A Method to the mAbNESS?

”I’m thrilled that we already have so many monoclonal antibodies for cancer patients,” says Louis M. Weiner, MD, chairman of medical oncology at the Fox Chase Cancer Center in Philadelphia. “These are remarkable agents that target cancer biology in ways that weren’t previously available.” Next year marks the 30th anniversary of the discovery by Milstein and Köhler of a technique for producing monoclonal antibodies (mAbs). This achievement, for which they won the 1984 Nobel Prize, also helped launch the biotech industry. With the recent approval of ImClone’s Erbitux and Genentech’s Avastin, 17 mAbs have been approved by the U.S. Food and Drug Administration for therapeutic use. What’s the rationale behind mAb treatment? A METHOD TO THE mAbNESS? BY JACK McCAIN Contributing Editor A ntibodies are tiny wisps of protein that are capable of mounting formidable attacks on virtually any foreign protein that invades the body. The immune system that deploys these weapons usually can distinguish friend (self) from foe (nonself), using its “memory” to keep track of potential targets. Immunologic memory is acquired through encounters with foreign proteins that are catalogued in B cells and T cells. Each of these lymphocytes is programmed during its development to recognize one and only one antigen. More precisely, an antibody binds to a specific part of an antigen, known as an epitope. Antibodies of a single kind — monoclonal antibodies (mAbs) — can be produced outside the body, so that great numbers of them can be deployed against a highly specific target. To understand how mAbs work, it’s useful first to review the structure and function of the ordinary immune system-variety antibody. CREATING THE HYBRIDOMA In 1975, the Argentine immunologist César Milstein (1926–2002) and the German immunologist Georges J.F. Köhler (1946– 1995) were working together at the Medical Research Council Laboratory of Molecular PHOTOGRAPH BY DON TRACY Biology in Cambridge, England. As basic scientists, they were attempting to elucidate theories developed by the Swiss immunologist Niels Kaj Jerne (with whom they would share the Nobel Prize). Toward that end, they wanted to produce cells that would survive a long time in the laboratory and produce antibodies of a known and easily identifiable variety. Yet, though they could easily harvest B cells (from mouse spleens) that would produce a desirable antibody, owing to their exposure to a specific protein, these cells did not live long in culture. The researchers also had a line of cancerous B cells (myelomas) that were immortal but did not produce specific antibodies that were easily identifiable. By fusing the harvested B cells with the myelomas, the researchers created a hybrid cell that they called a hybridoma. The hybridoma combined the myeloma cell’s immortality with the B cell’s ability to produce a single kind of antibody. The hybridomas can be isolated and cultured, pumping out antibodies in vast quantities. The overarching principle behind mAb therapy is similar to the concept supporting antisense and RNAi — exploit the discoveries of molecular biology to home in on a target with great precision. In the case of anti- MAY 2004 · BIOTECHNOLOGY HEALTHCARE 53 Recognizing strangers – and fending them off A ntibodies generally are referred to as immunoglobulins (Igs). The Ig molecule is complex in structure, yet its complexity at the genetic level is what gives it versatility — allowing it to recognize one out of more than a million targets. The basic Ig structure is Y-shaped, consisting of four amino acid chains — the building blocks of any protein. Two identical heavy chains, or H chains (Table 1) extend from the base of the stem to the tip of the arms. A molecular hinge allows the arms to flare outward, forming the Y shape. The molecule can be cleaved at the hinge enzymatically, with papain, to form pieces designated as the crystallizable fragment (Fc) — the stem of the Y — and the antigen-binding fragments (Fab) — the arms. Two identical light chains, or L chains extend along each arm. Strong disulfide bonds provide structural support within and between the light and heavy chains. The enzyme pepsin can be used to break the disulfide bonds and separate an Ig molecule into light and heavy chains. Each heavy chain is produced by at least four different genes, each light chain by at least three. The tips of each chain contain variable regions that account for their remarkable specificity. When the stem cell that will become a B cell still is in the bone marrow, the DNA pertinent to the light chain encodes 300 variable (V) sequences and four joining (J) sections. At