Precipitation-based purification could help make therapeutic mAbs more accessible, according to the Gates Foundation, which has awarded $3.1 million to a team using the approach as part of an effort to cut the cost of such drugs to $10 per gram.
The grant will support scientists at Rensselaer Polytechnic Institute (RPI) in New York who are working on a low-cost alternative to the affinity-based purification methods used in the production of monoclonal antibodies (mAbs).
Affinity-based purification accounts for a significant proportion of costs, according to Todd Przybycien, PhD, professor, project leader, and head of RPI’s chemical and biological engineering department.
“Estimates from technoeconomic evaluations of state-of-the-art mAb manufacture put platform downstream processing costs in the range of 60–70% of the cost of goods. The biggest cost is the mAb capture purification step, which is based on affinity chromatography with recombinant protein A media,” he tells GEN.
Precipitation
The RPI team’s alternative is based on precipitation. They use a combination of zinc chloride and polyethylene glycol 3350 as “synergistic precipitants,” which cause mAbs to precipitate out of the process stream.
The approach is selective due to the presence of histidine residues on mAbs, which serve as reversible ligands for Zn2+ ions. In addition, due to their large size, mAbs are sensitive to volume exclusion, a process whereby molecules are forced out of solution in the presence of crowding agents like polyethylene glycol.
A key aim is to develop the process for industrial use, according to Przybycien, who cites reproducibility as a major focus.
“We perform precipitation continuously using tubular contactors, giving rise to reproducible and readily scalable precipitate particle morphologies which, in turn, facilitate precipitate slurry washing and dewatering in hollow fiber microfiltration modules. Redissolution is facile and rapid: we drop the pH below 5, where the protons out-compete Zn2+ ions for binding with histidine residues. After the precipitates are redissolved, we go on to a pair of orthogonal flow-through chromatography steps to remove residual impurities.”
The approach is particularly suited to intensified production, according to Przybycien, who says, “The whole purification process is fully continuous and operates without surge tanks. Residence times from the addition of harvested cell culture fluids to the collection of purified mAb are about 30 min.
“The process is fully compatible with continuous low pH viral inactivation, viral filtration, and solution conditioning operations.” And, at commercial scale, the cost-saving potential of the approach is significant, Przybycien says.
“We anticipate that a large part of the cost benefit will be realized via intensification: we intend to pre-concentrate mAbs from harvest streams, perhaps to as much as 20 to 50 g/L.
He adds, “Since precipitant usage to set solubility is essentially independent of mAb concentration, by processing more highly concentrated mAb streams, we drive precipitant and buffer usage per kilogram of mAb down and we also drive process equipment volumes per kilogram of mAb processed down. Feedback from technoeconomic evaluations will be used to drive further cost optimization.”
IP access
Appropriately for a project designed to make mAbs cheaper and more accessible, Przybycien and his team intend to share their findings.
“We have not and will not seek an IP position on this process R&D work. We will present and publish everything we do. We want to push this process technology as hard as we can to see if it can be part of the solution to bring low-cost mAbs into the hands of those who need them globally. That would be the greatest reward to me,” he said.