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Rituxan Therapy


Rituxan Enhancements

Date: August 18, 2003

by Chaya Venkat

Approaches to Turbocharge Response

shell glass 

Optimizing CD20 Expression to Improve Response to Rituxan

As you are aware, unlike non-Hodgkin's lymphoma patients, the response rates are lower with CLL patients upon treatment with Rituxan. This is attributed to lower expression of CD20 antigen on CLL cells, the target marker for Rituxan. In fact, a direct correlation has been established between the  density of CD20 markers per CLL cell, and its likelihood of lysis upon treatment with Rituxan.

It follows that there is a great deal of interest in pre-treatment of CLL patients with immunomodulatory cytokines to increase expression of CD20 marker on the B-cells. The article below cites that the cytokines effective in this regard are IL-4, TNF-alpha and GM-CSF (but not GCSF). 

A little while ago I discussed an immunomodulatory compound called beta-glucan, derived from baker's yeast. I am pleased to note that beta-glucan causes significant up-regulation of TNF alpha (Figure 3, Dr. Ross's paper in Immunopharmacology 42 (1999) page 68, the URL to this very interesting paper was cited in my previous articles), and therefore probably contributes to up-regulation of CD20 expression on CLL cells. One more reason why pre-treatment with beta-glucan might make subsequent therapy with Rituxan more effective in this disease. It is interesting to note that the other cytokine that up-regulates CD20 expression is Granulocyte-Macrophage-Colony-Stimulating-Factor (GM-CSF), and one of the effects of beta-glucan administration is activation of macrophages.  

As always, I am not suggesting you guys go out there and buy beta-glucan for self medication, please do review the detailed cautions I posted a few days back on use of these compounds.

The abstract below describes the effect of GM-CSF, IL-2 and TNF-alpha on up-regulating CD20 expression on B-cells. This underlines a point I was trying to make the other day, in my article on CTL therapy. There are a number of autocrine and paracrine feedback loops (cross-talk, chit-chat going on all the time) in the various cell lines of the immune system, and the cytokines that control the whole process of communication. 

Even in the case of patients with adequate levels of neutrophils and other effector cells to provide CLL cell kill assist in Rituxan therapy, there may still be benefit in using GM-CSF, especially in CLL patients, because of the added advantage of up-regulating CD20 expression. In fact, I have heard several researchers discussing whether resistance to Rituxan after repeated use, in terms of decreased antigen expression, can be reversed by co-administration of GM-CSF. 


Leuk Res 2000 May;24(5):411-5

Effects of cytokines on CD20 antigen expression on tumor cells from patients with chronic lymphocytic leukemia. 

Venugopal P, Sivaraman S, Huang XK, Nayini J, Gregory SA, Preisler HD. 

Rush Cancer Institute, Rush Presbyterian St. Luke's Medical Center, 1725 West Harrison Street, Suite 809, Chicago, IL.

Anti-CD20 antibody is an established treatment for low-grade non- Hodgkin's lymphoma (NHL). Augmenting the expression of CD20 antigen on the tumor cells may increase the cell kill and therefore increase the effectiveness of the antibody. To study this, we incubated peripheral blood lymphocytes from CLL patients with the following cytokines: EPO, SCF, TNFalpha, TGFbeta, GMCSF, TPO, IL-1, IL-2, IL-3, IL-4, GCSF. CD20 expression was studied by flow cytometry at baseline, 24 and 72 h after exposure to these cytokines. Upregulation of CD20 antigen expression was observed with IL-4, TNFalpha and GMCSF. 

PMID: 10785263

Adhesion and Residence Time

One line of research is trying to make the anti-CD20 monoclonal "stick better" to the B-cell once it has tagged the CD20 marker. The result of this would be that the drug will stay attached longer to the cancer cell, not come un-glued shortly after tagging. The longer the drug stays on the CLL cell, the better the chance it will get killed by complement mediated cytotoxicity (CDC) or antibody mediated cytotoxicity (ADCC). Another advantage of keeping the drug stuck to the cancer cell for longer periods is that it will take longer for the body to get rid of it in urine and feces. Longer residence time of the drug in the body means it will keep working longer. For many conventional chemotherapy drugs with significant toxicity profiles, longer residence time in the body is a mixed blessing: longer for the drug to act, but also longer for the drug to do damage to the body. This is not the case with drugs like Rituxan, since they have such a low toxicity profile.