random, one of the 300 V sections is matched with one of the four J sections, and the remaining DNA is eliminated through excision and splicing. This allows for 1200 (300 x 4) different combinations to be created, one combination per stem cell. Imprecise recombination of the DNA increases the number of variants in the L chain to about 3,000. A similar process of random recombination occurs in the DNA for the H chain. When the resulting antibodies are put on display by the immature B cell, any B cells having antibodies that bind with an antigen are eliminated on the basis that the antigen is “self.” VIRGIN B CELLS The immature B cells that survive the winnowing process are assumed to have produced antibodies that will respond only to foreign antigens, and are released into circulation as “virgin” B cells. Thus, through this process of random recombination of DNA within immature B cells, each surviving B cell contains genes encoding a single antibody, and the virgin B cells as a group can watch for more than 1 million different antigens, though the human genome consists of far fewer protein-coding genes, perhaps as few as 23,000. The humoral immune response, as the antibodybased defense is known, necessitates that B cells and T cells work in concert to recognize an invader. A virgin B cell has numerous antibodies mounted on its surface, like so many tiny antennae all tuned to the same wavelength. The stem of the Y is embedded in the cell membrane with the two arms protruding above, waiting. When the right antigen wanders by, the antibody binds to the epitope. The ensnared protein is taken into the cell’s interior, where it is cleaved into peptides. The fragments are returned to the cell surface by certain proteins (class 2 major histocompatibility complex [MHC] glycoproteins). At this point, the B cell is activated but not quite ready to begin producing antibodies. That requires another set of events involving the T cells or, more precisely, type 2 helper T cells (TH2 cells) that also have been activated. CHOPS IT UP TH2 activation occurs after a macrophage known as an antigen-presenting cell (APC) engulfs the same kind of antigen as has been trapped by the B cell. Like the B cell, the APC chops the antigen into pieces and displays the fragments on its surface with the assistance of MHC class II proteins. This trophy of war is recognized by a T cell receptor (TCR) that is capable of binding with that particular antigen and the associated MHC molecule. Another protein on the surface of the T cell, CD4, abets the binding. After such an interaction with an APC, the T cell is activated. When the activated TH2 cell comes into contact with an activated B cell displaying the same MHC-antigen assembly that the APC displayed, the T cell binds to the B cell and releases chemical signals known as cytokines. The cytokines stimulate the B cell to multiply and differentiate into memory cells and plasma cells. The memory cells are long-lived and stand on the ready, to mount an extremely rapid immune response in the event of a repeat infection. Meanwhile, the plasma cells secrete armies of antibodies that bind to the epitopes of the invading antigens. In some instances, antibodies kill invading cells by inducing programmed cell death (apoptosis). Because an antibody molecule bears at least two binding sites, a single antibody can bind with at least two epitopes simultaneously. Some antigens have two or more identical epitopes, making it possible for cross-linked antibodies to assemble rafts of antigens. Many antigens have different epitopes against which different antibodies have been generated. When a network of antibodies and antigens is formed, the complement system of blood proteins is activated. A bacterium studded with antibodies can become swathed in complement proteins, killing it via the process known as complement-dependent cytotoxicity (CDC). In addition, large antibody-antigen complexes are more readily identified and destroyed by phagocytes, in a process known as antibody-dependent cell-mediated cytotoxicity (ADCC). The cell surface of a phagocyte bears receptors for the “stem” of the antibody, or its Fc fragment. sense and RNAi, the target is messenger RNA (see “Renewing the Attack on mRNA,” BIOTECHNOLOGY HEALTHCARE, March 2004); with a monoclonal antibody, the target is a protein. Yet along with the ability to create mAbs comes the ability, in theory, to direct them against any protein. If you know the amino acid sequence in a protein of interest, you can synthesize a peptide and inject it into a mouse. Then you draw some serum from the mouse and test it with your synthetic peptide to see if the mouse has