Ocular Preservatives: Associated Risks and Newer Options

Cutaneous and Ocular Toxicology, 2009; 28(3): 93–103

CRITICAL REVIEW

Ocular preservatives: associated risks and newer options

Indu Pal Kaur, Shruti Lal, Cheena Rana, Shilpa Kakkar and Harinder Singh

University Institute of Pharmaceutical Sciences, Panjab University, Chandigarh, India

Abstract Presence of a preservative in an ocular medication has often been considered a culprit in damaging the epithelium. However, the inclusion of a preservative is equally necessary, especially in multiple-dose containers, in order to protect against dangerous organisms accidentally gaining access during instillation. Benzalkonium chloride (BAK), chlorobutanol, chlorhexidine acetate (CHA), and phenylmercuric nitrate or acetate are some commonly used preservatives in eye preparations. New preservatives with a wide range of activity and good safety profiles have been introduced in the market, such as stabilized oxychloro complex (SOC), sofZia, and sodium perborate. In the present review, we discuss various conventional and newly proposed and patented preservative molecules for ocular use. Reasons for discontinuing traditional preservatives and the need for less-toxic molecules are discussed at length, along with newer options coming up in this area. Keywords: Preservative; ocular; newer; toxicity

For personal use only. Introduction and stabilizers, and a vehicle by which all the above ingredients are “carried.” Of these, the preservative In chronic eye diseases, for effective therapy, ophthalmic has most often been considered the culprit in damag- medication is to be given regularly for longer durations ing the corneal epithelium, leading to disruption of than usual dosing regimens. Typical examples of such the glycocalyx, when drops are used beyond the rec- chronic pathologies are dry eye, glaucoma, and certain ommended dosing. This sequence of repeated dosing eye infections, especially fungal infections. Preservative leaves the epithelium unable to keep the tear film in is an important constituent of multiple-dose containers place and can lead to ocular surface disease. This is that helps to minimize the risk associated with acci- especially true for patients who overuse their artificial dental microbial contamination, but its frequent use tear products, use multiple ocular medications, suffer has been associated with alteration in the precorneal from chronic eye diseases like dry eye or glaucoma, or film. In patients suffering from dry eye, preservatives require postsurgery dosing of medication. In a way, Cutaneous and Ocular Toxicology Downloaded from www.informahealthcare.com by HINARI on 08/21/09 tend to aggravate the already existing problem; and in it is not incorrect to say that the preservatives are a glaucoma patients, the prolonged use of eye drops with “necessary evil.” preservatives has been associated with changes in ocu- Today, ophthalmologists have numerous antiglau- lar surface accompanied by inflammation. In fact, con- coma medications and artificial tears from which to junctival biopsies in patients suffering from glaucoma choose. Although topically administered medications have revealed an increased number of immune cells and are increasingly used with apparent safety and good fibroblasts (1,2). tolerance, there is growing evidence that long-term use Ocular medications are composed of unique of topical drugs can induce changes in the ocular sur- mixtures of the active drug, a preservative, the drug face and may often produce damage to conjunctival and delivery system, viscosity-increasing agents, buffers corneal epithelial cells. There have been several reports

Address for Correspondence: Indu Pal Kaur, University Institute of Pharmaceutical Sciences, Panjab University, Chandigarh-160014, India. Tel.: 91 172 2534113 (0); Fax: 91 172 2541142; E-mail: [email protected] (Received 06 January 2009; revised 24 April 2009; accepted 25 April 2009)

of the toxic effects of prolonged topical treatments, cannot neutralize chemical preservatives, so the pre- partly due to the preservatives associated with the for- servative is incorporated into the cell and causes cellular mulation of such treatments (3,4). In the eye, preserva- damage (21). tive turnover is very slow, and quaternary ammonium Oxidants—Oxidative preservatives are usually small molecules can be retained in ocular tissues for up to 7 molecules that penetrate cell membranes and inter- days (5). The lipophilic nature of some preservatives fere with cellular function (22). They can destabilize causes them to bind to the ocular tissues immediately cell membranes, but to a lesser degree than detergent after topical application. Previous studies by Burstein preservatives. Stabilized oxychloro complex (SOC) (4,5) have shown that topically applied benzalkonium and sodium perborate are 2 examples of oxidative pre- chloride (BAK), the most commonly used preservative servatives. At low levels, oxidative preservatives have in ophthalmic solutions, can cause morphologic disrup- an advantage over detergent preservatives because tion of the corneal epithelium at high concentrations. In they can provide enough activity against microorgan- addition, there is evidence that clinical concentrations isms while having only negligible toxicity on eukaryo- of BAK may change the ionic resistance of the cornea by tic cells. This is because many microorganisms do not intercalating into cellular membranes, which results in have the ability to cope with oxidative stress, whereas increased permeability (7). mammalian cells are equipped with antioxidants, oxi- Three types of mechanisms have been described: dases, and catalases to neutralize the effect of a low- detergent effects causing loss of tear film stability, toxic level oxidant. effects to the corneal and conjunctival epithelia, and This article concentrates on some of the most com- immunoallergic reactions (5,6). Furthermore, repeated monly used detergent and oxidative preservatives, doses of preserved eye drops can have a cumulative namely, BAK, phenyl mercuric nitrate or acetate, chor- effect, because the preservatives are in prolonged con- hexidine acetate (CHA), sorbic acid, chlorobutanol, tact with the epithelium. Several studies have confirmed sodium perborate, SOC, and polyquaternium-1. Other the participation of preservatives in induction of ocular preservatives found in ophthalmic preparations include surface inflammation (7,8), allergy (8), fibrosis (9), and benzododecinium bromide (BDD), cetrimonium chlo- dry eye syndrome (10,11). Preservatives are also sus- ride, thiomersal, methyl parahydroxybenzoate, poly- pected of strongly increasing the risk of failure of trab- quaternium ammonium chloride, polyaminopropyl eculectomy in glaucoma (12,13). biguanide, and hydrogen peroxide. Tables 1 and 2 con- In vitro models have been developed to predict the tain a list of commonly used products and associated

