Webinar: Hairy Cell Leukemia, Diagnosis & Targeted Therapy
July 22, 2020
Hosted by the Hairy Cell Leukemia Foundation with Dr. Clive Zent from the University of Rochester Medical Center
Presentation Materials
View a recording of Dr. Zent’s presentation (audience Q&A is in the transcript only). >>
Transcript of Presentation and Question & Answer
The following is a transcript of the 1.)Presentation given by Dr. Zent and 2.) Question & Answer session with participants.
Presentation by Dr. Clive Zent
Dr. Clive Zent: I appreciate the invitation and happy to have so many people listening. I hope that we'll be able to make this a worthwhile experience. So today we're going to talk about the diseases that constitute the hairy cell leukemia spectrum, and also talk a little bit about how targeted therapy is becoming more and more useful in treating this disease.
So the overview of what we're going to talk about today. So a little bit of physiology, because if you don't understand how the immune system works and how your blood cells are made, it's very hard to understand how lymphomas occur and how hairy cell leukemia develops and presents. Then we'll talk a little bit about B-cell lymphomas, because the hairy cell spectrum is a subgroup of B-cell lymphomas. Then talk a little bit about the hairy cell leukemia themselves, and then targeted therapy. The hairy cell leukemia spectrum, as you all know, comprises of classical hairy cell leukemia, hairy cell leukemia variant, and the very long named lymphoma, which is one of the most rare of all of lymphomas, which really is the small red pulp splenic B-cell lymphoma. The targeted therapies we are going to cover today are the purine analogs, which actually targeted chemotherapies, the BRAF inhibitors, the antibodies, both MEK and the CD20 antibodies, the CD22 conjugated antibodies, and then touch a little bit on the Bruton's tyrosine kinase inhibitors.
Dr. Clive Zent: So let's start off at the beginning because I think it's a good place to start. And as a hematologist, this is the basis of almost everything I do. So bone marrow is the cavity of the bones in the trunk and the beginning part of the limbs, which houses the very important organ called the bone marrow which makes your blood cells. It is essentially the fundamental factory of your immune system at the same time. So at the top of this figure, right at the top here, you have the illusive stem cell, which is something that we have our whole lives.
These STEM cells divide to provide a daughter cell and to replace themselves, and those daughter cells divide and differentiate into millions upon millions of cells. They have two lineages. On the left side of the screen over here, we have the Myeloid stem cells that give rise to progenitor cells to make a karyocyte, which provide platelets, that plug up holes in the circulation, as they occur, giving us a self-sealing circulatory system. The red cells, which carry hemoglobin, which is the oxygen carrying a molecule of the body, which allows us to respire and therefore live. And the granulocytes and the macrophages, which are not here on this picture, which essentially provide us with our innate immune system. So these are the immune cells, which protect us from infection. They are pre-programmed they're really, to the best of our knowledge, not programmable, and they provide an important innate immune system for all of us.
Dr. Clive Zent: This other side here, the lymphoid side, is really where the problems start that lead to hairy cell leukemia. So the lymphoid cells of importance are the natural killer cells over here, the T lymphocytes and the B-lymphocytes, the T cells as most of you know are the controlling cells, the regulators of the adaptive immune system, which is what lymphocytes provide us with. They control the B cells and they control the natural killer cells. The natural killer cells are the sort of targeted assassins of the immune system. They can go after targeted cells and kill them. And the B lymphocytes over here are the precursors of the plasma cells, which make antibody. And hairy cell leukemia is a disease of a B lymphocyte. So it's a B cell malignancy and a B cell lymphoma. So just very quickly, you're B-cell starts as a stem cell. It takes about two weeks to become a mature naive B-cell in the bone marrow, which is the primary organ of that part of the immune system. And when a B-cell comes out of the bone marrow, it contains a fully active B-cell receptor on its surface. And B-cell receptor contains an antibody, which is pretty much unique to that particular B-cell. That B-cell then travels in the blood system to a lymph node or other part of the lymphoid system, where it basically does sentry duty for about a week. And during that time it interrogates all the fluid coming through the lymphatics, which we're going to talk about in a moment, for anything that had recognizes by shapes. So the antibody has a specific shape and that specific shape can bind to another specific shape in a complimentary way. And if that happens, the B-cell gets activated. It divides really fast and undergoes mutations, which help that either make better antibody or worse antibody. And eventually it becomes a very, very good antibody producing cell.
Now that's the exception. Under most circumstances that antibody looks for something it can recognize for that week. And at the end of that week, it doesn't find anything. Then a biological clock clicks over, and that cell undergoes programmed cell death. It dies by a by activation of program, which makes it a very neat ecologically friendly system, so that doesn't disturb any other cells around it. It basically gets recycled. So the cell that did get activated goes back into the bloodstream, it circulates around and it settles down in the bone marrow as a plasma cell, which becomes an antibody producing factory. The antibodies go out throughout your body. They bind to the target where ever, they see it and together with the other members of the innate immune system, especially your nutraphyls and your macrophages, will destroy their target, whatever it be. Bacteria, fungus, or viral infected cell. And you clear out the infection. Now you've got lots and lots of cells sticking around that no longer needed, they're redundant. And they undergo program cell death and disappear. If we're a few cells that stick behind with the memory, the genetic memory, to make that highly efficient antibody. And those are called memory cells. And those memory cells can stay with you for as long as you're healthy for as long as you're living. And that provides a very, very important part of your immune repertoire, which can protect you from infection in the future. So that is very simply biology of the adaptive immune systems’ humoral branch, in a nutshell. And so if any questions, I'd be happy to take them after this talk.