generated antibodies. If the test is posi- tive, B cells are removed from the mouse’s spleen and then fused with myeloma cells to create a hybridoma. After further screening, the mAbs ultimately are injected into a murine model of the disease in question. If the test is positive, human trials may be warranted. If not, it’s back to the peptide synthesis lab and more mice. mAbs IN THERAPY The mAbs available in the U.S. market are more or less evenly divided between indications for cancerous and noncancerous condi- tions. Rituximab was the first in the former group, gaining FDA approval for treatment of nonHodgkin’s lymphoma in 1997. Since then, six other mAbs have been approved for treating cancer patients, more than keeping pace with the number of the new smallmolecule cancer treatments that have come on the market. “I’m thrilled that we already have so many monoclonal antibodies for cancer patients,” says Louis M. Weiner, MD, chairman of medical oncology at the Fox Chase Cancer Center in Philadelphia. “These are remarkable agents that target can- MAY 2004 · BIOTECHNOLOGY HEALTHCARE 55 PHOTOGRAPH BY LANNY NAGLER For patients who respond to mAb treatment, “The difference is like night and day,” says Ann Parke, MD, a rheumatologist at the University of Connecticut Health Center. “There has been a huge explosion in mAb utilization, and they have made a considerable difference. We’re not seeing the horror stories of 20 years ago.” mAbs cer biology in ways that weren’t previously available.” Weiner explains that it was the relatively recent development of “humanized” mAbs that has allowed mAbs to start coming into their own as cancer therapies. The problem with mAbs of murine origin is that, not surprisingly, they trigger an immune response when administered to humans. That is, human antibodies are generated in response to the murine antibodies. These are known as HAMA, or human anti-mouse antibodies. This characteristic can be exploited in cancer therapy, however, so that a murine mAb bearing a radioisotope (e.g., tositumomab, ibritumomab) doesn’t linger in the body too long after delivering its dose of radiation to the target cell. The targets of the radioisotope-carrying mAbs also need to be carefully selected, so as to pose no threat to normal tissue. In contrast, mAbs that do not bear a radioactive payload have a different set of ideal characteristics, Weiner says. In addition to being humanized, they should bind to a target that is important to the cancer cell’s growth or function, they should mediate antibody-dependent cellmediated cytotoxicity and should cause no significant side effects. In contrast to murine mAbs, a chimeric mAb, such as rituximab or infliximab, contains both murine and human regions. Being approximately one third murine and two thirds human, these chimeras elicit a HACA (human antichimeric antibody) response, which is similar to the HAMA response that is seen with purely murine mAbs. TABLE 1 Properties of immunoglobulin classes IgG IgA IgM IgD IgE Gamma Alpha Mu Delta Epsilon Percent of total Ig 80 13 6 0–0.04 0.00002 Weight (kDa) 190 160 (monomer), 400 (dimer) 900 180 200 Serum halflife (days) 23 5.5 5.0 2.8 2.0 Y-shaped monomer Dimer (secretions) or monomer (serum) Pentamer Monomer Monomer Predominant locations Blood, lymph, cerebrospinal fluid, peritoneal fluid External secretions (saliva, mucus, sweat, gastric fluid, tears) Intravascular space, surface of mature B cells Surface of B cells Surface of basophils and mast cells in saliva and nasal secretions Biologic functions Neutralizes virus, bacteria, toxins; activates complement; directs antibodydependent, cellmediated cytotoxicity; confers maternal immunity on fetus Protects against local infections First Ig class to respond to infection: agglutinates antigens, activates complement Promotes maturation of B cells Protects against parasitic infections H chain Structure SOURCE: Benjamini E and Leskowitz S. Immunology. A Short Course. 2nd Ed, New York: Wiley-Liss. 1991. 