The use of centrifuges to remove cells in combination with depth filters to remove colloids has become widespread. High colloid concentrations and allowance for process variability can lead to large single-use filter assemblies of several hundred square meters. This study measures the performance differences and variability between multilayer depth filters of different formats and sizes using a Chinese hamster ovary (CHO) centrate challenge.
Cell debris and colloids are removed from centrifuged harvested cell culture fluid (HCCF) by depth filters to extend the capacity of the downstream sterile filters. The successful efforts to improve factory productivity by increasing bioreactor titers often leads to higher cell densities, cell lysis, and higher colloid concentrations that can lower depth filter capacity. Allowance for process variability and scaling can require large, single-use assemblies of several hundred square meters.
Depth filters that show high capacities for this application are composed of cellulose fibers impregnated with diatomaceous earth and a polymeric amine binding resin. These depth filters are typically fed by the centrifuge and operated at a constant flow. As particle laden fluid is passed through these depth filters, the submicron colloids are removed and the depth filters show an increase in pressure drop across them. When the pressure drop reaches an operational limit, here taken as 15 psi, the filters are considered to have plugged. The amount of fluid that has passed through the filter is then considered to be the design capacity, expressed here as liters per square meter of filter area.
MATERIALS AND METHODS
This study measures the performance differences and variability between multilayer depth filters of different formats and scales using a CHO centrate challenge. Variability from other sources was minimized to see the effect of scale and format more clearly. A single 12,000-L bioreactor production-scale batch of Chinese hamster ovary (CHO) cells expressing a monoclonal antibody product was used to challenge all the filters. Use of a single lot removes lot-to-lot harvest variability so that capacity differences between filters can be seen more readily. The centrifugation of the harvest took several hours, and therefore, controls were taken to assess if differences in hold time before centrifugation affected filter capacity.
Figure 1. Depth filters in a) Lenticular stack, b) Pod, c) Mini capsule formats
The different filter formats and sizes used in this study are shown in Figure 1 and Table 1. The Millistak+ A1HC filter used in this study contains multiple layers of cellulose filtration media and a cast membrane. A single manufacturing lot of filter cellulose media was used for this study. Use of a single lot removes lot-to-lot filter media variability so that capacity differences between filters of different formats and scales can be seen more readily.
Table 1. Test filter device formats and sizes
A parallel run between a 1.8 m2 lenticular stack device (Stack) and a 1.65 m2 Pod filter (Pod) assembly (one 1.1 m2 Pod device and one 0.55 m2 Pod device in parallel), allowed a more direct comparison of the different formats at comparable sizing. Large-area filters and assemblies were run in the manufacturing plant while small area filters were run in the pilot plant. Replicate 0.11 m2 Pod controls were run in both the pilot and manufacturing plants to assess any differences in performance between the two test locations.