Systemic mastocytosis (SM) encompasses a group of heterogeneous myeloproliferative neoplasias (MPNs) characterized by excessive proliferation of neoplastic mast cells that accumulate in 1 or more organs, most frequently in hematopoietic tissues like bone marrow, spleen, and lymph nodes, and in the skeletal system and liver, among others.1, 2 The clinical course of patients with SM is highly variable and ranges from indolent disease, with normal life expectancy, to highly aggressive disease, which is associated with multisystem involvement and poor overall survival. SM has been associated frequently with the presence of somatic mutations in the v-kit Hardy-Zuckerman 4 feline sarcoma viral oncogene homolog (KIT) oncogene, which encodes a transmembrane receptor protein with kinase activity whose ligand is the stem cell factor (SCF). It is believed that KIT mutations play a critical role in the pathogenesis of SM and have potential as diagnostic markers and therapeutic targets. Herein, we discuss controversial issues regarding diagnostic criteria and taxonomic aspects of SM and provide an account of recent therapeutic developments in the field.
The term systemic mastocytosis (SM) encompasses a group of hematopoietic malignancies characterized by excessive proliferation of neoplastic mast cells that accumulate in the bone marrow and visceral organs. Most patients with SM, particularly those who present with aggressive clinical courses, carry somatic mutations of the v-kit Hardy-Zuckerman 4 feline sarcoma viral oncogene homolog (KIT) gene. KIT mutations are considered central events in the pathogenesis of SM and serve as diagnostic markers and putative therapeutic targets. The heterogeneity in the clinical course of patients with SM and recent advances in the genetic and immunophenotypic characterization of neoplastic mast cells may help to improve current diagnostic, taxonomic, and therapeutic approaches in SM. Cancer 2011;. © 2011 American Cancer Society.
Classification of SM
Mastocytosis can be classified into several subtypes according to the 2008 World Health Organization (WHO) classification system: 1) cutaneous mastocytosis (limited to the skin); 2) extracutaneous mastocytosis (unifocal mast cell tumor with low-grade cellular atypia and nondestructive features); 3) mast cell sarcoma (unifocal mast cell tumor with destructive features and poorly differentiated mast cells); and 4) SM, which almost invariably involves the bone marrow and is the most frequently diagnosed mast cell disorder diagnosed in adults.3 SM can be subdivided in 4 subcategories: 1) indolent systemic mastocytosis (ISM) (no evidence of organ dysfunction), which includes 2 provisional, indolent subvariants—isolated bone marrow mastocytosis (BMM) and smoldering SM (SSM); 2) SM associated with another clonal, hematologic nonmast cell lineage disease (SM-AHNMD), most commonly chronic myelomonocytic leukemia (CMML); 3) aggressive SM (ASM), which is characterized by life-threatening impaired organ function from mast cell infiltration; and 4) mast cell leukemia (MCL), which is a very aggressive form of SM.3 Although the WHO classification segregates relatively discreet subtypes of SM, an outstanding question is whether such groups also represent distinct prognostic categories.
Validation and Refinement of the WHO Classification of SM
The prognostic significance of the WHO classification recently was validated in 342 patients who were treated at the Mayo Clinic.4 In such series, the percentages of patients with SM classified as ISM, ASM, and SM-AHNMD were 46%, 12%, and 40%, respectively. The median survival of patients with ISM (198 months) was better than the median survival of patients with ASM (41 months), SM-AHNMD (24 months), and MCL (2 months). Shorter survival was associated independently with advanced age, WHO subtype, weight loss, anemia, >5% bone marrow blasts, and low platelet or albumin levels.4 The 2008 WHO classification recognizes 2 different variants of ISM: SSM, which is characterized by high mast cell burden, and isolated BMM, which is characterized by bone marrow involvement but no cutaneous involvement.3 A recent report identified SSM in 14% of patients and BMM in 23% of 159 patients with ISM who were analyzed.5 The remaining 63% of patients were classified with ISM-other. Patients with SSM had a higher incidence of constitutional symptoms, anemia, mast cell mediator levels, and older age at presentation. It is noteworthy that mast cell mediator symptoms were more frequent in patients with BMM. The median survival was 301 months for patients with ISM-other, 120 months for patients with SSM, and not reached for patients with BMM (P < .01).5 In another series, BMM was diagnosed in 46 of 99 consecutive patients with ISM, supporting the notion that this subcategory of SM largely has been under diagnosed.6