So lots of things can go wrong. It's a very complex process. This it involves cells, develop things. Cells undergoing gene rearrangement in the bone marrow and subsequent mutation in the lymphoid tissue. It involves cells that have to grow very fast. They divide every four hours, which means a single cell can become a million cells in about three days. They're cells that have to die on command, and lots of things go wrong. And we can now recognize 70 different B-cell lymphoid malignancies along this pathway. And one of them is classical hairy cell leukemia. The other one is hairy cell leukemia variant. And then there are red pulp splenic lymphoma. And they occur because there’s a cell that is probably in the memory cell stage of its life doesn't die when it is no longer required or has a defect and accumulates over a long period of time and can cause trouble over decades and longer because of accumulation of these abnormal cells.
Dr. Clive Zent: The next thing I just wanted to go over as part of the basic preparation for this talk is the lymphoid system. The picture on the left there shows your lymphatic system. So the lymphatic system is a system of simple ducts from all of you that drain excess tissue fluid from all over your body, back to the heart. This lymphatic duct system has checkpoints on it where immune cells can accumulate. And this is called the lymph node. So you can see lymph nodes here in the groin, under the arms, in the neck and in various other places, especially at the back of the abdomen and the chest where immune cells can congregate, interrogate the fluid coming through then lymphatic ducts, which includes some fluids that's coming from some pretty seriously bad neighborhoods. Like your gut full of your biome, your skin covered with whatever's in your environment and your upper respiratory tract. This provides us with a very good sentry system to be able to respond to all sorts of infections.
Now, not everything that's called lymphoid tissue is directly connected to the lymphatic system. And on the right is just an emphasis on that. So organs that are not necessarily lymph nodes, but are important part of the lymphoid system are the spleen, which is over here under your diaphragm, in the abdomen on the left. And the spleen is a very interesting organ, which is very important for immune response to infections. So that's one function. The other it is an organ that basically monitors the health of your circulating cells, especially the red cells and the platelets, and will take them out when they get old and ineffective, or if they're diseased in any way.
We also have lymphoid tissue in our upper airway, the adenoids in the nose, the tonsils at the back of the throat. And then we have lymphoid tissue along our gut, especially in the small intestine called the Peyer's patches. And not to forget that there's an important lymphoid organ in the front of the chest and the upper neck which is called the thymus, which is where our T cells mature.
Dr. Clive Zent: Okay. So that's the basis for what we're going to talk about for the rest of the session. We are going to talk about three diseases, as I've said before, they're not very common diseases. This is the current US incidence, so the number of events that people that are diagnosed with each of these diseases in one year. For classical hairy cell leukemia it's just over 1,000 people, for variant it's just under 1,000. And for this very rare splenic red pulp, we don't even know because it's really not being counted. So you're really talking about 2,000 people in this country, in the United States being diagnosed, I would expect that the European Union would have a similar incidence. And fortunately people with these diseases often live a very long time. So this translates into a lot of people with a disease, but not a lot of them getting it every year.
Dr. Clive Zent: So let's go back and see what we mean by hairy cell leukemia. Why does this disease have what could be considered a very strange name. And the reason isn't what they did when it was first recognized in the late 1950s, which in terms of medicine and immunology is a very, very long time ago. It was recognized as an abnormal cell under the microscope. So these pictures here show us a peripheral blood smear. That's where you take a drop of blood, smear it off on a glass slide and then stain it. So you can see the red cells over here and the white cells over here. And this white cell is a lymphocyte, has a big nucleus, not much cytoplasm. What's very unusual about this particular cell is it has these very interesting strands of the cytoplasm going out and around it.
This was thought to look like hair, and hence it was called hairy cell leukemia. But these hairy cells actually occur in quite a few diseases. It occurs in classical hairy cell leukemia, which I know this is because that's what the diagnosis was of this patient. But it also occurs in hairy cell leukemia variant. So if you're a specialist, a hematopathologist, you might be able to tell that these cells are different from these cells, but a lot of us can't. If you look at this more magnified one over here, it also has strands of cytoplasm making it hairy. And sides that I don't have for another disease known as splenic marginal zone lymphoma, which can have these hairy cells as well as red pulp lymphoma. So those are the four diseases that this could be. This just tells you have to look further.
Dr. Clive Zent: And how do we look further? We do something called flow cytometry. And hopefully this is not as scary a picture to anybody. I'm going to go through each of these components one by one. But essentially what we do here is we take blood or bone marrow aspirate, or any other group of cells. And we put them through a machine called a flow cytometer, which has a very, very fine capillary in it, through which fluid flows in a very organized way. And it's set up so that the cells of interest will flow through this capillary one at a time with an even distance between them at a regular speed. And these cells can then be interrogated by a series of lasers for whatever you want to do.
So, for instance, in this picture over here, we have a cell that has been stained for antibody on its surface. And these antibodies, the light chains can either be kappa, which is on this side over here or lambda. And they can't be both. So an individual cell either has kappa or lambda. And in normal people, the B lymphocytes will have two kappa for one lambda. And here you can see that almost all the cells of interest, which are stained red are kappa. So this tells us that a lymphocyte is abnormal and that we can imply from that, that this is monoclonal or that all the cells are the same. So there's something abnormal there. So we can go and look at some other characteristics by staining for surface proteins. We can see that these cells are all CD 79-B, which makes them B cells. That's part of the B-cell receptor.
And they also have a marker called CD11c, and here, we've got another B cell market called CD19, and these cells are negative for CD25. And over here, you can see, again, that they're negative for CD25 because a positive would be over here, but they're positive for CD103, and they are positive for CD20 and negative for CD200.
So what I can tell from this is that hairy cell that we are studying in this flow pattern is more compatible with this cell over here. In other words, it's hairy cell leukemia variant rather than hairy cell leukemia of the classical type. And that is how we go through the next step. So flow cytometry is incredibly important in being able to identify different groups of B lymphocytes. Sometimes this can be confirmatory of a diagnosis, sometimes suggestive.
