The past failure of monoclonals stemmed in part from the way they were originally made. The classic manufacturing technique was devised in 1975 by immunologists Georges J. F. Köhler and César Milstein of the Medical Research Council's Laboratory of Molecular Biology in Cambridge, England, who were awarded the 1984 Nobel Prize in Physiology or Medicine for their innovation. The basic process involves injecting an antigen--a substance the immune system recognizes as foreign or dangerous--into a mouse, thereby inducing the mouse's antibody-producing cells, called B lymphocytes, to produce antibodies to that antigen. To harvest such antibodies, scientists would ideally pluck only the B cells that make them. But finding the cells and getting them to make large quantities of the antibodies takes some doing.

Part of the complex procedure involves fusing B cells from the mice to immortalized (endlessly replicating) cells in culture to create cells called hybridomas [see illustration]. The drawback of these particular hybridomas is that they produce murine antibodies, which the human immune system can perceive as interlopers. Patients who have received infusions of murine monoclonals have experienced a so-called HAMA response, named for the human anti-mouse antibodies they generate. The HAMA response includes joint swelling, rashes and kidney failure and can be life-threatening. It also destroys the antibodies.

To avoid both the HAMA response and the premature inactivation of mouse antibodies by the immune system, scientists have developed a variety of techniques to make murine antibodies more human. Antibodies are Y-shaped molecules that bind to antigens through the arms, or FAb regions, of that Y. The stem of the Y, the Fc region, interacts with cells of the immune system. The Fc region is particularly important in eradicating bacteria: once antibodies coat a bacterium by binding to it through their FAb regions, the Fc regions attract microbe-engulfing cells to destroy it.

One approach involves replacing all but the antigen-binding regions of murine monoclonals with human components. Four of the monoclonals now for sale in the U.S. are such chimeric--part mouse, part human--antibodies. Among them is ReoPro, made by Centocor in Malvern, Pa., which prevents blood clots by binding to a specific receptor on platelets; it had sales last year of $418 million. (The body usually doesn't make antibodies targeted to healthy tissues, or autoimmune disease would result. But such antibodies, delivered as drugs, can help treat certain disorders.)

Another strategy, called humanization, is behind five more products on the market, including Herceptin, the breast cancer-targeting monoclonal antibody developed by Genentech. Humanization entails using genetic engineering to selectively replace as much as possible of the murine antibodies--including much of their antigen-binding regions--with human protein [see illustration].

Campath--thought by its maker, Millennium Pharmaceuticals in Cambridge, Mass., to be the first humanized antibody ever made--received FDA approval in May for people with B cell chronic lymphocytic leukemia for whom other therapies haven't worked. Campath binds to a receptor found on various types of normal and cancerous immune cells, but patients make more of the normal cells after treatment ends. The other monoclonal on the market is a purely murine antibody.

After more than 25 years of trying, researchers have also finally fused human B cells to immortalized cells to create hybridomas that generate fully human antibodies. In February, Abraham Karpas of the University of Cambridge and his colleagues reported accomplishing the feat, although it is too soon to tell whether the monoclonals made using human cells will be safer, more effective or cheaper to manufacture than those generated using other technologies.

Medarex and Fremont, Calif.-based Abgenix have devised ways to induce mice to produce fully human antibodies. The companies genetically alter the mice to contain human antibody genes; when they inject the mice with antigens, the animals produce antibodies that are human in every way. "The technology to humanize or make fully human monoclonal antibodies was one of those changes that made [monoclonals] more commercially viable," suggests Walter Newman, senior vice president of biotherapeutics and nonclinical development for Millennium Pharmaceuticals, which is also developing monoclonal antibody therapeutics.

Abgenix is conducting clinical tests of a fully human antibody against interleukin-8 (IL-8), a naturally occurring chemical known as a cytokine that normally activates cells of the immune system. When the body produces too much IL-8, inflammatory autoimmune diseases such as rheumatoid arthritis or psoriasis can result. Medarex has a variety of clinical trials of fully human monoclonals ongoing for cancer and autoimmune conditions. It is also developing so-called designer antibodies that have been engineered either to deliver a toxin directly to a diseased cell or to recruit immune cells specifically to attack tumors.