56 BIOTECHNOLOGY HEALTHCARE ·MAY 2004 mAbs “Humanized” mAbs, like trastuzumab or omalizumab, are sufficiently human (90 to 95 percent) to generate little or no HACA response. Adalimumab is touted as the first “fully human” mAb, meaning that all its recombinantly engineered components are of human origin. Another problem associated with administration of mAbs is that they are large molecules. At 150,000 Daltons, the typical mAb molecule is 300 times larger than a small molecule such as fluvastatin or fluticasone, with molecular weights of 433.46 and 500.6, respectively. Administration of a mAb, therefore, must be either intravenous, intramuscular, or subcutaneous. IDENTIFYING APPROPRIATE TARGETS A therapeutically viable mAb candidate must be directed against a unique accessible target. “The trick is finding the right target,” says Matthew Rasband, PhD, a neuroscientist at the University of Connecticut Health Center, who makes his own mAbs for basic research on the function of the myelin sheath. If your interest is in developing a mAb to treat cancer, for example, you must identify proteins that are unique to the cancer cell. Moreover, the protein must be on the cell’s surface, because proteins in its interior are inaccessible to a mAb. Hence, the preponderance of therapeutic mAbs (Table 2) that target various CD proteins, which by definition are found on a cell’s surface. CD20, for example, is the molecular target of tositumomab, rituximab, and ibritumomab, all of which are used to treat non-Hodgkin’s lymphoma. This protein is found on more than 90 percent of B-cell non-Hodgkin’s lymphomas. In general, solid tumors offer a less hospitable terrain for mAbs than the more-accessible cancers that arise in the hematopoietic system, Weiner, the oncologist, points out. While solid tumors may produce as much target protein as the hematologic tumors, the solid tumor cells are better protected by other tumor elements such as collagen, malignantly co-opted fibroblasts, and other normal cellular elements. This impairs antibody delivery into larger tumors. But if the mAb’s target is not unique, adverse events become a risk. For example, TNF-alpha is a proinflammatory cytokine that is implicated in Crohn’s disease and rheumatoid arthritis (RA). Infliximab and adalimumab disrupt the signaling ability of TNF-alpha by binding to it, thereby preventing it from binding to its own receptor. Even as TNF-alpha’s pathologic role in RA is thwarted, however, its normal role as a defender against infection could also, in theory, be disrupted. The package inserts for both of these mAbs thus contain “black box” warnings stating that cases of tuberculosis have been seen in patients treated with these products. According to Ann Parke, MD, a rheumatologist at the University of Connecticut Health Center, infection indeed sometimes does complicate mAb therapy for her patients with RA. For example, sinusitis sometimes arises, necessitating the addition of an antibiotic. The target of omalizumab is another antibody, immunoglobulin E (IgE). Like TNF-alpha, IgE has an important role in a disease process while also being part of the body’s natural defenses. IgE makes its presence unpleasantly known to people with asthma and allergic rhinitis when, on exposure to an allergen, it binds to mast cells and triggers the release of histamine, leukotrienes, and other substances that promote bronchial constriction, sneezing, and wheezing. When omalizumab binds to IgE, the IgE is prevented from binding to its receptors on mast cells. Nevertheless, IgE also protects people against parasitic infections, so its use also presents at least a theoretical risk. COSTS AND COVERAGE By the time physicians begin to contemplate prescribing a mAb for a given patient, the risk-benefit ratio usually has tilted pretty far to the benefit side. That’s because mAb therapies are so expensive; they tend to be reserved for patients who have exhausted the more conventional treatments. A single dose of ibritumomab (full course), which is indicated for non-Hodgkin’s lymphoma, is about $28,000. Tositumomab, also for non-Hodgkin’s lymphoma, is comparably priced. A year’s worth of psoriasis treatment with efalizumab runs about $14,000. Treating RA with adalimumab or infliximab might cost from $16,000 to $24,000 per year. These products tend to be approved by the FDA for use only after most other treatments have failed, not that the official indication prevents some mAbs from being used as first-line therapy or as off-label treatment of other conditions. For example, efalizumab is the only mAb with an indication for psoriasis, but infliximab and adalimumab also have been used to treat psoria- MAY 2004 · BIOTECHNOLOGY HEALTHCARE 57 TABLE 2 Monoclonal antibodies with FDA approval for therapeutic use Generic name Trade name Abciximab Adalimumab Therapeutic indications Company mAb type ReoPro Centocor (Johnson & Johnson) Chimeric Humira Abbott Fully human Rheumatoid arthritis Platelet aggregation inhibition Antigen FDA approval Glycoprotein IIb/IIIa receptor 1994 TNF-α 2002 Alemtuzumab Campath Millennium/Ilex Humanized Chronic lymphocytic leukemia CD52 (found on mononuclear blood cells) 2001 Basiliximab Simulect Novartis Chimeric CD25 (IL-2 receptor-α chain, expressed on activated T cells; IL-2 stimulates proliferation of activated T cells) 1998 