(202c) Mechanistic Modeling of the Loss of Protein Sieving Due to Internal and Externalfouling of Microfilters

Most antibodies are currently produced in fed batch or perfusion cell culture bioreactors. Both fed batch and perfusion cell culture processes can utilize microfilters (MF) for product harvest.

Conventional tangential flow filtration (TFF) operations continuously pump feed from the bioreactor across a filter channel and back to the bioreactor while cell-free permeate is drawn off and collected. Alternating tangential flow (ATF) systems use an alternating flow diaphragm pump that pulls and pushes feed from and to the bioreactor while cell free permeate is drawn off.

Both TFF and ATF operations can be limited by membrane fouling, which is detected by pressure increase at constant flow, flow rate decrease at constant pressure, or by loss of product sieving. Variable or declining product sieving can impair process robustness and economics.

It is surprising that microfilters, with typical pore sizes between 0.1 and 1 micron, retain ~10 nm diameter antibody molecules. It is likely that foulants shrink membrane pores, or cake layers build up on membrane surfaces, leading to antibody retention.

There have been limited efforts to theoretically model the loss of sieving with fouling. In this study, new explicit mathematical models of sieving loss due to internal membrane fouling, external membrane fouling, or a combination of the two were generated. The models accounted for membrane and cake structures and hindered solute transport.

Internal membrane fouling was assumed to occur due to the accumulation of foulant on either membrane pore walls (pore-retention model) or membrane fibers (fiber-retention model). External cake fouling was assumed to occur either by the growth of a single incompressible cake layer (cake-growth), or by the accumulation of a number of independent cake layers (cake-series).

The pore-retention model was combined with either the cake-series or cake-growth models to obtain models that describe internal and external fouling occurring either simultaneously or sequentially. The sieving models incorporated the same parameters as those used in the corresponding flux decay models.

The models were tested using literature and in-house sieving decline data available in the literature. The cake-series and cake-growth models provided good fits of sieving decline data during the microfiltration of a perfusion cell culture.

The new models provide insights into the internal and external mechanisms of fouling that result in the loss of product sieving. The models can be used to fit sieving data to determine the mechanisms underlying sieving loss, to extrapolate sieving data beyond experimental filtration volumes, and to predict the effects of changing operational parameters.