The number of protein-based pharmaceuticals reaching the marketplace has dramatically increased over the past few years. New developments in protein expression and bioprocessing technologies have led to significant advancements in protein manufacturing and production. The development of highly efficient downstream protein purification processes, however, has not kept pace with the significant increases in recombinant protein expression from bacterial and yeast systems and high titers of monoclonal antibody from recombinant mammalian cell culture.
Inducible expression systems enable routine overexpression of fusion-tagged proteins at up to 50% of the total cell protein, and monoclonal antibody titers can exceed 10g/L using specialized cell lines and fed batch cultures. This dramatic rise in protein productivity has been a mixed blessing for biomanufacturers. Although desirable from a cost-of-goods perspective, these huge productivity increases stress the capabilities of all downstream steps originally designed for much lower titers. The biomanufacturing bottleneck has shifted further away from upstream protein expression productivity and toward downstream protein capture and purification.
Protein purification using affinity media in a single-column bind-wash-elute chromatography mode is a well-established technique that has been pushed to its productivity limit by expression increases upstream. Throughput at high target protein concentration requires larger chromatography columns, more process buffer, additional purification steps, or multiple purification campaigns translating into higher cost, longer lead times, and decreased efficiency.
Some of these process problems have been overcome by employing high-capacity, high-flow affinity supports such as monoliths, membranes, and perfusion particles. Ion exchange, mixed-mode, and precipitation methods are less expensive alternatives to protein A capture, but antibodies purified by these methods require additional polishing steps to achieve the same level of purity possible with highly specific protein A resins.
Recombinant proteins purified through histidine (His) or glutathione-S-transferase fusion tags often are insufficiently pure for functional characterization, x-ray crystallography, or drug target validation. Improvements in purity and efficiency at any stage of these purifications would increase throughput and lower costs.
Simulated moving bed chromatography (SMBC) combined with high capacity, high linear velocity chromatography supports can help relieve the bottleneck created by upstream protein increases. The countercurrent flow, central to SMBC, enables the highest yields of purified proteins with the smallest investment in chromatography resins and mobile phases, bringing dramatic increases in operational efficiency and productivity. In SC systems, separation only occurs in a small fraction of the column at any one time, with the rest of the column performing no function other than occupying solvent and broadening bands. Repeated stacked injections necessary when crude product mass exceeds SC capacity amplify process delays.
With SMBC, a series of small columns is used instead of one large column. Typically 50–70% of the affinity media’s capacity is actively engaged in the separation, while the rest of the column media is being prepared for the next cycle of purification. Moreover, SMBC essentially provides an “infinite” column bed length for increased resolution in theoretical plate-dependent separations without the costs associated with obtaining, operating, and maintaining much larger single columns.
Finally, SMBC purification functions continuously until the desired volume of feedstream has been loaded. These advantages of SMBC over SC purification translate into 10- to 20-fold increases in throughput and productivity, respectively, and corresponding reductions in labor and QC costs.
SMBC was developed by D. B. Broughton and C. G. Gerhold almost 50 years ago and commercialized as the Sorbex process by UOP for industrial petrochemical separations. Since then, SMBC has been successfully applied to separations of sugars, chiral compounds, organic molecules, nucleic acids, and proteins. To date, the SMBC process has been successfully utilized in the large-scale production of several single enantiomer active pharmaceutical ingredients (APIs).
Although applications of SMBC for bioseparation have been increasing, it has not been used frequently for protein purifications. SMBC has primarily been used to separate binary component mixtures, and the complex mixture of molecules in biological feed streams is certainly not binary. However, affinity fusion tags and highly selective affinity chromatography media can be used to simplify SMBC purification of proteins. Highly specific affinity interactions simplify the fractionation behavior of complex biochemical feed stocks to binary mixtures for purposes of SMBC protein purification.