. Additionally, the widespread adoption of hydrogel contact lenses prohibits the use of preservatives for these wearers because the preservatives bind to the lenses and intensify toxicity. The frequency of complications attributed to the use of preservatives in contact lens wearers is difficult to obtain, but most manufacturers and practitioners estimate it to be in the vicinity of 5% to10%.

In this article, I will document the deleterious effects of preserved tears and suggest a safer, nonpreserved alternative with packaging designed to prevent contamination.

The most common preservative in ophthalmic preparations is benzalkonium chloride (BAK). It has been well demonstrated that this chemical, while moderately bactericidal, is highly toxic to the cornea and conjunctiva as well as to nasal mucous membrane.

When solutions containing this preservative are frequently applied, serious tissue damage has been reported.1-4 Reports of eye surface pathology related to the use of BAK have been extensive, based on the use of scanning electron microscopy to document epithelial destruction due to application of commercial eyedrops.5 Moreover, recent reports of the effect of the most commonly used anti-glaucoma drops, timolol and latanoprost, combined with BAK produce damage to lens epithelial cells.6,7

Miyake et al and Brandt demonstrated that BAK not only caused cataract formation but cystoid macular edema (CME) as well.8,9

The accumulative dose of BAK may also contribute to both cataract formation and maculopathy. It is ironic that this preservative is still the most commonly used in eyedrops and nasal sprays given the significant body of data identifying such toxicity. Glaucoma medication is inevitably a lifetime course and the gradual tissue absorption of these proinflammatory and proapoptotic chemicals is inevitably destructive.

Effects on the Conjunctiva

Although eyedrops are mostly well tolerated, especially when they are used in short-term treatments, in certain circumstances patients may complain of frequent stinging, burning, discomfort, irritation or dry eye. However, with long-term treatment with multiple eyedrops, corneal damage and chronic conjunctivitis may occur. In many cases, these adverse reactions cannot be attributed to the active ingredient. Because preservative toxicity is well documented, especially by experiments in vivo and in animals, the preservative is suspect.

Among preservatives, mercury derivatives are rarely used and induce mainly allergic reactions. Quaternary ammonium salts, particularly BAK, are the most commonly used preservatives. They are also the most toxic for the ocular surface, even at low concentrations. As quaternary ammonium molecules, they have detergent properties and can easily be incorporated into the membranes of epithelial cells by their lipophilic chains, and create inroads for aqueous or ionic substances into the intracellular space. They can also break up the intercellular junctions, letting through aqueous or ionic substances.10 BAK is responsible for modification of the tear film, causing dryness and patient discomfort. Ironically, BAK is still the most frequently used preservative in eyedrops.

It is well understood that the tear film has a protective and nutritional role that is essential to the health of the cornea and conjunctiva. When BAK-preserved eyedrops are introduced, surface disruption is created due to BAK's effect on the lipid components of the tear film. This detergent effect causes increased tear evaporation, exacerbating dry eye syndrome.

With frequent installation of preservative-laden eye drops, the resulting toxicity produces a conjunctival inflammatory response with epithelial metaplasia affecting goblet cells and transmembrane mucins.11 This was dramatically noted by Garcher,12 who demonstrated a modification of the structure of mucin chains in glaucoma patients treated by a preservative-containing timolol.

It has been frequently noted that glaucoma patients on long-term therapy are concurrently treated for dry eye syndrome. This comes as no surprise, as the frequent application of a preserved glaucoma medication includes a build-up of toxicity that causes loss of goblet cells with subconjunctival fibrosis.11 A reduction of 50% of the density of these cells has been observed in biopsies from glaucoma patients under extended treatment.13 Significant decreases in Schirmer test values and reduced tear breakup time in patients with primary open-angle glaucoma compared to healthy controls have been documented.14

In another study, it was demonstrated that 0.005% latanoprost (one drop per day for 2 weeks) containing 0.02% BAK produced conjunctival injury and reduced tear film to a greater degree than 0.5 timolol instilled twice daily for 2 weeks containing half the concentration of BAK (0.01%).15 A significant study by Baudouin et al16,17 demonstrated that the degree of overexpression of antigens HLA-DR at the surface of epithelial cells was higher in patients receiving preserved eyedrops. By contrast, HLA-DR expression was not significantly increased in patients receiving a preservative-free beta-blocking agent compared with healthy subjects with no ocular disorders, suggesting a direct involvement of the preservative in the inflammatory reaction.

In sum, preservatives induce an infiltration of the conjunctiva by inflammatory cells (lymphocytes and macrophages). The intensity of this inflammatory response is related to the duration or number of preserved eyedrops used. There have been extreme cases reported where pseudo-pemphigoid has developed after prolonged administration of preserved eyedrops. Patients receiving preservative-free eyedrops exhibit no inflammation.

