The second biological activity of DNA that should be considered is its potential infectivity. If a genome of a DNA virus or the provirus of a retrovirus is present in the cell substrate used for vaccine manufacture, then the residual DNA has the potential, upon inoculation into the vaccine recipient, to produce infectious virus from this DNA and thus establish a productive infection.
In 1954, during discussions surrounding the development of adenovirus vaccines for use in the military, the U.S. Armed Forces Epidemiology Board (AFEB) recommended the use of “normal cells” as the substrate for vaccine production rather than cell lines established from human tumors. This decision was based on concerns about the possibility that human tumor cells might be contaminated with occult oncogenic agents that might be transferred to vaccine recipients, an event which might in turn increase the risk of cancer and other neoplastic diseases in vaccinees. As evidenced by current regulatory guidelines and activities of control authorities worldwide, the precedent set in 1954 by the AFEB remains an important factor in the acceptance of all substrates for vaccine manufacture. Currently, the only cultured animal cells that have been used as substrates in U.S. licensed viral vaccines have been primary cells (e.g., derived from monkey, chick, mouse), diploid cell lines (e.g., WI38, MRC-5, FRhL-2), or immortalized (continuous), non-tumorigenic cell lines (e.g., VERO).
Over the past 47 years, two important factors have emerged that warrant serious consideration of the use of immortalized tumorigenic cell lines for viral vaccine production. The first of these factors is that certain novel virus vectors that are presently under development for high-priority target diseases, most notably AIDS, cannot feasibly be propagated in traditionally acceptable cell substrates. The second factor is that scientific understanding of neoplastic processes and viral-induced carcinogenesis has rapidly advanced, as has the ability to detect and identify infectious, oncogenic agents and other types of adventitious agents that may potentially contaminate cell substrates. These factors underscore the need for developing a regulatory framework in which the relative benefits and risks in using tumorigenic cell lines for vaccine production can be carefully and cautiously revisited.
FDA would like the VRBPAC to consider the potential risks in using two novel cell substrates, 293 cells and PER.C6 cells. These cell lines were developed by transforming human embryonic kidney cells (293) and human embryonic retinal cells (PER.C6) with the transforming early region 1 (E1) of adenovirus type 5 (Ad5). Since cell lines such as 293 and PER.C6 express the Ad5 E1 region, they are able to complement the growth of defective Ad5 vectors which have been “crippled” by deletion of E1. Defective Ad5 vectors have been engineered to express foreign genes such as those from human immunodeficiency virus (HIV), the causative agent of AIDS, and vectors of this type are thought to have significant potential for vaccine development because of their demonstrated ability to generate cell-mediated immune responses to HIV. However, a feature of regulatory importance associated with Ad5-transformed cells is their capacity to form tumors in immunodeficient animals such as nude mice.
In considering potential risks associated with the use of these so-called Designer Cell Substrates – i.e., neoplastic cells derived from normal human cells transformed by defined viral or cellular oncogenes or by immortalizing cellular genes (e.g., telomerase) – OVRR/CBER is considering the approach outlined below within the framework of a “defined-risks” assessment (see enclosed reference Lewis et al., “A defined-risks approach to the regulatory assessment of the use of neoplastic cells as substrates for viral vaccine manufacture”, In Evolving Scientific and Regulatory Perspectives on Cell Substrates for Vaccine Development. Brown, Lewis, Peden, Krause (eds.) Develop. Biol. Stand. [in press]). This framework is intended to examine, and wherever possible, to quantify the potential risk of “transmitting” the tumorigenic components of the cell substrate used for vaccine production, and determine whether that “transmission” might pose a risk, particularly an oncogenic risk, to vaccinees. Factors that could influence the risk associated with the use of Designer Cell Substrates include (1) the known mechanism of cell transformation leading to the development of tumorigenic cells; (2) residual cell substrate DNA; and (3) the presence of adventitious agents, especially oncogenic viruses. These three factors are discussed in more detail below.