Bevacizumab Avastin Genentech Humanized Metastatic colorectal cancer Vascular endothelial growth factor 2004 Cetuximab Erbitux ImClone Chimeric Epidermal growth factor receptor 2004 Daclizumab Zenapax HoffmanLa Roche Humanized Prophylaxis of acute renal allograft rejection CD25 (IL-2 receptor-α chain) 1997 Efalizumab Raptiva Genentech Humanized Psoriasis CD11a (α subunit of leukocyte- 2003 function antigen-1 [LFA-a], found on all leukocytes) Gemtuzumab ozogamicin Mylotarg Berlex, ILEX, Millennium Humanized Acute myeloid leukemia CD33 (found on surface of leukemic myeloblasts) 2000 Ibritumomab tiuxetan Zevalin Biogen Idec Murine Follicular non-Hodgkin’s lymphoma, used in conjunction with rituximab CD20 (found on normal and malignant B cells) 2002 Infliximab Remicade Centocor (Johnson & Johnson) Chimeric Crohn’s disease, rheumatoid arthritis TNF-α 1998 Muromonab -CD3 Orthoclone Ortho OKT3 Biotech (Johnson & Johnson) Murine Acute allograft rejection in kidney, heart, or liver transplant patients CD3 (found on all T cells) 1986 Omalizumab Xolair Tanox / Genentech / Novartis Humanized Asthma IgE 2003 Palivizumab Synagis MedImmune / Abbott Humanized Respiratory syncytial virus An antigenic site of F protein of the virus 1998 Rituximab Rituxan Biogen Idec / Genentech Chimeric Follicular non-Hodgkin’s lymphoma CD20 1997 Follicular non-Hodgkin’s lymphoma, refractory to rituximab CD20 2003 Human epidermal growth factor receptor 2 (HER2) 1998 Tositumomab Bexxar and iodine I 131 tositumomab Corixa / Murine GlaxoSmithKline Trastuzumab Genentech 58 Herceptin Prophylaxis of acute kidney transplantation rejection Metastatic colorectal cancer Humanized Metastatic breast cancer BIOTECHNOLOGY HEALTHCARE ·MAY 2004 mAbs sis — in advance of any FDA indication. Infliximab is administered via intravenous infusion, which makes it eligible for reimbursement under Medicare’s current rules, while efalizumab and adalimumab are administered via subcutaneous injection, so they do not qualify for Medicare reimbursement. “Medicare reimbursement definitely can be a deciding factor behind the selection of infliximab,” says Parke. The advantage of being covered by Medicare soon may disappear. Concerned about the effects on their budget, come 2006 — when Medicare begins providing broad drug reimbursement — Medicare officials recently began a review of the off-label use of high-priced drugs. Two mAbs, tositumomab and ibritumomab, were included in the initial review. Both agents are indicated for non-Hodgkin’s lymphoma after other regimens have failed, but they sometimes are used as first-line therapy. As of press time, no decision had been made regarding the curtailment of reimbursement for off-label therapies in the Medicare population. In the meantime, adalimumab’s manufacturer, Abbott, has devised a program to help Medicare enrollees who qualify for adalimumab treatment to obtain the drug at no cost. Other patients with financial constraints can apply for help through the company’s patientassistance programs, which are offered by most pharmaceutical companies to provide needy patients with access to expensive prescription medications of all kinds, not just mAbs. In addition to generating goodwill among patients, the phar- maceutical companies stand to benefit as satisfied patients tell their friends and relatives about their experiences taking a new drug, and as their physicians grow more comfortable prescribing the product. “NIGHT AND DAY” From the perspective of the patient or the physician, mAb therapy might be well worth the going price (and the hassle of obtaining prior authorization, which MCOs invariably require), and then some. Consider the social stigma that leads some people with severe psoriasis to wear concealing clothing yearround, regardless of season — or even to contemplate suicide. Or consider that patients with severe RA may find it difficult or impossible to perform routine activities such as dressing and eating, let alone pursuing a livelihood. “For patients with rheumatoid arthritis who respond to mAb treatment, the difference is like night and day,” says Parke. “There has been a huge explosion in mAb utilization, and they have made a considerable difference. We’re not seeing the horror stories of 20 years ago.” Rheumatologists are caught in a bind, though, Parke says. “We’re bombarded with literature from the biotech companies that suggests we are remiss if we are not using biologics. The argument is that if you can prevent bone erosion, you can prevent joint destruction. Thus the biologics should be used sooner rather than later. But insurers usually won’t approve a biologic unless the patient already has failed on