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Introduction to Monoclonal Antibodies

Date: December 2, 2002

by Chaya Venkat

Magic Bullets and Target Practice

Related Articles: Review of Monoclonal Antibodies in Development

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What Exactly Is a Monoclonal Antibody?

Here is a short summary of the history and science of monoclonal antibodies, affectionately called MABs or MOABs.

MABs have been around for a while, since the 1980's, at least in concept and theory. They were supposed to act like smart bombs, homing in on malignant cells and wiping them out while leaving normal cells intact. There was the additional possibility of arming the MABs with radio-isotope tips, and once the MAB had homed in on the cancer cell, the radio-isotope would selectively poison and destroy the cancer cell. The main attraction of MABs for pharmaceutical companies is that they are easier to develop than traditional drugs composed of small, inorganic molecules. Estimated cost to bring a monoclonal antibody through the various steps of clinical testing to the point where the FDA approval can be sought is about ten times less than that needed for a traditional small molecule drug.

Life and fight against cancer is never simple: in the early trials, people who received MABs often developed their own antibodies against the MABs, and the side reactions of this were often quite serious. Another problem was that the patients' livers often targeted the MABs as dangerous and foreign objects, and went about destroying and excreting them before the drugs had a chance to do their job. We learned a lot from those early trials, and it now looks like the day of the MABs is finally come. About a dozen MABs have reached the market, including the first one, Zevalin, that would be equipped with radioactive isotope of yttrium.

This is the general process of how MABs are made: let us say we are interested in creating a MAB to attack a specific antigen. This antigen is injected into a mouse. The mouse's immune system, its B-cells, recognize this antigen as dangerous and foreign and produce antibodies to fight it. Researchers then isolate these antibodies, and fuse them onto endlessly replicating cells in a culture, creating what is called a hybridoma. It is the hybridomas that produce the MABs.

In our example, the MAB produced is a "murine" (fancy word for "mouse") antibody. Sometimes, the murine MABs can cause severe side effects such as rashes, swelling, and kidney problems due to rejection of the MAB by the patient. This effect is called HAMA response, or "human anti-mouse antibody" effect. Researchers have developed many ways of making the murine MABs more human, by replacing much of the antibody with human components. MABs which have both mouse and human components are called "chimeric" MABs. Another approach is to use genetically altered mice so that they produce antibodies that are human in every way.

Part of the problem is that the state-of-the-art production facilities for MABs is severely limited. Some companies are attempting to make the MABs by using genetically transformed animals and plants. It may be possible, for example, for a transgenic mammal to produce MABs in its milk, at a significant reduction in cost.

In my next few articles, I will try to discuss how Campath and Rituxan work, what makes them different from straight chemotherapy, their advantages and potential drawbacks. You can read more about MABs in general at the site below. These are very readable and informative articles in the Scientific American. A basic introduction is provided in the Wikipedia link.

Magic Bullets Fly Again (Scientific American feature article by Claire Panosian dated October 2001)
Scientific American: The Mice That Warred (Scientific American feature article from June 2001 on the business of developing antibodies). Natural selection picks the best antibodies to fight invading microbes — and it also determines who survives to sell these molecules as drugs.
Wikipedia Entry on Monoclonal Antibodies: This resources has lots of links and references.
Monoclonal Antibodies from Kimball's Biology Pages: Lecture format discussion of various aspects of monoclonal antibodies.

Monoclonal Antibodies in Action

We have come a long way from the non-selective chemotherapy drugs of past decades, drugs that killed the cancer cells but in the process caused so much damage to the rest of the systems. Very often, the cure was almost as bad as the disease. Cancer patients were taken to the very brink of death, in order to kill as much of the cancer as possible, with doctors keeping their fingers crossed that the game of chicken comes out in favor of the patient, that the cancer dies before the patient does. 

