NEWS

New Discoveries Still Abundant in Monoclonal Antibody Research

Ken Garber

In the quarter century since the appearance of the original Nature paper on monoclonal antibodies, optimism about their therapeutic value has waxed and waned. These key immune system proteins have evolved from laboratory novelties to overhyped "magic bullets" to discards on the garbage heap of failed therapies. Only in the last few years, with U.S. Food and Drug Administration approval of Rituxan and Herceptin, has their image as therapeutic agents begun to recover the old shine. Burnished by a series of scientific innovations, monoclonal antibodies may be poised for a renaissance.

Like many breakthroughs, the discovery of monoclonal antibodies was serendipitous, not intentional. César Milstein, Ph.D., an expatriate Argentine immunologist working at the Medical Research Council laboratories in Cambridge, England, was trying to decipher the mechanism of antibody diversity—how the body quickly generated antibodies to foreign substances, or antigens. He used antibody-secreting mouse myeloma cells because normal B cells do not survive long in culture.

Swiss immunologist Georges Kohler, Ph.D., after joining Milstein’s laboratory, tried to grow myeloma cells capable of recognizing antigens (he failed) and then fused myeloma cells to see if they produced antibodies from both parent cells (they did). It then occurred to Milstein and Kohler to try fusing normal B cells with myeloma cells to immortalize the B cells and produce a steady stream of identical (monoclonal) antibodies.

"To our surprise, the experiments were a resounding success," Milstein later recalled. "If my research aim . . . had been the production of monoclonal antibodies, I would never have stumbled on the idea."

‘A Huge Future’

The Nature paper generated little excitement at first, but some researchers immediately grasped the potential of monoclonal antibodies. "I was just over the top when I read it," recalled Ellen Vitetta, Ph.D., an immunologist at the University of Texas Southwestern Medical Center in Dallas. "I read it three times. . . . I realized there was going to be a huge future in using these reagents both diagnostically and clinically." The Nature paper’s last sentence ("Such cultures could be valuable for medical and industrial use") Vitetta called "the understatement of the century."



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Dr. Ellen Vitetta

 
The Medical Research Council, not seeing any commercial potential in monoclonal antibodies, chose not to patent the discovery. "It was a very good thing they didn’t, because it let the whole technology really flourish," said Vitetta. "It probably launched a gazillion dollar industry."

The event that sparked the epochal hope (and hype) of the 1980s was the treatment by Ronald Levy, M.D., of a gravely ill lymphoma patient with a tailor-made mouse anti-idiotype antibody in 1981. The patient made a complete recovery, and is still alive today. Although Levy, a professor at Stanford University’s School of Medicine, later abandoned the anti-idiotype approach, he had shown that a monoclonal antibody could cure cancer. Companies formed to develop monoclonal antibody-based therapies, and the National Institutes of Health poured money into academic translational research. The press touted monoclonal antibodies as "magic bullets" that could target tumors without side effects.



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Dr. Ronald Levy

 
A Shortcoming

Unfortunately, most monoclonal antibodies were a colossal failure as therapy. Truly unique tumor antigens do not exist, so side effects were inevitable and often serious. Antibodies’ large size kept them from penetrating deep into tumors. Attempts to conjugate antibodies with various toxins to add potency made them unstable in the bloodstream and often blocked the antigen binding site, rendering these "immunotoxins" useless.

The worst problem was that monoclonal antibodies came from mice. (Fusing human B cells with mouse myeloma cells does not work because the human chromosomes disappear, and so far no suitable human myeloma cell line has been found.) Mouse-derived antibodies did not mobilize the human immune system very well, and they provoked a strong immune reaction against themselves when injected into people. This blunted their effectiveness, precluded repeat treatments, and made patients sicker.

Once these facts became apparent, industry soured on monoclonal antibodies. "Interest diminished and diminished," recalled National Cancer Institute immunotoxin researcher Ira Pastan, M.D. The early monoclonal antibody companies sold out in the mid-1980s. Failures in sepsis trials and a disastrous 1993 trial of the monoclonal antibody Campath 1H for arthritis further sullied the reputation of monoclonal antibodies. Only one therapeutic monoclonal antibody, OKT3 for the prevention of organ transplant rejection (1986), received FDA approval before 1994.



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Dr. Ira Pastan

 
By the early 1990s, however, innovations in the laboratory had started the pendulum swinging back. Scientists, including Pastan, developed better ways of using recombinant DNA technology to link antibodies to toxins. They also reduced their size to better penetrate tumors. Most importantly, genetic engineering enabled a frontal attack on the mouse problem: In 1984 "chimeric" antibodies, consisting of a mouse variable region and human constant region, were unveiled.

Three years later saw the arrival of "humanized" antibodies, which are entirely human except for the precise antigen-binding site. Finally, in 1996, two companies—Abgenix, Foster City, Calif., and GenPharm, Mountain View, Calif. (now Medarex, Annandale, N.J.), created fully human antibodies from transgenic mice engineered to express human immunoglobulin gene sequences.