For personal use only. cytotoxic potential of preservatives. These models were preservatives. essentially based on corneal epithelial cells (14,15) or on other epithelial systems with characteristics similar Table 1. Preservative-containing common glaucoma medications. to those of the superficial layer of the corneal epithelium Trade name of (Madin–Darby canine kidney cells) (16). The human Sr. No. glaucoma formulations Manufacturer Preservative continuous conjunctival cell line has also been useful 1. Alphagana Allergan 0.005% BAK for ocular toxicologic studies (17,18). It has been shown 2. Alphagan Pa Allergan 0.005% SOC that BAK is a strong proapoptotic agent in Chang’s con- 3. Azopta Alcon Laboratories 0.01% BAK junctival cells (19). 4. Betagana Allergan 0.005% BAK In the present review, we discuss conventional, 5. Betoptic Sa Alcon Laboratories 0.01% BAK newly proposed, and patented preservative molecules 6. Cosopta Merck & Co. 0.0075% BAK for ocular use. Reasons for discontinuing traditional 7. Resculaa Novartis Ophthalmics 0.015% BAK Cutaneous and Ocular Toxicology Downloaded from www.informahealthcare.com by HINARI on 08/21/09 preservatives and the need for less-toxic molecules are 8. Timoptica Merck & Co. 0.01% BAK discussed at length, along with newer options coming 9. Timoptic-XEa Merck & Co. 0.012% BDD up in this area. 10. Trusopta Merck & Co. 0.0075% BAK 11. Xalatana Pfizer 0.02% BAK 12. Humorsolb Merck & Co. 1:5,000 BAK Mechanism of action of preservatives 13. Iopidine eye dropsb Alcon Laboratories 0.01% BAK 14. Lumiganb Allergan 0.005% BAK Preservatives can be classified into 2 main categories, on 15. Ocupressb Bausch & Lomb 0.005% BAK the basis of their mechanism of action: 16. Optipranololb Bausch & Lomb 0.004% BAK Detergents—Detergent preservatives, such as BAK, 17. Propineb Allergan 0.04% BAK a act upon microorganisms by altering cell membrane Noecker R. Effects of common ophthalmic preservatives on ocular health. Adv Ther 2001; 18:205-215. permeability and lysing cytoplasmic contents (20). bhttp://www.rxlist.com. Accessed September 2008. Ocular toxicity can occur because some detergent pre- BAK = benzalkonium chloride; BDD = benzododecinium bromide; servatives can affect eukaryotic cells. Mammalian cells SOC = stabilized oxychloro complex. Ocular preservatives: risks and new options 95