Remember, antibodies such as Rituxan are huge "Y" shaped molecules, with each of the arms of the "Y" designed such that they have exactly the right shaped pincers to latch on to the CD20 marker on B-cells. The stem of the "Y" is also important, it is what attracts and binds to immune system cells like T-cells, NK-cells etc in the body. The more attractive this tail of the Y looks to the body's immune system cells, the more likely they are to attach to it, and begin the process of destroying the cell that has been tagged by the arms of the "Y". A number of companies are working on making new versions of anti-CD20 monoclonals that are more strongly attractive to immune cells, in other words the stem of the "Y" is made to be more 'antigenic'.

Complement-mediated Cell Kill

A third approach is to make the complement mediated cell kill more effective by temporarily blocking the function of markers such as CD45, CD55 and CD59, all of which are implicated in interfering with complement mediated cell kill. We have also discussed the role of immunomodulatory substances such as beta-Glucan, that short circuit the function of complement inhibitory molecules such as CD55 and CD59, by triggering what is called the Lectin mode of activating neutrophils and macrophages. (See Beta Glucan, Complement and Rituxan). Also, I discussed the role of Fludarabine in blocking the effect of some of these inhibitory markers (Rituxan plus Low Dose Fludarabine). If indeed Fludarabine helps to down regulate the inhibitory proteins such as CD55 and CD59 that prevent cell kill by Rituxan based therapy, the 64 thousand dollar question is how much Fludarabine does one need to get this effect going? Is a smaller dose of Fludarabine enough to serve as the enabling drug that makes Rituxan more effective, without getting in the way by suppressing the very immune system that we are counting on to kill the cells tagged by the Rituxan? This was the whole basis of a series of articles on the concept of "RF Lite", where the dose of Fludarabine co-administered with the Rituxan could be smaller than in conventional RF therapy, just enough to get the synergy going between Rituxan and Fludarabine, negate the effects of the inhibitory markers and make the Rituxan therapy that much more effective. The first PubMed citation attached below discusses this important finding, the down-regulation of complement inhibitors by Fludarabine.

All of these approaches have one thing in common, namely the tagging of the CD20 marker by a monoclonal antibody. They are all looking to make the tagging more effective, but the target is still CD20. That is the common feature of all this work. I have no doubt some of these avenues of research will pan out, and we will have improved versions of Rituxan in clinical testing within the next two or three years. There is just too much money to be made by anyone who can replace Rituxan with a better anti-CD20 monoclonal.

The reason for this fixation on CD20 marker is simple: it is a very good target, in the sense that only mature B-cells express it. Therapy based on anti-CD20 monoclonals is likely to be quite specific to mature B-cells. Precursors to B-cells, stem cells, other immune system cells such as T-cells, neutrophils, monocytes etc do not have the CD20 marker and will therefore not likely to be killed. This is the single biggest difference between Rituxan and Campath, this incredible selectivity of Rituxan. That is also what makes Rituxan so much easier to take, with so many fewer side effects, and so much less immune suppression or worries about getting opportunistic infections, than Campath. 

Most of the recent work seems to suggest that both percentage of CD20 positive cells, as well as the intensity with which they express the marker are important factors in determining response to Rituxan therapy. Below is the URL to a recent paper in Blood, which gives a lot of the details of the complement inhibitory markers such as CD45, CD55 and CD59. It also shows clearly that the cell kill by Rituxan is quite strongly influenced by the number of cells that are CD20 positive, and also the intensity of that positivity. If you are not in the mood for heavy duty techno-jargon, skip to Figure 3A of the paper. It shows that cell kill increases nicely when the intensity (Mean Fluorescent Intensity or "MFI") for CD20 increases. 

Blood Journal Article (full-text): CD20 levels determine the in vitro susceptibility to rituximab of B-CLL.

