An Evaluation of Preservative Adsorption onto Nylon Membrane Filters

Abstract

Pharmaceutical drug products often contain antimicrobial agents as a preservative in their formulation. These excipients are required to destroy or impede the growth of microorganisms that inadvertently enter the product during manufacturing. Unfortunately, these preservatives may also interfere with microbiological assays used to determine product sterility or bioburden levels. The extent of interference by these preservatives can be quite significant, but varies depending on the method used. The most frequently used method for testing parenteral drug products is the membrane filtration technique. Membrane filters are composed of a wide variety of materials such as cellulose, polycarbonate, acrylic polypropylene, Teflon, and nylon. This study evaluated the adsorption characteristics that nylon filters, obtained from five different manufacturers, had on the filtration of solutions of four different antimicrobial compounds {phenol, methylparaben, propylparaben, and benzalkonium chloride). The adsorption properties were determined using both HPLC and microbiological assay techniques. The data revealed that there was a wide range in the amounts of antimicrobial agent (2.3 to 94.1%) bound to the membrane filters when direct product filtration was used without a subsequent rinse step. However, when a rinse step is included, only propylparaben showed any significant “true” adsorption (< 1 to 33.3%), but showed only marginal bacterial inhibition. Interestingly, the microbiological assays indicated that with a saline rinse step, only benzalkonium chloride was lethal for the two challenge organisms even though the percent adsorbed as measured by HPLC was below 1 %. This discrepancy is significant because it demonstrates the analytical limitation when using HPLC to detect minimal concentrations of benzalkonium chloride that may be deleterious to microorganisms. Finally, the level of adsorption (with no rinse) was lowest for the Gelman brand which showed a range of 2.0 to 6.4% when compared to the overall range of 5.4 to 83.2% obtained with the other membrane filters. However, because of membrane stiffness and the difficulty of obtaining full contact with the agar surface, its application for direct plate counts for product evaluations appears to be minimal.