Table 2. Multidose, over-the-counter medications for dry eye with or without preservatives. Sr. No. Trade name Manufacturer Active ingredients(s) Preservative 1. GenTeal Milda Novartis 0.2% Hydroxypropyl methylcellulose GenAqua (sodium perborate) 2. GenTeal PFa Novartis 0.3% Hydroxypropyl methylcellulose None 3. Minidrops Eye Therapya Optics Laboratory Polyvinylpyrrolidone, polyvinyl alcohol None 4. Preservative Free Bausch & Lomb 0.95% Propylene glycol None Moisture Eyesa 5. Refresh Plusa Allergan 0.5% Carboxymethylcellulose None 6. Refresh Tearsa Allergan 0.5% Carboxymethylcellulose Purite (stabilized oxychloro complex) 7. Refresh Celluvisca Allergan 1% Carboxymethylcellulose None 8. Refresh Enduraa Allergan 1% Polysorbate 80, 1% glycerin None 9. Refresh Liquigela Allergan 1% Carboxymethylcellulose Purite (stabilized oxychloro complex) 10. Refresh PMa Allergan White petrolatum, mineral oil None 11. TheraTearsa Advanced Vision Research 0.25% Carboxymethylcellulose Sodium perborate 12. TheraTears PFa Advanced Vision Research 0.25% Carboxymethylcellulose None 13. Systane Ultrab (High Alcon 0.3% Propylene glycol, Polyquad Performance) 0.4% polyethylene glycol 400 Systane Preservative Freeb Alcon 0.3% Propylene glycol, None 0.4% polyethylene glycol 400 Systaneb (long lasting) Alcon 0.3% Propylene glycol, Polyquad 0.4% polyethylene glycol 400 Systane Nighttime Lubricant Alcon 3% Mineral oil, 94% white petrolatum, None Eye Ointmentb 3% anhydrous liquid lanolin 14. Moisture Eyes PMa Bausch & Lomb White petrolatum, mineral oil None 15. Lacrisertc Aton Pharma Hydroxypropyl cellulose 5 mg None 16. Murine Tears Plus Lubricant Prestige Brands 0.5% Polyvinyl alcohol, 0.6% povidone, Benzalkonium chloride Redness Reliever Eye Dropsc 0.05% tetrahydrozoline hydrochloride 17. Murine Tears Lubricant Medtech 0.5% Polyvinyl alcohol, 0.6% povidone, Benzalkonium chloride Eye Dropsc sodium bicarbonate For personal use only. 18. AMO Blink Gel Tears Amo Sales and Service Inc 0.25% Polyethylene glycol 400 None Lubricating Eye Dropsc 19. Murocel Methylcellulose Bausch & Lomb 1% Sodium chloride, sodium borate, Methylparaben, Lubricant Ophthalmic Pharmaceuticals propylene glycol propylparaben Sterile Solution USPc 20. Clear Eyes for Dry Eyes Plus Prestige Brands 0.012% Naphazoline hydrochloride, Benzalkonium chloride ACR Reliefc 0.2% glycerin, 0.25% zinc sulfate 21. Allergan Optive Lubricant Allergan Pharmaceutical 1.4% Polyvinyl alcohol, 0.6% povidone None Eye Dropsc 22. Tears Renewed Ophthalmic Akorn. 0.3% Hydroxypropyl methylcellulose, Benzalkonium chloride Solutionc 0.1% dextran 70 23. Soothe Emollient Alimera 0.4% Polysorbate 80, Restoryl Polyhexamethylene (Lubricant) Eye Dropsc (consists of 15.1% Drakeol and biguanide Cutaneous and Ocular Toxicology Downloaded from www.informahealthcare.com by HINARI on 08/21/09 4.5% Drakeol-35) 24. Isopto Tearsc Alcon 0.5% Hydroxypropyl methylcellulose Benzalkonium chloride 2910 25. Puralube Opthalmic Fougera 42.5% Mineral oil, 56.8% white Chlorobutanol Ointmentc petrolatum 26. Rohto V Arctic Cooling Mentholatum Inc Hypromellose, tetrahydrozoline Benzalkonium chloride Lubricant/Redness Reliever hydrochloride Eye Dropsc 27. Lacri-Lube S.O.P. Lubricant Allergan 42.5% Mineral oil, 56.8% white Chlorobutanol Eye Ointmentc petrolatum 28. Allergan Optive Lubricant Allergan Pharmaceutical 1.4% Polyvinyl alcohol, 0.6% povidone None Eye Dropsc 29. GenTeal Lubricant Eye Gel Novartis Pharmaceutical 0.3% Hypromellose None for Severe Dry Eye Reliefc Table 2. continued on next page. 96 I. P. Kaur et al.

Table 2. Continued. S. No. Trade name Manufacturer Active ingredients(s) Preservative 30. Visine Tears Lasting Relief Pfizer Consumer Glycerin, hypromellose, polyethylene Ascorbic acid, Lubricant Eye Dropsc glycol 400 benzalkonium chloride 31. Hypo Tears Lubricant Novartis 1% Polyvinyl alcohol, 1% polyethylene Benzalkonium chloride Eye Dropsc glycol 400 32. Bausch & Lomb Soothe Bausch & Lomb Personal 0.6% Glycerin, 0.6% propylene glycol None Lubricant (Preservative Product Free) Eye Dropsc 33. Visine Pure Tears Lubricant Pfizer Consumer 0.2% Glycerin, 0.2% hypromellose, None Eye Drops for Dry Eye Reliefc 1% polyethylene glycol 400 34. Bion Tearsc Alcon Laboratories 70.01% Dextran, 0.3% hydroxypropyl None methylcellulose 2910 aOptometric Study Center. How to Grade Dry Eyes. February 2006. http://www.revoptom.com/index.asp?ArticleType=SiteSpec&page=osc/1051 82/lesson.htm. Accessed September 2008. bhttp://www.systane.com. Accessed April 2009. chttp://www.americarx.com. Accessed September 2008.

Some of the commonly used ocular preservatives decreased rate of basal tear turnover (34). In one study, it has been shown that ocular cells repeatedly exposed to BAK can overexpress the cell marker Apo 2.7, which Benzalkonium chloride has been implicated in apoptosis (32). BAK is a quaternary ammonium compound and is the The epithelial cells of cornea are cemented together most common antimicrobial preservative used in topical by the apical portions of their plasma membranes. The multiuse ophthalmic preparations (23,24). The reasons cells of corneal epithelium are tightly bound together for the frequent use of BAK as a preservative include its with intercellular cement similar to that of desmosomes. extreme efficacy in combating microbial contamination The surfactant abilities of BAK solubilize the intercellular of bottles and its ability to break cell–cell junctions in the cement of the corneal epithelium, thereby increasing its corneal epithelium, thus allowing for antimicrobial and penetration (3,26) and subsequent accumulation in the antihypertensive drops to enter the anterior chamber, as ocular tissue when used for relatively lengthy periods