So what do we do next? We can do a bone marrow study. And what we're seeing over here is a core of a bone marrow. So this is a core biopsy. We put a hot hollow needle into the bone marrow cavity, and then take a core of the bone, fix it, and then cut it into very, very thin slices and stain it with various stains. And this on the left here is the Hematoxylin eosinophil, here where nuclei are stained blue, cytoplasm that's pink. And you can see here that there's not very much fat - bone marrow should have a lot more fat than this. A lot of cells, they're very packed in together and they all look very much the same. So there's something very wrong with this bone marrow. And if you look on the right hand side, this bone marrow has been stained with an antibody versus CD20. So these are all B cells. So this is packed out with B cells, something very wrong with it.
So you can go to the next group of stains. So here you can stain for various things. And the one thing you can stain for is BRAF, which is not shown here. And if this was positive for BRAF, this would be highly likely to be a hairy cell leukemia.
So what we are doing here that I'm showing you is that you can do a whole lot of sequential tests until you get to a final diagnosis.
Dr. Clive Zent: This is the picture of spleens from people with different types of these hairy cell leukemia type of syndromes. This shows you a person who has splenic marginal zone lymphoma. And if you look over here, which is showing the macrophages of the red pulp and the white pulp, you can see that this is a disease of the white pulp of the spleen, which is where most of the lymphocytes normally reside. Whereas these other three diseases over here, classical hairy cell leukemia, the variant and the splenic red pulp, are all diseases of the red pulp. So you can differentiate between disease that involves the white pulp of the spleen, or the red pulp. This is another differentiating characteristic that we use to try and make a definitive diagnosis.
Dr. Clive Zent: So diagnosis is very important and an accurate diagnosis is essential for being able to tell you what disease you've got, and what treatment is likely to work. But then we have some additional factors that we look at, which involve looking at the antibody that is specific to this lymphoma. And we can look at the variable region of the antibody over here, which determines its shape for the sequence. And the sequence tells us which of our many options for this variable region has been used in this particular antibody, and whether that has undergone special mutations that allow it to be a better antibody. So in general, cells that have hypermutations, somatic hypermutation, tend to be less aggressive. And there certain family members, for instance this VH4-34, which if the hairy cell leukemia variant is using that, tends to be more aggressive. Separate from this sequencing, this gene for the antibody, we can sequence genes for P53 and other molecules that can tell us whether the disease is more or less likely to be aggressive.
Dr. Clive Zent: So if a person presents with classical hairy cell leukemia, they would be having a median, or mid group of the age would be 58 years. There's a male predominance. It can present as being asymptomatic, just a incidental finding because the person has an abnormal blood count during a routine check, or patients can present because their immune system's not working and they are getting infection. Because they're not making red blood cells and therefore they're feeling weak or fatigued. They can have an absence of particular blood cells, red cells causing anemia, platelets, causing thrombocytopenia and bleeding.
And one of the particular characteristics, which you don't really understand, but we recognize well is that if you have classical hairy cell leukemia, your monocytes disappear when your disease progresses. You can have big organs, especially the spleen. And if any of these things are happening, you would look at either circulating or bone marrow lymphocytes for a particular pattern of proteins, which I partially showed you, CD25, CD103, CD123, which would be characteristic of most classical hairy cell leukemias.
And then the BRAF mutation either by sequencing it or by staining for it in the bone marrow would finalize the diagnosis. Because this a mutation occurs in well over 90% of people with classical hairy cell leukemia.
Just to review hairy cell leukemia variant. This occurs in older people than people who get classical hairy cell leukemia. There is a male to female predominance, but not nearly as striking as for classical hairy cell leukemia. And this can present in more protean ways, sometimes not enough blood cells, quite often a very big spleen. Don't really often have a leukemic phase of the disease. In other words, you don't often see these cells in the blood and they don't have a low monocyte count. And in general, this is a more aggressive disease that doesn't respond quite as well to treatment. It has features on the flow cytometry, which are different. CD103 is positive, but usually it's negative for these other two markers, CD125, CD123 and CD200. And then in this particular disease, it's probably a little bit more important to look for additional factors, which predict how well are people going to respond to treatment.
Dr. Clive Zent: So one of the things that's been really very promising in treatment of these diseases has been the emergence of targeted therapies rather than more generalized therapies. And as everybody knows, the purine analogs, generally are still the most important group of disease therapies for classical hairy cell leukemia. And what people don't always remember is that these drugs are actually somewhat targeted. Most chemotherapy will damage DNA or block metabolic pathways in every cell in the body. But the purine analogs like cladribine and pentostatin are actually relatively specific for lymphocytes. The lymphocytes are much more sensitive because of the specific reliance on a group of pathways, which are detailed in this slide. And so they are semi-targeted therapies, but that's all, really, I'm going to say about this and to move on to the more recent developments.
So this is a pathway which is always a way to confuse an audience. So I've tried to find the easiest one I could. And what this really shows is an important signaling pathway in the maintenance of the classical hairy cell leukemia cell. So what happens is a surface protein receptor is activated and it activates a series of molecules that each activate the next one. So RAS will activate the B type of RAF, which will activate MEK will activate, ERK. ERK then goes into the nucleus and alters how the DNA is transcribed into proteins, which will affect what happens to that cell.
And in 2011, this BRAF mutation was found to be a cardinal feature of classical hairy cell leukemia. And fortunately, subsequent to that, small molecule drugs have been developed that stop the function of this pathway. And that has become a very important treatment. Just while we're here I just want to show you that BRAF, the next molecule down here is called MEK or MAP kinase 2. And that is also a targetable pathway component, which can be effective in treatment of hairy cell leukemia.