So, the million dollar question is this: how high is high? You might be surprised to know that there can be literally thousands of CD20 markers per cell. The second PubMed abstract below (Keating, et. al.) describes the average numbers of CD20 markers per cell in CLL patients, also called "APC" or Antigens Per Cell. There are about 9000 CD20 markers per B-cell on cells found in the peripheral blood of CLL patients. The APC is much lower, only about 4000 CD20 markers per cell, when they examined CLL cells found in the bone marrow, and about the same in CLL cells in the lymph nodes. Interesting fact, this might explain why it is so much easier to clean out the peripheral blood with the higher APC, than the bone marrow and the lymph nodes; it also explains why Rituxan monotherapy works better on patients without bulky lymph nodes, and relatively low infiltration of the bone marrow.

How does this antigens per cell for CLL patients compare with other B-cell lymphomas? Researchers have studied this, and came up with a measure of how much Rituxan can be bound per cell (this then reflects the number of CD20 markers per cell) for a variety of B-cell cancers. The units are not quite the same as the APC I discussed above, but the trends in the numbers below track the APC quite closely.

Cellular Antigen Density by Phenotype
L. Ginaldi, et al., J. Clin. Pathol. 1998;51;364-369
Antigens per Cell
Normal B Cells
Mantle Cell Lymphoma
Pro-lymphocytic Leukemia
Splenic Lymphoma
Hairy Cell Lymphoma
  Chronic Lymphocytic Leukemia  

Bacterial DNA to Enhance Response to Rituxan

Over the millennia, mammals have developed immune systems that recognize bacterial DNA snippets as potent danger signals that trigger immediate activation. About a hundred years ago, a New York physician named Coley tried to use this immune system activation in fighting diseases like cancer. "Coley's cocktail" was a mix of smooshed up bacterial brew, and it actually helped some patients! Some of you old timers on CLL Topics may remember the piece(s) I did on yeast based beta Glucan in bringing about potent immune system responses. This is the same sort of an approach, instead of yeast based material, we are talking about material of bacterial origin.

Bacterial extracts such as Coley’s toxins have demonstrated the concept, the extremely potent immune response can be triggered by this method. However, this does not make for a practical drug that can be used today, unless it is possible to identify the specific components within bacterial extracts that reproduce these immunostimulatory effects without the very dangerous consequences from impurities. Coley Pharmaceutical Group (Wellesley, MA) is a company that has been working on this. It turns out that our immune systems have evolved complement based innate immune defenses that detect unmethylated CpG motifs within bacterial DNA. In vertebrate DNA, these CpG motifs are suppressed and methylated. Synthetic oligodeoxynucleotides (ODN) containing CpG motifs act as "danger signals," inducing strong innate and acquired immune responses". If you search on PubMed with the term "CpG ODN", you will get a huge number of hits, this is a hot field!

An area of obvious interest to us is the use of CpG ODN in cancer immunotherapy, in combination with monoclonal antibodies. Remember, we discussed that one of the mechanisms of cell kill in Rituxan monotherapy is ADCC (Antibody Dependent Cellular Cytotoxicity), where the B-cells tagged by the Rituxan are killed by the other cells of the immune system. Part of the problem with CLL is that as the disease progresses, there is such a confusing mix of cytokine chemical signals put out by the cancer that the rest of the immune system is often "anergized", becomes ineffective and unmotivated. This lack of effective ADCC and complement dependent cell kill (CDC, CDCC) are some of the reasons Rituxan therapy less than perfectly effective in CLL patients. It has now been shown that administration of CpG ODN dramatically activates ADCC effector cells. When this is followed by infusion of a monoclonal such as Rituxan that targets and tags the CLL cells, dramatic increases in biologic activity are seen. Regression can be achieved with large tumors that would not normally respond to antibody therapy, as well as with tumors that only express the target antigen at a low concentration. In preliminary human clinical trials, CpG ODN appear to be generally well tolerated and have shown efficacy in enhancing innate and acquired immune responses. Its efficacy for cancer immunotherapy in humans is currently under investigation.

