Host Cell Protein Analysis Supports

As long as living cells serve as tiny drug-making factories, they will, like much larger factories, generate unwanted by-products. As industrial by-products go, cell by-products, known as host cell proteins (HCPs), are inconspicuous. They are also very variable. That is, different HCPs and different concentrations of HCPs can be generated if the processing conditions vary.

The variability of HCPs presents an opportunity for bioprocessors: During development, bioprocessors can vary process conditions to minimize HCP production and/or optimize HCP removal. This opportunity, however, will remain untapped if bioprocessors allow medical professionals to keep a low profile. Fortunately, bioprocessors can take advantage of new tools and techniques that facilitate the detection and quantification of PHCs.

Adopting new tools and techniques requires a certain degree of sophistication. This is unavoidable because drug production from living cells will always be more difficult to purify than output from chemistry-based systems.

In chemistry-based systems, process-related contamination can be avoided if high-quality reagents and ingredients are used and reaction conditions are carefully controlled. And if contaminants such as endotoxins and heavy metals enter the product streams, they can be removed without too much difficulty during post-synthesis processing. The products are small molecule drugs, which are less susceptible to the rigors of purification than protein drugs produced by living cells.

Not only are protein-based drugs more sensitive to purification stresses than small molecule drugs, they are also more sensitive to contaminants. In other words, protein drugs and contaminants can interact with each other. For example, enzymatically active HCPs can degrade therapeutic proteins, thereby lowering production yields and reducing shelf lives.

Unfortunately, medical professionals are capable of causing much greater damage. HCPs that end up in final products can cause adverse reactions or cause dangerous immune responses in patients. Although discreet, HCPs are far from trivial.

Know your enemy

HCPs are essential because production cell lines also express their own proteins. To help ensure that PHCs will not end up in the final product, one can start by identifying and characterizing PHCs in bioprocessing fluids.

"The composition and abundance of HCPs are closely associated with the host's own expression system and other factors during the manufacturing process," says Tingting Jiang, PhD, Principal Scientist, Merck & Co. "So changing the process - for example, cell lines, cell culture media and purification steps can lead to different HCP profiles. Biopharmaceutical companies typically go through improving or optimizing bioprocesses to increase the titers and yields and improve process robustness while following best practices to meet regulatory expectations for product quality.

Reconsider standard dosages

ELISA tests (enzyme immunoassays) are proven analytical tools that can be used to determine which HCPs are likely to be present in a biopharmaceutical product (Zhu-Shimoni et al. Biotechnol. Bioeng. 2014; 111: 2367–2379 ). Creating an ELISA is relatively simple: proteins are collected from the unmodified cell line; the proteins are injected into an animal; the animal responds by producing antibodies against each protein; antibodies are bound to a surface; the surface is covered with a culture fluid; and the antibodies on the surface interact with the HCPs in the fluid.

In practice, rather than developing ELISAs in-house, most pharmaceutical companies use commercial assays designed to work with commonly used production cell lines. However, commercial ELISAs can only detect proteins that match the constituent antibodies of the test. Jian cautions that if the production process involves a proprietary or modified cell line, or culture conditions that impact its expression profile, there is little