Monoclonal antibodies have changed the rules of the game in our favor. It is now generally accepted that the majority of the cell killing gets done by the body's own immune system, or if that is not up to the task, by co- administered chemotherapy agents such as Fludarabine. The targeting power of monoclonal antibodies is very powerful. The great value of Rituxan lies in the fact that the CD20 marker, which is the marker tagged by this monoclonal antibody, is expressed only by mature B-cells. Immature B-cells, plasma cells, other immune system cells such as T-cells, neutrophils etc, or red blood cells, platelets or most importantly, the stem cells, do not express this CD20 marker and are therefore spared. 

We have seen that there is exceptionally good toxicity profile for Rituxan when it is used as a single agent. However, it has also been shown that Rituxan does not give very good results in late stage and heavily pre-treated CLL patients, probably because these patients have been through the wars and their immune system is no longer efficient in killing the cancer cells tagged by the Rituxan. Hence the need for combo therapies such as RF and RFC for this patient population. Rituxan also does not work well in patients whose CLL cells do not have sufficient density of expression of the CD20 marker. 

Two drugs are on the market now, called Zevalin and Bexxar, which take the concept of the anti-CD20 monoclonal one step further: both of these monoclonals attach themselves to CD20 positive cells, just like Rituxan, but in addition they also carry a pay load of radioactive material. It is logical that in this approach the cell kill does not need the assistance of the immune system or co- administered secondary drugs, the radioactivity associated with the Zevalin or Bexxar is enough to do the job. 

Both of these new drugs are showing good promise in NHL, but are contra-indicated for CLL if the patient has significant bone marrow involvement. That is easy to understand: the radioactivity associated with both of these drugs kills not just the CD20 positive cell to which the drug is attached, but also any neighboring cells that happen to be close to it. If your bone marrow has a lot of cancer cells in it which carry the CD20 marker, it is clear that the Zevalin or Bexxar will home in on those cells, and in the process of killing the cancer cells, most likely the drugs would also kill too many valuable stem cells. In the case of NHL, where there is usually only very small level of bone marrow infiltration, this is less of a problem.

Now comes a new approach. What if the payload attached to the monoclonal is not a radioactive material, capable of killing everything in the neighborhood, but a poison, that kills only the local cell to which it is attached? This way we can get around needing an assist from the immune system to do the killing, as Rituxan does, and away from the non-selective killing imposed by drugs such as Bexxar and Zevalin. 

Many lymphomas and leukemias express the marker CD25 (also called IL2 receptor). These include Hodgkin's disease, chronic lymphocytic leukemia, adult T cell leukemia, hairy cell leukemia, and cutaneous T cell leukemia. To create a cytotoxic agent that will eradicate these tumors, genetic engineering techniques were used to fuse a portion of an antibody that binds to the CD25 marker on such cancer cells, with a genetically modified form of a powerful bacterial toxin (Pseudomonas exotoxin A). This new drug, termed LMB-2 has been designed and produced at the National Cancer Institute and is now undergoing clinical trials at the NIH. So far, the results have been quite impressive in T-cell leukemia, and hairy cell leukemia ( see second abstract below). But in the case of CLL, the results are not as good because the CLL cells do not have as high a density of CD25 markers per cell as the other two cancers. 

My sense of optimism regarding the future rests not on the easy victories we have won, but the way we are able to build on each hard won battle, and how we are able to use each piece of information we  obtain to cross the next hurdle. Not enough CD25 expression? Not a problem, we now seem to know how to pre-treat the CLL cells with a drug called DSP30, so that the cells express more CD25 markers per cell. The idea is to light up the CLL cells with much higher density of CD25 markers per cell, then hit them with the LMB-2. Up- regulation of the CD25 marker by DSP30 made the CLL cells very much more susceptible to killing by LMB-2 (see abstract below). Since most normal B-cells and T-cells do not exhibit CD25 marker, LMB-2 has the potential of being even more selective and targeted in its killing action than Rituxan, and likely to be of therapeutic value even in heavily pre-treated patients with damaged immune systems. Best of all, we seem to have a lead on how to improve the CD25 expression on CLL cells, so that they become better targets for this valuable drug. 

This development is worth watching.