The first abstract below is a cell based study: interesting to note that CpG ODN administration increased CD20 expression (making it easier to tag them with Rituxan), increased CD40 expression and MHC (Major Histocompatibility Complex) I and II expression, making the cells much more attractive targets to attacking T-cells, and best of all, these effects were most pronounced in CLL cells. 

The second abstract confirms the findings of the first one, this time in mice. Large tumors in mice that could not be cured by monoclonal antibody therapy, or by CpG ODN, were cured by a combination of both materials, underlining the synergy between the immune response stimulator and monoclonal antibodies like Rituxan.

The third abstract also discusses murine (mice) studies, the reason I included it is because it comes with a link to the full article, which you can access free of charge. I am getting increasingly frustrated with making-do with brief and tantalizing abstracts, it is nice to come across an article where you can read the whole thing for free.

This approach has progressed beyond cell studies and mice studies. A Phase - I clinical trial has been announced, sponsored by the National Cancer Institute for varieties of NHL patients. You can read more about it at the URL below. Whether they will include SLL ( = CLL) patients in their study is unclear. But I would be willing to bet other studies are going to be initiated soon that will include CLL patients, we are the ones most likely to benefit by the goosed up response to Rituxan, since we are the ones that need the extra help most. 
"RATIONALE: Monoclonal antibodies such as Rituximab can locate cancer cells and either kill them or deliver cancer-killing substances to them without harming normal cells. Biological therapies such as CpG 7909 use different ways to stimulate the immune system and stop cancer cells from growing. Combining CpG 7909 with Rituximab may kill more cancer cells".

Last but not least, the Lymphoma Research Foundation has scheduled an education program on April 26, at the Manhattan Beach Marriott in Manhattan Beach, California. Attendance is free, but you have to pre-register. While the specific aim of the program is to talk about clinical trials for NHL patients, see my correspondence below with their representative. While I did not get an unequivocal "yes" to my question about inclusion of CLL patients, I did not get the door slammed in my face either, some of us may be able to get in because of re-classification as SLL patients. You can find out more about these clinical trials at their website, I am particularly interested in the two clinical trials I discussed with the lady, both dealing with Rituxan in combination with immune system stimulants. If any of you happen to be in the general area, do attend and let us know what you find out.


J Leukoc Biol 2001 Jan;69(1):81-8

CpG DNA increases primary malignant B cell expression of costimulatory molecules and target antigens.

Jahrsdorfer B, Hartmann G, Racila E, Jackson W, Muhlenhoff L, Meinhardt G, Endres S, Link BK, Krieg AM, Weiner GJ.

Department of Internal Medicine, University of Munich, Germany.

Multiple factors, including expression of costimulatory molecules, antigen-presenting molecules, and target antigens, likely impact the efficacy of antibody therapy and other approaches to the immunotherapy of B cell malignancy. Unmethylated CpG-dinucleotides in select base contexts ("CpG motifs") that resemble sequences found in bacterial DNA are potent immunostimulatory agents capable of inducing a complex immune response, including a strong B cell stimulus. We examined the effect of a potent human CpG oligonucleotide (CpG ODN 2006) on different types of primary human malignant B cells and reactive follicular hyperplasia. CpG oligodeoxynucleotide (CpG ODN), but not control (non-CpG ODN), increased the expression of costimulatory molecules (CD40, CD80, CD86, CD54) on malignant B cells without altering the phenotype of B cells obtained from reactive follicular hyperplasia. CpG ODN also enhanced expression of class I and class II MHC in most samples. CD20 expression was increased in response to CpG ODN, most notably in B-CLL and marginal zone lymphoma. An inverse correlation was found between baseline expression of CD20 and CD40 and their expression after exposure to CpG ODN, thus the most significant increase in expression of these molecules was found in those samples that had the lowest baseline levels. In conclusion, CpG ODN can lead to increasing expression of molecules involved in costimulation, antigen presentation, and as targets for antibody-based therapy and deserve further evaluation as potential immunotherapeutic agents for B cell malignancy.

PMID: 11200072

Semin Oncol 2002 Feb;29(1 Suppl 2):93-7

Synergism between cytosine-guanine oligodeoxynucleotides and monoclonal antibody in the treatment of lymphoma.

