While there is great interest in alternative production hosts, such as yeast, transgenic animals, and bacteria, the only FDA-approved antibody products are generated in mammalian cells grown in bioreactors. Options discussed at the meeting included Lemna, or duckweed, and the use of transgenic goats, corn, and chicken eggs.
In Centocor's case, the commitment to build a large-scale production facility was made years ago, during Phase I evaluation. "Even in the best of situations there always is an element of risk," Dr. Siegel said. "However in this instance it turned out to be a good choice."
The regulatory picture today is especially complex because of the need to meet both European and American criteria. "While the U.S. regulatory situation is streamlined, the European regulatory climate can be challenging due to the inconsistent implementation of new regulations among all member states."
Dr. Siegel expressed praise for the FDA as a good business partner, and feels FDA officials have gone out of their way to make the process as efficient as possible. The FDA will face the challenge of evaluating alternative antibody production technologies in the coming years, as transgenic animals and plants are considered.
"Prions are an issue of great concern in proteins produced in transgenic mammals," he said. "There probably won't be an acceptance by the public until a transgenic protein has moved through the entire regulatory process to the market place."
Dr. Siegel's remarks were reinforced by comments made by Jim Cornett, Ph.D., vp for business development at Medarex (Princeton, NJ). "When it comes to antibody targets, the low hanging fruit is gone," he stated, "companies need to be more innovative in identifying novel targets for new antibody-based therapeutic products."
By developing simpler technologies, such as disposable inserts for bioreactors, antibody expenses may be driven down considerably. In this respect, transgenic plants may prove to be of particular value in squeezing the cost of antibody production.
"We're also concerned with purification costs," Dr. Cornett added. "Protein A is costly, and alternate chromatographic methods could greatly lower downstream purification outlays."
Dr. Cornett is particularly intrigued over the prospect of polyclonal recombinant antibodies. "The technology under development by Symphogen and Merus is especially appealing, as it combines the virtues of polyclonal antibody's potential efficacy with the ability to engineer recombinant antibodies," he said.
"This may be a promising technology for the area of infectious diseases as well as cancer. In the latter area, a cure is not required if the anticancer therapy can extend the patient's life to a normal lifespan."
One of the main barriers to effective antibody therapy is the lack of specific antigenic targets in cancer cells. Whereas cancer cells express a variety of proteins not seen in normal cells, these differences are mainly quantitative, and obtaining truly cancer-specific markers is virtually impossible.
An unusual approach to new anticancer therapeutic antibodies was discussed by Jon Weidanz, Ph.D., chief scientist at Receptor Logic (Amarillo, TX).
"We have approached the problem of developing antibodies that behave as T-cell receptor mimics," stated Dr. Weidanz. The major histocompatibility complex proteins, located on many cell types, display specific peptides, forming a unique complex that has the potential to behave as a specific cancer marker.
"An antibody generated against a peptide-MHC complex will behave as a mimic of the T-cell receptor (TCR). This is because in the course of a normal immune response, protein antigens are broken into peptides that bind to the major histocompatibility complex proteins.
"This complex, in turn, is recognized by one of myriad different T-cell receptors, and the association of the MHC-Peptide with its matched T-cell receptor sets in motion a cascade that will result in the immune response.
"We reasoned that antibody mimics to the T-cell receptor could be used in a wide variety of therapeutic functions, including cancer cell targeting for therapy and imaging, and presentation of drugs to tumor cells," Dr. Weidanz stated.
It had previously been shown that a translation initiation factor known as eIF4G was overexpressed in many cancer cells, with the result that peptides from its breakdown traveled to the cell surface bound to the major histocompatibility complex proteins. Dr. Weidanz and his coworkers were able to engineer similar complexes and employ them as antigens.
In the past, the approach of targeting the MHC-receptor complex in cancer cells has not been particularly successful, so the Receptor Logic group was determined to use a new technique. The Receptor Logic strategy of selecting promising peptides, such as the eIF4G peptide, provides a unique way for isolating responsive epitopes.
The Receptor Logic team generated antibodies against the complexes using a proprietary method, which included a specially formulated peptide-MHC immunogen, an optimized number of immunizations, and development of highly specific and sensitive screening assays.
While over production of the MHC-eIF4G complex is a recognizable feature of HIV infection, antibodies to the MHC-eIF4G complexes bound to only a limited number of cancer samples, as demonstrated by flow cytometry, using the antibody mimic.
Another well-known marker, Her2/neu, present in a wide range of breast cancer cells, was selected as a candidate. A TCR mimic antibody was generated against one of the Her2/neu peptides-MHC complexes, and the antibody was used to predict the presence of Her2/neu epitopes in a sample of cancer cell lines. In theory, a number of overexpressed cancer-related antigens could behave as markers for the T-cell receptor.
The antibodies that mimic the T-cell receptor are novel tools for target validation, and they have the potential to perform as a unique therapeutic tool, by arming them with drugs that when internalized, kill the appropriate cancer cell. The Receptor Logic group is pursuing this tactic with both cell culture and animal model investigations.