Blood 2002 Feb 15;99(4):1320-6

Sensitization of B-cell chronic lymphocytic leukemia cells to recombinant immunotoxin by immunostimulatory phosphorothioate oligodeoxynucleotides. 

Decker T, Hipp S, Kreitman RJ, Pastan I, Peschel C, Licht T. 

3rd Department of Medicine, Technical University of Munich, Munich, Germany. 

A recombinant anti-CD25 immunotoxin, LMB-2, has shown clinical efficacy in hairy cell leukemia and T-cell neoplasms. Its activity in B-cell chronic lymphocytic leukemia (B-CLL) is inferior but might be improved if B-CLL cells expressed higher numbers of CD25 binding sites. It was recently reported that DSP30, a phosphorothioate CpG- oligodeoxynucleotide (CpG-ODN) induces immunogenicity of B-CLL cells by up-regulation of CD25 and other antigens. The present study investigated the antitumor activity of LMB-2 in the presence of DSP30. To this end, B-CLL cells from peripheral blood of patients were isolated immunomagnetically to more than 98% purity. Incubation with DSP30 for 48 hours augmented CD25 expression in 14 of 15 B-CLL samples, as assessed by flow cytometry. DSP30 increased LMB-2 cytotoxicity dose dependently whereas a control ODN with no CpG motif did not. LMB-2 displayed no antitumor cell activity in the absence of CpG-ODN as determined colorimetrically with an (3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-  tetrazolium, inner salt (MTS) assay. In contrast, B-CLL growth was inhibited in 12 of 13 samples with 50% inhibition concentrations (IC (50)) in the range of LMB-2 plasma levels achieved in clinical studies. Two samples were not evaluable because of spontaneous B-CLL cell death in the presence of DSP30. Control experiments with an immunotoxin that does not recognize hematopoietic cells, and an anti- CD22 immunotoxin, confirmed that sensitization to LMB-2 was specifically due to up-regulation of CD25. LMB-2 was much less toxic to normal B and T lymphocytes compared with B-CLL cells. In summary, immunostimulatory CpG-ODNs efficiently sensitize B-CLL cells to a recombinant immunotoxin by modulation of its target. This new treatment strategy deserves further attention.

PMID: 11830482

The anti-CD25 recombinant immunotoxin LMB-2 shows antitumor activity in patients with cutaneous T-cell lymphoma. 

Year: 2002 Abstract No: 33 Category: Molecular/Ligand Targeted Therapies 

Author(s): David R Squires, Wyndham H Wilson, Diana O'Hagan, Thomas A Waldmann, Ira Pastan, Robert J Kreitman

National Cancer Institute,    Bethesda, MD. 

LMB-2 is an anti-CD25 recombinant immunotoxin containing variable domains of MAb anti-Tac and truncated Pseudomonas exotoxin. LMB-2 induced major responses in patients with CD25+ chemotherapy-resistant hematologic malignancies, including 4 with hairy cell leukemia and one each with adult T-cell leukemia, Hodgkin's disease, chronic lymphocytic leukemia, and cutaneous T-cell lymphoma (CTCL) (Kreitman et al., J Clin Oncol, 18:1622, 2000). New patients have been enrolled to investigate the use of LMB-2 combined with prophylactic IV fluid to prevent hypotension and renal toxicity as a result of 3rd spacing of fluid. A total of 12 patients have received the maximum tolerated dose of LMB-2, 40 ug/Kg IV given every other day for 3 doses (QOD x3), all without dose limiting toxicity. The most common adverse events are transient fever and transaminase elevations. Two out of 2 patients with CTCL responded to LMB-2. One CTCL patient had Sezary syndrome (stage IVa) with erythroderma, pruritis, and a circulating Sezary count of 1000 cells/ul. She responded to the first cycle of LMB-2 with a 97% reduction of sezary cells by day 3 and regression of symptomatic disease. The response was maintained > 6 months with 6 repeated cycles of LMB-2. Ex vivo incubation of the patient's peripheral blood mononuclear cells (containing 50% sezary cells) with LMB-2 resulted in a maximum protein synthesis inhibition of 33% with half-maximal inhibition at 0.2 +/- 0.09 ng/ml. This was comparable to the IC50s of LMB-2 toward hairy cells, and ~0.1% of plasma levels of LMB-2 achieved in patients. Cytotoxicity was specific for CD25 and not produced by BL22, an anti-CD22 immunotoxin. A second CTCL patient (stage IIb) with 8 years of pruritis throughout previous therapy had resolution of pruritis by day 3, flattening of tumors and 50% clearing of malignant cells by skin biopsy obtained on day 8. The Cmax values of LMB-2 were 300-450 ng/ml with half-lives of 206-228 minutes in the CTCL patients. Because LMB-2 is cytotoxic to cells expressing CD25 without the other IL-2 receptor subunits needed to form the high affinity receptor, CD25+ CTCL patients are good candidates for further testing.