"Humanization has really rewritten the book on these [monoclonal] agents," said Ed Sausville, M.D., Ph.D., associate director for NCI’s Developmental Therapeutics Program. "You can give them repeatedly, they are well-tolerated, they successfully deal with the anti-antibody responses, in most cases."

Slow in Popularity

Acceptance took time. In the late 1980s, "there were pockets of people who believed that an antibody could be an effective therapy," recalled John Mendelsohn, M.D., now president of the University of Texas M. D. Anderson Cancer Center, Houston. But the believers, mostly in academia, faced heavy resistance from companies. The story of how oncologist Dennis Slamon, M.D., Ph.D., of the University of California at Los Angeles, personally drove the development of Herceptin despite long hesitation on the part of Genentech, South San Francisco (which owned the drug), is now well known.



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Dr. John Mendelsohn

 
Mendelsohn also struggled to interest a company in his own antibody, a cousin to Herceptin. In the early 1980s, he theorized that an antibody blocking the EGF receptor, a key signaling molecule overexpressed on the surface of about one-third of epithelial tumors, could arrest tumor growth.

"There was a period of about 4 years where we were delayed because the company [Eli Lilly] that had the license really didn’t want to take it forward," Mendelsohn recalled. Eventually, Lilly (Indianapolis) reassigned the patent rights to the University of California. A small New York biotech company, ImClone, licensed the drug, C225, and began clinical trials in the early 1990s.

Now in phase III trials, C225 may prove even better than Herceptin. In one phase II trial for head and neck cancer, C225 combined with radiation achieved a 100% response rate, including complete remissions in 13 of 15 patients. It also worked well combined with chemotherapy. Different treatments may synergize, killing tumors through separate apoptotic pathways, said Mendelsohn.

Wave of Approvals

If C225 wins FDA approval, it will ride a cresting wave of product launches. Since November 1997, the FDA has approved eight monoclonal antibody-based drugs for marketing. The latest is Wyeth-Ayerst’s (Madison, N.J.) Mylotarg, an anti-CD33 antibody linked to the antibiotic calicheamicin, for treating relapsed acute myelogenous leukemia.

Others are on the way. Two radiolabeled antibodies, IDEC Pharmaceuticals’ (La Jolla, Calif.) yttrium-linked Y2B8 and Coulter Pharmaceuticals’ (South San Francisco) iodine-conjugated Bexxar, have completed impressive phase III trials for non-Hodgkin’s lymphoma. Bexxar, for example, has an overall response rate of 70% with complete remissions in 30% of relapsed patients with the especially deadly low-grade form of the disease. (As first-line therapy, Bexxar is even better.) "It is too soon to tell if Bexxar cures anybody, although we have seen . . . ongoing complete remissions of 5 to 7 years in chemotherapy-failed patients," says Bexxar co-inventor Mark Kaminski, M.D., of the University of Michigan, Ann Arbor.

Antibodies linked to natural toxins like ricin or Pseudomonas toxins are not nearly as far along, but some of these immunotoxins have done well. Pastan’s BL22 recently produced complete remissions in six out of seven patients with hairy cell leukemia and could begin a pivotal trial in various leukemias and lymphomas soon.

But serious obstacles remain in linking toxins to the antibodies. Although the linked antibodies are better, "stability of the linkage is going to be an issue" when mass-producing immunotoxins, said Sausville. Many immunotoxins still cause serious toxicity, especially "vascular leak syndrome," a devastating inflammation of the blood vessels. Naked antibodies like Rituxan and Herceptin are safer, but not very effective alone.

"As single agents, they don’t have a lot of activity," said Sausville, who sees far more potential in combination use with chemotherapy and radiation, and eventually together with small molecule drugs and vaccines. Those trials will be expensive and could take decades to complete.

The pharmaceutical industry, historically averse to large molecule drugs like antibodies, may be changing. Abgenix supplies its "Xenomouse" to virtually all big drug companies. "I see [antibody research] only continuing to grow," said Matt Sherman, M.D., head of clinical oncology R&D for Wyeth-Ayerst. "If you have an antigen, and generate an antibody, that very often is the final product. You can generate that quickly, and have it available for clinical testing. And you don’t need to know what the target actually does."

What of the future? Many variations on the monoclonal antibody theme are in advanced development: bispecific antibodies that attach both to tumor and to an effector cell or toxic molecule; antibody dimers, which deliver strong apoptotic signals to the target; new drug-antibody conjugates like Mylotarg; and antibody mimetics, synthetic peptides that more easily penetrate tumors.

"We’ve now got one that’s better than Herceptin," said the University of Pennsylvania’s Mark Greene, M.D., Ph.D. The field "has just become potentially more rich and diverse in terms of the opportunities than certainly we dreamed of even a short time ago," said Sausville. "We’re really limited only by our imagination."

Many of the pioneers, like Vitetta, Mendelsohn, and Greene, are still working. "What keeps me going? I know this is going to work," said Vitetta. "I can see it in my mice, I can see it in my cells. And I know it’s going to help humans. And I just can’t let it go until I prove that."


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