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In the past, only tablet dosage forms of enzymatic cleaners were available. They required soaking lenses in solutions prepared from a tablet for a period of 15 minutes to more than 2 hours before disinfecting the lenses. Although this process provided sufficient time for cleaning, it was a cumbersome process and required multiple steps. A complicated or cumbersome process inevitably leads to poor user compliance. Enzymes in aqueous liquid compositions are inherently unstable. New technological advances have led to the stabilization of enzymes in liquid vehicles, which are compatible with soft and RGP contact lenses (393). The newer products are either in a tablet or a solution product form. Simultaneous cleaning and disinfection can be achieved, which reduces care time and the need for multiple steps (394).

Contact lenses are contaminated with microorganisms during lens handling and lens wear. They must be disinfected to prevent ocular infections, especially from pathogenic microorganisms. The two disinfection methods used are thermal and chemical. In thermal disinfection systems, lenses are placed in preserved or unpreserved solution in a lens case and then heated sufficiently by a device to kill the microorganisms. The current FDA requirement for thermal disinfection requires heating at a minimum of 80°C for 10 minutes. The unpreserved salines are either packaged in a unit-dose or an aerosol container, and they do have some antimicrobial activity (395). Preservatives must be used in salines packaged in nonaerosol multidose containers. The type and names of preservatives and antimicrobial disinfectants commonly used in lens care products are listed in Table 11. Thimerosal and sorbic acid are commonly used preservatives in these products; however, concerns of sensitization potential and discoloration of lenses has led to the introduction of new and safer molecules like polyquad (a polymerically bound quaternary ammonium compound) and dymed. Specifically, polyquad is resistant to absorption into the lenses; thus, there is little to diffuse out of the lens into the eye, leading to corneal toxicity, an inherent problem associated with nonpolymerically bound quaternary ammonium compounds. The FDA and the USP have specific standards for preservative effectiveness that these products must meet. The FDA standards detailing the method were published in 1985 (396). Oxidizing agents and nonoxidizing chemical disinfectants that are nontoxic at product concentrations are used to disinfect lenses chemically. Primarily, hydrogen peroxide is used as an oxidizing agent (397). It is used in concentrations of 0.6% to 3.0%. Peroxides are very toxic to the cornea of the eye. After the disinfection cycle, and before placing the lens in the eye, hydrogen peroxide must be completely neutralized by reducing agents, catalase, or transition metals, such as platinum.

An ideal chemical-disinfecting agent would have the following properties: (i) it should be nonirritating, nonsensitizing, and nontoxic in tests for cytotoxicity; (»') it should have an adequate antimicrobial spectrum and be able to kill ocular pathogens during a

Table 11

Antimicrobial Agents Commonly Used

in Lens Care Products

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