The initial capture and purification step using Protein A affinity chromatography yields a product with >90% purity, but further
polishing is required to reduce impurities and viruses to acceptable levels and conform with regulatory guidelines. The dual
Sartobind Q/S disposable system tackles both positively and negatively charged impurities in a single step. The Q membrane
is able to remove nearly 400 µg of DNA, 1.2 mg of Protein A, and nearly 30,000 EU of endotoxin (Sartobind Q5, surface area
5 cm2 equivalent to a 0.137 mL bed volume). Experiments to test the system for virus removal capacity are in progress.
The reproducibility of these results has been tested by monitoring the yield (calculated as the amount of Teleukin obtained
after polishing as a percentage of the input amount) and the elution conductivities of several runs. The results, as presented
in Table 1, confirm the robustness of the process. The elution conductivities are highly reproducible both among lots and
between runs, and the yield ranges from 82.2 to 99.9%, which shows that the process, when optimized, can recover nearly all
of the input product. The Q/S dual adsorber system is therefore suitable as a manufacturing process for this product, and
probably all MAbs.
Table 1. Reproducibility and robustness of the dual Q/S polishing system, as determined by measuring yield and elution conductivity.
Yield is expressed as [D]/[PD]%, where D is the amount of antibody dimer recovered after polishing and PD is the amount of
antibody dimer submitted for processing. Elution conductivities (MC, mSi/cm) represent values between fraction T collection,
and the value in parentheses represents conductivity at the edge of the chromatogram peak.
This final step in the process is a nanofiltration operation to mechanically remove endogenous or adventitious virus particles
<20 nm in diameter. A 4-inch NFP Millipore cartridge filter (Millipore, Billerica, MA) is used for this step, and the efficiency
of this device has been confirmed for a sample load up of up to 1.6 g of Teleukin in a volume of up to 6.5 L, loaded at a
constant pressure (4 bar) using the Akta Pilot chromatographic system. The filtration flow rate is automatically adjusted
by the Akta system to keep the pressure value constant. The flow rate usually ranges from 100 to 300 mL/min and sample recovery
after filtration is usually very high (>90%).
Several comparative evaluation studies have been carried out to determine the relative performance of membrane adsorbers and
traditional packed-bed columns in flow-through mode, although direct comparisons can be difficult because different standards
are used to measure process capacity and flow rate.12
When such issues are taken into account, the performance of Q adsorbers is encouraging, but until now there has been no similar
evaluation of either Q or S adsorbers in retention mode.
The process outlined above demonstrates that a membrane adsorber in retention mode can be used efficiently and robustly in
a commercial antibody manufacturing process. The reproducibility analysis shows that the dual adsorber format operates successfully
to remove both positively and negatively charged impurities from the feedstream, and because of the in-series linkage between
the devices the dual system can be regarded as essentially a single, integrated polishing step. As membrane adsorbers become
more widely accepted in antibody purification, it seems that not only flow-through but also retention chromatography steps
in antibody polishing could be replaced by disposable membrane devices. Even so, the superior capacity of column chromatography
for the capture of antibodies and other recombinant proteins means that resins are unlikely to be replaced for the foreseeable
future in the primary capture step, which is the most significant bottleneck of all.13,14 .
Leonardo Giovannoni, PhD, is the head of CMC production and Marco Ventani is the head of downstream processing, both at Philogen S.p.A., Sienna, Italy, +39 0577 588539, email@example.com
Uwe Gottschalk, PhD, is vice president of purification technologies at Sartorius Stedim Biotech GmbH, Göttingen, Germany. Gottschalk is also
a member of BioPharm International's Editorial Advisory Board.