Dr. Clive Zent: So just to give you a graphic example of this, this is a bone marrow, and you can recognize it from earlier on. Very extensively involved by hairy cell leukemia. And if you look at the CD20 marker over here, which shows you all the B cells, it's basically full of abnormal B cells. Patient was given eight weeks of Vemurafenib, which is a BRAF inhibitor, an oral medication. And this bone marrow has got much more fat in it. There is a much wider spectrum of these cells, including cells that are making platelets and making all the other good things a bone marrow's supposed to make. This patient has done very well. And if you look at the bottom here, you see only scattered B-lymphocytes, which means that most of the cells that are present in the bone marrow before the treatment have disappeared. This is a very good result.
Dr. Clive Zent: Just very quickly move on to the next targeted therapy. So the other targets we can use, we can target with an antibody. An antibody can be made as part of the immune system, but it can also be used for therapy. And there are ways of making antibodies against specific human proteins, especially on the surface of cells. And this is based on a technology that won the Nobel prize back in 1975. We don't have time to go through it all, but antibodies can either have a little bit of another animal's protein, like say rituximab where the variable region comes from a mouse, but the constant region from a human. Or even less for some of the more sophisticated antibodies and these antibodies have a lot of working components that can be modified to make them different types of antibodies.
Dr. Clive Zent: So antibodies can really come in two types. They come in the antibodies, which you call unconjugated, where the antibody binds to the target. For instance, CD20. And by binding to it, identifies it to the immune system as something abnormal and the immune system then kills it, either by activating a series of proteins called compliment, by activating natural killer cells, which will then make the tumor cell undergo apoptosis, or activating macrophages in the liver and spleen, which will then eat the target cell in something known as phagocytosis.
The best known of these drugs in treating the hairy cell leukemia is rituximab, it binds the CD20. It can be given either intravenously or subcutaneously, and it's been used quite successfully in combination with the purine analogs, cladribine and pentostatin, and with vemurafenib. And other than the first dose reaction, which can be a problem because of the immune system being switched on quickly by the antibody, these drugs are reasonably well tolerated other than immune suppression, which can cause infections. So they've been effective therapies, not so much by themselves, but mostly in combination with other drugs.
Now, the next thing you can do with an antibody is you can use it as a delivery system for something that is toxic to the tumor and provide a targeted toxicity. And this has been developed by Dr. Kreitman and his colleagues at the National Institutes of Health. They've developed a targeted antibody, which binds to CD22, which is another protein that is expressed at high level by hairy cell leukemia cells. And this delivers a toxin, which is actually part of the toxin made by the bacteria called pseudomonas. And it has this very complicated name called moxetumomab pasudotox, but most people call it moxe. This is now FDA approved for use in the United States. It's an IV infusion and it's licensed for use as third or later treatment for hairy cell leukemia. It works for both classical hairy cell leukemia and hairy cell leukemia variant. Toxicities are related to the pseudomonas toxin. It can cause a syndrome known as hemolytic uremic syndrome, which can be serious, but they always try and mitigate against this. And it can also cause fluid leakage into the tissues called capillary leak. It's not very widely used, but it's an important drug to treat people who are no longer responding to other conventional therapies.
Dr. Clive Zent: So in summary, classical hairy cell leukemia, we still treat generally with cladribine or pentostatin, although in the time of the coronavirus pandemic, this has been modulated by some people because of concern about immuno-deficiency that occurs with these drugs. The drug vemurafenib with or without rituximab is a very good second line therapy or therapy for people that are not fit for purine analogs. The MEK inhibitors are being used for people that no longer respond to vemurafenib. Moxe is a good backup drug, and there's some limited but interesting data from ibrutinib tyrosine kinase inhibitor. For hairy cell leukemia variant, one of the therapies that is still used because it is often a good diagnostic test is splenectomy. Unfortunately, you can't safely biopsy the spleen, and it's often not possible to get a diagnosis from the bone marrow only. Cladribine is usually used with rituximab these days, but it's not nearly as effective as it is in classical hairy cell leukemia for most people. Moxe works and there's some isolated experience with the MEK inhibitors, which can also work.
So the last treatment slide here is something that really you should never forget. That is looking after yourself, which is very, very important in people with chronic diseases that suppress your immunity. Infections, second cancers are a problem and surveillance, especially for skin cancer can be very helpful. Avoiding carcinogens and especially stopping smoking, weight control because being seriously overweight can limit your treatment options and give you complications which make treatment difficult. Limit alcohol, especially if your bone marrow is damaged because the bone marrow doesn't do well if you have high alcohol levels, and keep yourself fit with exercise.
Dr. Clive Zent: I want to thank you for this opportunity to talk to you and remind you that all the things we do in medicine are the result of a good team. And our team consists of two excellent pathologists, Dr. Evans and Dr. Burack. We have a research scientist in our lab, Dr. Chu, who is trying to make progress in understanding the disease better. My colleague, Dr. Paul Barr, who runs a service, a nurse doctor, Sharon Lewinsky, and our nurse practitioner Tania. Philip Meacham, who has been very involved in the hairy cell leukemia program as an epidemiologist. And I'm out of Africa and I always have to have something out of Africa to finish up my slides. So this is a conference in Africa.
2. Question & Answer Session
Anna Lambertson (relaying questions from webinar participants): Can you tell us what you know about cross resistance to purine analogs, cladribine and pentostatin, in hairy cell leukemia?
