Normal flow filters are used widely in biopharmaceutical operations to remove colloidal material, bacteria, and viruses from
growth media, buffers, and process intermediates. A modern biopharmaceutical process typically contains 40-50 normal flow
filtration operations from seed culture propagation to final vial filling. As shown in Figure 1, normal flow filtration accounts
for approximately one-fourth of the total cost for downstream processing. Therefore, the choice of normal flow filter(s) has
a potentially large impact on the total production cost for a biotherapeutic.
Figure 1: The relative cost of biopharmaceutical separations. (CREDIT_PHOTO)
The variety of filter materials available to process development scientists is large—from depth media containing nominally-rated
micron-sized filtration-matrices to validated sterile filtration membranes containing submicron-sized pores. The criteria
by which one chooses the optimal filter is commonly application-specific, and it is therefore important to understand these
criteria when designing experiments, analyzing data, and comparing product attributes.
Quick guide to the selection of normal flow filters
In general, normal flow filtration operations can be divided into three main categories:
- Cell culture media sterilization
- Buffer filtration
- Product-stream filtration.
Figure 2 shows a typical biopharmaceutical process and highlights where each of these filteration steps occurs.
Figure 2: Normal flow filtration in biopharmaceutical production.
Cell culture media sterilization
One of the first unit operations in any biopharmaceutical operation is the preparation and sterilization of cell culture media.
Cell culture media are nutrient-rich, buffered solutions containing amino acids, salts, vitamins, and energy sources (e.g.,
glucose)—all of which are essential components for the culture of healthy cells. Over the past several decades, formulations
have evolved from generic basal media supplemented with animal-derived sera, to more cell-line specific formulations that
are serum-free, animal-derived component-free and chemically defined. The sterilization of these media is critical to successful
cell growth and protein expression.
There are many characteristics one should look for when selecting a cell culture media filter. The following paragraphs describe
some of the most important features.
The term "sterilizing-grade filter" is defined in the FDA's document Guidance for Industry: Sterile Drug Products Produced by Aseptic Processing — Current Good Manufacturing Practice (1), which describes a sterilizing-grade filter as a filter that is "validated to re-producibly remove viable microorganisms
from the process stream, producing a sterile effluent." The validation of sterilizing-grade membranes is commonly performed
using the procedure documented in ASTM F838-05, "Standard Test Method for Determining Bacterial Retention of Membrane Filters
Utilized for Liquid Filtration" (2). To be labeled "sterilizing-grade," a filter must produce a sterile effluent when challenged
with Brevundimonas diminuta (B. diminuta) at a minimum concentration of 107 colony forming units (CFU) per square centimeter (cm2 ) of membrane area. Sterile filters are nearly always constructed of one or more sheets of polymeric membrane, either in
pleated or flat-sheet form.