5.3.4.3 Viruses

Biosimilarity: The FDA Perspective

232 patterns are highly conserved among strains and secondly, a generic HCP assay will be a strong tool for detection of variations in the process, which could lead to a different impurity profile. Process-specific assays are directed against HCPs copurifying with the target protein. The assays are typically developed for purifying the cell lysate without target protein using the exact process to be used for the licensed product. By definition, the process-specific assay cannot be developed before the process is locked. The assay, which typically takes one to two years to develop, may thus be a time-delaying factor. The process-specific assays provide a much narrower window than generic assays do and their value as lot-release test has recently been challenged because these assays will most probably fail to detect atypical HCP contaminants. There are some significant differences between WB technology and immunoassays. In WB, the denaturation and solubilization steps can destroy some native epitopes while the immunoassay technology relies on reaction with the native protein. The immunoassay technology provides an objective result while the WB depends on a subjective interpretation. Finally, the sensitivity of immunoassays is generally higher than that of WB assays. None of the methods mentioned are quantitative. They are at best semiquantitative. Even the commonly used HCP immunoassay lack the linear accuracy applied to single analyte immunoassays, due to variance in relative affinities between antibodies and HCP antigens and HCP concentrations. Instead, assay results are correlated with clinical conditions and to process control. The amount of HCP to be accepted depends on the antigenicity of the copurifying proteins. Recognizing the complexity of the task and the inability of quantitative measurements it is not possible to state a generally acceptable level.

5.3.4.3 Viruses Virus contamination comes from the host cell, the culture medium, and the infections during manufacture. The host cell may contain a genomic virus or virus vectors used to transform the cell line. The type of viral genome and/or vector depends on the cell line history. Continuous cell lines are extensively characterized, but chronic or latent viruses may be present. The retroviruses associated with continuous cell lines are noninfectious but oncogenic. The Epstein–Barr virus or the Sendai virus is often used for cell transformations. Contaminants such as bovine viral diarrhea virus, infectious bovine rhinotracheitis, reovirus, PI-3, bovine leukemia virus, and bovine polyomavirus should be expected from serum-supplemented media. Virus control is executed on several levels. The cell line history reveals all information on the origin and identity of the cell line and the host genome vectors used to establish the cell line. The master cell bank is extensively characterized using viral identity tests, in vitro tests, and in vivo tests to assure freedom of adventitious viruses. The end-of-production test of the cell culture is carried out to assure that the cells are free of viruses. Viruses are brought into the process from the environment because of either contaminated equipment, infected raw materials, water, or because of