However, there are other tenuous issues regarding the WHO classification of SM, such as the lack of a clear recognition of splenic, occult, and well differentiated mastocytosis and those categorized as mast cell activation syndrome with clonal mast cells,7 which resembles a forme fruste of SM. Immunophenotyping of neoplastic mast cells may prove an invaluable tool to further define these rarer forms of SM and to delineate subsets of SM with common pathogenetic features and similar outcomes within the categories currently recognized by the WHO classification. In addition to coexpression of cluster of differentiation 25 (CD25) and CD2, it was demonstrated recently that neoplastic mast cells frequently over express CD63, CD69, CD58, CD33, and several complement-associated molecules (eg, CD11c and CD35).8 Three distinct immunophenotypic patterns that were identified in 123 patients with SM by flow cytometry analysis8 were associated with distinct markers of mast cell clonality and clinical outcomes. A more immature immunophenotype is present in ASM (CD25+CD2−CD63+CD69+) and MCL (CD25+CD2−CD63−CD69−), whereas mature activated (ISM/clonal mast cell activation disorder) or resting (well differentiated SM, typically CD25−CD2−) bone marrow mast cell phenotypes, which depend on the presence or absence of the aspartic acid-to-valine KIT mutation at codon 816 (D816V), are associated with better prognostic categories of SM. The diagnosis of each specific subtype of SM (well differentiated vs ISM/clonal mast cell activation disorder vs ASM/MCL) can be made with high sensitivity (67%, 86%, and 100%, respectively) and specificity (100%, 86%, and 88%, respectively).8 The use of immunophenotypic information may improve future taxonomic and diagnostic efforts in SM. A prognostically important subtype of ISM recently was described.9 Patients who had this subtype were characterized by the presence of KIT mutations in all hematopoietic lineages, increased serum β-2 microglobulin levels, and a high risk of progression and early death.9 Those results suggested the presence of different subgroups of patients with distinct clinical and prognostic characteristics.
Criteria for the Diagnosis of SM
The diagnosis of SM is based on a set of diagnostic criteria set forth originally by the WHO in 2001. A diagnosis of SM requires the presence of 1 major criterion and 1 minor criterion or a combination of 3 minor diagnostic criteria (Table 1).10 Ultimately, the diagnosis of SM is underpinned by the identification of tissue mast cell infiltration. Bone marrow examination is imperative to establish a diagnosis of SM, because bone marrow involvement is easily identifiable in most adults with this malignancy. Approximately 20% of patients with SM are diagnosed after a bone marrow biopsy prompted by an abnormal blood count.11 Bone marrow examination also assists in the diagnosis of SM-AHNMD.11 This is important, because the long-term prognosis of these patients relies heavily on the associated hematologic malignancy, with a worse prognosis conferred by the presence of acute leukemia, followed by CMML, myelodysplastic syndrome (MDS), and MPN.12 Neoplastic mast cells exhibit an atypical spindle shape and form multifocal aggregates consisting of 15 or more cells13 that frequently express the surface markers CD2 and/or CD25.14 A high prevalence of eosinophilia (34%) has been reported in patients with SM-AHNMD, particularly in those with SM-MPN, but has no prognostic implications.12 Serum tryptase levels and urinary histamine levels (both released by mast cells) typically are elevated. Screening for the KIT D816V mutation confirms its presence in approximately 90% of patients with SM and should be considered when a diagnosis of SM is entertained. Similarly, the presence of eosinophilia should prompt testing for the Fip1-like 1-platelet-derived growth factor receptor alpha (FIP1L1-PDGFRα) oncogene. However, most FIP1L1-PDGFRα carriers will not fulfill criteria for SM and are classified with “myeloid and lymphoid neoplasms with eosinophilia and abnormalities of PDGFRα, PDGFRβ or fibroblast growth factor receptor (FGFR),” in accordance with the 2008 WHO classification.3, 15
|A. Major criteria|
|Multifocal, dense mast cell infiltrates (≥15 mast cells in clusters) in bone marrow biopsy sections and/or in other extracutaneous organ(s)|
|B. Minor criteria|
|1. Greater than 25% mast cells in bone marrow or other extracutaneous organ(s) showing an atypical (spindle-shaped) morphology|
|2. KIT mutation at codon 816 is present in extracutaneous tissues, bone marrow, or blood|
|3. Mast cells in bone marrow coexpressing CD117 and either CD2, CD25, or both, as assessed by flow cytometry|
|4. Serum tryptase persistently ≥20 ng/mL (not applicable to patients with an associated clonal hematologic nonmast cell disorder)|
Once a diagnosis of SM is made, patients with this malignancy are categorized further according to the presence of “B and C findings,” which assess disease burden and disease aggressiveness, respectively (Table 2). Patients who have SM with no findings are categorized with ISM; those who have B findings are categorized with SSM, which is a subtype of ISM that has a more aggressive clinical course; and those who have C findings are categorized best with ASM and are candidates for immediate therapeutic intervention.
