ReviewMembrane separations in biotechnology
Introduction
Membranes have traditionally been used for size-based separations with high-throughput but relatively low-resolution requirements. These uses include microfiltration for clarification and sterile filtration, and ultrafiltration for protein concentration and buffer exchange. Current research and development efforts are directed toward drastic improvements in selectivity while maintaining the inherent high-throughput characteristics of membranes. This is critical if membrane processes are to satisfy the new purification and process economic challenges posed by the advent of high-dose chronic therapies using recombinant DNA derived antibodies. This article reviews recent developments and trends in microfiltration, normal flow clarification, ultrafiltration, virus filtration, high-performance tangential flow filtration (HPTFF) and membrane chromatography. Particular emphasis is placed on new membrane materials, modules and process design strategies that provide opportunities for the development of high-throughput and selectivity purification schemes.
Section snippets
Microfiltration
Tangential flow microfiltration (MF) competes with centrifugation, depth filtration and expanded-bed chromatography for the initial harvest of therapeutic products from mammalian, yeast and bacterial cell cultures. In contrast to centrifugation, MF using 0.2 μm-rated membranes generates a particle-free harvest solution that requires no additional clarification before subsequent purification. However, many processes now employ larger pore size membranes to improve product yield and throughput,
Normal flow clarification
Sterile filtration is performed in a normal flow, or dead-end, configuration using 0.2 μm pore size membranes that have been validated for the absolute removal of Brevundimonas diminuta [10]. However, there is evidence that sterilizing filters can pass very small microorganisms under some process conditions, causing some users to employ 0.1 μm-rated filters to provide enhanced sterility assurance in pharmaceutical processes [11].
Normal flow filters are also used to remove bacteria and particles
Ultrafiltration
Ultrafiltration (UF) has become the method of choice for protein concentration and buffer exchange, largely replacing size-exclusion chromatography in these applications [15]. Recent work has also demonstrated the use of UF for the purification of plasmid DNA [16] and virus-like particles [17]. UF membranes are cast from a variety of polymers, with polysulfone, polyethersulfone and regenerated cellulose being of greatest interest in biopharmaceutical applications [18]. Although synthetic
Virus filtration
Mammalian cells used in the manufacture of recombinant DNA products have been shown to contain endogenous virus-like particles [25]. In addition, mammalian cell cultures are susceptible to contamination with adventitious viruses introduced during processing. The safety of mammalian cell-derived products is regulated by requirements to obtain less than one virus particle per million doses. This is achieved by designing purification processes that include validated viral removal using multiple
High-performance tangential flow filtration
High-performance tangential flow filtration (HPTFF) is an emerging technology for protein purification 19, 30., 31.. HPTFF is a two-dimensional unit operation that exploits both size and charge mechanisms. In addition, protein concentration, purification and buffer exchange can be accomplished in a single unit operation. HPTFF is capable of separating monomers from dimers based solely on size differences and can separate proteins of equal size on the basis of charge differences. Despite
Membrane chromatography
Membrane chromatography was explored many years ago without any substantial commercial success. Recent developments [33] have generated renewed interest in this technology and several membrane chromatography products will be brought to market in the next few years. In contrast to conventional bead chromatography, membranes have the inherent benefit of not being diffusion limited. Binding capacity is therefore independent of flow rate. The challenge has been to achieve binding capacities that
Conclusions
Membrane processes are currently used throughout downstream purification processes. However, the development of high-dose chronic therapies using recombinant DNA derived antibodies poses a range of new separation challenges for the biotechnology industry. The need to reduce production costs coupled with projected increases in batch size requires the development of improved separation technologies with high-throughput and selectivity. Membrane systems have the potential to meet these challenges.
Acknowledgements
The authors thank Vinay Goel, Herb Lutz and Rich Levy (Millipore Corporation), David Patterson (Mensco), and Jerry Martin (Pall Corporation) for their kind assistance with literature references.
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
of special interest
of outstanding interest
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