Product yield and purity are often monitored to ensure that product degradation is not induced by repeated use of a membrane.
For cases when the filtration step is used for clearance of host cell and process-related impurities, it is important to monitor
clearance at appropriate intervals during reuse studies. Such monitoring can help ascertain whether the efficacy of the clearance
step is undermined by reuse.
Figure 1. Data from membrane reuse studies for prospective validation: A) normalized water permeability; B) step yield; C)
endotoxin levels; and D) conductivity at retentate end
Examining filter integrity can identify problems such as macroscopic holes in the membrane, cracks in the seals, or improperly
seated modules, which can lead to product leakage and loss, or unsatisfactory clearance of impurities. A variety of methods
like air diffusion tests are available for this purpose.
Transmembrane pressure (TMP) is the force by which the liquid moves through the membrane. The TMP-versus-flux curve often
serves as a qualitative indicator of the performance of a membrane step. A carryover of product or impurities often results
in decay of the TMP-versus-flux curve. Spectroscopic methods such as Fourier-transform infrared (FTIR) and Raman spectroscopy
can be used to analyze the membrane post-use to quantify the buildup or absence of protein.
Performing periodic blank runs at appropriate intervals is a common method used to evaluate cleaning efficacy and potential
for product carryover. In a blank run, the membrane step uses load material that does not contain any product, and the resulting
pool is analyzed for product-related or other impurities. Analytical techniques often used for this purpose include high-performance
liquid chromatography (HPLC) assays, product-specific enzyme-linked immunosorbent assays (ELISA), sodium dodecyl sulfate polyacrylamide
gel electrophoresis (SDS-PAGE) assays, and total organic carbon (TOC) analysis.
Table 1. Results from the scale-down modeling and qualification
APPROACHES FOR ESTABLISHING MEMBRANE REUSE
Concurrent validation and prospective validation are two widely accepted approaches for establishing membrane lifetimes.
Concurrent validation involves performing cycling studies solely at large scale. Carryover and performance are assessed at
preset frequencies, and a determination is made on the acceptability of membrane reuse. Lots manufactured in the meantime
are quarantined and released once the carryover and performance data have been reviewed and found acceptable. For example,
to establish 100 reuses at large scale, a strategy could be to evaluate carryover and performance data for every 10 runs,
and to release all 10 lots after the evaluation is complete and successful, including the blank run performed after the tenth