Host cell proteins (HCPs) are process-related protein impurities that are produced by the host organism during biotherapeutic manufacturing and production. During the purification process, a majority of produced HCPs are removed from the final product (>99% of impurities removed). However, residual HCPs still remain in the final distributed pharmaceutical drug. Examples of HCPs that may remain in the desired pharmaceutical product include: monoclonal antibodies (mAbs), antibody-drug-conjugates (ADCs), therapeutic proteins, vaccines, and other protein-based biopharmaceuticals.

HCPs may cause immunogenicity in individuals or reduce the potency, stability or overall effectiveness of a drug. National regulatory organisations, such as the FDA and EMA provide guidelines on acceptable levels of HCPs that may remain in pharmaceutical products before they are made available to the public. Currently, the acceptable level of HCPs in pharmaceutical drugs range from 1-100ppm (1–100 ng/mg product). However, the accepted level of HCPs in a final product is evaluated on a case-by-case basis, and depends on multiple factors including: dose, frequency of drug administration, type of drug and severity of disease.

The acceptable range of HCPs in a final pharmaceutical product is large due to limitations with the detection and analytical methods that currently exist.Analysis of HCPs is complex as the HCP mixture consists of a large variety of protein species, all of which are unique to the specific host organisms, and unrelated to the intended and desired recombinant protein.Analysing these large varieties of protein species at very minute concentrations is difficult and requires extremely sensitive equipment which has not been fully developed yet. The reason that HCP levels need to be monitored is due to the uncertain effects they have on the body. At trace amounts, the effects of HCPs on patients are unknown and specific HCPs may affect protein stability and drug effectiveness, or cause immunogenicity in patients.[6][7] If the stability of the drug is affected, durability of the active substance in the pharmaceutical product could decrease. The effects that the drug is intended to have on patients could also possibly be increased or decreased, leading to health complications that may arise. The degree of immunogenicity on a long-term basis is difficult, and almost impossible, to determine and consequences can include severe threats to the patient’s health.

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