Introduction
Virus filtration is widely used as a necessary virus removal step for biotherapeutics and protein drug products derived by plasma fractionation (Besnard et al., 2016; Grein et al., 2014; Miesegaes et al., 2010; Strauss et al., 2017). Virus filters have multi-layered membranes that allow for separating viruses from protein solutions, which have only a small size difference (Manabe et al., 1987, 1989). While virus removal from protein solutions is made possible by the more sensitive separation membrane, larger sized proteins and aggregates in particular have been confirmed to cause clogging in virus filters (Barnard et al., 2014; Bieberbach et al., 2019; Chen & Chen, 2015; Genest et al., 2013). To improve the capacity of the virus filtration step, processing the feed solution through one of several commercially available prefilters has been confirmed to be effective in removing substances that cause clogging and has been widely implemented (Brown et al., 2010; Genest et al., 2013; Roederstein & Thom, 2013). Alternatively, column chromatography has also been shown to effectively remove aggregates (Yigzaw et al., 2009).
Examination of filtration behavior and, in particular, changes in filtration behavior due to clogging can be used to elucidate the clogging mechanisms and to extrapolate filtration capacity in well characterized systems. The theoretical relationship between filtration volume and filtration throughput for a given feed solution can be fitted to one of four established clogging models for filter membranes (Grace, 1956): cake filtration, intermediate blocking filtration, standard blocking filtration and complete blocking filtration. Maximum filtration volume (Vmax) was theoretically determined using the standard blocking model based on filtrations conducted with a microfilter at constant pressure, and the data were then used for facility scale-up recommendations as reported in Badmington et al. (1995). Following this same strategy, the standard blocking model has been applied in nanofiltration to determine Vmax of virus filters under constant pressure (Bolton et al., 2004). Thus, small volumes of feed solution samples can be used to effectively evaluate the filtration volume or filter size that will be used in the process and additionally quantitatively compare the filterability of process solutions. Further, there have been attempts to combine several clogging models into a theoretical equation in order to match complex filtration behavior results to theoretical values (Ho & Zydney, 2000; Bolton et al., 2006; Bolton & Apostolidis, 2017).
In this report, monoclonal antibody (mAb) and plasma IgG each spiked with aggregates prepared from the respective solution were processed by various chromatography resins or prefilter and run on a virus filter under constant pressure to determine the relationship between filtration volume (throughput) and flow rate (flux). In order to analyze the experimental results, filtration behavior results for solutions spiked with aggregates as well as several column chromatography outputs were applied to the clogging models of cake filtration, intermediate blocking, standard blocking and complete blocking, the model that best fits the filtration behavior data was identified, and clogging models were used to extrapolate filtration behavior beyond the experimental range.