|B Findings: Indication of high mast cell burden and expansion of the genetic defect into various myeloid lineages|
|1. Infiltration grade of mast cells in bone marrow >30% on histology and serum total tryptase levels >200 ng/mL|
|2. Hypercellular bone marrow with loss of fat cells, discrete signs of dysmyelopoiesis without substantial cytopenias, or World Health Organization criteria for myelodysplastic syndrome or myeloproliferative disorder|
|3. Organomegaly: Palpable hepatomegaly, splenomegaly, or lymphadenopathy (>2 cm on computed tomography or ultrasound) without impaired organ function|
|C Findings: Indication of impaired organ function because of mast cell infiltration (confirmed by biopsy in most patients)|
|1. Cytopenia(s): Absolute neutrophil count <1000/μL, or hemoglobin <10 g/dL, or platelets <100,000/μL|
|2. Hepatomegaly with ascites and impaired liver function|
|3. Palpable splenomegaly with hypersplenism|
|4. Malabsorption with hypoalbuminemia and weight loss|
|5. Skeletal lesions: large osteolyses or/and severe osteoporosis causing pathologic fractures|
Caveats of the WHO Diagnostic Criteria
Although the criteria issued by the WHO remain the gold standard for the diagnosis of SM, several criticisms can be made that challenge their currency. The detection of multifocal, dense aggregates of mast cells in bone marrow (confirmed by immunohistochemistry and/or flow cytometry), elevated serum tryptase levels, and KIT exon 17 mutations is diagnostic in 90% of patients with SM. However, atypical mast cell morphology can be identified in all tumors, and an aberrant immunophenotype is present in 96% of patients with suspected SM, thus highlighting the sensitivity of these pathologic features in the diagnosis of SM. However, such features are categorized as “minor” only by the WHO diagnostic system. It must be emphasized that tryptase immunostaining on a bone marrow biopsy or clot specimen is not adequate to reliably assess mast cell morphology, and the latter is assessed best on a bone marrow aspiration specimen.16 The presence of a substitution of valine for aspartic acid at codon 816 (D816V) of the KIT gene is an important diagnostic criterion.17 However, in our experience, the presence of KIT D816V was detected in only 75% of patients with suspected SM, which is highly reliant on the sensitivity of the technique used and the type of sample used for testing.16 KIT mutations typically are assessed by direct sequencing techniques, which have a sensitivity of only 10% to 15%, resulting in an inferior sensitivity for the diagnosis of SM to that of the assessment of CD25 on the surface of aberrant mast cells.16 It is noteworthy that sensitivity for detecting KIT D816V reportedly was increased when testing was carried out in bone marrow mononuclear cells that were enriched for CD25+ cells that contained the aberrant mast cell population.18 A contentious issue regarding KIT D816V detection in SM is the frequency of this mutation in patients with SM-HNMD. The prevailing view is that the majority of patients with SM-AHNMD will carry KIT D816V mutations. However, in a recent series that used laser microdissection to separate the mast cell and the AHNMD components, it was demonstrated that the frequency of this mutation varies significantly depending on the type of hematologic malignancy associated with SM.19 Eighty-nine percent of patients who had SM associated with CMML carried the KIT D816V mutation, whereas the frequency decreased to 20% and 30% among patients who had SM associated with MDS or acute myeloid leukemia, respectively, and could not be detected in any patients who had lymphoproliferative AHNMD associated with SM.19
Widely considered a marker of mast cell clonality, the specificity of CD25 positivity in SM has been challenged, because it has been demonstrated that a fraction of patients with idiopathic anaphylaxis express CD25 on the surface of mast cells in addition to exhibiting at least 1 minor criteria of SM and other markers of clonality, such as KIT D816V.18 In addition, as discussed above, some patients with SM lack CD25 expression. The recent discovery that neoplastic mast cells from patients with advanced SM coexpress CD30 suggests that the detection of this surface molecule may be used as a grading marker for diagnostic purposes and as a therapeutic target.20 The presence of a serum tryptase level persistently ≥20 ng/mL is another minor criterion for the diagnosis of SM. In our experience, this criterion is present in 85% of patients. However, the specificity of this parameter is limited by the finding that marked elevations in tryptase have been reported in other myeloid malignancies or in severe allergic reactions.21 Furthermore, the utility of this criterion is limited further by its unreliability in patients with SM-AHNMD, which precludes its use in that particular subtype of SM. Perhaps the specificity of serum tryptase levels could be improved by raising the threshold in patients with SM-AHNMD, because most present with levels >100 ng/mL. The above-described limitations of the 2008 WHO diagnostic criteria result in a fraction of patients with clinically suspected SM in whom a diagnosis cannot be definitely made exclusively on the basis of such criteria. In those patients, clinical features of mast cell activation become particularly relevant. Improvements clearly are needed to increase the sensitivity and specificity of criteria for the diagnosis of SM.