Other investigators are attempting to mass-produce monoclonals without the aid of mice. Cambridge Antibody Technology in England and MorphoSys AG in Munich are using a technique called phage display that does just that--and also helps to find the most specific monoclonals against a particular antigen [see illustration].

Phage display takes advantage of a long, stringy virus called a filamentous phage that infects bacteria. Researchers can isolate DNA from human B lymphocytes (each cell of which makes antibodies against only one antigen), insert this DNA into bacteria such as Escherichia coli and then allow filamentous phages to infect the bacteria. As the phages produce new copies of themselves, they automatically make the proteins encoded by the antibody genes of the various B lymphocytes and add them to the surfaces of newly forming phage particles. Scientists can then use the antigen they intend to target, such as a receptor on cancer cells, to fish out the phages containing the gene for the most specific antibody to that antigen. To produce a lot of that antibody, they can either have one phage infect more bacteria or insert the antibody gene into cultured cells.

Zeroing In on the Targets

Together the newer forms of monoclonals--chimeric, humanized and human--are looking good against an array of diseases. Two such drugs, if they pass muster with the FDA this year as expected, will finally fulfill the dream of deploying so-called conjugated monoclonals--ones that carry radioactive chemicals or toxins directly to tumors as a new form of cancer therapy. Zevalin (developed by San Diego-based IDEC Pharmaceuticals and Schering AG) and Bexxar (devised by Corixa in Seattle and GlaxoSmithKline) both target an antigen called CD20 on the surfaces of B lymphocytes, cells that grow uncontrollably in the cancer known as non-Hodgkin's lymphoma. And both pack a hot punch: Zevalin totes an isotope of yttrium (90Y), and Bexxar carries a radioactive form of iodine (131I).

Many other monoclonals now in clinical trials also target molecules on immune cells that play a role in a variety of diseases. For example, Genentech is in the late stages of testing Xanelim, its monoclonal against CD11a. This protein exists on the surfaces of T lymphocytes and helps them to infiltrate the skin and cause the inflammation of psoriasis, which afflicts an estimated seven million people in the U.S. In a study of nearly 600 psoriasis sufferers that was reported at the American Academy of Dermatology conference in July, researchers found that 57 percent of patients on the highest dose of the drug experienced at least a 50 percent decrease in the severity of their disease. Several companies are also pursuing monoclonals against CD18, a protein on T lymphocytes that underlies inflammation as well as the tissue damage resulting from a heart attack.

A molecule called the epidermal growth factor (EGF) receptor is an additional tempting target for monoclonal developers. One third of patients with solid tumors make excess EGF receptors; indeed, the much heralded small-molecule drug Gleevec, sold by Novartis, interferes with the ability of cancer cells to receive growth signals from those receptors. Anti-EGF receptor monoclonals might best be administered in combination with traditional chemotherapies. At the American Society of Clinical Oncology conference in May, researchers reported that cetuximab, an anti-EGF receptor antibody produced by ImClone Systems in New York City, helped chemotherapy to start working again in 23 percent of patients with advanced colorectal cancer who had stopped responding to chemotherapy alone.

Other companies are focusing on making monoclonal antibodies to molecules on the surfaces of the cells that line the blood vessels. Certain types of these molecules, such as avb3, play a role in angiogenesis, the growth of new blood vessels, which is a crucial step in the development of tumors. A hugely successful monoclonal antibody drug now on the market, Remicade, targets tumor necrosis factor (TNF), a molecule the body uses to juice the cellular arm of the immune system but that also plays a role in inflammatory diseases. According to company reports, Remicade, which is on pharmacy shelves for Crohn's disease (an inflammatory bowel disease) and rheumatoid arthritis, made $370 million last year for its developer, Centocor. Therapies that wipe out TNF have a potential $2-billion annual market, according to Carol Werther, managing director of equity research at the investment bank Adams, Harkness and Hill. (Enbrel, an anti-TNF drug developed by Immunex in Seattle that was approved in 2000 for patients with moderate to severe rheumatoid arthritis, is not technically a monoclonal antibody, because only part of an antibody--the backbone--is used; that backbone is linked to a piece of another kind of molecule, the normal cellular receptor for TNF.)

