What’s New in HCL? Webinar Series: Vemurafenib plus Rituximab in Relapsed or Refractory HCL
August 11, 2021
Hosted by the Hairy Cell Leukemia Foundation with expert speaker Dr. Enrico Tiacci from the University and Hospital of Perugia in Italy. Moderated by Anna Lambertson, HCLF Executive Director.
Presentation Materials
Note: Slides and a recording of Dr. Tiacci’s presentation can be accessed using the links below. Each link will open the file via a new window. You do not need a Box account to view these files.
Transcript of Webinar Presentation and Q&A
Presentation from Dr. Enrico Tiacci
Dr. Enrico Tiacci:
I'm going to recapitulate the mechanism underlying hairy cell leukemia development. In 2011, we discovered that hairy cell leukemia is caused by a mutation of a certain gene called BRAF. It normally conveys within the cell, below its surface, signals that come from the surrounding environment and that tell the cell to survive and proliferate in a controlled way, just when it is necessary, and as long as the external signal is present.
BRAF, which is an enzyme, does so by phosphorylating, that is adding a phosphate group, labeled as P, to its target MEK, which then gets activated and also phosphorylates its target ERK, which ultimately activates several targets in the nucleus of the cell, to instruct it to survive and to proliferate.
But when BRAF acquires a particular mutation called V600E, which happens in several cancers and with the highest frequency in hairy cell leukemia, 97% of patients. BRAF continuously phosphorylates MEK, independent from upstream regulatory signals, resulting in survival and proliferation of the cell, which eventually becomes a tumor cell.
Dr. Enrico Tiacci:
However, BRAF and MEK can be blocked by orally available inhibitors, drug inhibitors, in particular Vemurafenib, which blocks the BRAF. These were being tested in patients with BRAF mutated metastatic melanoma in 2010 and 2011, when we were discovering the mutation in hairy cell leukemia.
And here you see what happens to our leukemic cell, with its typical and thin hairy like projection in green here. If you expose it in the laboratory to Vemurafenib, the cell first loses its hair after one or two days, becoming smooth, but still alive. And then after about another day it shrinks and dies, acquiring this red color, the red color of a death marker added to the culture in laboratory.
So mutant BRAF is the key genetic cause of hairy cell leukemia because it occurs in almost all patients. Its blockade first spoils nucleic cells of their hairy cell identity, and then kills them. This turned out to be useful as a non-invasive diagnostic test to distinguish in a simple blood draw, patients with hairy cell leukemia from patients with other leukemias that don't have the mutation, but that can mimic hairy cell leukemia. Although, currently they require different treatments.
Of course, the BRAF mutation lends itself to therapeutic targeting, especially in patients who have been treated multiple times with chemotherapy, cladribine or pentostatin. Indeed, patients who relapse become progressively less responsive to repeat treatment courses with these drugs, which are also myelotoxic and immunosuppressive. And this is of concern, especially in the setting of repeated relapses years apart from one another, while the patient continues to age and exposing him or her to another course of myelotoxic chemotherapy.
So, mutated BRAF can be attacked by Vemurafenib, this small drug here that displaces from BRAF the ATP molecule donating the phosphate group to BRAF to allow phosphorylation of MEK. The Vemurafenib is taken by mouth, and at the dose of 960 milligrams twice daily it has been approved for more than 10 years for the treatment of BRAF mutated metastatic melanoma.
We started in 2012 a clinical trial in patients with relapsed or refractory hairy cell leukemia that was published in the New England Journal of Medicine, together with a similar American study later initiated by Dr. Park in New York and other centers in the United States. And in these two trials, a total of 54 patients with an average of three prior therapies received Vemurafenib by the standard dose, 960 milligrams twice daily for an average of 16 to 18 weeks, that is four to five months.
Importantly contrary to chemotherapy, there was no significant myelosuppression, as Vemurafenib kills only BRAF mutant leukemic cells and spares normal bone marrow cells which don't have the BRAF mutation. Vemurafenib did have other side effects which are almost inevitable as with any drugs, and we will see them later. But these side effects were manageable and similar to those already observed in melanoma patients through Vemurafenib.
Vemurafenib produced a very high response rate, around 90% overall, with about 35% complete remissions (CR) and about 55% partial remissions (PR), usually obtained after eight to 12 weeks. But all patients, even those achieving complete remission, had residual disease, around 10% in complete responders. Residual disease in the bone marrow. It was not visible by general morphologic staining, something that is enough to call a complete remission by standard response criteria for hairy cell leukemia. However, residual disease in complete responders could be documented through more sensitive immunological staining or genetic assay for the BRAF mutation.
