Corneal Clarity

The Preservative Question

By Thomas John, MD

Our daily lives are full of preservatives. They may be in the food we consume, the paint we use to paint our homes, the wood in our utility poles, the cosmetics and pharmaceuticals that we use both systemically and topically in our bodies — and specifically in our eyes. As we know, however, many preservatives have been linked to harmful effects. So let's take a closer look at the world of preservatives, especially at those that are more relevant to the human eye. Should we continue to use preservatives or walk away from them? The real question is “P or no P”?

Ophthalmic Preservatives

Let us now enter our world of ophthalmic pharmaceuticals and preservatives. An ophthalmic, multi-dose medication consists of the active drug, viscosity-increasing agents, buffers and stabilizers, carrier vehicles, preservatives and the drugdelivery system. With all multidose, topical ophthalmic medications, the FDA and the US Pharmacopeial Convention (USP) mandate the use of preservatives (Table). Numerous studies have shown that ophthalmic medications in the absence of preservatives often get contaminated with bacteria within a week or two of b.i.d. usage.¹ Besides the antimicrobial activity in the container, preservatives can also prevent biodegradation or decomposition of the active medication and thus prolong its shelf-life.

The ideal ophthalmic preservative should have the following features:

1. Bactericidal and fungicidal properties, yet also be harmless to ocular surface tissues. In other words, it should have selective toxicity.
2. A good shelf life. That is, it should be non-irritating, be readily soluble in aqueous solution, effective within physiologic pH 6 to 8, nontoxic to corneal and conjunctival epithelia and corneal endothelium, and have prolonged chemical stability at physiologic pH in aqueous solution.
3. Chemical and thermal stability in aqueous solution, and be able to withstand autoclaving for 20 minutes at 120°C.
4. Be non-sensitizing, not absorbed systemically, and not have a negative impact on other components of the medication.
5. Resist absorption or adsorption into the polymers of eye-dropper containers.
6. Resist absorption into the soft contact lens polymer matrix, and resist significant adsorption onto the contact lens surface, which can disrupt the water wettability of the contact lenses.

IMAGE COURTESY OF THE AUTHOR

Corneal surface epithelial breakdown secondary to drug preservatives in a dry eye patient.

In the absence of such an option, we need to choose an ophthalmic preservative that is gentle on the corneal and conjunctival epithelia. Continued research has provided newer preservatives with relatively good safety profiles while maintaining efficacy. These include stabilized oxychloro complex, SofZia and sodium perborate. Although in vitro laboratory testing on cell cultures and animals may in some instances show significant toxicity of tested preservatives, this may be quite different in the real world, where tears immediately dilute an applied drop of a drug on the healthy ocular surface, and it subsequently washes away from the ocular surface, than the in vitro situation.

Types of Ophthalmic Preservatives

Ophthalmic preservatives may be divided into the following types:

► Chemical preservatives.
► Oxidative preservatives.
► Antioxidant preservative and others with a preservative activity.

Chemical preservatives alter the permeability of cell membranes and lyse the cytoplasmic contents. Examples include:

• Qauternary ammoniums (surfactant and disinfectants): BAK, polyquaternium-1 (polyquad), cetremide, miramine, etc.
• Mercurial derivatives: thimerosal, mercurobutol, etc.
• Amides: chlorhexidine digluconate, polyhexamethylene biguanide (PHMB).
• Alcohols: chlorobutanol, phenylethenol.

• Parabens or esters: parahydroxybenzoic acid, methylparaben, etc.

Oxidative preservatives penetrate the cell membranes and interfere with cell functions.

• Sodium perborate (GenAqua).
• Stabilized oxychloro complex (Purite).
• Stabilized oxyborate complex (Dissipate).

Antioxidant preservative and others with a preservative activity include:

• EDTA (ededate disodium).
• Sorbate (sorbic acid).
• Sodium silver chloride complex, silver sulfate.

My Take

Here's my own review of some common preservatives we run across in eye care.