Effects on the Cornea

Toxic keratopathies (prolonged superficial keratitis, corneal ulcers) attributed to preservatives have been described in various situations, i.e., contact lens wearers, ocular dryness, glaucoma and post-surgical patients.18-21 Whenever long-term use of a preserved eyedrop is maintained, superficial punctuate keratitis is commonly observed.

With extended exposure, a marked loss of epithelium, stromal edema and infiltration leading to corneal opacification may develop. Patients may develop a hyperemic painful eye and decreased visual acuity, and epithelial erosion and superficial corneal neovascularization may occur. With time, preservative toxicity may extend through the stroma and lead to endothelial damage.

Lemp et al19 reported a case of severe endothelial toxicity, clearly due to BAK, in a case of keratoconjunctivitis sicca treated for several years by installation of artificial tears containing BAK.

This patient showed an advanced corneal degeneration requiring keratoplasty. The histopathology of the excised corneal button revealed stromal fibrosis and corneal edema. After surgery, the patient's symptoms persisted until the preservative-containing medication was totally replaced by a saline solution containing no preservatives. A dramatic alleviation of the symptoms was observed after 2 weeks.

There have been numerous reports in the literature citing the toxic effects of ocular preservatives. Schwab et al22 report toxic keratopathies in patients who had instilled preserved eyedrops. Most of the patients demonstrated ocular surface disease. The corneal defects were mostly inferior or inferonasal, suggesting concentration of preservative inferiorly. Severe reactions have been reported post-cataract, obfuscating the diagnosis of infection vs. preservative toxicity or allergy. The use of preserved eyedrops may in certain conditions (e.g., contact lens wear, ocular dryness, anti-glaucoma treatment, during general anesthesia or after surgery) be the cause of severe cell toxicity resulting in prolonged superficial keratitis and ultimately corneal ulcers. Switching to a preservative-free formulation can improve/prevent these conditions.

Glaucoma and Cataract

It has been noted that when patients with primary open-angle glaucoma and a history of extended therapy come in for trabeculectomy, post-inflammatory fibrosis secondary to the extended use of preserved eyedrops may compromise the maintenance of a filtering bleb.23,16

The prolonged administration of preserved anti-glaucoma eyedrops increases the failure rate of filtration surgery. No active ingredient has been incriminated in these failures in any of the studies. Because the preservative increases fibrotic scarring, the hindrance to the flow of aqueous is secondary to BAK toxicity.

Miyake et al24, 25 point to the role of BAK in cataract surgery impacting on the macula and causing CME. They suggest that the the mechanism probably involves the release of pro-inflammatory mediators (prostaglandins, cytokines).

Miyake introduced the descriptive term "pseudophakic preservative maculopathy" for this complication. Additionally, it has been noted clinically that glaucoma patients on long-term eyedrop therapy are prone to cataract formation. Brandt9 notes that the incidence of cataract increases significantly in patients under long-term treatment with topical anti-glaucoma agents, compared with non-treated patients followed up for several years.

Recommendations

Chronic keratoconjunctivitis has been noted to result frequently from the use of preservatives. In addition, eye-drop preservatives have been demonstrated to cause ocular surface, lens and retinal pathology. Because of the obvious adverse effects of eyedrop preservatives, unit-dose, nonpreserved eyedrops have become quite popular despite objections relating to cost and inconvenience. An improvement in the form of a multi-dose preservative-free system would be highly desirable.

A recent report from the British Journal of Ophthalmology noted that a study using nonpreserved multi-dose eye drops revealed a high incidence of bacterial contamination, including a potent strain of Staphlycoccus aureus. The study emphatically demonstrated the danger of multi-dose nonpreserved eyedrops in a standard container.

However, the quest for a safe method to deliver eye- drops in a multi-dose system has great appeal. Realizing this medical need, Pfizer has introduced multi-dose nonpreserved Visine (without the vaso-constrictor) using the German Commode packaging system. This is a complex spring and one-way valve system with a collapsing inner container. It keeps out bacteria but allows air in, which ages the content. Pfizer makes the claim that "The Package is the Preservative." This is the first commercial use of package alteration to provide a multi-dose preservative-free delivery of eye drops.

While this format represents a significant breakthrough, the package is expensive and awkward to manipulate, especially for an arthritic senior. Future package modifications hold promise for a safe, economical, user-friendly, one-way valve system. Other safe and effective methods of multi-dose, preservative-free, sterile eye-drop delivery are currently under development, with at least one now in the pipeline for clinical use.

The sooner these innovative delivery systems are perfected, the safer the administration of artificial tears will become.

Herbert L Gould M.D., F.A.C.S., is associate clinical professor at New York Medical College. He can be contacted via e-mail at hlgould60@aol.com.

References

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