Warren TL, Weiner GJ.

Holden Comprehensive Cancer Center and the Department of Internal Medicine, University of Iowa, Iowa City, IA.

Synthetic oligodeoxynucleotides containing unmethylated cytosine-guanine dinucleotides (CpG ODN) are potent immunostimulatory agents that can activate various immune cell subsets. We have found that CpG ODN show a variety of effects that could be useful in enhancing the efficacy of antibody therapy of lymphoma. In a mouse model, CpG ODN alone had no effect on survival of animals inoculated with lymphoma. In contrast, CpG ODN plus monoclonal antibody (MAb) was more effective at inhibiting tumor growth than MAb alone or MAb plus control ODN. Cytosine-guanine ODN plus MAb cured mice with a large tumor burden that could not be cured with MAb therapy alone. We also evaluated the effects of CpG ODN on the phenotype of human malignant B cells. Cytosine-guanine ODN upregulated the expression of a number of antigens, including CD20. The upregulation of CD20 was most extensive in cells that had low baseline expression of this antigen. We conclude that CpG ODN enhances the efficacy of MAb in a murine lymphoma model, most likely by activating effector cells, and upregulates expression of CD20 on primary human malignant B cells. Given the effects of CpG ODN on both target antigen expression and effector-cell function, further evaluation of the combination of CpG ODN plus rituximab (Rituxan; Genentech, Inc, South San Francisco, CA, and IDEC Pharmaceuticals, San Diego, CA) and CpG ODN plus other MAbs is warranted. Copyright 2002 by W.B. Saunders Company.

PMID: 11842395

Article from Blood Journal (Free full-text article)

Blood 1997 Apr 15;89(8):2994-8

Immunostimulatory oligodeoxynucleotides containing CpG motifs enhance the efficacy of monoclonal antibody therapy of lymphoma.

Wooldridge JE, Ballas Z, Krieg AM, Weiner GJ.

Iowa City Veterans Administration, the Department of Internal Medicine, The University of Iowa College of Medicine, Des Moines, IA.

Bacterial DNA and synthetic oligodeoxynucleotides containing the CpG motif (CpG ODN) can activate various immune cell subsets, including natural killer cells and macrophages. We evaluated whether the combination of CpG ODN and antitumor monoclonal antibody is effective at preventing tumor growth in an immunocompetent murine lymphoma model. CpG ODN-activated murine splenocytes induced lysis of tumor targets more effectively than unactivated splenocytes. These effector cells were also superior to unactivated splenocytes or cells activated with a control methylated ODN at inducing antibody-mediated lysis of 38C13 murine lymphoma cells. In vivo, CpG ODN alone had no effect on survival of mice inoculated with 38C13 cells. However, a single injection of CpG ODN enhanced the antitumor response to antitumor monoclonal antibody therapy. Ninety percent of mice treated with monoclonal antibody alone developed tumor compared with 20% of mice treated with antibody and CpG ODN. These antitumor effects were less pronounced when treatment consisted of an identical ODN containing methylated CpG dinucleotides. A single dose of CpG ODN appeared to be as effective as multiple doses of interleukin-2 at inhibiting tumor growth when combined with antitumor monoclonal antibody. We conclude that immunostimulatory CpG ODN can enhance antibody dependent cellular cytotoxicity and warrant further evaluation as potential immunotherapeutic reagents in cancer.

PMID: 9108420

Rituxan Squared - Efficacy of Rituxan Dimer

The abstract below and the link to the full free article make it into the top 10 must read article for CLL patients interested in Rituxan therapy. 


Before we get into this very interesting approach to improving the efficacy of Rituxan, especially for poor responders to this drug, it might help to briefly recap how Rituxan works (or doesn't work) in CLL patients. As we have described several times before, Rituxan is a monoclonal antibody that targets the CD20 marker on B-cells. Fortunately for us folks with a B-cell CLL, the CD20 marker is exhibited only on mature B-cells, not on stem cells, not on precursors to B-cells, not on plasma cells, and not on any of the other immune system cells such as T-cells, neutrophils, monocytes, macrophages etc. This is what  makes Rituxan therapy so specific, since it is limited only to B-cells.