For personal use only. well as its familiarity among those formulating ophthal- (3,27). Formulations containing BAK have been shown mic preparations in industry. The regulatory approval to cause the duplex tear film to become unstable. The from the FDA and its current widespread use in more tear film comprises an oily layer and an aqueous layer than 70% of existing multidose bottles constitute another that further consists of water and mucus. It keeps the eye notable consideration. While the efficacy of BAK is well moist, creates a smooth surface for light to pass through known, a multitude of published studies document the the eye, nourishes the front of the eye, and provides detrimental effects of BAK (23,27,30–32). Benzalkonium protection from injury and infection. Tear film is com- is known to induce necrosis (at concentrations of 0.05– promised in patients suffering from dry eye syndrome, 0.1%) and cellular apoptosis (at concentrations of 0.01%) which makes the cornea more vulnerable to deleterious by way of disturbing the cellular membrane in bacterial substances like preservatives. The degree of instability cells (23). However, human ocular surface cells can also induced by BAK, however, may not be physiologically absorb this detergent, and its effects on ocular surface harmful under normal situations since the lipid layer of Cutaneous and Ocular Toxicology Downloaded from www.informahealthcare.com by HINARI on 08/21/09 cells are similar to those seen in bacterial cells. The the tear film is re-formed every 15–30 seconds. However, potential of BAK-induced damage is clinically important BAK-induced tear film instability needs to be consid- in patients who need eye drops several times a day for ered when treating patients with a compromised tear years, who use multiple eye drops, or who have compro- film as in the case of dry eye syndrome (32). It is also of mised corneas. The effects of the detergent are cumula- concern in situations where the medicament needs to tive and become more severe with more concentrated be instilled frequently and over a long period of time, as and frequent exposures (23). Breakdown of the corneal in conditions like glaucoma (where very frequent use is epithelium and increased permeability of the cornea required) and dry eye (where no drop dilution occurs). as a result of BAK toxicity are well documented (30). BAK is typically included in antiglaucoma medica- Ophthalmic preparations that contain high concentrations at a concentration of 0.01%, with a range of 0.004% tions of BAK interfere with the integrity of the superficial to 0.02% (35). BAK in antiglaucoma ophthalmic prepara- lipid layer of tear film and reduce its breakup time, thus tions does not alter the ability of the medication to lower affecting its stability (31–33). This is especially prob- intraocular pressure (IOP), but it can modify the ocular lematic in glaucoma patients, as they inherently have a surface (31). This effect has been demonstrated in many Ocular preservatives: risks and new options 97

studies and with several antiglaucoma medications that may be at greater risk of BAK-induced adverse effects. In contain BAK. For example, timolol maleate (Timoptic) these patients, the cornea is particularly susceptible to elicited cell damage along with a rapid decrease in the effects of preservatives because the corneal epithe- cell number and viability in a human conjunctival cell lium is an exposed surface and meets the full strength of line (32). Patients treated with timolol maleate exhib- topical ophthalmic preparations (20). Moreover, these ited BAK-induced ocular surface damage caused by patients may not produce sufficient tears to dilute a a decrease in the aqueous layer production rate and harmful preservative (20,29). Preservatives can disrupt impaired tear film mucus layer (36). Another study the precorneal tear film—which acts as a lubricant and found that 127 patients using BAK-containing glaucoma protective layer for the epithelium—and lead to damage drugs, alone or in combination, had a statistically sig- to the epithelial surface and worsening of the dry eye nificant degree of conjunctival metaplasia compared condition (29). One study showed that artificial tears with patients not using topical treatment (37). Another containing BAK increased corneal epithelial perme- study concluded that long-term use of antiglaucoma ability in patients with dry eye, indicating that BAK may medications containing BAK changes the conjunctival contribute to ocular surface disease (14). Often, allergic surface and tear film function. This may increase the risk complaints or chronic irritation of the conjunctivae and associated with future glaucoma therapy (38). eyelids declines when patients with glaucoma or dry eye In 1990, it was reported that a significantly greater stop using preserved eye medications (3). percentage of patients with aqueous tear-deficient dry In a very recent study, where the authors have com- eye than normal control subjects showed inflammatory pared the toxicity of commonly used preservatives using cell infiltration of their conjunctival epithelium, and immortalized human conjunctival and corneal epithelial based on this finding, it was speculated that inflam- cells, BAK (0.01%) showed a significantly higher toxicity mation plays an important role in the pathogenesis of than most of its alternatives, and the order of toxicity was the conjunctival epithelial squamous metaplasia that as follows: thiomersal (0.01%) > BAK (0.01%) > chlorobu- develops in dry eye (39). Three-fourths of the ophthal- tanol (0.5%) > methylparaben (0.01%) > sodium perborate mologists surveyed (70%) reported that the preserva- (0.02%) = ethylenediamine tetraacetic acid (EDTA) (42). tives in glaucoma medications are a significant cause of the ocular surface disease they see in glaucoma Phenylmercuric nitrate or acetate (0.002% w/v) patients (40). Most believe each of the following is related to the preservatives in glaucoma medications: Phenylmercuric nitrate and, to a lesser extent, phe-

