A guest blog by Barry Garfinkel Ph.D., who has recently retired from Merck & Co. Inc. During his 35 year tenure he worked in the areas of process development/improvement; Quality; and Strategy for vaccines and biologics. During this time he served as VP of Vaccine Quality; VP Vaccine Technology and Engineering; and VP Biological Sciences and Strategy. He now leads a consulting company Barry Garfinkle & Associates, LLC. Barry has also served on the USP council of experts and Chaired the Committee on Vaccines & Virology. He has written several dozen papers on vaccine manufacture as well as chapters on validation and aseptic manufacturing.
Currently there are some viral vaccines made in cell culture. Polio, rotavirus, and flu are currently made in cell culture and are non-recombinant. There is also an HPV vaccine made in cell culture using baculovirus technology. The more common viral vaccines: measles, mumps, rubella, varicella, hep A are currently made in cell culture BUT in either primary cells or diploid cell strains.
Issues: The regulatory authorities have approved the manufacture of viral vaccines in VERO, chick primary, duck primary, WI-38 and MRC-5 cells as well as insect cells (baculovirus). Initially only primary cells were acceptable because of the concern about oncogenes and or adventitious agents and all of these vaccines were injectable. Later some inactivated virus vaccines were approved for growth in VERO cells such as polio, where the reasoning was that the risks of oncogenes or adventitious agents would be reduced/removed by the inactivation process. More recently there have been some rotavirus vaccines approved for use which utilize VERO cells. Here, the VERO cells used are “passage controlled” and these vaccines are not injectable but oral dosage forms. Again, the risk was deemed lower with the oral route of administration.
The use of primary cells from SPF derived chicken flocks, or diploid cell substrates such as WI-38 and or MRC-5 remains the standard. All of these cell substrates are passage limited and tested extensively to show they are non-tumorigenic, are not shedding retroviruses, and have been shown safe over MANY years of usage and probably >1B doses administered. Another point to consider is that these vaccines (measles, mumps, rubella, varicella, polio, rotavirus are all administered to “healthy” young children so the hurdle for safety is very high and the need to change is not deemed a high priority. Historically, there was an episode where commercial polio vaccines were found to be contaminated with SV40 and even recently a commercial rotavirus was found to be contaminated with a porcine circovirus. This rotavirus contamination resulted in a temporary suspension in sales of the rotavirus product.
A disadvantage of the use of primary cells is that a “master cell bank” is not really feasible and therefore each time primary tissue is harvested there is risk of contamination. Similarly, diploid cell strains can utilize a master and working cell bank system which can be tested for adventitious agents but these can only be utilized for a limited number of cells passages/doubling levels before they senesce and die. Therefore, they will all need to be replaced in the relatively near future probably in the next 10-20 years. The amount of testing required for these banks is not trivial and includes adventitious testing, chromosome abnormality monitoring, and finally tumorigenicity testing before the banks can be certified according to current regulatory standards. This can be a substantial investment in the vaccine development cycle and the changing of a cell bank might trigger repeating the clinical work to license the vaccine.
So the advantages of a “continuous” cell line are that the utilization of a cell banking system is doable. The investment in adventitious agent testing, tumorigenicity studies and chromosomal monitoring may be done once in the life cycle of a vaccine vs. needing to repeat these studies if a master cell bank needs replacement. The disadvantages of a “continuous” cell line are regulatory concerns about the cell substrate activating an oncogene or causing cancer IF intact cells are inadvertently introduced into a vaccinee.
So to sum up: the live virus vaccines administered by injection are produced in primary and or diploid cell strains. The inactivated vaccines administered by injection are produced in VERO cells (continuous) but lower passage cells are utilized. The live virus orally administered vaccines have been approved for use with VERO cells. Finally, those recombinant viral vaccines produced in a baculovirus system (producing virus like particles, VLP’s) are produced in continuous insect lines. This last vaccine group is typically highly purified as well as perhaps having “inactivation” steps within the downstream processing of the vaccine, therefore mitigating the potential risk of using a continuous cell line.