Antibody Therapy in CLL

If you wish to know the state of the art in monoclonal therapy for CLL, this  paper has all the details. After reading it, I came away with mixed feelings. It is sort of like the half-full glass versus half-empty glass perspectives. 

It is encouraging to see how much work has already been done in testing usefulness of Campath (ant-CD52 monoclonal) and Rituxan (anti-CD20 monoclonal). Bulk of the paper is devoted to these two monoclonal antibodies. It is also disheartening to see how thin the trickle in the pipeline is, as far as other monoclonals are concerned. Rituxan and Campath are the half-full glass part of our present situation. But, as we have discussed in many previous articles, Rituxan does not work effectively for every one. Campath is yet to receive widespread use, because of its strong tendency to suppress the immune system. True, there are several promising combinations of Rituxan and/or Campath with standard chemotherapy drugs, where the combinations seem to be synergistic, more than just simply adding up the responses obtained by the individual drugs of the combinations. But if like me you thought there was a huge explosion of new and improved monoclonal antibodies in the pipeline, over and beyond Rituxan and Campath, this article confirmed my worries that the pickings are slim. 


CD52, the target marker of Campath, is strongly expressed by lymphocytes (B-cells and T-cells), as well as monocytes, macrophages and eosinophils. Stem cells, red blood cells and platelets do not express CD52 marker. The heavy-duty immunosuppression and myelosuppression seen in Campath therapy is due to the wide variety of immune system cells that are attacked by this monoclonal antibody, in addition to the B-cells. The present generally accepted dose for Campath is 30mg three times a week, for approximately 12 weeks. Drug administration may be at reduced levels initially, spaced out over a few days, till the infusion-related toxicity subsides. It is strongly recommended that the patient be protected against opportunistic infections during and after Campath therapy, some of the drugs used are trimethoprim, sulfamethoxazole and acyclovir. 

In late stage and previously treated patients, many of whom were refractory to other chemotherapy drugs, the overall response rate was 33%, with only 2% of the patients receiving a CR (complete response). CLL cells were rapidly cleared from the peripheral blood for most of the patients, and about a third of the patients also had clearance of the bone marrow. But lymph nodes were harder to handle, none of the lymph nodes larger than 5 cm were cleared, and only 15% of those between 2 cm and 5 cm were cleared of CLL. Infections were common, in about 55% of the patients. Half of these were mild to moderate, but the other half were serious infections. The median response lasted under 9 months. 

The story looks a lot better if the patient group is previously untreated. Here the overall response rate was 87%, with 19% CRs. Much better than the statistics I quoted above, and to be expected in this group of "healthier" patients. Also, since in this study Campath was administered subcutaneously, the infusion-related toxicity seems to be significantly reduced. You can get a lot more of the detailed statistics from the paper. 

