3.2.4. Viral clearance with higher spike concentration
As shown in the viral clearance study in the integrated process (Table 2), the load titer of X-MuLV is sometimes lower than that for MVM even with the same spike level. Lower load titer may cause a lower virus LRV than expected, and in this case, X-MuLV LRV of > 4 could not be achieved, despite that no virus was detected in the permeate, indicating complete clearance. The lower spike titer for X-MuLV may have been due to the instability of X-MuLV, and similar results have been reported previously (Shirataki et al., 2021a; Gefroh et al., 2014).
To test virus removal for a higher spike level, 10 mg/mL mAb spiked at 2% with X-MuLV was processed to virus filter at 40 LMH with a 60 min process pause. As shown in Table 4, much higher X-MuLV titer was achieved by increasing the virus spike to 2% (6.69 and 7.27 log TCID50/mL for Planova BioEX and S20N, respectively) compared to 5.84 log TCID50/mL for the 1% spike (Table 2). An X-MuLV LRV > 4 was achieved for both Planova BioEX and S20N filters with complete clearance for both the flow-through fraction and the flow-through fraction plus wash. Thus, the reason X-MuLV LRV did not exceed 4 in Table 2 and in the previous report (Shirataki et al., 2021a) was that the load titer was not sufficient.