For personal use only. ocular surface disease (83%), conjunctival inflamma- nylmercuric acetate both are extensively used in eye tion (75%), allergic reactions (71%), and dry eye (67%). drops. Their solubilities are adequate (acetate: 1 in 500; Chronic application of detergent preservatives can nitrate: 1 in 1,500) and they are nonirritant and stable. lead to chronic lymphocytic infiltration in conjunctival They are effective against a wide range of bacteria and stroma in addition to deleterious changes to corneal fungi but are rather slow in action, particularly against epithelium and tear film instability (41). heavy inocula of Pseudomonas aeruginosa (43,44). Their BAK use may be problematic in patients with dry eye main disadvantages are their incompatibility with hali- syndrome; they are often highly susceptible to its poten- des, strong absorption by rubber, and capacity to cause tially harmful effects, because they lack natural tears to mercurialentis. dilute BAK (23). With chronic, long-term exposure or in the setting of dry eye, BAK is reported to lessen the Chlorhexidine acetate (0.01% w/v) integrity of epithelial cells, increase the number of con- Cutaneous and Ocular Toxicology Downloaded from www.informahealthcare.com by HINARI on 08/21/09 junctival inflammatory cells, cause a loss of goblet cells CHA is a nonirritant bactericide of low toxicity. It is most (16), reduce tear function (31), and decrease the tear effective against gram-positive bacteria but inactive film breakup time (31). Research has shown that in cell against spores. The susceptibility of some Proteus and culture, animal, and human studies, BAK causes greater Pseudomonas species is low but is increased in the pres- cytotoxic effects than some of its alternatives, such as ence of EDTA. CHA’s highest activity is shown in solu- SOC (2). Although intermittent use of preserved eye tions with neutral or slightly alkaline pH (45). drops in healthy individuals is probably not harmful, high Organic matter such as serum and, particularly, concentrations of some preservatives can cause damage phospholipids seriously reduces its efficiency, and, like and irritation to the ocular tissue, particularly in patients BAK, it is incompatible with anionic compounds and, with dry eye. Frequent use of these can cause a higher because of binding, with methylcellulose and hypromel- incidence of epithelial damage, edema, and bullous lose. Cork causes inactivation (46) and should not be a keratopathy in patients with glaucoma, dry eye, infec- component of wads of closures. tions, or iritis, who need to use eye drops for a long period CHA forms sparingly soluble salts with bicarbonates, of time (3). Therefore, patients with dry eye syndrome borates, carbonates, chlorides, citrates, phosphates, and 98 I. P. Kaur et al.

sulfates, but precipitation (which is usually delayed for goblet cells, thereby decreasing aqueous tear film about 24 hours) does not occur unless the concentration production (53). of CHA is about 0.05%, except with sulfates, which cause difficulty even when the preservative concentration is as Sorbate (sorbic acid) low as 0.005%. Consequently, CHA is not used for eye drops containing the sulfates of atropine, neomycin, Sorbate generally has limited antimicrobial activity, and physostigmine, and zinc (47). it is not able to eradicate many organisms on its own. Sorbic acid molecules pass through the plasma membrane, dissociate in the cytoplasm, release protons, and Chlorobutanol inhibit growth via acidification. This may activate ener- Chlorobutanol is an alcohol-based preservative, with getically inefficient intracellular activities, such as energy- no surfactant actions (26). Although the antimicro- dependent ion pumps, wasting the cell’s energy stores. bial activity of chlorobutanol is extensive (27), its use Whereas adverse reactions appear to be infrequent, a has been limited because it becomes unstable when reaction consisting of punctate keratitis may rarely result stored at room temperature for extended periods of from the use of sorbate. Still, sorbic acid–preserved time. Unlike BAK, chlorobutanol does not act like a products are commonly promoted for sensitive eyes and surfactant (48). The method of action of chlorobuta- for contact lens wearers (54). nol is cell lysis by way of disruption of microbial cell membrane lipid configuration (48). Chlorobutanol Sodium perborate has been found to be safe in rabbit corneal cells, even at 100 times the concentrations found in commercial Sodium perborate (GenAqua) is a preservative contained products (8). in Genteal lubricant eye drops (Novartis Ophthalmics, Although chlorobutanol seems relatively safe as an East Hanover, NJ, USA). Sodium perborate was one ophthalmic preservative, chlorobutanol 0.5% in artificial of the first oxidative preservatives to be developed. It tears has been shown to cause irritation in more than destroys numerous types of bacteria and can destroy 50% of the subjects in a double-blind crossover study Aspergillus niger, a fungus that is otherwise difficult to (49). To this end, research has shown that in human kill (55). Sodium perborate is typically found in lubricat- corneal cells, 0.5% chlorobutanol caused cell retraction ing drops used to combat dry eye. When sodium perbo- and cessation of normal cytokinesis, cell movement, rate is combined with water, it is converted to hydrogen

