Polyhexamethylene biguanide (PHMB) is a broad-spectrum antimicrobial biocide used as a disinfectant and antiseptic. PHMB is efficacious against gram-positive and gram-negative bacteria including methicillin-resistant Staphylococcus aureus (MRSA) (Müller and Kramer, 2008), fungi, parasites, Aspergillus brasiliensis (mold), and certain viruses. In addition, PHMB eyedrops have been used as a treatment for Acanthamoeba keratitis (Larkin et al, 1992) and Candida albicans (Buchele et al, 2018). When used in contact lens care solutions, PHMB’s antimicrobial properties also prevent contamination of a care solution bottle’s contents.

PHMB is composed of repeating basic biguanidine units linked by hexamethylene hydrocarbon chains in a structure that is both cationic and amphipathic (Chindera et al, 2016). Of interest, resistance to PHMB has not been reported despite widespread use over several decades (Wessels and Ingmer, 2013).

Other types of modern disinfectants include hydrogen peroxide, polyaminopropyl biguanide, polyhexanide hydrochloride, polyquaternium-1 (PQ-1), and myristamidopropyl dimethylamine (MAPD). Hypersensitivity reactions occurred with first-generation disinfectants such as thimerosal or chlorhexidine used in higher concentrations (Gromacki and Ward, 2013). Current disinfectants such as PHMB have higher molecular weights that, as a result, avoid many tissue complications. They also do not penetrate the matrices of soft contact lenses as did earlier molecules such as chlorhexidine. PHMB and polyaminopropyl biguanide are types of biguanides and are chlorhexidine derivatives.

The traditional mechanism of action of these bactericidal biguanides is to disrupt the plasma membrane of a bacterial cell and eliminate the metabolic activity within the cell (Gromacki and Ward, 2013). One study examined the mechanism of how PHMB kills bacteria selectively over host cells and suggested an alternate mechanism (Chindera et al, 2016). Cell entry into a range of bacterial species was observed; treated bacteria exhibited cell division arrest and chromosome condensation. This suggests DNA binding as an alternative antimicrobial mechanism. This is the first example of any drug that binds and condenses bacterial chromosomes.

Effects of CLs on PHMB

A study evaluated the effects of eight different contact lens materials—both silicone hydrogel and hydrogel—on PHMB concentration in multipurpose solution (MPS) levels over time (Shoff et al, 2012). This study also explored the effect of contact lenses on lens solution microbial efficacy over time.

Lenses were soaked in polypropylene lens cases containing contact lens MPS for six to 168 hours. Cases without contact lenses that were filled with the same solution were controls. Solutions were assayed for activity against Staphylococcus aureus according to International Organization for Standardization 14729 (with modifications) and analyzed for PHMB concentration at each time point. Some contact lens materials significantly affected the PHMB concentration (P < 0.0001) and biocidal efficacy. After six hours of lens soak time, etafilcon A lenses significantly decreased PHMB levels. PHMB levels were not significantly decreased with enfilcon A, senofilcon A, and lotrafilcon B lenses.

In Summary

Thus, contact lens materials differ in their effect on PHMB concentration and the consequent efficacy of the MPS. Some lens materials may reduce the PHMB concentration and can significantly reduce the microbial activity against Staphylococcus aureus. This insight into PHMB may help with proper contact lens solution selection and mitigate potential contact lens and solution interaction complications. CLS

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