Dr. Clive Zent: Essentially the three purine analogs that are used to treat lymphomas in general as you can see from the slide are pentostatin, cladribine, and fludarabine. We don't usually use fludarabine very much in hairy cell leukemia. But cladribine and pentostatin are actually quite different. Pentostatin is an antibiotic, which means it's made by a fungus to kill bacteria for the fungus. And we've appropriated that activity. So it's a very different structure and molecule to cladribine, which is a synthetic molecule designed by adding chlorine to a purine analog, to stop the body from using a purine analog that looks the same. So it's a purine analog that's synthetic.
So these are very different drugs and pentostatin actually inhibits adenosine deaminase as one of its most important functions. Whereas cladribine has a bit more of a function that it actually gets incorporated into nucleic acids and interferes with a lot more enzymes. So these drugs do have different activities and different cross resistance, but we aren’t able to take some of these cells and actually test them in vitro in a meaningful way in the lab.
So yes, they are very different drugs and people will often respond to one when they haven't responded to the others. Dr. Grever at Ohio state has developed these drugs and is the world expert in it. He usually uses cladribine first because it's more convenient. And then we'll treat people who don't do well with cladribine or relapse early with pentostatin. And they're shown that it actually works very well. So the answer is, there might be some degree of cross resistance, but in general, people who don't respond to the one will often respond to the other. And we just don’t know who those people are. So we generally try that if it's appropriate.
Anna Lambertson (relaying questions from webinar participants): This individual says that he is CD25 negative, but has the BRAF mutation. How rare is this and what are the clinical implications?
Dr. Clive Zent: If you do have the BRAF, the V600E mutation, the good news is that you probably would respond to vemurafenib, which is great. Being CD25 negative. So one of the things they teach us when we do flow cytometry for the first time is that no one marker is ever definitive. So we have all these panels of markers, and we have a particular panel of positive and negative markers that we say is characteristic of a particular disease. And for instance, for classical hairy cell leukemia, you'd expect a person to have B cells with CD19 and CD20 positive. CD20 is usually quite bright.
And then the specific markers that you want to be positive are CD25, CD103, CD123 and CD200. And if one of them is negative, we usually don't take it seriously if the other ones are positive. And if everything else fits for classical hairy cell leukemia, except CD25, and the BRAF is mutated with that specific mutation, then I think most people would agree this is classical hairy cell leukemia, which for reasons we don't understand doesn’t express CD25, which is actually the protein that has a receptor for interleukin-2. So it's not something that you need to worry about and providing everything else fits with classical hairy cell leukemia, that would be an acceptable diagnosis.
Webinar participant: You spoke about cladribine and pentostatin, and I know that from past experience with my father, 70 years old when he was diagnosed and he was treated with cladribine. He was BRAF positive. And after two years he had to be retreated, and he was retreated with cladribine and rituximab. He started rituximab after one month from cladribine. Now today, he's 75, 76 years old. Now I heard that if you do it concurrently, the rituximab and cladribine, it might be even better. But my question is when should you decide to move to Vemurafenib, now we see that you're doing it concurrent together with rituximab or you can go ahead and do it again, the cladribine.
Dr. Clive Zent: Hopefully your dad's going to do well, and he won't need treatment for a long time, in which case, whatever I say now will be irrelevant because things will have changed. So what I'm going to say is only really appropriate for right now. I do not use purine analogs for a third time, for a couple of reasons. First of all, there's the law of diminishing returns. The cells that are surviving to cause relapse didn't get killed the first time. So they're obviously resistant.
And secondly, this is a very immunosuppressive drug. So you can have long-term immunological complications. Third is that it is damaging to those STEM cells. And when you're 75, your stem cells are sort of not in great shape to start off with. And you hit them three times with cladribine, you might land up with a problem in one of two ways. Either you can not be able to make blood in the future, or thirdly, there's always a risk that these sort of drugs can make those cells mutant, and you can get myelodysplastic syndrome, which can result in acute leukemia.
So the question is, the cladribine, I would not use a third time. When somebody has had a purine analog twice would you change to another purine analog like pentostatin? There's a better argument for that because pentostatin doesn't hit the rest of the stem cells quite as hard. You'd probably want to use it in a lower dose than we usually do. And you probably want to use rituximab again, because there's no real cumulative toxicity in the same sort of way for rituximab. But if your dad does have the disease, first of all, he needs to be evaluated. If his blood counts drop, you've got to be sure that it's not a long-term complication of the cladribine. He needs a bone marrow study. If it's recurrence of his classical hairy cell leukemia, then this is a time to think about more targeted therapies like vemurafenib. The vemurafenib by itself in heavily pretreated patients gives a median duration of response for approximately two years.
The initial study showing that if you add rituximab it can last for longer. And the toxicity in terms of suppressing the bone marrow is much less. So I would seriously think about that. Using something like moxe in somebody who's 75 years old probably increases the toxicity in that, hemolytic uremic syndrome and the capillary leak syndrome are not common. But if you're 75 years old, they're probably more serious. So that's the sort of approach I would have for your father if he hypothetically needed treatment now.
Anna Lambertson (relaying questions from webinar participants): We've received a number of questions both through the chat and the Q&A related to COVID-19, particularly about patients who are in remission. Is there any new information or any light that you can shed on the situation with COVID-19, particularly for patients who are in remission?
Dr. Clive Zent: So the disclaimer is that we know almost nothing about COVID-19. There's a lot of stuff going on all over the place about it, but in terms of our usual due diligence in scientific research, we know very little that has been established with good science and confirmed with separate good science about COVID-19. However, that said, if you have hairy cell leukemia, if you've been treated for hairy cell leukemia, or you have any other B-cell lymphoma, you really don't want to get COVID-19 because you are profoundly immunocompromised and you're not going to generate a good immune response. You're less likely to have a asymptomatic infection. And if you do develop multi-organ damage, you're going to have a higher risk of secondary infections and other complications. So I think that the only really sound advice that I can give you is social distancing, wear a mask, avoid anybody who has any signs of infections, and get medical care immediately if you're not well. We really have no other way of preventing this infection. We have almost no effective treatment. And unfortunately, vaccination for Coronavirus has never previously been successful. We are all hoping for a novel approach that will overcome that burden, but we don't have any really good data.