We have also said that once the B-cell expressing CD20 is tagged by Rituxan, very little of the actual killing of the cell is done by the Rituxan itself. The major mechanisms of cell death after getting tagged by Rituxan is via ADCC (Antibody dependent cellular cytotoxicity) or CDC (Complement dependent cytotoxicity). In other words, other parts of the immune system are needed to make the cell kill happen. This may be one reason why heavily pre-treated patients respond less well than "naive" patients to Rituxan therapy. Perhaps in patients who have been through several bouts of chemotherapy, the immune system is not as effective any more and plays a poorer part in providing the necessary assist to Rituxan mediated cell kill. 

However, even some frontline and naive CLL patients do not respond very well to Rituxan therapy. This could be due to poor antigen expression. In other words, their CLL cells express CD20 marker only dimly, and not enough Rituxan gets bound per cell to trigger strong and potent ADCC and CDC cell kill. There has also been recent work demonstrating that some patients have complement inhibitory proteins, CD45, CD56 and CD59 among others, that block the complement mediated routes for cell kill, even when the CD20 expression is quite adequate and a substantial amount of Rituxan is bound to the cell targeted for death. 

One way of getting away from all these issues is to make the Rituxan some how self-sufficient in its ability to not only tag the CD20 positive B-cells, but carry out the cell kill as well. In other words, it is not only the cop that nabs the thief, but carries out the execution as well. Two new drugs on the market, Zevalin and Bexxar, take this approach. The anti-CD20 monoclonal also carries a payload of radioactive material. Once the CD20 marker on the B-cell is tagged with the drug, the radioactivity carried by the drug kills the cell. Only problem with this approach is that it is not very selective. These "smart bombs" are not quite that smart, there is so-called collateral damage. The radioactivity they carry kills not just the cell to which the drug is attached, but any other quite innocent cells that might be in the vicinity. This is particularly a problem for CLL patients, since we have significant bone marrow involvement. In order to clean out the bone marrow, drugs such as Zevalin and Bexxar need to enter the bone marrow, and their non-specific cell kill can easily do a  great deal of harm in killing irreplaceable stem cells. 

Rituxan "Squared" 

We have described before what some of these monoclonal antibodies look like: they are "Y" shaped molecules, with the tips of the arms of the "Y" being the business end. These tips are shaped just right to attach themselves to a specific marker on the target cell. That is what makes Rituxan an anti-CD20 monoclonal, the tips of its arms are just the right shape and size to "mate" with the CD20 markers on B-cells. 

Now researchers have found an elegant way of hooking up two molecules of Rituxan together, so that instead of having just two paws with which to hold the B-cell, this new molecule (called a "dimer") has four paws with which it can attach itself to the target cell, holding on at four different points and tagging four different CD20 groups on the surface of the cell. (As you may remember from previous articles, even on B-cells with dim expressions of CD20, there are hundreds if not thousands of CD20 markers per cell). 

Continuing our analogy, this dog with four paws is very good at hanging on, and not letting go. One of the aspects of drug development is to make sure the drug stays put on the target, not get washed off into the general milieu where it can do little good. In the case of Rituxan dimer, compared to the standard single molecule monomer, the dimer does a much better job of sticking on to the target cell, until death do them part. Smaller doses of the Rituxan dimer may be able to do a better job, since less of the drug is wasted just sloshing around in the blood plasma. 

Besides hanging on better to the target cell, the multi-pronged attachment of the dimer to the B-cell has another very important effect. It causes the cell to die. That's right, here is a situation where it has been demonstrated that cell kill happens when the Rituxan dimer is attached to the cell, with no assist required using ADCC or CDC mechanisms, or any need for radioactive payloads. There is some indication that the mechanism involves calcium ions. Without getting into very complex biochemistry, the Cliff Notes version is that there is an optimum level of calcium ions needed inside the cell, for it to continue working right. When Rituxan dimer is attached to the cell, it appears to function as a calcium channel, pumping more and more calcium ions from the outside through the cell membrane and into the cell. The calcium overload is very quickly lethal to the cell and it dies on the spot. 