Benzalkonium chloride: BAK is the most common antimicrobial preservative. It is used in multidose, topical, ophthalmic solutions and is considered the gold standard of preservatives. Currently, more than 70% of multidose, ophthalmic medications contain BAK, with concentration ranging from 0.004% to 0.02%.² A quaternary ammonium compound, BAK is often used along with disodium EDTA. Further, it is a chemically stable, detergent preservative that acts quickly and highly effectively against many microorganisms by altering their cell membrane permeability and lysing the cytoplasmic contents.

BAK is effective against bacteria, some of the viruses, fungi and protozoa. Gram-positive bacteria are usually more susceptible than gram-negative bacteria. Because it can cause corneal epithelial separation, BAK can increase the corneal penetration of some topical ophthalmic medications. However, BAK can accumulate in ocular tissues and cause cell death, especially if used frequently.

It is worth noting that some strains of Pseudomonas aeruginosa are resistant to BAK and can grow in BAK-containing solutions. However, adding EDTA can eliminate this acquired resistance of P. aeruginosa, and therefore it is often used as a preservative aid. This property is attributed to EDTA having the ability to chelate divalent cations.

Polyquad (polyquaternium-1): This polymeric quaternary ammonium, antimicrobial preservative has less effect on corneal epithelial cells than BAK. It is somewhat new to ophthalmic preparations, and has been used in contact lens care solutions. Compared to BAK, polyquad causes less corneal damage with regard to breakdown of cell junctions. This may be due to its relative inability to penetrate eye tissues, especially the cornea.

Stabilized oxychloro complex (Purite): This broad-spectrum microbicide possesses a very low toxicity to mammalian cells. Although sodium chlorite has been used in water purification processes since the mid-1940s, it was not until 1996 that it was introduced to ophthalmic preparations. It has a broad spectrum of antimicrobial activity, including Aspergillus niger. Purite preservative finally breaks down to normal tear components, namely, water, oxygen, sodium and chloride ions, without causing any significant ocular irritation or ocular surface cellular damage.

Sodium perborate: Used in dental hygiene solutions since the 1950s, this was one of the first oxidative preservatives used in ophthalmic solutions. On contact with an aqueous environment, sodium perborate is catalyzed into hydrogen peroxide, water and oxygen. The hydrogen peroxide kills microbes. It is present in laundry detergents, laundry bleaches, cleaning products, and in some tooth-bleaching formulas. It is used as a “disappearing” preservative in some eye drops, e.g., GenTeal (Table).

Sorbate (sorbic acid): An ophthalmic preservative with low toxicity, sorbate is a relatively weak static inhibitor of several bacteria and fungi.

SofZia: This ionic buffer contains sorbitol, borate, propylene glycol and zinc. These substances do not cause significant ocular surface toxicity,³ yet maintain an antibacterial environment within the container.

Determining Efficacy

The FDA uses the Preservative Effectiveness Test (PET) as a minimum standard of preservative performance. The microorganisms used for the PET are as follows:

Bacteria:

1. Staphylococcus aureus ATCC 6538.
2. Pseudomonas aeruginosa ATCC 9027.
3. Escherichia coli ATCC 873.9
Fungi:
1. Aspergillus niger ATCC 16404.
2. Candida albicans ATCC 10231.

These five indicator microorganisms are used for challenging the preservative in a product. They are ATCC cultures and have to be harvested under current USP guidelines to ensure viability. The product is inoculated (contaminated) with 1x10⁵ (100,000) to 1x10⁶ (1,000,000) colony-forming units (CFU) per mL of the test product. PET then compares the number of microorganisms found on a control sample against the test sample over the course of 28 days. Following bacterial inoculation of the tubes, they are incubated at 20°C or 25°C for four weeks, with weekly examinations.

The requirements for opthalmic preparations include:

• At 7 days — viable bacterial concentration should not be >10% of the initial concentration.
• At 14 days — viable bacterial concentration should not be > 0.1% of the initial concentration.
• At 28 days — a further decrease in bacterial count.
• At 7, 14, and 28 days — no increase in yeast and mold counts from the initial count.

A preservative may be considered effective if it reduces the bacterial concentration to 0.1% or less of the initial concentration after two weeks and keeps the fungal concentration of yeasts and molds at or below their original concentration for the remaining two weeks (total of 28 days).

