Guidelines for Selecting Normal Flow Filters - Pharmaceutical Technology

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Guidelines for Selecting Normal Flow Filters
Proper selection of normal flow filters leads to increased process efficiency from early phase product development through to full-scale biopharmaceutical production.


Pharmaceutical Technology



Figure 3: Comparative performance of multilayer sterilizing-grade filters for cell culture media filtration (shorter bars represent better performance). {{ART: x axis should be labeled "Media type"}}
Each of the media was tested on a panel of sterilizing-grade membranes from Pall (East Hill, NY), Millipore (Billerica, MA), Sartorius (Goettingen, Germany), and GE Healthcare. Solutions were filtered at a constant pressure of 10 psid, and the volume filtered as a function of time was recorded until the flow rate had decayed by at least 50% or until the solution was exhausted. Based on the test results, estimations of the required number of equivalent 10-in. filter cartridges were made for a 12,000 L batch of media filtered in 2 h. Results are presented in Figure 3.

Buffer filtration

Buffer solutions are used widely in nearly every step of bio-pharmaceutical production processes. In fact, buffer filtration is the most commonly performed filtration operation in any biopharmaceutical process. During operation of the bioreactor, buffers are used to control pH and osmolality of the cell culture media. At cell harvest, buffers are used to precondition filters and to assist in product recovery operations. Chromatography steps use numerous buffers for such operations as column conditioning, column elution, and column regeneration. Once a biopharmaceutical is ready for formulation, buffers become a key ingredient in the bulk drug substance. Finally, buffers are used throughout the process for clean-in-place operations.

Biological and particle contaminants present in buffers can have a large impact on process efficiencies and final product quality. Therefore, normal flow filtration is one of the first steps (after dissolution) in any buffer preparation process. Buffer filtration is key to protect chromatography columns and ultrafiltration operations and to produce an endotoxin-free final product.

The following paragraphs describe the key characteristics of a buffer filter.

Validated 0.2-m membranes. Buffer filtration is commonly done with 0.2-m membrane filters to reduce bioburden or to achieve sterility of the buffer and to remove particulate contaminants. The choice between a sterilizing-grade and a bioburden-reduction filter* often depends on the final use of the buffer. For example, sterility is a requirement for buffers used as additions to the bioreactor to prevent contamination of the cell broth, while bioburden reduction may be sufficient for buffers used in chromatography operations, which are often not aseptic processes. Bioburden reduction filters are generally less expensive (per unit membrane area) and require less filtration area for a given batch size (thereby improving their economics even further). In either case, it is important for regulatory purposes that the membrane is validated for retention of bacteria and that the retention can be correlated to an in-process integrity test.

Chemical compatibility. Buffer filters must have broad chemical compatibility, because buffers used in biopharmaceuti-cal production span a wide range of pH levels (1-14), and must withstand exposure to alcohols and (occasionally) other organic chemicals.

Physical robustness. Filters used in buffer preparation must withstand the rigors of steam-in-place and/or autoclaving. They must also be validated for multiple sterilization cycles since buffer preparation areas may be designed to re-use filters. Buffer filters should remain integral within a wide range of operating conditions in order to avoid filter failures which can lead to batch reprocessing, lost product and/or costly regulatory investigations.

High permeability. Buffer filtration is a high-volume, short-time operation. Because buffers are generally fluids with low particle loading, they do not tend to plug membrane filters. Therefore, permeability (rather than capacity) becomes the key determining characteristic in the size of the filtration system. Using high permeability membranes can significantly reduce the amount of filter area needed for buffer preparation. Small filtration footprints are desirable because they are not only cheaper in terms of consumables, but they also require smaller capital investments and reduce the risk of filter integrity failures.

Survey of filters for buffer filtration. GE Healthcare performed a study to evaluate the most common filters that are used for buffer filtration. In this study, eight common buffers spanning a range of concentration, pH, and organic content were prepared and tested on a panel of buffer filtration membranes from Pall, Millipore, Sartorius, and GE Healthcare. The tested buffers are shown below:

  • 1 M hydrochloric acid (pH = 1)
  • 1 M sodium hydroxide (ph = 13)
  • 50 mM Tris-buffered saline (pH = 8)
  • 10 mM phosphate-buffered saline (pH = 7)
  • 50 mM phosphate-citrate buffer (pH = 5)
  • 0.1 M acetic acid
  • 10% ethanol
  • 6M urea.


Figure 4: Comparative performance of sterilizing-grade and bioburden reduction filters for buffer filtration (shorter bars represent better performance).
Solutions were filtered at a constant pressure of 10 psid, and the volume filtered as a function of time was recorded until the solution was exhausted. Based on the test results, membrane permeability for each filter tested was calculated and estimations of the required number of equivalent 10-in, filter cartridges were made for a 12,000-L batch of buffer filtered in 1 h. Results are presented in Figure 4.


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