The fact that healthy people get some sort of immune response to the vaccines as reported from Oxford, England recently is hopeful. Because without that, you're not going to have a vaccine, but there are many other vaccines that have generated an immune response in the past, but never actually became useful. And we also know that patients who have hairy cell leukemia and being treated for it, never recover fully immune function. And they don't really respond to vaccines as well as people who haven't had lymphomas. So I think, take care to not get infected is the most important thing to do.
Anna Lambertson (relaying questions from webinar participants): When it comes to patients who are refractory, can you speak to current thinking on the treatment course of purine analogs, cladribine or pentostatin, combined with rituximab. Currently, combining rituximab with a purine analog is not generally used as first-line treatment, but do you think there's a chance that it could be in the future? And what is your thinking on whether rituximab should be given concurrently or sequentially?
Dr. Clive Zent: And we're talking about classical hairy cell leukemia now, because for variant, almost everybody is combining rituximab with cladribine. So, there was a time when standard therapy for diffuse large B-cell lymphoma, there was a big question of whether you should give it together or afterwards. And the idea of using it afterwards was that it was less toxic and people were very worried about the first dose effect. And it was never really shown conclusively for diffuse large B-cell lymphoma, even for some of the other lymphomas, that there was any real difference between concomitant or sequential therapy. Based on what we've learned since then, I think most of the data would support concurrent therapy, if you're going to do it. There really isn't a whole lot of overlap with cladribine, because cladribine, if you give it is usually five days of therapy. And you can give rituximab with it if you want to, but the cladribine has got a short half-life. So a day after your last dose, there's no cladribine around. And so subsequent rituximab given weekly or monthly is not really actually being given with the cladribine. So I'm not sure there's a big difference between giving it concurrently or sequentially.
For pentostatin, it's a bit different because we give pentostatin every two weeks. And there's really no data that I know of to give rituximab after pentostatin, so most people do it together. Purine analog therapy for classical hairy cell leukemia in a good number of patients has been so good in terms of what we had before we had cladribine, that it's really hard to show that adding rituximab helps. So if you have a treatment that has a median duration of response in excess of 10 years, it's very hard to do a study that's going to show that it goes from 10 years to 12 or 15 years, because first of all, it's going to take you 20 years to find out. And by then, hopefully we'll have much better therapies. And secondly, it's hard to prove that that sort of change actually happens.
Adding rituximab to cladribine for first line therapy, just on first principles, I'm not sure that we really have a very good reason for doing that because rituximab is not a trivial drug. People get into trouble the first dose reactions, people get a suppression of immunity that can give them opportunistic infections. And it also causes prolongation of neutropenia, which is a problem with cladribine already. So I think that using cladribine by itself for most people with first treatment of hairy cell leukemia is still reasonable. And adding rituximab is very unlikely to be proven to be better in the sort of standard randomized controlled trials that we'd like to use to base our therapy. So I'm sorry, I can't give you a definitive answer on either, but I have reasonable surety that there isn't a definitive answer out there and not going to come soon.
Webinar participant: One question, if I could. Could I ask you to look into the future and talk a little bit about some of the therapies or drugs that you're particularly excited about? You referenced the world being very different 10 to 20 years out. Could you be more specific there?
Dr. Clive Zent: I think that working with the drugs we have already is going to make a big difference. So if you look at the lymphomas that are curable today, they’re curable because we've been able to use more than one drug at the same time. So if you look at a tumor burden in a person who is needing treatment for lymphoid malignancies, we're talking about a tumor burden somewhere in the order of 10 to the 11, to 10 to the 12 cells. This is a massive number of cells. This is kilograms, many pounds of cells. And the reason we can't cure a lot of these diseases is because there's a very small subpopulation of those cells that doesn't get killed by the therapy we use. And that cell, even if it occurs in one in 10 million, okay, there's still 100,000 cells there.
So we are expecting our drugs to perform miracles in that they are going to have an effectiveness of a 99 with 10 nines past it or better. That doesn't exist. But fortunately, resistance to cells is generally drug specific if you use drugs that are different enough. So in other words, let's say we use something we talked about now, cladribine and rituximab. The reason cladribine doesn't work perfectly is because there are some cells in their clone that don't care whether the adenosine deaminase inhibitor marker because they don't need it. They have a mutant form that either doesn't get inhibited by cladribine, or they have a redundant pathway that allows them to survive without that pathway. And let's say we're using rituximab. Rituximab, why don't all cells get killed by rituximab? Well, some cells, they don't have much CD20 on their surface, or they have very high levels of molecules that prevent compliment killing or something else. There will be some cells that will not get killed by that drug. In addition, antibodies have limited ability to kill because they use your innate immune system and your innate immune system is not infinite in its ability to kill cells.
So if you use two drugs that have different mechanisms of action, you're going to get less cells left over at the end of it. And those cells are unlikely to be resistant to both drugs. And if you do the math, the magic number is somewhere between four and five different drugs. I think if we can combine four or five different drugs that have different targets and different mechanisms of action, we might actually be able to cure somebody with hairy cell leukemia in the future. So we need to carefully learn how to use these drugs together.
The second thing we need to do is learn how to fix the immune system. So with chronic lymphocytic leukemia, hairy cell leukemia, marginal zone lymphoma, all these slow growing or indolent B-cell lymphomas, we are reasonably good compared to 50 years ago at treating these diseases. But we can't fix the underlying immune defects that are caused by the diseases and often exacerbated by our treatments.
