Biopharmaceutical drugs, often called biologics, are a class of treatments derived from living organisms. Their production involves using host cells to generate specific therapeutic proteins. Host cell proteins (HCPs) are proteins produced by these host cells during the manufacturing process. While host cells create the desired therapeutic protein, they also produce their own native proteins to support normal life functions. These HCPs are not part of the intended drug but are unavoidable byproducts that become mixed with the therapeutic product. Consequently, manufacturers must remove these residual proteins to ensure the quality, purity, and safety of the final medicine.
The Origin of Host Cell Proteins
The creation of biologic drugs relies on biomanufacturing, where living cells act as microscopic factories. Scientists genetically engineer specific cells, such as Chinese Hamster Ovary (CHO) cells or the bacterium Escherichia coli (E. coli), to produce a particular therapeutic protein. These cells are selected for their ability to grow rapidly and produce large quantities of complex proteins that can function as medicines.
To survive and function, these host cells must also produce a wide array of their own proteins for everything from energy metabolism to cellular repair. When the time comes to harvest the biologic drug, the host cells are often broken open in a process called cell lysis to release the therapeutic protein contained within. This action unavoidably spills the cell’s internal contents, including its functional proteins, into the mixture. The resulting solution is a complex blend of the desired drug and thousands of different host cell proteins, which are now considered impurities.
Risks Associated with HCP Contamination
The presence of residual host cell proteins in a biologic drug poses potential risks to patients, with the main concern being immunogenicity. This is the potential for a patient’s immune system to recognize these non-human proteins as invaders, triggering an immune response. This reaction can manifest in several ways, from mild injection-site reactions to more significant clinical consequences.
An immune response against HCPs can compromise the effectiveness of the drug. If the body develops antibodies against these impurities, it may lead to faster clearance of the drug from the bloodstream, reducing its therapeutic effect.
Beyond immunogenicity, HCPs can also affect the drug’s stability. Certain proteins, such as proteases or glycosidases, can degrade the therapeutic protein over time, shortening its shelf life or altering its structure. This degradation can impact the drug’s biological activity, so regulatory agencies require manufacturers to reduce HCP levels to the lowest practical amounts.
Detection and Quantification of HCPs
To manage risks from host cell proteins, manufacturers must use reliable methods for detecting and measuring them. This monitoring helps assess the effectiveness of purification strategies and ensure the final product meets purity standards. The goal is to quantify the total amount of HCPs present, often measured in parts per million relative to the drug protein.
The most widely used method for quantifying total HCPs is the enzyme-linked immunosorbent assay, or ELISA. This technique uses antibodies that bind to the various HCPs present in a sample, generating a detectable signal proportional to the total HCP concentration.
For a more detailed analysis, manufacturers may use mass spectrometry. This technique can identify and quantify individual HCPs by separating proteins based on their mass-to-charge ratio, creating a detailed profile of specific contaminants. This information is useful for identifying HCPs that are difficult to remove or pose a higher risk, enabling a more targeted approach to risk assessment.
Purification Processes to Remove HCPs
Removing host cell proteins is accomplished during downstream processing. This manufacturing phase involves a series of purification steps to separate the therapeutic protein from HCPs and other impurities. The central technology used is chromatography, which separates molecules based on differences in their physical and chemical properties.
A common technique is affinity chromatography, which often serves as the initial purification step. In this method, the protein mixture is passed through a column containing a resin that specifically binds to the therapeutic protein, allowing most HCPs to be discarded.
Following this capture, subsequent chromatography steps are employed to further refine the product. These methods can separate proteins based on characteristics like size (size-exclusion chromatography) or electrical charge (ion-exchange chromatography). This sequential, multi-column approach is fundamental to consistently reducing host cell protein levels to the trace amounts required for a safe biologic drug.