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.