The treatment of hairy cell leukemia is one of the great success stories in hematologic malignancies, with patients now having a survival that is only slightly inferior to an age-matched normal population. Purine analogs, such as cladribine, are the mainstay of first-line therapy, with approximately 80% of patients achieving complete remission.1,2 Remissions are generally highly durable, with the majority of patients remaining progression-free 10 years after a single course of cladribine.3
Nonetheless, treatment is not curative, and eventually many patients will relapse and require retreatment. Most patients who relapse after initial treatment with cladribine will respond to retreatment with the same drug; however, invariably, subsequent responses are shorter in duration than the initial response. In addition, there are cumulative toxicities associated with the repeated use of purine analogs, including immunodeficiency and damage to bone marrow stem cells.4 For this reason, despite the generally favorable outcomes, novel treatment approaches are required both in first-line treatment and in relapsed or refractory disease.
Key Driver Mutation
An activating mutation in the BRAF gene (BRAF V600E) is a key driver mutation in classic hairy cell leukemia5,6 and the hallmark of the disease; it results in constitutive activation of the mitogen-activated protein kinase (MAPK) pathway. Elegant work published previously by Dr. Tiacci’s group4 demonstrated the central biologic importance of activating BRAF mutations in classic hairy cell leukemia.
Specifically, treatment of classic hairy cell leukemia cells (but not cells from similar diseases such as splenic marginal zone lymphoma with villous lymphocytes or hairy cell leukemia variant) either in vitro or in vivo with BRAF or MEK inhibitors resulted in dramatic morphologic and molecular changes: “Hairy” cytoskeletal projections were lost; dose-dependent downstream MEK and ERK dephosphorylation was seen, indicating silencing of BRAF kinase activity; and there were dramatic changes in gene expression profiling, including downregulation of targets of BRAF-MEK-ERK signaling and, interestingly, genes encoding diagnostic markers in hairy cell leukemia, such as CD25 (interleukin 2 receptor alpha) and tartrate-resistant acid phosphatase (TRAP).7 Cell death by apoptosis occurred. Notably, in vitro, both MEK-ERK dephosphorylation and apoptosis could be partially rescued by stromal cell co-culture.
A Closer Look at Vemurafenib
BRAF inhibitors achieve high response rates in malignant melanoma carrying the BRAF V600E mutation; hairy cell leukemia is considerably less genetically complex than malignant melanoma, and therefore targeting the mutant BRAF is attractive.
As summarized in this issue of The ASCO Post, Tiacci and colleagues recently reported the results of two phase II studies of the BRAF inhibitor vemurafenib (Zelboraf) in classic hairy cell leukemia.4 A total of 54 patients with relapsed/refractory classic hairy cell leukemia and confirmed BRAF V600E mutation were enrolled in the two studies. Patients had high-risk disease, as indicated by purine analog-refractory disease or early relapse, or multiple relapses post purine analog therapy; all patients had cytopenias indicating the need for treatment. Unlike in melanoma, treatment in hairy cell leukemia was given briefly, for 12 to 24 weeks, rather than indefinitely until disease progression.
The overall response rate was remarkable: 96% to 100% of patients responded, despite 41% to 54% having disease refractory to the most recent prior therapy; 35% to 42% had a complete remission. Treatment was associated with rapid improvement in hematologic parameters. Median relapse-free survival was 9 months in the Italian study, and progression-free survival at 1 year was 73% in the U.S. trial. Achieving complete remission was associated with longer progression-free survival.
Vemurafenib was associated with frequent toxicities but no grade 4 or 5 adverse events; dose reductions were required in over half the patients (most commonly by 25% to 720 mg twice daily) and were not associated with inferior progression-free survival. Frequent adverse events included rash, arthritis and arthralgia, pyrexia, and abnormal liver function tests. Two patients developed clinical pancreatitis. Five patients developed nonmelanoma skin cancers, and one patient developed melanoma.
The precise relationship of the cutaneous malignancies to vemurafenib could not be determined, since the majority of patients had a history of similar skin tumors. However, vemurafenib and the BRAF inhibitor dabrafenib (Tafinlar) have been associated with the development of squamous cell carcinomas of the skin in studies in malignant melanoma.8-10
In the Italian study, downregulation of CD25 and ultrastructural changes with loss of “hairy” cytoplasmic projections were seen, similar to previous observations11; the loss of CD25 expression is important, since it likely renders soluble CD25 a less reliable marker of residual disease during BRAF inhibitor therapy. Notably, in the Italian study, 13 of 26 patients had immunohistochemical analysis for phosphorylated ERK performed on bone marrow biopsies, the day after completion of therapy. Of these 13 patients, 6 had residual phosphorylated ERK–expressing hairy cell leukemia cells present; although numbers were small, those patients with residual phosphorylated ERK expression appeared to have inferior progression-free survival.
A previously published case report demonstrated residual ERK phosphorylation in a patient treated with vemurafenib for hairy cell leukemia, despite an excellent response.12 Combined, these data suggest that there is an alternative mechanism of BRAF-MEK-ERK pathway activation, which may lead to treatment resistance; notably, during in vitro treatment with BRAF inhibitors, co-culture of classic hairy cell leukemia cells with bone marrow stroma appears to antagonize both apoptotic effects of treatment and dephosphorylation of MEK-ERK.