For personal use only. and mitotic activity (50). It also caused degeneration of peroxide, an effective antimicrobial agent. human corneal epithelial cells and the formation of con- Sodium perborate oxidizes cell walls or membranes, spicuous membranous blebs (48). Chlorobutanol does affects membrane-bound enzymes, and disrupts cel- not, however, affect the stability of the lipid component lular function, such as protein synthesis. Once sodium of the tear film (48). perborate enters the eye, it is decomposed to water and As compared with 0.004–0.02% BAK, 0.2–0.5% chlo- oxygen by catalase and other enzymes present in the robutanol is less toxic to rabbit corneal epithelial cells conjunctival sac (55). The hydrogen peroxide allows low in vitro (51). In human corneal epithelial cells, the levels of sodium perborate to be effective at destroy- cytotoxic effects of chlorobutanol occur less rapidly ing microbes while hydrogen peroxide as a byprod- than those of BAK, and the toxicity changes are less uct also permits ocular comfort. However, hydrogen severe (50). Therefore, ophthalmic solutions preserved peroxide levels above 100 ppm or 3% can cause ocular with chlorobutanol may be less damaging. stinging (56,57). Cutaneous and Ocular Toxicology Downloaded from www.informahealthcare.com by HINARI on 08/21/09 Although gentler than some other preservatives, sodium perborate may still cause ocular toxicity. Patients Polyquaternium-1 with dry eye may frequently instill a preserved solution, Polyquaternium-1 (Polyquad) is a polymeric quaternary which increases the chance of aggravating the condition ammonium antimicrobial preservative. It is used in con- by disrupting the precorneal tear film (29). In limited tact lens solutions and the artificial tear product Tears testing, sodium perborate was found to be a direct-acting Naturale II (Alcon Laboratories, Fort Worth, TX, USA). in vitro mutagen (58). It can also destabilize cell walls Polyquaternium-1 has been proven to have less of an and membranes, but to a lesser degree than other types effect on corneal epithelial cells than BAK. One rabbit of preservatives because it is an oxidative preservative. study showed that polyquaternium-1 produces a lesser uptake of dye into the cornea than BAK and only super- Stabilized oxychloro complex ficial epithelial damage compared with BAK (52). The main detriment associated withpolyquaternium-1 SOC (Purite, Bio-Cide International Inc., Norman, OK, is its tendency to reduce the density of conjunctival USA) was introduced into ophthalmic medicines in Ocular preservatives: risks and new options 99

the mid-1990s under the trade name Purite. SOC is an safe and well tolerated when administered frequently, oxidative ophthalmic preservative that destroys many as demonstrated in a 62-patient study in which Refresh types of bacteria and can destroy the fungus Aspergillus Tears containing an SOC-based preservative was admin- niger (56). SOC consists of an equilibrium mixture of istered 4 to 8 times daily for a 4-week period (61).

oxychloro species: 99.5% chlorite (ClO2), 0.5% chlorate None of the currently available glaucoma medica-

(ClO3), and trace amounts of chlorine dioxide (ClO2), tions contains SOC. However, the safety profile of SOC which have bactericidal, fungicidal, and virucidal activ- makes it likely that novel ophthalmic products contain- ity (59). While the antimicrobial efficacy of SOC supports ing SOC will be developed. On March 19, 2001, the FDA its use as a preservative agent, its mechanism of action is approved brimonidine tartrate ophthalmic solution not fully elucidated. Most likely, the mechanism is due to 0.15%, preserved with SOC. This is the first glaucoma the oxidation potential of chlorite and possibly from the medication to contain SOC. generation of chlorine dioxide in the presence of micro- A number of other preservatives have also been bial acidic environments, which leads to the disruption investigated for use in eye drops and are discussed in of protein synthesis in the bacteria and ultimately its anti- Table 3 (46). microbial activity. SOC is capable of destroying microorganisms in fish, fruit, and vegetables without altering the nutritive and organoleptic properties of the food (59). Comparing safety/toxicity of preservatives SOC dissipates by converting into components normally found in tears, such as sodium ions, chloride ions, oxy- A direct comparison between preservatives can be seen gen, and water (56,59). This unique quality makes SOC via scanning electron microscopy in a 2001 study con- an unusually gentle preservative that is ideal for chronic ducted by Allergan (62). To evaluate toxicity, rabbits use. For this reason, products containing SOC may have were treated 4 times a day with products preserved with an improved tolerability profile, reduced toxicity, and, 0.001% polyquaternium-1, sodium perborate, or 0.005% ultimately, better patient compliance. SOC, and the corneas were examined after 1 week of Although SOC is a chemical oxidant, there is no in treatment (62). The untreated rabbit corneal epithelium vivo or in vitro evidence of its mutagenicity or carci- had extensive microvilli and tight intercellular junc- nogenicity (60). Accordingly, SOC has mild cytotoxic tions. The rabbit eye treated with the product preserved effects and an excellent safety record. It has been given with SOC looked nearly identical to the untreated rabbit an Environmental Protection Agency Category II rating eye, and the eye treated with the perborate-preserved

For personal use only. as a mild eye irritant based on rabbit studies (60). SOC product also had a mostly normal epithelium with is efficacious at low concentrations (0.005% w/v). SOC is extensive microvilli and tight epithelial junctions,

Table 3. Obsolete preservatives and reasons for their discontinuationa. Sr. No. Preservative Properties 1. Hydroxybenzoates • Included in eye drops in 1949 but displaced in 1963. • Slow antibacterial activity, particularly against Pseudomonas aeruginosa. • Irritant to the eye. 2. Chlorocresol (0.05% w/v) • Replaced solution for eye drops in the 1963 British Pharmaceutical Codex, but reports of eye damage led to its deletion within a few months. • It is a rapidly acting bactericide and fungicide, but a 0.05% solution is no more than marginally effective against Pseudomonas aeruginosa.