And indeed, the definition of response in hairy cell leukemia is somewhat tricky as complete remission requires a normalization of all blood counts and no enlarged palpable spleen, and no hairy cells visible in the bone marrow or the blood, just using morphologic stains. But complete remissions without minimal residual disease, abbreviated as MRD, also requires the absence of hairy cells and the BRAF mutation when using these more sensitive assays like immunohistochemistry and PCR for the BRAF mutation.
Finally, in a partial remission, blood counts have normalized, but there is gross residual disease in the bone marrow and blood. And/or the spleen is still enlarged, although at least 50% less than before therapy.
Dr. Enrico Tiacci:
So, this persistent leukemic residue in patients treated with Vemurafenib was the reservoir for subsequent disease relapse after stopping the drug. And indeed, in our trial we did a longer follow up, the patients relapsed on average nine months after stopping treatment overall, and this occurred later in patients achieving complete remission after 19 months on average. Then, in patients achieving just a partial remission, who had a greater residual disease burden at the end of treatment, and relapsed on average after six months.
To get rid of these Vemurafenib resistant cells, we then performed another clinical trial at our center, also published in the New England Journal of Medicine about three months ago. Using Vemurafenib to block mutated BRAF inside the cell as before, but now adding another target agent, the monoclonal antibody rituximab, to attack the hairy cell from the outside as well in various ways and also harness the patient immune system in order to combine these two non-chemo therapeutic drugs, known to be effective against hairy cell leukemia on their own through distinct and therefore potentially complementary mechanisms. And rituximab has some activity as single agent in relapse or refractory patients, producing frequent responses, but few complete ones.
Vemurafenib and rituximab were given in the second trial for two cycles, each one comprising 28 days of Vemurafenib by the standard dose, plus two concomitant doses of rituximab on days one and 15. Then after the second cycle, four additional infusions of rituximab were to be given as a kind of consolidation.
We enrolled in this trial 31 patients, with a mean age of 61 years, who were refractory to chemotherapy in 35% of cases. And they were quite heavily pretreated with on average three prior therapies. Toxicity was as expected from either drug when used alone, it was largely of low grade, that is grade one or two, in a scale of one to five. Again, no myelosuppression.
And for rituximab, there were some reactions to the infusion of this drug, which is given intravenously, and some transient reductions of the neutrophil count. For Vemurafenib, mainly transient joint pain. Anthralgia you see here. Cutaneous rash, photosensitivity, and asymptomatic increase of bilirubin and of pancreatic and hepatic enzymes, all reversible.
Dr. Enrico Tiacci:
No patients discontinued Vemurafenib prematurely, but its dose was readily frequently reduced in about half of patients, yet mostly by just a fourth from 960 milligrams to 720 milligrams twice daily. And it was often followed by re-escalation to 960 milligrams twice daily, such that the relative dose intensity received by the patient was high, at an average of 92% of the planned dose. Attesting to the overall good tolerability of the standard Vemurafenib dose, especially when delivered for this short duration of eight weeks.
So, four of the 31 patients did not get the whole planned treatment for various reasons, largely unrelated to drug toxicities, and were therefore not well available for treatment efficacy. And among the 27 evaluable patients, strikingly all but one obtained a complete remission, that is 97% rate of complete remission, or 87% by intention to treat when including also the non-evaluable patients.
And complete remission was obtained in all five evaluable patients that had been previously refractory to rituximab, and in all seven evaluable patients who had relapsed after responding to a previous therapy with single BRAF inhibition. But mostly responding with just a partial remission.
Dr. Enrico Tiacci:
And the presence of an enlarged spleen or the prior removal of the spleen did not compromise the response to treatment with Vemurafenib plus rituximab. Most patients entered complete remission already after cycle one, that is after four weeks of Vemurafenib and two concomitant rituximab infusions. And very importantly, almost two thirds of available patients obtained a complete remission and was negative for MRD, for minimal residual disease. And MRD negativity was achieved even before sequential rituximab consolidation in about half of patients. So right after cycle two. Whereas the other half of patients cleared MRD, minimal residual disease, after rituximab consolidation.
So this is the type of complete remission produced by Vemurafenib alone in about 35% of patients in our first trial. Usually it takes eight weeks to get from this heavy infiltration of the bone marrow by the red leukemic cells, to a complete remission that however always carried a leukemic residue. About 10% of cells are masked here, you see them in red, by immunostaining for the leukemic marker CD20.