So using immunotherapy to clear the disease and perhaps maybe developing immunotherapy in the future to restore immune function would be very helpful. And I think these are things we need to think about. Talking about immunotherapy, sort of the next generation. The next paradigm leap with antibodies was to use them to make chimeric antibody expressing T cells and other effector cells. So this is something that might become useful for hairy cell leukemia in the future. There's no reason to believe that CAR-T, if it becomes less onerous, especially in terms of toxicity, couldn't be used later on to clear out a good part of the remaining immune cells.
But remember that all these diseases come back. If you have one replication, competent cells leftover. These are cells that occur because one cell became mutant cell. So you've got to kill every single cell that is capable of regeneration, or you have to restore the immune system so when those cells start dividing, your own immune system takes care of them. So I think this should be the focus going forward. And I think using targeted therapies rather than more general chemotherapies and using them together in smart combinations and learning how to restore immune surveillance to prevent recurrence and to prevent all the other complications is what I would like to see happening going forward. But I don't have ideas how to fix all those things. Just a few.
Anna Lambertson (relaying questions from webinar participants): We've received a few questions about secondary cancers. You had mentioned that in your slides. Can you share any more information about the types of secondary cancers that patients with hairy cell leukemia may be at greater risk for? And there's one individual who's asked specifically about colon cancer and whether there's a connection.
Dr. Clive Zent: So the answer to the question is skin, skin, skin. It's our biggest organ. It's irradiated all the time by ultraviolet at various levels. And in the low grade lymphomas, there's overwhelming evidence that the biggest risk in terms of second cancer, numerically, is skin cancer. The best data comes from CLL, chronic lymphocytic leukemia, but there's mounting evidence that this is a problem in patients with all the indolent B-cell lymphomas, including the hairy cell spectrum.
The reason is that your immune surveillance that will destroy any mutant cells in the skin is not working. And in fact, almost everybody who's immunocompromised, whether it be post-transplant because of a long-term use of immunosuppressant therapy, are at increased risk of skin cancer. So that is the most important thing. And it's both squamous cell and basal cell and melanoma. So ultraviolet is avoidable, or you can at least modulate its effect. You can get regular skin checks to yourself and with a dermatologist and you need aggressive treatment for any skin cancer you get.
When it comes to other tumors, we don't have a lot of data about hairy cell leukemia. Some of the older work showed that there's a general increase in non-hematological diseases. I don't remember if colon was particularly high, but as far as I'm aware, there's not a particular concern about colon cancer. So coming back to the other cancers, besides skin that are concerning, the one that is a problem is myeloid malignancies because of use of drugs, like purine analogs, which can increase your risk of getting myelodysplastic syndrome.
And the other thing is we are becoming increasingly aware that people who get one B-cell malignancy have an increased risk, about a five-fold increased risk of getting a second lymphoid malignancy, possibly because of predisposition or maybe an environmental event or events that are making them more predisposed to B-cell malignancies. And then of course the last thing here is just sort of somewhat speculative. There's some data that there's a predisposing lesion, perhaps a BRAF mutation itself, that could cause people to get hairy cell leukemia again, and maybe some of the people that get hairy cell leukemia decades later have got a separate clone from the original one. So those are the second malignancy problems that I'm aware of. And the one that I think people should really try and mitigate as best they can is skin cancer.
Webinar participant: I understand that post-cladribine, even if you're in remission, that your immune system is typically compromised for sort of six months to two years. Is there any test that sort of gives you a good idea how well your immune system is recovering? So whether it's nearer to six months or two years. Maybe a CD4 test or something?
Dr. Clive Zent: So the simple answer is yes, the CD4 test is probably the best way of seeing if there's recovery from Cladribine induced immunosuppression. People usually use something in the range of an absolute counter between 0.2 and 0.4. Sometimes it's called 200 to 400. Obviously your neutrophils coming back in the early phases is good for your innate immune system. Your monocytes coming back as also important, that usually happens at about six to eight weeks.
But the important thing is that hairy cell leukemia itself is immunosuppressant. And the damage done to the immune system by the disease does not get reversed by cladribine ever. So people who've been treated effectively with cladribine, pentostatin or anything else have a immune defect, which we can't fix yet, we're just not able to fix it. So you had higher risk of infections with opportunistic infections, and you're also probably at higher risk of infection with encapsulated organisms forever. And that's things like pneumococcus and staph and strep. So you've got to be aware of that and take due care. And also you probably have a higher risk of complications from influenza. So vaccines like the influenza vaccine, pneumococcus vaccines, opportunistic infections like shingles can be a big problem. So the Shingrix vaccine is all things you can take to mitigate it. I don't know of any really good tests of recovery of your own immune system. Unfortunately, the functional event is what really tests it is when you get these infections.
Anna Lambertson (relaying questions from webinar participants): We've received a number of questions about how frequently one should have a bone marrow biopsy done. What do you recommend for your patients?
Dr. Clive Zent: Outside of a clinical trial, obviously a lot of people will need a bone marrow to confirm the diagnosis. And then subsequent bone marrows in people who are doing well are not obligatory. So what we usually do is follow the guidelines and suggest the bone marrow in patients who've done well after cladribine or pentostatin, after cladribine after six months to see how well they've done, is there residual disease, and how well is bone marrow functioning, but it's not obligatory. I think you can treat people, manage people very well without doing it.
The one time that you do really need a bone marrow is when you're using pentostatin. Because really the best way to treat people with pentostatin is to treat them until they have a negative bone marrow and CAT scan them, and then give them two more cycles. So I guess that's part of the management. But I don't think most people need routine bone marrows. On the other hand, when you relapse and need treatment, again, I would generally recommend a bone marrow. Because there are a lot of causes of not making cells in the bone marrow and only one of them is recurrence of your disease. So you really want to be sure you're treating the right disease. Because the treatments for this disease are pretty specific and non-trivial in terms of their toxicity.
