Use of Madin-Darby Canine Kidney (MDCK) Cells for Manufacture of

Inactivated Influenza Vaccines:Introduction

Philip R. Krause, M.D.

Outline

• Definitions and brief introduction to the issues

• Recent history of thinking about neoplastic cell substrates

• Summary of scientific concerns regarding use of neoplastic cell substrates

• Brief description of the plan for today’s meeting

Definitions

• Neoplastic cells (continuous cell lines)– Cells that are immortalized and can progress along the

pathway to tumor formation

• Tumorigenic cells– Neoplastic cells that form tumors when injected into

susceptible animals– Highly tumorigenic cells- form tumors at low cell

doses, or form metastatic tumors– Weakly tumorigenic cells- form tumors only at higher

cell doses

Cell substrates used for vaccine production

• Primary cells & Tissues (1954)– Calf lymph for smallpox vaccines – Monkey kidney cells for polio vaccines– Embryonated hens’ eggs for influenza, yellow fever vaccines– Chicken embryo cell culture for measles, mumps vaccines– Mouse brain for inactivated JEV vaccine

• Human diploid cells (introduced in 1960s)– MRC-5, WI-38 for rubella, varicella vaccines

• CHO cells for highly purified, subunit investigational vaccines (1980s)• Vero cells at non-tumorigenic passages for highly purified, inactivated

vaccine (IPV) (1980s)• Vero cells at non-tumorigenic passages for live-attenuated vaccines (late

1990s)• In vitro transformed human cells (e.g., 293, PER.C6) for replication-

defective recombinant vaccines (early 2000s)

History of MDCK cells• Multiple, relatively independent derivatives of

the MDCK cell line have been described

From Gaush, PSEBM, 1996 122:931

1958, MDCK established from healthy female cocker spaniel

MDCK, ATCC

1963 MDCK-USD 1961 MDCK-NIH

Why are MDCK Cells being considered for use in manufacture of inactivated influenza vaccines?

• Virus growth advantage• More rapid scale-up• Ability to bank & thoroughly characterize

cells• Adaptation to serum-free growth

Manufacturers will provide more detail

Issues with MDCK Cell Substrates

• Original line of MDCK cells was non tumorigenic• Some MDCK derivatives have been found to be

highly tumorigenic • Highly tumorigenic cell substrates have never

been used to manufacture viral vaccines• Highly tumorigenic cell substrates pose significant

regulatory challenges

Introducing tumorigenic cell lines for vaccine development

• Expands the repertoire of neoplastic cells that can be used in development of new vaccines, including:– genetically engineered viral vectored vaccines– HIV vaccines– Influenza vaccines

• annual

• pandemic

Why are there concerns about tumorigenic cells?

• Potential for increased risk of adventitious agent contamination

• Potential for increased risk associated with residual DNA

• Potential for increased risk associated with virus/cell interactions

• Potential for other increased risks?• Perception of increased risk

The next slides will describe the past 10 or so years of CBER thinking about introduction of neoplastic cell substrates

Neoplastic cells in use in 1995 for production of biologicals

• Namalwa cells for interferon• Rodent cells for monoclonal antibodies (hybridomas),

therapeutic proteins (CHO and BHK), including CHO cells for investigational protein subunit vaccines– These cells are tumorigenic– Non-infectious retroviruses are present– High amounts of viral elimination/inactivation (clearance) are required

• at least 6 logs clearance in excess of known retrovirus burden; can generally only be demonstrated with multiple “orthogonal steps”

• Vero cells, at non-tumorigenic passages, used for production of inactivated poliovirus vaccines– Stringent limitations on DNA content – Used only for inactivated vaccines

VRBPAC discussions regarding neoplastic cell substrates

• Based on the premise that full public discussion of transition to use of neoplastic cell substrates is important

• 1998: Initial discussion with committee• 1999: International cell substrate meeting &

report to VRBPAC• 2000: Discussion of the use of Vero cells• 2001: Discussion of 293 and PER.C6 cells

11/19/1998 VRBPAC:Initial Discussion with Committee

• Committee recommended:– Development of a document describing a proposed approach to

addressing use of neoplastic cells in vaccine manufacture

– A workshop to obtain public discussion of this document and additional scientific input on these issues

– Continued dialogue with the advisory committee

– Research to provide scientific foundation for decision-making regarding use of neoplastic cells in vaccine manufacture

September 1999: International Meeting: EvolvingScientific and Regulatory Perspectives on Cell

Substrates for Vaccine Development

• Sponsored by: CBER, IABs, NIAID, NVPO, WHO• Summarized at 9/14/99 VRBPAC• Goals:

– Identify concerns & issues– Identify approaches to determine levels of risk associated

with those issues

• Discussion of CBER document prepared in response to 11/98 VRBPAC– Presentation of “Defined-Risks Approach” as a conceptual

framework for considering the issues

Defined risks approach• Represents an attempt to establish, where possible, a

quantitative conceptual framework for estimating upper bounds on potential risks– Identifying the possible risk event – Estimating or determining the frequency with which the

risk event might occur or has been observed to occur, either in nature or under experimental conditions

– Estimating the possible frequency of the risk event per dose of vaccine

– Developing and determining the sensitivity of one or more assays that can be used to detect the risk event

– Developing and validating one or more processes that can be used to establish a product-specific safety factor

1999 meeting: Scientific conclusions

• Multi-factor nature of carcinogenesis suggests very low risk of oncogenicity from cellular components other than oncogenic viruses

• Unrecognized adventitious agents may be the major concern with neoplastic cell substrates– Primary cells present greater risks for adventitious agents than neoplastic cells