The key points to take away from this section:

  1. Campath targets CD52 marker, which is exhibited by many of the immune system cells in addition to the B-cells.
  2. The generally accepted standard dose is 30mg, three times a week, for 12 weeks. 
  3. Deep immune suppression is a fact of life with Campath therapy and severe, opportunistic infections are possible unless the patient is carefully supported during and after Campath therapy with anti-bacterial, anti-viral and anti-fungal medications. This is not something to take lightly, people. If you are going to go for Campath therapy, be very sure to follow your oncologist's recommendations on taking these supporting medications, no ifs or buts. 
  4. If you have large and bulky lymph nodes, larger than 5 cm, it is unlikely that Campath will give you a clean bill of health on these sites of the cancer. It does a better job on cleaning out the bone marrow. 
  5. As is to be expected, Campath gives much better response statistics in "naive" (previously untreated) patients than in those who have been through prior chemotherapy. But I guess the question here is why would a "naive" patient go for Campath as frontline therapy, without exploring other, less immunosuppressive options; this might be the case if one is truly naive, I expect. 



The next section deals with the details of the many clinical trials carried out using our favorite monoclonal, Rituxan. Since many of these have been discussed in previous articles, I will not go into the details, but this paper will be a handy reference for you if you want to have the details nicely tabulated in one place. Specifically, the clinical trials conducted by John Hainsworth ("naive" CLL patients), Susan O'Brien (previously treated patients, dose escalation), John Byrd (increased number of doses of Rituxan per week) and Deborah Thomas (early stage and naive patients) are discussed in detail. 

Compared to the Campath therapy, there were a lot less infusion related toxicity, and a lot less immunosuppression or myelosuppression. The response rates were much better in naive and early stage patients, than in late stage and previously treated patients. There was slightly higher response at higher dose levels, but not enough to justify the added cost of this very expensive drug. 

The paper also discusses the Rituxan plus Fludarabine therapy, where co-administering the two drugs had more than additive benefit. This work was reported earlier in the paper by Byrd, and was discussed at length in CLL Topics. 

The popular combination "RFC" (Rituxan, Fludarabine and Cyclophosphamide) yielded only 7% CR in patients who were refractory to Fludarabine. But in naive patients, this combination yielded an impressive 66% CRs, more than half of them "PCR negative", indicating a deep remission. These sorts of statistics are truly a new milestone in CLL therapy. Neutropenia, thrombocytopenia and major infections were "tolerable". The therapy consisted of six cycles, one per month. Please refer to the paper for detailed information on dosages of the three drugs. 


Other monoclonal drugs in the pipeline: 

This whole section in the paper was less than one page long, and I found it a big disappointment. 

Epratuzumab (also known as LymphoCide) is an anti CD-22 monoclonal antibody. There is no published information of how this drug works for CLL patients. Even the information on NHL is sketchy phase-1 information. In aggressive NHL, the response rate was 10%. Underwhelming, to say the least. 

Apolizumab (also known as Hu1D10, and Remitogen) went through Phase - 1 and Phase - 2 trials for NHL, sponsored by the National Cancer Institute. Clinical trials of this monoclonal for CLL are still recruiting patients, I understand. But, before you sign up, be aware that the manufacturer of this drug, Protein Design Labs, has just announced that because of disappointing clinical trial results, they have decided to discontinue their efforts with this drug. Another much heralded idea bites the dust. 

IDEC-152 (anti-CD23 monoclonal, from IDEC pharmaceuticals, the manufacturer of Rituxan) is going through Phase - 1 clinical trials for CLL right now. Mouse studies and laboratory studies with cell lines have shown good synergy between this drug and Rituxan. It is the wild card in our future, no one knows for sure exactly how this monoclonal antibody will work in real life, with real live CLL patients. Particularly interesting would be the combination of this monoclonal with Rituxan. Indications to date are such a combination is likely to retain the low toxicity profile we have come to appreciate with Rituxan. (Editor's Note: See our later articles on Lumiliximab.)

Three immunotoxin drugs (combination of a monoclonal antibody with a payload of a carefully chosen toxin) are discussed. Ontak and BL22 are two of these, and we have discussed the logic of these drugs in prior articles. This paper does not provide many additional details. 

Along the same lines, another approach to making the monoclonals more effective is to combine them with radioactive payloads, as in Zevalin, Bexar. These two drugs may have significant value for lymphoma patients, but their use in CLL is problematical. CLL patients usually have extensive bone marrow involvement. Both of these radioisotope labeled drugs kill not just the cell to which the monoclonal antibody attaches itself, but several of the perfectly innocent by-stander cells. This can be a real hazard in the bone marrow, since this non-specific cell kill will likely kill the very important stem cells as well. 