Cutaneous and Ocular Toxicology Downloaded from www.informahealthcare.com by HINARI on 08/21/09 • Its efficiency is increased in acid solution. • It is more irritating than most preservatives. 3. Thiomersal (0.01% w/v) • Its stability is low in acid solutions, and it is not widely used in eye drops. • Its antibacterial activity is slow, and its absorption by rubber is very high. • Less stable in solution (protection from light and heavy metals is necessary) and does not cause mercurialentis. • Highly toxic to corneal and conjunctival cells.b 4. Phenylethyl alcohol • Particularly effective against gram-negative bacteria, its action on Pseudomonas aeruginosa is rather slow. (0.5% w/v) • Recommended as an eye drop preservative in the U.S. Pharmacopeia. • Often it is combined with another bactericide, such as benzalkonium chloride, phenylmercuric nitrate, or chlorbutol, to confer enhanced activity against gram-negative organisms. • The solubility of chlorbutol is improved when used with phenylethyl alcohol. aCooper JW, Gunn C. Ophthalmic products. In: Carter SL, ed. Dispensing for Pharmaceutical Students. 12th ed; Delhi, India: CBS Publisher and Distributors. 2000:634-662. bEpstein EP, Ahdoot M, Marcus E, Asbell PA. Comparative toxicity of preservatives on immortalized corneal and conjunctival epithelial cells. J Ocul Pharmacol Ther 2009; 25:113-119. 100 I. P. Kaur et al.

90.00% Sustained-release formulations 80.00% One approach is to decrease the cumulative toxic 70.00% effects, hence a once-daily form of timolol maleate (Timoptic-XE) with 0.012% BDD, a preservative similar 60.00% to BAK, was developed. In place of bromide, the benzo- 50.00% dodecinium cation may be used with chloride or another 40.00% anion. Chemically BDD is dimethyldodecylbenzylam- monium bromide and, like BAK, is a quaternary ammo- % Viable Cells 30.00% nium compound having a property of cationic surfactant 20.00% and is primarily active against gram-positive microbes. However, the gel-forming preparation may prolong 10.00% contact of the preservative with the eye, contributing to 0.00% greater toxic effects to the cornea. In this regard, it may 24 48 be noted that BDD, a preservative related to BAK, has Time (h) been shown to induce corneal epithelial damage when incorporated into a gel (35). However, the concept of Sodium Perborate BAK using sustained release formulations may be extended to newer and safer preservatives. Figure 1. Toxicity profile of benzalkonium chloride (BAK) and sodium perborate.

while the same workers reported that the rabbit eye Safe, mild, and less-toxic preservatives treated with polyquaternium-1-preserved product had All of the original formulations of the glaucoma medica- extensive superficial epithelial erosion and lack of pro- tions commonly used today contain BAK. Several agents truding microvilli. This finding is in agreement with available in the United States have been reformulated another study that found that 0.001% polyquaternium-1 without BAK, like SOC (Purite)-preserved brimonidine caused a small but significant degree of epithelial dam- tartrate (Alphagan P). These formulations appear to be age (52). However, another study has shown that 0.001% benign to the ocular surface. However, the efficacy of polyquaternium-1 has a low potential for ocular irritation the brimonidine class of antiglaucoma drugs is less than

For personal use only. (63). As compared with the untreated eye, damage to the that of prostaglandins, which are probably the first-line corneal epithelium was highest with polyquaternium-1, monotherapy today; hence, it is important to reformu- followed by the sodium perborate product, and then by late these agents as well. the SOC-preserved product. Of the artificial tears com- The preservative sofZia (Alcon) is a relatively new, pared in this study, the product with SOC caused the proprietary, ionic, buffered solution consisting of zinc, least damage to corneal epithelial cells (62). borate, propylene glycol, and sorbitol—chemical enti- The superiority of sodium perborate has also been ties that are themselves not significantly toxic to the established by us (64) using SIRC cell lines in a cell prolif- ocular surface. sofZia, however, maintains an antimi- eration assay. Figure 1 depicts the comparison between crobial environment in the bottle, such that it meets the sodium perborate and BAK. The percentages of viable U.S. Pharmacopeia standards for antimicrobial activity. cells that remain after 24 hours and 48 hours of expo- It produces more than a 3-log reduction (99.9% kill) sure (positive control = 100% and negative control = 0%) in surviving organisms after 8 days of incubation (65). Cutaneous and Ocular Toxicology Downloaded from www.informahealthcare.com by HINARI on 08/21/09 are significantly higher for sodium perborate than the Recently, sofZia-preserved travoprost (Travatan Z, same concentration of BAK (p < .05). The 48-hour study Alcon) was introduced into the American market. This was designed to investigate possible delayed cytotoxicity formulation appears to have the same IOP-lowering effi- of preservatives and/or the recovery of damaged cells. It cacy as BAK-preserved travoprost in controlled clinical was observed that after being incubated for 48 hours the trials (66). Arguably, this formulation is the most potent toxicity of sodium perborate decreases (p < .05%), which non-BAK-containing glaucoma eye drop currently indicates that the initial damage caused by sodium per- available. borate is repairable with long-term use. Preclinical studies comparing sofZia-preserved travoprost and latanoprost, which contains the highest amount of BAK, demonstrated the relative safety to the Future directions ocular surface of the non-BAK formulation (67). Cell culture models and confocal studies of rabbit corneas The following methods are reported in the literature to have shown that BAK-containing latanoprost caused help reduce the toxic effects of preservatives: significant corneal epithelial cell death and loss with Ocular preservatives: risks and new options 101