• Risks from residual DNA were perceived to be low, although there is a need for more scientific data to verify this perception

• Virus/cell interactions:– Risk must be considered based on specific virus/cell substrate combinations,

and any selective pressures in the cell culture system • Concern was raised that neoplastic cells could contain abnormal PrP

genes, of unclear significance• Designing cell substrates using defined mechanisms of transformation

should be considered

VRBPAC 2000: Issues/Topics regarding Vero cell use for vaccine manufacture

Vero cells– Non-tumorigenic, but have capacity to become

tumorigenic upon repeated passage– Mechanism of transformation is unknown– Substantial experience exists using Vero cells

in research and diagnostics– High level of testing detected no evidence for

the presence of adventitious agents

VRBPAC 2000: Conclusions/Recommendations on Vero cell use for vaccine manufacture

• Importance of assuring removal of intact cells from vaccine

• More concern about parenteral than mucosal vaccines produced in Vero cells

• Significant concern expressed about use of Vero cells at tumorigenic passage levels

• Some members expressed concern about using cells with the potential to become tumorigenic

• Limit DNA to 10 ng for vaccines produced in Vero cells at non-tumorigenic passages

• 293, PER.C6 cells for gene therapy products, investigational live adenovirus vectored vaccines– These cells allow replication of defective adenovirus

vectors (PER.C6 designed to minimize RCA formation)

– Defined mechanism of transformation (Ad5 E1)

– These cells are weakly tumorigenic

– Extensive testing detected no evidence of the presence of adventitious agents

VRBPAC 2001: Issues/Topics on in vitro-transformed neoplastic cells to produce

replication-defective vaccines

VRBPAC 2000: PER.C6 and 293 cells for vaccine manufacture

• Discussed value of these cells for manufacturing vectored viral vaccines

• Discussed role of known mechanism of transformation- including some skepticism that this provided a clear safety margin

• Discussed importance of minimizing steps (i.e. initiation events) toward oncogenesis in vaccine recipients (even if oncogenic outcome is not directly correlated with use of neoplastic cells, it is important to assure that vaccine recipients are not “primed”)

VRBPAC 2001: PER.C6 and 293 cells for vaccine manufacture (continued)

• Discussed adenovirus E1 gene: – low likelihood of uptake in a significant number of cells– involvement in apoptosis– unlikelihood of reaching tumor cell threshold dose

necessary for clinical impact• Discussed whether degree of tumorigenicity was

important– varying opinions

• Discussed approach to TSE issues in neoplastic or retinal cells

• Conclusion that these cells could be used for manufacture of replication defective adenovirus vaccines, with appropriate limitation on residual DNA.

The next slides will summarize the concerns that may be associated with introducing new neoplastic cell substrates

• Tumorigenic cells may form tumors if transferred to a recipient– Has been reported with human cells– Unlikely if cells are non-human, due to

immunological xenograft rejection mechanisms – Addressed by assuring (via validated methods)

absence of intact cells in final product

Concerns regarding use of neoplastic or tumorigenic cells-1

• Special considerations regarding adventitious agents– Adventitious agents that may have induced the original

neoplastic or tumorigenic phenotype may be present in the cells

• Some viruses are known carcinogens in animals and humans

– Neoplastic or tumorigenic cells may have expanded capacity to support viral replication, and thus be more likely to contain agents

– Addressed to date by:• Limiting use of tumorigenic cells to investigational inactivated

vaccines for which high levels of purification is performed

• Expanded testing for oncogenic and other agents

Concerns regarding use of neoplastic or tumorigenic cells-2

• Residual DNA from tumorigenic cells may be infectious or oncogenic– Addressed to date by:

• In vivo oncogenicity testing on cell substrate DNA

• Limitation on quantity of DNA

• Limitation on biological function (i.e., size, other properties) of any residual DNA

Concerns regarding use of neoplastic or tumorigenic cells-3

• Virus-host and Virus-cell interactions: Vaccine virus may package cell DNA or incorporate cell elements that could be oncogenic, thus limiting ability to eliminate these theoretically oncogenic agents from vaccines– Addressed to date by:

• Demonstration that final vaccine preparations do not contain transforming DNA

• Not an issue for cytoplasmic RNA viruses like influenza

• In some cases, inactivation of viral vaccine

Concerns regarding use of neoplastic or tumorigenic cells-4

• Some other mechanism (e.g., oncogenic proteins, RNAs, or other factors that could induce heritable epigenetic changes) associated with immortalization or tumorigenicity could present a risk to the recipient of a vaccine manufactured in tumorigenic cells– Addressed to date by

• scientific consensus that other such mechanisms are very unlikely

• use only of weakly tumorigenic cells

• In vivo testing of cell lysates

Concerns regarding use of neoplastic or tumorigenic cells-5

Concerns regarding the tumorigenicity testing of neoplastic

cells

• Previously used tumorigenicity assays may not adequately define the tumorigenic phenotype or the risk associated with use of tumorigenic cells

Goals for this meeting

• Discussion of the use of MDCK cells, including those that are highly tumorigenic, in manufacture of inactivated influenza vaccines

• Discussion of OVRR approach to evaluate the safety of tumorigenic cells for use in vaccine production

• Discussion of any additional steps CBER should take to address issues associated with the use of neoplastic cell substrates

Today’s talks

• Andrew Lewis: Regulatory implications of neoplastic cell substrate tumorigenicity

• Arifa Khan: Adventitious agent testing of novel cell substrates for vaccine manufacture

• Keith Peden: Issues associated with residual cell substrate DNA

• Manufacturers– Chiron– Solvay