With the ability to prevent microbial growth, preservatives play a vital role in so many day-to-day and pharmaceutical products. We only have to think of multi-use items such as a nasal spray or eye drop product, which can pick up microorganisms from repeated contact with the nasal passage or eyelashes, for example, to realise the importance of these ingredients in keeping pharmaceutical products safe and effective.
A wide range of preservatives are used in topical and ophthalmic formulations, from alcohols and parabens to EDTA, chlorohexidine, and quaternary ammonium compounds (Quats). A type of Quat known as Benzalkonium Chloride, often shortened to BKC or BAK, has become the preferred choice for ophthalmic formulations due to its high antimicrobial efficacy and relatively low corneal toxicity. It is also used widely in nasal and topical products.
There are stringent regulatory requirements concerning preservative use, with the United States Pharmacopeia requiring the ingredient to achieve 1.0 and 3.0 log reductions by days 7 and 14 respectively. Even stricter requirements are in place in the EU. Benzalkonium Chloride can reach the required level of antibacterial activity at very low concentrations ranging from 0.002% w/w (20 ppm) to 0.025% w/w (250 ppm).
Assaying Benzalkonium Chloride
Analytical testing is an essential part of pharmaceutical drug development and manufacturing. However, analysing these very low BKC/BAK concentrations can be challenging.
The usual method for assaying Benzalkonium Chloride in drug formulations is through a reverse phase isocratic high-performance liquid chromatography (HPLC) step, with UV detection at 254nm. Separation is reached at 30°C.
A method for routine analysis of BKC in dosage forms has been validated. The method requires an L10 column using a mobile phase consisting of a phosphate buffer and acetonitrile mixture. Flow rate is 1 ml/min and detection wavelength is 210nm.
However, Benzalkonium Chloride can present some challenges during filtration. Since BKC is a positively charged (cationic) surfactant, it can bind to surfaces during processing and analysis. Because cationic molecules are attracted to anionic ones, this binding effect means that any filter membranes made with natural or cellulosic fibres should not be used. Polymer substrates made from polypropylene or polyester, for example, are far more appropriate.
Studying the filtration challenge
Researchers have also investigated the effect of different formulations and process parameters on the adsorption of Benzalkonium Chloride to filter membranes. Six different filter membranes were analysed, with results showing how the adsorption properties of each membrane were related to its composition. Membranes that were both hydrophilic and nonionic or hydrophilic and cationic adsorbed the least BKC, while membranes that were hydrophobic or anionic had the most significant adsorption levels.
When it came to formulation-related factors such as the addition of various tonicity-modifying or chelating agents, the presence of sodium chloride and edetate disodium in combination with membranes with cationic sites led to the most significant adsorption levels. The effects of flow rates, temperature and pre-saturation of the membrane with BKC were also studied.
When using BKC in your formulation, it is always recommended to perform adsorption studies. A solution with a known BKC concentration should be filtered, and the filtration and substrate can then be assayed through HPLC to determine the amount adsorbed. This helps process developers decide on the most suitable substrate for their analytical processes.
To learn more about analytical testing challenges and validated methods for Benzalkonium Chloride, please download the whitepaper below.
References
Bin T, Kulshreshtha AK, al-Shakhshir R, Hem SL. Adsorption of benzalkonium chloride by filter membranes: mechanisms and effect of formulation and processing parameters. Pharm Dev Technol. 1999 May;4(2):151-65. doi: 10.1081/pdt-100101350. PMID: 10231877.