Emerging Issues

With all these good opportunities facing them, biotechnology and pharmaceutical companies might be expected to be ramping up their production lines in anticipation of a big market surge. But worldwide just 10 large-scale antibody plants are now operating.

Part of the problem is financial: banks don't want to lend the hundreds of millions of dollars it takes to build a state-of-the-art monoclonal production facility unless the likelihood that the plant will generate profits is all but guaranteed. Many of them look back on the 1980s, when drug manufacturers built facilities that operated for years at only partial capacity.

The gold standard for producing monoclonals from hybridomas relies on enormous tanks called bioreactors. V. Bryan Lawlis, chairman of Diosynth ATP in Cary, N.C., estimates that one giant, 60,000-liter bioreactor plant would be able to (hypothetically) accommodate only four products. Assuming that 100 monoclonals will be on the market by 2010, as analysts predict, Lawlis calculates that the industry will need to build at least 25 new facilities or "we can't satisfy all the needs." Those production plants would require $5 billion or more and between three and five years to be built and certified by the FDA--a prospect no one thinks is going to happen.

To fill the void, some companies are turning to transgenic animals and plants, those organisms engineered to carry genes for selected antibodies. Transgenic mammals that secrete monoclonals in their milk can generate one gram of antibody for roughly $100--one third the cost of traditional production methods, industry officials say. Centocor and Johnson & Johnson are looking into producing Remicade using transgenic goats, and Infigen in DeForest, Wis., intends to make monoclonals in cow's milk, although no such products have yet received FDA approval. Moreover, it isn't clear how many companies will be willing to turn to these transgenic options over the standard bioreactors.

Newman concedes that transgenic animals are attractive alternatives, but he adds that companies must still undergo the sometimes tedious step of isolating the monoclonals from the milk proteins. "You have purification problems, but you don't have the expense of 10,000-liter bioreactors," he says. Genetically engineering and breeding the animals can also take years.

Mich B. Hein, president of Epicyte in San Diego, sees green plants as the answer to the monoclonal production shortfall. "It's pretty clear that the production facilities will not meet the demand for even the most promising molecules," he says. Plants have the advantages of being economical and easily scalable to any level of demand: they can yield metric tons of monoclonal products. But purification problems remain, and it is still unclear how the FDA will regulate pharmaceuticals produced by transgenic plants.

Epicyte has teamed up with Dow to produce corn plants able to generate monoclonal antibodies that will be formulated as creams or ointments for mucosal surfaces such as the lips and genitalia or as orally administered drugs for gastrointestinal or respiratory infections. Hein predicts that by the end of next year Epicyte will seek FDA clearance to begin clinical trials of corn-produced monoclonals to prevent the transmission of herpes simplex between adults and during childbirth. The company is also developing monoclonals that bind to sperm as possible contraceptives, as well as antibodies that could protect against human papillomavirus, which causes genital warts and cervical cancer.

Whether they come from cattle, goats, corn or bioreactors, monoclonal antibodies are set to become a major part of 21st-century medicine.

Further Information:

Monoclonal Antibodies: A 25-Year Roller Coaster Ride. N. S. Halim in The Scientist, Vol. 14, No. 4, page 16; February 21, 2000.

A Human Myeloma Cell Line Suitable for the Generation of Human Monoclonal Antibodies. A. Karpas, A. Dremucheva and B. H. Czepulkowski in Proceedings of the National Academy of Sciences USA, Vol. 98, No. 4, pages 1799-1804; February 13, 2001.

Biotech Industry Faces New Bottleneck. K. Garber in Nature Biotechnology, Vol. 19, No. 3, pages 184-185; March 2001.

Abstracts of scientific presentations at the 2001 annual meeting of the American Society of Clinical Oncology are available at virtualmeeting.asco.org/vm2001/

The Author

Carol Ezzell is a member of the board of editors.