methotrexate.” Parke says methotrexate is a very good drug and not difficult to use. Even if she puts patients on a bio- logic therapy, she tends to keep them on methotrexate, too. In patients receiving infliximab, concomitant methotrexate therapy seems to eliminate the HACA response, she notes. In cancer treatment, combining mAbs with existing chemotherapeutic regimens has been found to be extremely beneficial, Weiner says. Numerous combinations of chemotherapy for aggressive lymphoma have been tried during the past 25 years without finding any regimen that represents an improvement over the others. But when rituximab is added to aggressive chemotherapy, patients’ prospects improve considerably. Likewise, in a woman newly diagnosed with metastatic breast cancer expressing HER-2, which is indicative of a more-aggressive cancer, adding trastuzumab to standard chemotherapy enables the patient to live longer and better, even if her cancer is not cured. Trials are in progress to determine whether trastuzumab therapy is beneficial as early treatment for women with HER-2–expressing breast cancer that is likely to become metastatic. Nevertheless, even as Parke rejoices in the sometimes dramatic benefits of mAb therapy for RA, she expects that long-term experience with the agents will uncover some unpleasant surprises. “We’re going to pay a price somewhere down the road,” she says, which might become manifest in, say, an increase in the incidence of lymphoma. “These are lifelong diseases, and lifelong manipulation of the immune system is bound to have some consequences.” In cancer treatment, Weiner says, MAY 2004 · BIOTECHNOLOGY HEALTHCARE 59 mAbs TABLE 3 Theoretical patient populations for monoclonal antibody indications Therapeutic indication Trade name of mAb(s) with indication Estimated upper limits to U.S. market Generic name ONCOLOGIC AND HEMATOLOGIC INDICATIONS Metastatic colorectal cancer 90,000 (150,000 cases of colorectal cancer at any stage, including 60% that are metastatic on detection) Avastin Bevacizumab Erbitux Cetuximab Metastatic breast cancer expressing HER2 217,000 cases of breast cancer diagnosed annually; 25% of metastatic cases express HER2 Herceptin Trastuzumab Follicular non-Hodgkin’s lymphoma 54,000 new cases of non-Hodgkin’s lymphoma annually; >300,000 prevalent cases, all with tendency to become treatment refractory Zevalin Ibritumomab tiuxetan Rituxan Rituximab Bexxar Tositumomab and iodine I 131 tositumomab Follicular non-Hodgkin’s lymphoma, refractory to rituximab Acute myeloid leukemia 12,000 new cases annually Mylotarg Gemtuzumab ozogamicin Chronic lymphocytic leukemia 8,000 new cases annually Campath Alemtuzumab Rheumatoid arthritis, moderate to severe 2 million prevalent cases, with tendency to progressively worsen Humira Adalimumab Remicade Infliximab Psoriasis 1.5 million U.S. adults with moderate to severe psoriasis Raptiva Efalizumab Platelet aggregation inhibition 600,000 percutaneous coronary interventions performed annually ReoPro Abciximab Crohn’s disease 500,000 prevalent cases Remicade Infliximab Multiple sclerosis 400,000 prevalent cases Antegren (in phase 3) Natalizumab Respiratory syncytial virus (RSV) >300,000 pediatric patients with high risk of RSV infection, mostly owing to premature birth Synagis Palivizumab Asthma (moderate-to-severe persistent asthma, inadequately controlled by inhaled corticosteroids) About 80% of the 17 million Americans with asthma have moderate-to-severe persistent asthma, but only a small subset of these patients fail to respond to inhaled corticosteroids Xolair Omalizumab Acute allograft rejection in kidney, >18,000 transplants annually; heart, or liver transplant patients 60,000 patients on waiting lists Orthoclone OKT3 Muromonab-CD3 Prophylaxis of acute kidney transplantation rejection Simulect Basiliximab Zenapax Daclizumab NONCANCER INDICATIONS 15,000 kidney transplants annually; 52,000 patients on waiting lists patients and physicians are more inclined to accept the risks associated with mAbs, especially in palliative situations, than are clinicians providing decades of treatment for psoriasis or RA. Additionally, the 60 setting of metastatic disease affords opportunities to explore and learn about the characteristics of mAbs, he says, which will become increasingly important as their utilization is extended into patient BIOTECHNOLOGY HEALTHCARE ·MAY 2004 populations with a relatively good long-term prognosis. MORE mAbs ON THE WAY The newest mAbs to receive FDA approval are cetuximab (Erbitux) mAbs and bevacizumab (Avastin), each for treatment of metastatic colorectal cancer. Avastin previously failed a phase 3 trial for metastatic