Biopharmaceutical production in transgenic plants: Toward development of regulatory guidelines

The potential of crop plants in the production of recombinant biopharmaceuticals is beginning to be realized. The ease by which foreign genes can be introduced, the use of novel promoters for controlling protein expression, the relative speed at which a single plant can be scaled up to cover acres of land, and the very low cost of biomass creation are all contributing to rapid development of both seed-based and leaf-based crops as bioreactors. At least three plant-produced biologics have entered human clinical trials, and that number will increase dramatically over the next couple of years (1).

Such developments not only have important implications for large-scale biopharmaceutical manufacture but also for the regulation of biological products by public health agencies around the world. Fortunately, much is already known about the genetics and biology of crop plants and about the manufacture of recombinant biopharmaceuticals in mammalian and microbial cells. Regulatory guidance exists on the use of transgenic animals for manufacture of such products.

In June 1998, 23 individuals representing 18 companies and institutions met in Las Vegas, at a workshop to discuss and formulate a white paper document that could serve as a basis for initiating discussions with the Center for Biologics Evaluation at Research (CBER) at FDA. Dubbing itself the "Human Therapeutics in Transgenic Plants" (HTTP) industry group, the participants created a white paper draft, drawing its structure primarily from recently issued Points to Consider (PTC) documents including the 1995 PTC for transgenic animals (2-6). The HTTP group met again in San Francisco during September 1998, and through email and phone conferences, it approved a draft white paper entitled "The Manufacture and Testing of Medicinal Biological Products for Human Use as Derived from Transgenic Plants" in November 1998.

In January 1999, representatives from CropTech Corporation (Blacksburg, VA), Dow AgroSciences LLC (Indianapolis), Monsanto (St. Louis), and Planet Biotechnology (Mountain View, CA) presented the draft white paper to a CBER panel of scientists chaired by CBER director, Kathy Zoon. The draft white paper covered a number of topics, some of which are listed here:

Construction and characterization of the transgene (coding and regulatory sequences);

Creation of transgenic plant seed stock (host plant characterization, method of transgene introduction);

Characterization of plant seed stock (analytical techniques, structural stability of the transgene, stability of protein expression);

Production of transgenic plants (monitoring at greenhouse or field sites including standard agronomic practices and disposal according to USDA/APHIS regulations);

Purification of the transgenic product (recovery from transgenic plants, initial extraction procedures, defining a production lot); and Characterization of the transgenic product (endogenous and exogenous agents in the host plant and product source tissue, whether leaf or seed; analysis of identity, purity, potency and heterogeneity; and lot release testing).

Consensus points emerged from discussions among the HTTP group representatives and CBER scientists. First, seed stock for plant production strains should conform to predetermined expression patterns and levels. Transgene stability studies can be restricted to the particular plant tissue used for production, but if the production process is transferred to another tissue type or changed to include another tissue, new manufacturing specifications (including transgene stability) will have to be determined. Stability of the transgene through subsequent plant generations should be established.

On potential product contamination from insect parts and exogenous infectious agents, the threat is minimal if plants are properly washed before their initial processing. Levels of other chemical agents, such as pesticides, can be measured adequately with existing techniques and should be lower than acceptable levels for food products. All downstream processing after initial processing steps (harvest of plant materials and initial extraction steps) should be performed under full CGMPs. Manufacturers should demonstrate that protein products remain stable during natural "hold" periods in production: for example, field harvest and transport to processing facilities.

The HTTP draft white paper was a good first step in establishing a dialogue between CBER and a new segment of the biotechnology industry. A continuing dialogue is planned, addressing manufacture in which gene expression is virally mediated or clonal propagation is used instead of seeds, and expansion of the scope of the document from transgenic plants to whole plants used as vaccines and therapeutics.

Acknowledgments

The author wishes to acknowledge the HTTP industry subgroup responsible for finalizing the draft white paper: Neil Cowen, Dow AgroSciences; Kent A. Croon, Monsanto; Frank E. Hagie, Applied Phytologics; and Jeffrey Price, Planet Biotechnology.

[Reference]

References

(1) T. Arakawa and W.H.R. Langridge, "Plants Are Not Just Passive Creatures!" Nat. Med 4, 550-551 (1998).

(2) Center for Biologics Evaluation and Research, Points to Consider in the Production and Testing of New Drugs and Biologicals Produced by Recombinant DNA Technology (FDA, Rockville, MD,1985).

(3) Center for Biologics Evaluation and Research, Guidance for Human Somatic Cell Therapy and Gene Therapy (FDA, Rockville, MD, 1998).

(4) Center for Biologics Evaluation and Research, Supplement to the Points to Consider in the Production and Testing of New Drugs and Biologicals Produced by Recombinant DNA Technology: Nucleic Acid Characterization and Genetic Stability (FDA, Rockville, MD, 1992).

(5) "International Conference on Harmonisation. Guidance on Quality of Biotechnological/ Biological Products: Derivation and Characterization of Cell Substrates Used for Production of Biotechnological /Biological Products; Availability" Federal Register 63, 182 (21 September 1998).

(6) Center for Biologics Evaluation and Research, Points to Consider in the Manufacture and Testing of Therapeutic Products for Human Use Derived from Transgenic Animals (FDA, Rockville, MD, 1995).

[Author Affiliation]

-Brandon J. Price is chief executive officer of CropTech Corporation, Blocksburg, VA.