Instead, in our subsequent trial with Vemurafenib plus rituximab, complete remission was obtained in almost 100% of patients, not just 35%, and mostly after just half the duration of Vemurafenib and with the addition of two concomitant rituximab infusions.
And CR were mostly MRD negative, 0% hairy cell leukemia, and these red cells here are normal plasma cells, expressing CD79A. Here CD20 staining was completely negative, and we used CD79A just to show that the staining was in principle working.
Dr. Enrico Tiacci:
So these deep responses translated into a survival free from relapse, that is free from low blood counts of 85% at almost three years of average follow-up from the end of treatment in the 26 patients who responded. And you see it here in this line where each drop, each stair over time, this is expressed in months, in the result axis. So each drop represents one or more patients experiencing disease relapse, and therefore decreasing the percentage of patients free from relapse, that is represented in the vertical axis. We observed four relapses during this follow-up.
And even when considering all 30 hairy cell leukemia patients enrolled, this including also those not receiving the whole planned treatment, survival free from worsening of blood counts, from disease progression, was still almost 80% at slightly more than three years from the start of treatment.
And the MRD negativity status was quite durable in that among the 17 patients who reached negativity for minimal residual disease, MRD, and who underwent subsequent bone marrow evaluations during an average of 28 months of follow up from achievement of MRD negativity, no patient lost the MRD negativity status. So the line is flat at 100%.
Dr. Enrico Tiacci:
So we observed only four patients relapsed, and all of those four patients had not cleared minimal residual disease at the end of treatment, so they were MRD positive. Indeed survival from relapse or low blood counts was longer in patients achieving MRD negativity, than in those still achieving complete remission but with the presence of minimal residual disease. Three of the four patients who relapsed had previously received therapy with a BRAF inhibitor, such that survival free from relapse was also longer in patients naïve to a BRAF inhibitor, here in red, than in patients with prior exposure to a BRAF inhibitor, here in blue.
And interestingly, relapse free survival in patients naïve to a BRAF inhibitor, red, remain longer than the cumulative relapse free interval the patients previously treated with a BRAF inhibitor enjoyed both following single therapy with a BRAF inhibitor RFS1, plus its subsequent combination with rituximab, RFS2, potentially suggesting that Vemurafenib should be used directly in combination with rituximab rather than being delivered first alone, and then added to rituximab at relapse. Instead, prior rituximab exposure in the patients did not seem to impact relapse free survival after Vemurafenib plus rituximab, which was good irrespective of prior treatment with Vemurafenib.
So, in an indirect comparison with previous trials of Vemurafenib alone, addition of rituximab greatly increased the rate of complete remission and even more the clearance of minimal residual disease. It significantly shortened treatment duration, while prolonging survival free from disease progression and relapse substantially, as you can see here in this comparison. And this rate of complete remission, MRD clearance and relapse free survival of Vemurafenib plus rituximab, compare indirectly well with the standard treatments approved in hairy cell leukemia, and with the same average number of prior therapies, that is three.
In particular, chemotherapy with Purine Analogs plus rituximab, a regimen which in the London series of Dr. Else has a longer follow up of more than seven years, compared to the Vemurafenib plus rituximab, and 87% of patients were still free from relapse at more than seven years in this series. But this series does not include patients refractory to a Purine Analog chemotherapy, who were instead 37% in the trial of Vemurafenib plus rituximab.
And regarding the immunotoxin moxetumomab pasudotox, which is approved for relapsed or refractory hairy cell leukemia in the US, but not elsewhere, by the way. Its anti-leukemic activity is notable for a single drug. But appears slower compared to these other two regimens combining two drugs.
Dr. Enrico Tiacci:
So in summary, Vemurafenib plus rituximab is a safe and tolerable chemotherapy free regimen, quickly producing deep and durable responses in relapse and refractory patients. And since it is not myelotoxic nor immunosuppressive, we and others have used it successfully also in patients with active infections. So it can be very helpful in this difficult setting, as it allows the delivery of an effective and definitive treatment during an infection that can be severe.
It has the further important and distinct advantage of having a fixed and short duration, and therefore of being quite well tolerated by patients. It is clearly superior to historical results obtained with Vemurafenib or rituximab alone, and compares well with all other treatment options, whether conventional or investigational, that are available in the relapse or refractory setting.