Anna Lambertson (relaying questions from webinar participants): We also received a question about ibrutinib, what are your thoughts on ibrutinib as an option for patients, maybe who received previous treatments and who have relapsed?
Dr. Clive Zent: Ibrutinib, there's been a few clinical trials. None of them have been reported outside of meetings. And the answer is that ibrutinib works for a lot of people who do have hairy cell leukemia. It's a very slow response, but it does work, and it's generally quite well tolerated. So I think that as a salvage therapy, especially for people who are not likely to tolerate or respond to more toxic therapies, it's certainly a reasonable option. In that vein, there's a couple of drugs that are molecular targeted therapies that could also be considered. We don't know whether the other BTK inhibitor acalabrutinib is useful, but it's sometimes tolerated better by older folks. And it would be interesting to know what venetoclax, BCL-2 inhibitor does. And I'm sure we'll have some data on these at some stage in the future.
Anna Lambertson (relaying questions from webinar participants): We've received a number of questions through the Q&A about what causes hairy cell leukemia. What can briefly share on that regarding our understanding of the cause?
Dr. Clive Zent: So there are two sort of components. First of all, there's a reasonably strong familial tendency in hairy cell leukemia. So people who have hairy cell leukemia, classical hairy cell leukemia, have a higher risk of a first degree relative. And in people who have first degree relatives, that's your parents, siblings, or children, who have hairy cell leukemia or another B-cell lymphoma, like CLL, marginal zone, these sort of types of diseases, have a much higher risk of developing these diseases. So I think it's your genetic repertoire. We all come from a genetic heritage and our antibody determining genes, which there's enormous diversity among different people. The set of antibody determining genes in a particular person depends on where their ancestors came from and the infections that that population was exposed to and how they responded to those infections. So for many of these diseases, these lymphomas in general, the sort of spectrum of lymphoma from people that say come from the Eastern parts of China versus parts of Africa or come from Europe is actually very different. So I think the biggest risk is your genetic heritage. And then for the rest, we really don't know. We have very little data on what causes people to have hairy cell leukemia or related lymphomas. It's possible that dioxin like substances and herbicides and pesticides and other synthetic chemicals might be important. There's not a lot of data on radiation. There's very little data on infectious cause. It really looks like it's mostly a genetic predisposition.
Anna Lambertson (relaying questions from webinar participants): We have a question with regards to classic hairy cell leukemia and partial response versus complete response. How common is it for a patient with the classic form to have a partial response rather than a complete response to treatment with cladribine?
Dr. Clive Zent: It depends how you classify partial response. Partial response can mean that the lymph nodes and the spleen don't go back to normal size. It could mean that there's still some disease in the bone marrow or occasionally in the blood. It's very hard to tell. So the first thing that's important about this is that you really don't want to check for response for six months after cladribine. Because if you check too early, there might be areas of disease that are slowly dying off, that you're going to call residual disease when in fact it's going to be gone at six months or later. So I think many people would only do a bone marrow on somebody who's been treated with cladribine when they've recovered their counts. And they're six months out.
With that disclaimer, you need to remember that our classification of partial response is very crude. If you have lymph nodes that are still big, that's a very large number of abnormal cells. On the converse sometimes lymph nodes don't go back to their normal size because there's scar tissue there, the structural tissue in the lymph node or the spleen, if it's big, doesn't always resolve. Organelles and organs don't always go back to the normal size and then come to the bone marrow.
The bone marrow testing can be different between different institutions. So for instance, you can just look at the bone marrow under the microscope and see if the lymphocytes are more than 30%. That is a very crude mechanism today. Because the 30% of lymphocytes in the bone marrow out of all the cells could be 90% hairy cell leukemia, and you're still going to call that a complete response. Then you can stain, as I showed you in the presentation for CD20 or for PAX-5 or even for BRAF, and see whether you can see anything left there. That will give you a resolution down to about 1%. And flow cytometry on the peripheral, on the bone marrow aspirate, will also give you resolution down to 1%. 1% of million cells are still a very large tumor burden left behind. And there's no standard clinical method of testing for better than that.
So what I'm trying to say in a very complex way is that there's a continuum of responses, but we know from history that we cannot prove a cure for anybody who got cladribine at this stage, because we are seeing relapses at 20 and 25 and 30 years. 30 years is about how long it's been used. So my answer is that you can define the level of residual disease with different degrees of resolution, but it's pretty unlikely that we get rid of all the cells.
Now, what you really want to know as a patient is how long before your disease comes back. And that's determined by two things. First of all, how much disease you've got rid of in other words, how much is left at the end of treatment and how fast that disease comes back. And that is an intrinsic component of the biology of the disease not probably affected that much by treatment. So the answer is you as a person are going to do better with less disease after you're treated. But if you compare person to person, some people with more disease at the end of treatment are going to have a longer time before they need treatment than other people who might have a complete response, but have faster growing disease. So it's very, very hard to answer the equation in a meaningful way. Because one, we don't have a good way of measuring residual disease. And two we have no way that I'm aware of knowing how fast their disease is coming back. It's not a very satisfactory answer, but the best I think we can do.
Anna Lambertson: We have reached the end of our time and I want to thank you on behalf of the Hairy Cell Leukemia Foundation and everyone who joined the webinar today for taking the time out of your very busy schedule, to speak with us and share this very important information. We will be sharing with Dr. Zent's permission, his presentation, along with the transcript of the discussion on our website. And we will email those who are registered to let you know when that's up on our website.
Dr. Clive Zent: Thanks for the opportunity. I appreciate it.
This transcript has been edited for clarity.