prolonged exposure times, whereas BAK-free travoprost invasion through inherent material properties independ- was fairly benign to these surface cells. Ongoing stud- ent of traditional preservatives (41). However, it may be ies with more chronic dosing seem to confirm these more difficult for a patient to use a unit-dose -vial cor effects (68). rectly, affecting compliance. Poor compliance may hinder Travatan Z was introduced as the first prostaglandin a nonpreserved drug’s effectiveness, even if it is more analogue to be preserved with a substance other than comfortable to use. This can be especially important BAK. The sofZia system effectively preserves the medi- when it is used concomitantly with multidose glaucoma cine while it is being stored; however, when the drug is medications, for which compliance is vital. Unit-dose introduced into the eye, it is modified into harmless ele- vials are also more expensive than multidose containers. ments that are gentle on the ocular surface (64). In addition, patients with advanced rheumatoid arthritis may find it difficult to squeeze the drops from the single- Nonpreserved formulations use vials. They may be tempted to use the multidose vials. While it is true that a preservative may be the cause of an Preservative-free medications may eliminate the risk of allergic reaction, it is difficult to determine if the problem toxic side effects, which can make them attractive treat- is caused by the preservative, the drug, the drug delivery ment options. For example, in a 1992 study conducted to system, the buffers, or the stabilizers. Hence, nonpre- assess the corneal epithelial toxic effects of preservative- served preparations still hold some risk. free tear preparations, researchers used scanning electron microscopy to evaluate the corneal epithelium of rabbit eyes after the administration of 2 preservative-free Conclusions ocular lubricants. The 2 preservative-free preparations were shown to be nontoxic to the corneal epithelium in At present, despite the numerous antiglaucoma medica- both the mild- and exaggerated-use protocols. The epi- tions and artificial tears in the market, none is risk-free. thelial changes observed were no different from those In the future, manufacturers may reformulate exist- seen in control eyes. The study supported the belief that ing products with less-toxic preservatives, offer less- preservative-free preparations are safe to use in patients, concentrated forms of current preservatives, or develop especially with frequent dosing (20). new ones. It has been reported by Pisella et al. (35) that using unpreserved timolol may be beneficial for the long-term

For personal use only. treatment of glaucomatous patients because it increases Acknowledgments tear film stability and decreases epithelial permeability and stromal aggression of the cornea. Baudouin and de Declaration of interest: The authors report no conflicts Lunardo (31) confirmed that carteolol is well tolerated, of interest. either with or without preservative. The preservative- free group showed better stability of the tear film, without loss of effect on IOP. Berdy et al. reported that References with frequent-dosage regimens, preservative-free Hypo Tears PF and various forms of Refresh formulation(s) by 1. Noecker R. Ophthalmic preservatives: considerations for long- term use in patients with dry eye or glaucoma. Rev Ophthalmol Allergan (Table 2) are free of the toxic effects associated 2001; 8:73–79. http://www.revophth.com/2001/june/cme0601_ with preserved solutions (29). article.htm Nonpreserved artificial tears have an extra advantage 2. Noecker R. Effects of common ophthalmic preservatives on ocu-

Cutaneous and Ocular Toxicology Downloaded from www.informahealthcare.com by HINARI on 08/21/09 lar health. Adv Ther 2001; 18:205–215. over preserved ones: they may be the best choice for 3. Gasset AR, Ishii Y, Kaufman HE, Miller T. Cytotoxicity of oph- patients immediately following eye surgery. Some non- thalmic preservatives. Am J Ophthalmol 1974; 7(1):98–105. preserved artificial tears include GenTeal, TheraTears, 4. Burstein NL. Preservative cytotoxic threshold for benzalkonium chloride and chlorhexidine digluconate in cat and rabbit cor- Refresh Tears, Moisture Eyes, and Bion Tears. These are neas. Invest Ophthalmol Vis Sci 1980; 32:2259–2265. the most highly preferred ones because they contain 5. Burstein NL. The effects of topical drugs and preservatives on either no or mild and less-toxic preservatives like SOC the tears and corneal epithelium in dry eye. Trans Ophthalmol Soc 1985; 104(Pt 4):402–409. (Purite) and sodium perborate. Table 2 lists some of the 6. Champeau EJ, Edellhauser HF. Effects of ophthalmic preserva- most common over-the-counter medications for dry eye tives on the ocular surface: conjunctival and corneal uptake along with the preservative used. and distribution of benzalkonium chloride and chlorhexidine digluconate. In: Holly FJ, Lamberts D, Mac Keen D, eds. The Nonpreserved drugs are available only in unit-dose Preocular Tear Film in Health, Disease, and Contact Lens Wear; vials and, as a result, the use of unit-dose bottles that Lubbock, TX: Dry Eye Institute. 1998:292–302. do not require preservatives will increase and become 7. Gassett AR. Benzalkonium chloride toxicity to the human cornea. Am J Ophthalmol 1977; 84(2):169–171. more cost-effective while newer technologies will enable 8. Fisher AA. Allergic contact dermatitis and conjunctivitis from multidose bottles to be constructed to inhibit microbial benzalkonium chloride. Cutis 1987; 3(5):381–383. 102 I. P. Kaur et al.

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