PRESERVATIVES in contact lens care products provide a means of preventing microbial contamination and growth, as well as an avenue to disinfect worn lenses so that they can be reused. This article will discuss the interaction of preservatives, past and present, found in multipurpose solutions (MPSs) with soft contact lenses.
THIMEROSAL AND CHLORHEXIDINE
Thimerosal is a mercury-containing compound that was one of the first ophthalmic biocides in contact lens care products. Its mechanism of action is primarily through influx of calcium into cells (Bradley et al, 2021). As a small molecule, thimerosal can be both absorbed into and released from contact lenses, leading to ocular surface complications. Thimerosal can cause a delayed hypersensitivity reaction that is not confined to the ocular area: reactions can occur anywhere on the skin exposed to the molecule (Wilson et al, 1981; Zeigen et al, 1981).
Chlorhexidine was another early biocide for contact lens care systems. This small molecule primarily has bacteriostatic activity and works to disrupt the cell membrane. Due to its small size, chlorhexidine can be taken up by contact lenses; however, it is thought to be released slowly and unlikely to have toxic effects on the ocular surface (Refojo, 1976). Since newer biocides have come on the market, both thimerosal and chlorhexidine have been withdrawn as a preservative in modern contact lens solutions (Bradley et al, 2021).
PHMB
Polyhexamethylene biguanide hydrochloride (PHMB) is a cationic biguanide and was one of the first “modern” contact lens solution preservatives. The molecule has a broad spectrum of activity against bacteria, viruses, and Acanthamoeba. It primarily exerts antimicrobial action by binding to and disrupting cell membranes (Niro et al, 2023).
Several studies have investigated the retained antibacterial and antifungal activity of MPSs preserved with PHMB after soaking with lenses over specified periods of time. Between 30% and 60% of the PHMB in a solution can be depleted by etafilcon A lenses within six hours of soaking, impacting its ongoing fungicidal activity (Rosenthal et al, 2006). Soaking for three days with high-water-content lenses (55%) can also deplete PHMB such that the solution no longer retains bactericidal activity (Rosenthal et al, 1997). This has obvious implications for the recommended frequency of solution replacement.
POLYQUATERNIUM
Polyquad (polyquaternium or PQ) is also a commonly used modern preservative (Bradley et al, 2021). It is a quaternary ammonium compound with activity against bacteria but limited activity against fungi, necessitating its combination with other agents (Bradley et al, 2021). It works through denaturation of proteins within the cell membrane (Bradley et al, 2021).
In contrast to PHMB, little or no PQ is taken up by soaking lenses, allowing for continued antimicrobial activity (Rosenthal et al, 1997; Rosenthal et al, 2006; Morris et al, 2018). There are conflicting reports as to whether PQ or PHMB is safer or more biocompatible with the ocular surface, with both molecules being reported to cause some corneal staining in some studies (Bradley et al, 2021).
CONCLUSIONS
It is interesting to observe the development of preservatives in MPSs. Molecules have been introduced and withdrawn as newer preservatives are released to address perceived shortcomings. Clinicians should remind patients to change their contact lens solutions daily, even if they are not wearing their lenses, to ensure continued antimicrobial activity, regardless of which modern MPS they ultimately end up using. CLS
Acknowledgments: The author would like to thank Alison Ng, PhD, BSc(Hons), MCOptom, for her helpful comments in preparing this article.
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