It seems more effective when delivered directly to patients never exposed to a BRAF inhibitor, rather than relapse after therapy with a BRAF inhibitor. It has the potential to challenge the chemotherapy-based standard of care even in the frontline setting, that is in newly diagnosed patients that have to receive their first treatment. Something that we're indeed trying to pursue in the near future.
So let me thank several people in my group who contributed to our laboratory research program in hairy cell leukemia, in particular Valentina Pettirossi, Alessia Santi and Gianluca Schiavoni. And for the clinical trials, I'd like to thank at our center Brunangelo Falini, with whom I co-lead the clinical research program on hairy cell leukemia. And also Luca de Caloris, as well as other colleagues. Many collaborators throughout Italy who enrolled or sent us patients for our trials. And our funding sources, with a special acknowledgement to the Hairy Cell Leukemia Foundation for its continued support to our research for more than 10 years.
With that I conclude, I thank you very much for your attention and I'm happy to answer your questions.
Question and Answer Session with Webinar Participants
Anna Lambertson:
One of the questions that came up is related to the dose. You had talked earlier about the dose that you used in this trial. Some patients are interested in the effectiveness if an alternative dose of Vemurafenib is used. For example, one patient said that they were only able to tolerate 240 milligrams of Vemurafenib two times a day because they experienced a very bad rash. Do you still find a lower dose of Vemurafenib to be effective for patients based on your experience in these trials?
Dr. Enrico Tiacci:
Yes, this is a very important question. We do not have a direct experience with a lower dose of Vemurafenib because we always use both in our trials and also off trials, the standard dose. And even in patients that had to lower the dose for toxicities, we never went below 480 milligrams twice daily. We treated more than 100 patients, and we didn't need to go below this half dose.
But the literature is rich with patients that have been treated with doses even lower than 480 milligrams twice daily, including 240 milligrams twice daily, like the patient that is asking this question. I know very well the data of my colleagues in Europe and in the US that have used these lower doses. And lower doses are also effective to some extent, definitely, but it seems that going below 480 milligrams twice daily might be associated to a shorter time to next treatment. So as if leukemia would relapse earlier.
So our approach is to start with the standard dose of 960 milligrams twice daily, which is relatively well tolerated by half of patients for eight weeks. And then in those patients that don't tolerate, to step down the dose gradually and then try to re-escalate the dose as soon as it is possible. For example, by adding some drugs like low dose Prednisone, low dose corticosteroids that can really ameliorate cutaneous rash, and arthralgia or joint pains, those are the main side effects that are troublesome for patients.
Anna Lambertson:
It's our understanding that the vast majority of patients with the classic form of HCL have the BRAF mutation, and those with the variant form of the disease do not.
We did have someone ask if it's possible for someone with the variant form to have a BRAF mutation.
They're really trying to understand whether this could benefit them. If you can quickly clarify whether this combination could be used for somebody with the variant form.
Dr. Enrico Tiacci:
The straight answer is that the overwhelming majority, almost all patients with hairy cell leukemia variant, do not have the BRAF mutation. And those, the very small minority that do have the mutation, then probably the diagnosis should be reviewed and they might well have classic hairy cell leukemia, not variant hairy cell leukemia.
These are very rare cases that need really a thorough review of the diagnosis and of the genetic makeup of the leukemia to really understand whether they would benefit or not from the Vemurafenib.
Anna Lambertson:
Many individuals joining the webinar today are thinking to the future. They’re asking what treatments they could talk to their doctor about should they relapse. Some of them have been treated with cladribine alone, and they're in remission. Others, there's at least one person who did have this combination, Vemurafenib and rituximab, not at first line but in subsequent treatments after other things didn't work.
Let's say somebody has already had Vemurafenib and rituximab, because they didn't respond well to other treatments, but then after a period of remission they relapse again. Is Vemurafenib and rituximab a combination that you would recommend for a patient like that? And then for somebody who did well with cladribine previously, is in remission and relapses, would you say, "Great, try Vemurafenib and rituximab at second treatment"? Or would you be directing them back to the standard therapies?
Dr. Enrico Tiacci:
So regarding the scenario of previous chemotherapy that produced a good and complete remission that is lasting several years, here upon relapse, which may well occur 10 years after cladribine, I would most likely repeat cladribine whether alone or adding rituximab, because the addition of rituximab is also a powerful tool to improve efficacy of chemotherapy, not only of Vemurafenib. If the first course of chemotherapy lasted several years, then I think that the best choice is to repeat chemotherapy alone or in combination with rituximab.