Precise understanding of the mechanism of residual MEK-ERK activation during treatment with BRAF inhibitors is essential to allow the rational design of combination therapies, which may enhance activity and prevent resistance. One patient in the U.S. study, who was refractory to vemurafenib retreatment after relapse, was found to have two activating, subclonal KRAS mutations; this is a previously described mechanism of resistance to BRAF inhibitors in melanoma13 and confers resistance through rephosphorylation of MEK and ERK.
Challenges for Researchers
The availability of a highly active oral therapy that has precisely targeted the key driver mutation responsible for hairy cell leukemia is a major breakthrough for patients with refractory disease. However, the challenge for researchers is now how best to apply this therapy.
There are many questions to be answered by subsequent, well-designed studies. First, the optimal dose and duration of therapy for relapsed/refractory patients are unclear from the existing data. Patients on these studies had durable remissions, even after cessation of treatment, suggesting that time-limited therapy may be possible, although it is unclear whether continued maintenance therapy (perhaps at a lower dose to limit adverse effects) could produce a prolonged remission duration; additionally, many patients required dose reduction due to toxicity, which did not appear to negatively impact progression-free survival; this finding may suggest that lower doses could have been used with similar efficacy and, potentially, reduced toxicity.
The ability to deliver time-limited therapy (and/or lower doses) is of major importance, given both the side effects associated with the drug (particularly the concern regarding pancreatitis and malignant cutaneous tumors) and the very high cost of such novel therapies for both patients and society. The issue of cost will assume even more importance if the use of BRAF inhibitors is contemplated earlier in the disease course, when more patients will likely be treated for longer durations.
Second, vemurafenib monotherapy may not achieve optimal MAPK pathway inhibition, as shown by the demonstration of residual phosphorylated ERK expression in residual hairy cell leukemia cells in the bone marrow in almost half the patients on the current study. This argues for the use of a more potent BRAF inhibitor, such as dabrafenib, or potentially dual BRAF/MEK inhibition, which achieves superior progression-free survival compared with BRAF inhibition alone in malignant melanoma.9,10
In vitro data in classic hairy cell leukemia certainly provide a solid rationale for the use of dabrafenib alone or in combination with MEK inhibitors: Dabrafenib is more capable of overcoming the protective effect of bone marrow stroma than is vemurafenib, and its ability to overcome this protective effect, to induce dephosphorylation of MEK and ERK and to induce apoptosis of classic hairy cell leukemia cells is further enhanced by its use in combination with the MEK inhibitor trametinib (Mekinist).11 Additionally, the combination of dabrafenib and trametinib significantly reduced the incidence of cutaneous squamous cell carcinomas during treatment of BRAF-mutated malignant melanoma, which would be an added advantage of combination therapy.9,10 A phase II study of dabrafenib plus trametinib for patients with relapsed/refractory classic hairy cell leukemia is currently ongoing.
Third, it is important to identify the mechanisms of rephosphorylation of MEK and ERK in the presence of bone marrow stromal cells. Identifying the specific activated pathways and/or the development of mutations involving other kinases such as KRAS is essential to plan subsequent therapies for patients developing BRAF inhibitor resistance and BRAF inhibitor–based combination strategies.
First-Line Setting
In other indolent lymphoproliferative disorders, such as chronic lymphocytic leukemia, the development of highly effective targeted therapies for relapsed patients has been rapidly followed by studies of novel therapies in the first-line setting, in an effort to replace conventional therapy. However, given the generally very favorable outcomes of patients with purine analog-based therapy and the unique toxicity profile of BRAF inhibition (particularly the development of cutaneous squamous cell carcinomas), we would anticipate that its use in the earlier disease setting may initially be restricted to elderly patients or those who have comorbidities that may limit the tolerability of purine analog therapy.
Perhaps a more important question is whether BRAF inhibitors can be integrated with existing therapies in first-line therapy or at the time of first relapse to improve progression-free survival. Detailed preclinical studies are required to understand how best to combine or sequence these treatments both to enhance efficacy and limit toxicity.
Minimal Residual Disease
Interpretation of any future studies comparing novel treatments such as BRAF inhibitors or BRAF inhibitor–based combinations with existing purine analog-based treatments will be complicated, due to the very long progression-free survival achieved with first-line purine analog therapy. As a result, surrogate primary endpoints for progression-free survival, most notably minimal residual disease, will be required.
We and others have previously shown that minimal residual disease can be detected in most patients following cladribine therapy.14 Given the known association between the depth of remission and progression-free survival, we anticipate that achievement of minimal residual disease negativity through the use of consolidation treatment will translate into a progression-free survival benefit, with fewer patients consequently requiring salvage therapy. However, long-term follow-up will be required to prove this theory.
The availability of potent, molecularly targeted therapy in classic hairy cell leukemia represents a great advance for the few patients with disease refractory to purine analogs. We anticipate that integration of BRAF inhibitors with existing treatment in earlier stages of the disease and use of more potent BRAF inhibitors or combinations of BRAF and MEK inhibitors will further improve results in the future. Given the effectiveness of nucleoside analogs with or without monoclonal antibodies in the majority of patients, cost and convenience considerations are likely to play a major role in the adoption of these agents. ■
Disclosure: Drs. Thompson and Ravandi reported no potential conflicts of interest.
References
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