Continuous cell substrate considerations.

The debate over the potential risk of tumorigenicity attributable to the use of CCL substrates for biologicals production has continued for over 30 years and may continue for some time to come. Manufacturers and regulatory agencies are developing scientifically based guidelines for such products. It is currently possible to follow these guidelines to prepare recombinant biologicals and monoclonal antibodies in CCLs which do not pose unreasonable risks. This chapter has attempted to describe the scientific tools available to evaluate the putative risk of tumorigenicity due to potential virus DNA and protein contaminants. No theoretical or experimental basis exists to hypothesize that residual cellular protein might present a significant risk of tumorigenicity. The tools are certainly adequate for characterization of putative risks due to viruses and DNA but are not sufficiently powerful by themselves to assure product safety. The subsequent chapter on process validation describes how adequate assurances of safety ultimately can be obtained for products of CCLs against theoretical risks of tumorigenicity due to putative viruses and DNA. In addition to these safeguards, no evidence of tumorigenicity has been found in human or livestock animal recipients of the products prepared in CCL substrates. Many patients have received inoculations of tissue plasminogen activator, erythropoeitin, factor VIII, soluble CD4, GM-CSF, hepatitis B surface antigen vaccine, and various monoclonal antibodies and other recombinant products of continuous cell lines in clinical trials. For tissue plasminogen activator, large doses of 100 mg per patient or more have been used. At the time of writing over 10 kg of CHO-derived tissue plasminogen activator has been sold since late 1987 for administration to over 100,000 human patients. For recombinant factor VIII, erythropoeitin, and soluble CD4 proteins, chronic administration has been employed. Millions have received polio and rabies vaccines prepared in continuous Vero cells. In addition to this human experience, livestock animals have received annual inoculations of foot-and-mouth virus vaccine prepared in BHK-21 (a highly tumorigenic CCL) for up to 14 years without effect (69). No effects have been reported which might be attributed to oncogenic factors. Thus, scientific tools of characterization and principles of process validation are available to protect patients from putative risks of tumorigenicity associated with products prepared in CCLs. Increasing clinical experience also supports this conclusion.