Immunotoxins: Monoclonals At Work In Fight Against Cancer

Since the discovery of monoclonal antibodies in 1975 (see pages 22 and 29), scientists have attempted to harness their power to bind selectively with substances in the body. In the late 1970s and early 1980s, researchers sought to combine potent cell toxins with a monoclonal antibody that would bind with cancer cells. They believed that the resulting immunotoxin (IT) could then be launched as a kind of surgical strike on cancer cells. But, as is the case with many new ventures in science, things didn't work out quite as smoothly as expected. "Back in the late 1970s, we thought that any toxin could be bound to any antibody, and the immunotoxin would be potent and specific," recalls Ellen S. Vitetta, director of the Cancer Immunobiology Center at the University of Texas Southwestern Medical Center in Dallas. Early studies, however, quickly indicated that IT therapy involves a variety of complicating factors. While not strictly immunotoxins, related targeted toxins using DNA techniques are beginning to appear. These third-generation molecules use the diphtheria toxin. Scientists have been able to delete the gene segments encoding the cell-binding domains of these toxins. They then splice the genes to other genes encoding certain growth factors, such as melanocyte stimulating hormone (MSH), epidermal growth factor (EGF), or interleukin-2 (IL-2). The newly formed hybrid genes are then inserted in E. coli, which produces the cell receptor specific toxin in large quantities. The growth factor portion of the toxin acts as the guidance system, leading the toxin to specific cancers or other diseases with cells that have growth factor receptors on their surface. For example, the IL-2 receptor is expressed on adult T-cell leukemia cells and the MSH receptor is expressed on melanocytes, the cells that cause malignant melanoma. Some of these genetically engineered growth-factor toxins may prove to be more effective than the monoclonal antibody immunotoxins because when they bind to the target cell, the cell always ingests them and carries the toxin across the cell membrane. Monoclonal antibody ITs are not absorbed in the same way, and that makes their cytotoxicity variable. Moreover, a genetically engineered toxin is easier to produce consistently, while a chemically coupled IT can combine in different configurations in some cases and result in ITs with different efficacies. "We now have the technology to build genes encoding proteins that have never been made before," says Jack Murphy of Harvard University Hospital in Boston. Murphy has developed a diphtheria/IL-2 toxin. "By design, these proteins can do a specific task - one that can't be done by naturally occurring molecules." --L.H. Many of the earliest ITs were prepared with the toxin ricin, which is produced by the beans of the plant Ricinus communis. Ricin consists of two chains, A and B. The A-chain kills a cell by interfering with protein synthesis, while the B-chain contains the toxin binding site, allowing ricin to bind to any cell. In creating the early ITs, scientists separated the binding B-chain of ricin from the toxic A-chain. They then joined the A-chain to a monoclonal antibody through disulfide linkage. The monoclonal antibody was to act as the toxin's guidance system, leading the IT to tumor-specific cells. Although these first-generation ITs did well in vitro, early animal studies were not always successful. Investigators discovered that enzymes in the blood or tissues cleaved the disulfide link, which stripped the poison from the antibody guidance system. Researchers then designed stronger chemical ties between the antibody and the toxin. Another difficulty encountered was the liver's propensity to steal the ITs away from circulation before they could reach the target cells. Liver cells have receptors for mannose-rich carbohydrates, which are found on the A-chain of the ricin toxin. Researchers overcame this difficulty by enzymatically removing the carbohydrate residues from the ricin A-chain. ITs of this type have greatly reduced liver uptake and, consequently, more of the IT can reach its target cells. A third problem was that different monoclonal antibodies had differing degrees of potency. Scientists solved this drawback by developing predictive screening assays to determine which antibodies would make the best immunotoxins. But perhaps the greatest problem in the use of ITs is immunogenicity. After being exposed to the IT, the body triggers an antibody response to both the monoclonal antibody and the toxin that renders the IT inactive after a single dose. One solution to this problem is administering immunosuppressive drugs to prevent the immune response. While immunosuppression may eliminate anti-IT antibody response, it may also reduce the ability of the host's immune system to fight remaining tumor cells. Studies are currently under way to establish a method of preventing antibodies from attacking ITs while also allowing the immune system to work with the immunotoxins to keep any remaining tumor cells in check. Another way to defeat the body's anti-IT response is the use of different types of ITs, sending a variety of toxins against the same tumor cells in a course of repeated therapy. Each dose would not interfere with the next round of IT administered. Many of the problems encountered early on have been overcome. "We've built better molecules," Vitetta says. "But," she cautions, "we haven't untangled all the complexities yet." Clinical trials of second-generation ITs are now in progress. In cancer therapy, ITs will probably be most successful in treating B-cell lymphoma and some leukemias - essentially those cancer cells that are accessible through the bloodstream. But ITs are also now being considered for therapeutic use against autoimmune diseases, graft-versus-host disease, inflammatory diseases, allergies, parasitic infections, and other ailments in which the cells of the immune system can be targeted. In these cases, it may not be necessary to kill every target cell to achieve a beneficial effect. Although the hoped-for single magic bullet (a term coined by German scientist Paul Ehrlich to describe a targeted cytotoxin) has yet to be found after a decade and a half of feverish research, scientists have clearly made great progress. The accompanying table, supplied to The Scientist by Vitetta, lists both significant developments and key papers in IT research over the last decade. The papers document the history of this research from the initial linking of a monoclonal antibody to the ricin A chain, to the use of carboxylic ionophores to enhance toxicity and disulfide linkages to increase stability, to the first clinical trials. It is important, Vitetta says, to understand that the papers listed represent a selection only; many others might have just as well been included, given more space. Those that are listed are ranked within each grouping by total citations recorded in the Philadelphia-based Institute for Scientific Information's Science Citation Index from the paper's year of publication through 1989. Plainly, the older a paper is, the more opportunity it has had to collect citations. "In the future," Vitetta says, "immunotoxins will probably be used in conjunction with other therapies to treat disease." As such, they will constitute a potent new weapon in the physician's arsenal. Lisa Holland is a freelance science writer based in New York City.

Immunotoxins: Monoclonals At Work In Fight Against Cancer

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