Leaving aside Rituxan and Campath, what struck me was how little additional detail there was in the development of the new drugs in the pipeline, several years after they were heralded as breakthrough new approaches. It underlines something I have been trying to say: new developments are worth watching, worth understanding. Someday, one of these new developments may become the next block-buster drug, the magic bullet that "cures" CLL with little or no toxicity. But you would be foolish to hold your breath waiting for these new drugs. If your CLL has progressed to a level where it is prudent to treat it, you would be making a big mistake if you procrastinate, try to keep yourself "pure for the cure", waiting for one of these early stage developments to come to fruition. You might be losing an extremely valuable window of opportunity, when perhaps your CLL can be effectively treated with low impact therapies, give you both quantity and quality of life for several years. Left untreated and allowed to gallop out of control, your choices may become more limited and less attractive. 

Recent advances in developing better prognostic indicators (ZAP-70, IgVH gene mutation status, CD38 and B2M levels etc) makes it a little easier to get a handle on your particular flavor of CLL. As in the case histories we discussed a couple of weeks ago, over-treating a smoldering variety with good prognosis is just as dangerous as under-treating an aggressive form of CLL that can rapidly take over your entire bone marrow capacity. This disease is clearly not a case of one-size-fits-all. It is up to you to stay sharp, learn as much as you can, and work with your oncologist to define the best possible therapy program custom-designed for you. 


I think this is an important article for one reason: many times our oncologists are reluctant to support use of Rituxan as single agent and frontline therapy, because of prior work showing less than impressive responses in heavily pre-treated patients. The story has been quite different in some of the trials by John Hainsworth where "naive" patients  who have not had prior therapy responded much better to Rituxan-only therapy. The results improved even more when early stage patients were treated.

This article brings all these different facts into focus in a single article, written by a respected member of the CLL researcher community. If you are having trouble convincing your oncologist or insurance company that Rituxan as single agent and frontline therapy is appropriate for CLL patients, this is an article you should have in your arsenal. Many times, just the fact that you have bothered to do your homework and read the relevant literature is sufficient to nip the argument in the bud. 

The abstract of the article is below. 


Link: JCO Article

J Clin Oncol 2003 May 1;21(9):1874-81

Monoclonal antibody therapy of chronic lymphocytic leukemia. 

Mavromatis B, Cheson BD. 

Georgetown University Hospital, Lombardi Cancer Center, 3800 Reservoir Rd., N.W., Washington, DC.

Chemotherapeutic approaches during the last decade have failed to result in major advances in the outcome of patients with chronic lymphocytic leukemia (CLL). The recent availability of an increasing number of active monoclonal antibodies, immunotoxins, and radioimmunoconjugates (RICs) has stimulated considerable interest in clinical research in CLL. Alemtuzumab was the first antibody approved for CLL on the basis of responses in one third of patients with advanced disease. However, infusion reactions and immunosuppression with opportunistic infections present a challenge that may be overcome with altered schedules and routes of administration. Rituximab has limited activity as a single agent in patients relapsed or refractory after prior chemotherapy; however, response rates seem to be higher in previously untreated patients. More importantly, combinations with chemotherapy drugs such as fludarabine are showing promise in early trials. Newer antibodies in development as single agents and in combinations include apolizumab (Hu1D10), a humanized antibody against an epitope of HLA-DR, and IDEC-152, a primatized anti-CD23 antibody. BL22, an immunotoxin with impressive activity in hairy cell leukemia, is in phase II trials in CLL as well. The safe use of RICs is complicated by the elevated peripheral blood B-cell count, and the extent of bone marrow involvement in CLL; studies will explore the use of agents to eliminate malignant cells from the bone marrow before RIC therapy. It is hoped that the rational development of combinations of the various promising antibodies with chemotherapy and each other will lead to more effective approaches for patients with CLL.

PMID: 12721266




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