One longstanding and ongoing challenge for bioprocessors is the removal of host-cell proteins (HCPs).
“The entire HCP field has taken off in the last several years and numerous groups have contributed to it,” says Abraham Lenhoff, PhD, Allan P. Colburn Professor of Chemical Engineering in the department of chemical and biomolecular engineering at the University of Delaware in Newark. HCPs “have historically been assayed as a combined group,” Lenhoff explains. “The more recent attention has been on individual HCPs that can be damaging even if present in only low concentrations.”
In manufacturing monoclonal antibodies (mAbs), for example, small amounts of HCPs may contaminate the product. Recently, Lenhoff and his colleagues discussed the identification and characterization of these HCPs. As they noted: “Some HCPs may evade clearance in multiple purification steps and reach the final drug product, potentially threatening drug stability and patient safety.”
Identifying the molecules
To improve the removal of HCPs in mAb products, scientists need better ways to identify the molecules. “There have been numerous efforts to look for biophysical properties of HCPs to correlate with persistence, but I’m not aware of any of these having been effective,” Lenhoff says.
Although he mentions that the molecular mass and isoelectric point (pI) of these HCPs, as well as some calculations of the docking of between mAbs and HCPs, have been examined, Lenhoff notes: “We’ve also done some of these and not found any smoking guns.”
One feature of HCPs in mAb products that draws Lenhoff’s attention is abundance. Based on a manuscript from Lenhoff’s lab that is in review, he says, “Essentially what has been seen in some published work and what we have also found is that HCPs don’t seem to bind to mAbs exceedingly strongly, so it would seem that binding is driven more by mass action than by binding affinity.” He adds, “This can explain why HCP abundance is an important factor.”
When asked about the best way to deal with HCPs that are hard to remove, Lenhoff says, “It’s not clear that there’s a single answer to this one.” For HCPs that persist because of associating with an mAb product, he explains: “What seems to be needed is to wash sufficiently well to remove enough of the bound HCP.” In addition, he points out that some additives and solution conditions, such as the pH, can disrupt binding between the product and even persistent HCPs. A significant additional factor, though, is that HCPs may be incorporated in mAb-rich aggregates that are not always easy to separate from free mAb.
Despite advances in removing HCPs from mAb products, Lenhoff concludes: “It seems that product association and aggregate clearance are appropriate focal points for further study.”