In Vivo Perspective

BAK has been claimed to have a synergistic enhancing effect on antibiotic efficacy of gatifloxacin ophthalmic solution 0.3% [preserved with 0.005% BAK (50 microg/mL), Zymar; Allergan]. This hypothesis was tested by Friedlaender et. al,⁴ who measured the concentration of tear film BAK at successive time points after topical administration of commercial gatifloxacin.

Ten subjects received five separate instillations of a single 35-mL drop of gatifloxacin 0.3% ophthalmic solution in each eye, and tear samples were collected at various intervals (30 sec., 1 min., 3 min., 5 min. and 20 min.). The study used a validated high-performance liquid chromatography method to measure the concentration of BAK in each tear sample. The results showed rapid BAK dilution to 6.4 microg/mL, 3.2 microg/mL, 1.4 microg/mL, below the detection limit, and below the detection limit at 30 sec., 1 min., 3 min., 5 min., and 20 min. post-drop instillation.

This study showed that due to rapid dilution, the concentration of BAK is reduced to near zero in minutes after ocular surface application. The authors concluded that BAK is not expected to have a clinically significant effect on enhancement of the antimicrobial efficacy of gatifloxacin on the ocular surface.⁴

It may be important to point out that in vitro laboratory studies of preservative toxicity may not manifest to the same extent in an in vivo situation due to the rapid dilution of the preservative on the human ocular surface, as this study showed.

Conclusions

Preservatives are an integral part of our daily life, whether it is the cosmetics we use, food we consume, or the ophthalmic medications we use on the ocular surface. They are an invaluable tool in man's continued efforts to keeping the enemy — the microorganisms — from harming us and threatening our very existence. One should recognize the multifaceted effects of ocular preservatives. While preservatives do present some complications, rejecting them introduces other hurdles that we would have to deal with, such as unit dose dispensers, and non-compliance with manufacturer's recommendation of discarding the vials after single use. Opening and reusing medications in these situations can lead to potential contamination and, subsequently, vision-threatening ocular infection.

So the answer to the question I posed in the beginning of this column is a “yes” to preservatives. That is, with selective situations of non-preserved unit-dose medication use in a subset of our patient population that can benefit from avoiding the potential toxicity of preservatives. Such cases are likely to arise for multi-medication use, prolonged use of topical medications, compromised ocular surface tissues, increased concentration of preservative, and/or increased frequency of topical drug. The subset of patients may include glaucoma patients, dry eye patients, those with existing, compromised, ocular surface tissues and decreased corneal sensation, or simply contact lens wearers.

We don't have to necessarily walk away from time-tested and proven ophthalmic preservatives, especially when dealing with a lower dosing schedule (q.i.d. or less/day), with otherwise healthy ocular surface, close clinical monitoring and a low threshold for management of any preservative-induced ocular toxicity.

Preservatives have a place in our daily existence; it is a question of using the preservative that is as close to an ideal preservative as currently possible, causing least harm, if any, to ocular tissues, while protecting and preserving vision.

So, “P” stays — at least for now. OM

References

1. Schein, OD, Hibberd PL, Starck T, Baker AS, Kenyon KR: Microbial contamination of in-use ocular medications. Arch Ophthalmol. 1992; 10: 82-85.
2. Kaur IP, Lal S, Rana C, Kakkar S, Singh H: Ocular preservatives: associated risks and newer options. Cutan Ocul Toxicol. 2009,28:93-103.
3. Kanamoto T, Kiuchi Y, Suehiro T, Nakano T, Nakano Y, Hirota A, Miyata A: Efficacy and safety of topical travoprost with sofzia preservative for Japanese glaucoma patients. Hiroshima J Med Sci. 2010;59:71-75.
4. Friedlaender MH, Breshears D, Amoozgar B, Sheardown H, Senchyna M: The dilution of benzalkonium chloride (BAK) in the tear film. Adv Ther. 2006;23:835-841.

Thomas John, MD, is a clinical associate professor at Loyola University, Chicago, and in private practice in suburban Chicago. E-mail him at tjcornea@gmail.com.