breast cancer. Bevacizumab is indicated as first-line treatment in combination with chemotherapy. In a phase 3 study, the bevacizumab regimen improved median survival time by 4.7 months compared with chemotherapy alone (20.3 months vs 15.6 months). Cetuximab has been shown to shrink tumors, but not to extend survival time. Cetuximab, of course, is the drug that led to legal problems for ImClone’s CEO and his friend Martha Stewart after the FDA declined to review it in 2001. Since then the company has submitted better clinical trial data that finally led to its approval for treating patients who have failed chemotherapy. In addition, Elan and Biogen Idec recently announced their intentions to file for FDA approval of natalizumab in mid-2004 for treatment of multiple sclerosis. Natalizumab, a humanized mAb, would be the first agent in the class of selective adhesion-molecule inhibitors. It is an antagonist of the glycoprotein alpha4 integrin, which is a subunit of alpha4beta1 integrin that is found on the surface of activated lymphocytes and monocytes and is involved in the adhesion and migration of these cells to regions of inflammation. Due to the role of alpha4 integrin in inflammation, natalizumab also is being studied in Crohn’s disease and RA. On a completely different front, Alexion Pharmaceutical’s pexelizumab is in the late stages of development as adjunctive therapy during coronary interventions. This mAb inhibits C5 complement, which mediates inflammatory damage. Alexion also is developing another C5 inhibitor, eculizumab, for paroxysmal nocturnal hemoglobinuria and possibly RA and other indications. Due to the diversity of their targets across multiple conditions, it’s difficult to make meaningful general statements about mAbs in therapy. Each mAb needs to be considered in the context of the full range of therapies available for a given disease. One characteristic that mAbs share with nonbiologic therapies is high research and development costs, but mAbs serve comparatively small potential patient populations (Table 3), so opportunities to recoup development costs are limited. Manufacturers of mAbs therefore try to expand the market by gaining new indications. Because psoriasis, RA, and Crohn’s disease all have inflammation as a common component, it stands to reason that a mAb that has shown efficacy in treating one of these diseases also might be appropriate for the others. This hypothesis has not always been borne out in clinical trials, however. In the meantime, while new indications are pursued, if physicians want to use a mAb for a less advanced stage of a disease than is indicated, or for a different disease altogether, the manufacturers are not likely to object to the off-label use, even if Medicare and MCOs balk. Another trait that mAbs share with other agents is that there is, alas, no way to tell whether they will work in a given patient. But whereas a trial to ascertain the effectiveness of a selective serotonin reuptake inhibitor or a statin in a patient might incur a financial outlay of a couple of hundred dollars, a course of mAb treatment can cost far more. None of this hurt Genentech’s bottom line much last year. Boosted by sales of rituximab ($1.5 billion, up 28 percent from 2002) and trastuzumab ($425 million, up 10 percent from 2002), Genentech’s revenues increased to $3.3 billion. Omalizumab (launched in July) and efalizumab (launched in November), have yet to make much of an impact, with 2003 sales of $25.3 million and $1.4 million, respectively. Investors have been rewarded, too, as Genentech’s stock reached a 52-week high, $99.85, in midFebruary, more than triple its 52week low of $31.53 last March. Meanwhile, Johnson & Johnson posted fourth-quarter sales of $456 million for infliximab, up 20 percent over the fourth quarter of 2002. So there is money to be made in the mAb market, at least for now. Pending changes in Medicare reimbursement policies could curb investors’ enthusiasm, though, either by restricting off-label utilization of mAbs or driving down prices, or both. Neither are competitors likely to let a mAb achieve dominance in a profitable field. They will either develop their own mAbs, or they will develop new biologic or conventional drug therapies that address the same target or pathway. As Centocor has learned, abciximab is not the only agent that inhibits glycoprotein IIb/IIIa. In the meantime, if mAbs make life easier and longer for patients with cancer and other devastating diseases, their grasp on the market will be strengthened as they demonstrate to patients, physicians, and payers that they represent true value. BH MAY 2004 · BIOTECHNOLOGY HEALTHCARE 61