But, if instead the remission lasted just one or two years, then at this point I would consider certainly a regimen including rituximab, and as a partner of rituximab, I would choose a BRAF inhibitor and not chemotherapy, because chemotherapy produced a very short-term remission.
And the same concept would apply for the scenario of a patient that has already received Vemurafenib plus rituximab, then relapsed. So, if the relapse occurred some years after finishing the treatment, then it is reasonable to consider retreatment with Vemurafenib plus rituximab, because one would assume that a patient that has already received a first course of these two drugs has already exhausted the chemotherapeutic standard of care that is available, like cladribine and pentostatin.
However, if the remission after Vemurafenib plus rituximab was short, like one year or one and a half year, then especially in the US where moxetumomab pasudotox is approved and is available. I would consider this known chemotherapeutic immunotoxin treatment, with moxetumomab pasudotox, if the patient has not received it before Vemurafenib plus rituximab, which I would assume to be the case in this question
So this drug (moxetumomab pasudotox ) is a very good drug and it's a pity that it's only available in the US, but it's something not to be forgotten, especially for American patients.
Anna Lambertson:
There have been a few questions about side effects related to rituximab. So, if an individual wasn't able to tolerate rituximab alone, would the combination treatment likely result in a poor response for them? Have you seen a patient unable to tolerate one of these two drugs alone, but able to tolerate the combination?
Dr. Enrico Tiacci:
Generally, if you had a severe reaction to rituximab alone, it's difficult for you to manage to have eight infusions of rituximab with Vemurafenib. But we had at least one exception. There was a patient coming to us from another center who had a history of a severe reaction to rituximab, but since he had no other options, we hospitalized him and started Vemurafenib plus rituximab, and we gave him rituximab at a very slow infusion rate overnight. And with close monitoring by physicians. And luckily he could tolerate the infusion of rituximab, he didn't have a reaction. The subsequent infusions of rituximab were given also slowly over eight hours during the day.
So, usually a patient that is allergic to rituximab won't be able to get it alone or a second time, or in combination with Vemurafenib, but there can be exceptions that could be verified by inpatient hospitalization, and inpatient infusion of rituximab, under strict and close medical control.
Anna Lambertson:
Earlier in your slides you pointed out that the combination was more effective for patients than either Vemurafenib or rituximab alone. There have been some questions from patients who have received Vemurafenib, and now they're looking to potentially add the rituximab. Could you speak to that a little bit in terms of timing? You touched on that a bit in terms of concurrent or sequential. But what is the best timing of adding rituximab to the Vemurafenib for the patient? Is it at the same time? Is it after the completion of Vemurafenib? Where do you see the greatest effectiveness for the patients in this combination?
Dr. Enrico Tiacci:
What we saw in our last trial, we have to remember that it refers to a handful of patients, 30 patients. So we're not talking about huge numbers, but this is the rule in a rare disease like hairy cell leukemia. With this caveat in mind, the results of our trial suggest that the highest effect of Vemurafenib plus rituximab is observed in patients never exposed to Vemurafenib. They could have been exposed to rituximab, but if they were exposed to Vemurafenib, they still would get into complete remission after Vemurafenib plus rituximab, but this complete remission could be frequently positive for minimal residual disease. Meaning that relapse will occur earlier.
So, if a patient becomes eligible, a candidate for therapy with a BRAF inhibitor, our practice is to deliver the BRAF inhibitor direct in combination with rituximab. Not treating first with a BRAF inhibitor, then having the patient in remission and waiting for relapse for adding the rituximab to Vemurafenib.
That said, we also observed that patients who were treated with Vemurafenib in their history, because that happened in earlier times when this regimen of Vemurafenib plus rituximab was not yet available or explored in clinical trials. So, patients that were treated in the past just with a BRAF inhibitor and responded to it but suboptimally, with partial remissions. When they got later Vemurafenib plus rituximab at relapse, even after a partial remission, they could get a better response with Vemurafenib plus rituximab, meaning a complete remission. And they could stay in remission for a longer time than what they obtained with the previous BRAF inhibitor, single therapy. Meaning that even a patient that has received Vemurafenib in the past that relapses, will benefit certainly from adding rituximab to another course of Vemurafenib.
Anna Lambertson:
We've unfortunately reached the end of our time. We're incredibly encouraged by the results that this combination is having for patients. We're excited to see what further discovery can be made in Vemurafenib plus rituximab, and we cannot thank you enough, Dr. Tiacci, for your research, for your dedication to rare disease, and for taking the time today to present and to answer some of these questions.
This transcript has been edited for clarity.