So, what does this mean for us? Well, for one, it might mean that Rituxan dimers may work better in heavily pre-treated patients, since this new version is not dependent on getting an assist from the immune system in doing the cell kill, ADCC is no longer the major mechanism for cell kill. Second, patients who would otherwise have been poor responders, because they have high level of complement inhibitory proteins on their surface can perhaps now get around that, again because Rituxan dimer does not depend on CDC for cell kill. There is also reason to expect that the four legged dog is better able to make use of the available CD20 markers per cell, and even patients with dim CD20 expression will respond to the dimer version. Last but not least , unlike Zevalin or Bexxar, the solo-act of Rituxan dimer is not dependent on radioactive payload, and therefore it retains all of the original selectivity in attacking only mature B-cells that express CD20 markers. No collateral damage problems, no worry about killing stem cells in the bone marrow. 

There is reason to hope that it might also be possible to fiddle with the size and molecular weight of the dimer, to make it stick around in the body for a lot longer. Small molecules with low molecular weight are soon cleared out of the system by the liver and kidneys, and this means expensive drugs are literally going down the toilet, before they get a chance to do their stuff. If on the other hand the drug molecules are too large, they have trouble getting into the nooks and crannies deep within bulky lymph nodes, spleen etc, and therefore do an inefficient job of clearing out all of the cancer cells. This whole area is called drug "pharmacokinetics", studying how long drugs stay within the body after they are injected, where the drug molecules go and hang out in the body, how the liver spleen and kidneys handle the by-products of the drugs as they break down, etc. 

And here is the last piece of the puzzle, why I really like the idea of Rituxan dimers. Since the FDA has approved Rituxan for B-cell malignancies, and it has now stood the test of time in several million human subjects, it might be easier to get the Rituxan dimer approved. Manufacturing monoclonal antibodies is no joke, and it takes a lot of money and skill and time to come up with good manufacturing techniques. But IDEC already knows how to make Rituxan! Approved manufacturing method for the monomer! Making the dimer from the monomer is something most chemistry grad students can do in the lab, with minimal fuss. So, one can hope the usual time delays in drug approvals and manufacturing bottlenecks can be avoided. 

This is 2001 paper. Why is the Rituxan dimer not in clinical trials yet? I guess lab mice have a union, and they need their job security too. Hopefully, this important new approach to selective and effective Rituxan based therapy will soon trickle down to us CLL patients. 


Blood Journal Article — Free full-text article.

Blood. 2001 Mar 1;97(5):1392-8.

Homodimers but not monomers of Rituxan (chimeric anti-CD20) induce apoptosis in human B-lymphoma cells and synergize with a chemotherapeutic agent and an immunotoxin.  

Ghetie MA, Bright H, Vitetta ES.  Cancer Immunobiology Center, University of Texas Southwestern Medical Center at Dallas, TX.

In 1997, a chimeric anti-CD20 monoclonal antibody (mAb) (Rituxan) was approved for the treatment of low-grade/follicular B-cell lymphoma. Rituxan has a long half-life and low immunogenicity, and it mediates effector function. Rituxan induces apoptosis in some tumor cell lines in vitro. Previous studies with mAbs that react with neoplastic B cells have demonstrated that homodimers of immunoglobulin G ([IgG](2)) often inhibit cell growth more effectively than their monomeric (IgG)(1) counterparts. In this study, the ability of IgG or F(ab')(2) homodimers vs monomers of Rituxan were compared for their ability to inhibit the growth of several different B-lymphoma cell lines in vitro. It was found that homodimers of Rituxan had superior antigrowth activity in vitro and that F(ab')(2) homodimers were the most active. Homodimers, but not monomers, of Rituxan induced both apoptosis and necrosis of several B-cell lymphoma lines in vitro; the inhibition of cell growth was not dependent upon the presence of Fc receptors or upon 10-fold or greater differences in the density of CD20 on the target cells. Rituxan homodimers, compared with monomers, also rendered drug-resistant CD20(+) B-lymphoma cells more sensitive to chemotherapeutic agents and synergized with an anti-CD22 immunotoxin in vitro. 

PMID: 11222385




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