Practical methods for the microbial validation of sterilizing-grade filters used in aseptic processing.

Microbial challenge test methods to validate sterilizing-grade filters were developed to facilitate the use of parenteral drug products as liquid vehicles. Scaled-down filtration systems were developed in response to processing parameters, e.g., flow rates, pressures, duration, and temperature, provided by customers. Accordingly, 47 mm disc filter holders were chosen to minimize the volume of product necessary for each test. A peristaltic pump was used to produce positive pressure within the test system, and a 47mm filter sampling manifold was used to assay the entire filtrate. The resultant system was able to accommodate a variety of flow rates while maintaining the viability of the retention test microorganism, Pseudomonas diminuta (ATCC Number 19146). Experiments were conducted by challenging sterilizing-grade (0.22 micron) and 0.45 micron polyvinylidene difluoride (PVDF) (Durapore) filter discs under simulated processing conditions with P. diminuta resuspended in the pharmaceutical products. The challenge concentration was at least 10(7) colony forming units per cm2 of filter area. The 0.22 micron filters were shown to be retentive in all cases, while the 0.45 micron membranes exhibited some degree of bacterial passage as expected. The results of these experiments were communicated to the customers for inclusion in their process validation documentation.