Pathogenesis of SM
Genetic screens in samples of patients with MPNs have unveiled the presence of recurrent, somatically acquired mutations, some of which encode protein products susceptible to pharmacologic inhibition. Somatic gain-of-function point mutations in the kinase domain of the protein encoded by the KIT oncogene, which encodes the receptor for SCF, are recurring genetic abnormalities present in the majority of patients with SM. KIT plays an important role in normal hematopoiesis, and its expression declines in hematopoietic cells with maturation, except in mast cells. Mice genetically lacking KIT or SCF expression are devoid of mast cells.22, 23 The most frequently detected single-point mutation in neoplastic mast cells is KIT D816V, which maps to the KIT tyrosine kinase domain.24, 25 However, it has been demonstrated that the KIT genotype varies significantly with the age of mastocytosis onset.26 Thus, mutations at KIT D816 are much more common in adult patients with SM versus pediatric patients with SM (77% vs 42%; P < .001).26 KIT D816V disrupts the hydrogen bond between residues D816 and N819, which destabilizes the inactive conformation of the kinase domain,27 thus leading to constitutive KIT autophosphorylation and deregulated mast cell proliferation.28 KIT D816V is present in 90% of patients with SM.29 However, KIT-independent pathways also have been implicated in the pathogenesis of SM, such as the activation of v-yes-1 Yamaguchi sarcoma viral-related oncogene homolog (LYN) and Bruton tyrosine kinase (BTK).30 Patients with MPNs also can acquire loss of heterozygosity and somatic deletions at chromosome 4q24, which involves the TET oncogene family member 2 (TET2), which frequently is mutated in patients with myeloid malignancies like MPN and MDS.31 The presence of TET2 mutations was investigated in 48 patients with SM, including 6 who had the FIP1L1-PDGFRα abnormality, using high-throughput DNA sequence analysis.32 Twelve patients (29%) patients with SM, but none with FIP1L1-PDGFRA, carried mutations in TET2. Six of those 12 patients (50%) with TET2 mutations also carried KIT D816V mutations, and 1 had the Janus kinase 2 (JAK2) V617V mutation. However, none of those 6 patients carried the KIT D816V mutation. The presence of TET2 mutations appeared to be prognostically irrelevant.
A recent single nucleotide polymorphism, array-based karyotyping analysis of 35 patients who had mastocytosis (9 with cutaneous mastocytosis, 14 with ISM, 9 with SM-AHNMD, 2 with ASM, and 1 with mast cell sarcoma) has unveiled 20 novel genetic lesions in 10 of them (1 cutaneous mastocytosis, 4 ISMs, 4 SM-AHNMDs, and 1 ASMs).33 Uniparental disomy was identified only in patients with SM-AHNMD and ASM and involved chromosomes 2p, 4p, 7p, and 13q. The presence of these lesions was not associated with different overall survival. KIT D816V, TET2, additional sex combs like 1 (ASXL1), and Cas-Br-M (murine) ecotropic retroviral transforming sequence (CBL) mutations were identified in 29%, 24%, 9%, and 3% of patients, respectively. Patients who carried KIT D816V and TET2 mutations had shorter overall survival (10 months) compared with those who carried KIT D816V mutations (not reached), TET2 mutations (32 months), or no mutations (not reached; P < .0001).33 Activating mutations in N-ras also have been documented in patients with advanced KIT D816V SM but not in those with KIT D816V ISM. It is noteworthy that N-ras mutations, but not KIT D816V mutations, were detected in the CD34+ compartment, suggesting that the former may precede the acquisition of KIT D816V in clonal development.34 Whole genome gene expression profiling on peripheral blood samples of ISM confirmed the overexpression of 2303 genes (compared with healthy controls) implicated in pathways like mitogen-activated protein kinase (MAPK), ubiquitin-mediated proteolysis, and JAK-signal transducer and activator of transcription (STAT).35 The challenge ahead will be to precisely define the exact pathogenetic role and the role as therapeutic targets for these abnormalities.
SM remains incurable, and therapy is largely palliative. Several support measures, such as the use of oral H1 antihistamines or epinephrine, cromolyn sodium, short courses of prednisone, or psoralen-ultraviolet A, have been used with different levels of success to attenuate symptoms caused by mast cell mediators like urticaria, anaphylaxis, or ascites.36 It has been demonstrated that omalizumab (a humanized murine monoclonal antibody that inhibits immunoglobulin E binding to mast cells and basophils) benefits patients with syncopal episodes related to mastocytosis37 and SM cutaneous manifestations.38 Cytoreductive agents like interferon-alfa (IFN-α) and cladribine typically are reserved for patients with ASM, MCL, and selected patients with ISM who have high mast cell burden, poor quality of life, and/or severe osteoporosis.36
Several groups have tested the activity of IFN-α in SM.39-41 In a retrospective analysis of 108 patients with SM who received cytoreductive therapy, 47 patients (11 with ISM, 14 with ASM, and 22 with SM-AHNMD) received IFN-α with (n = 20) or without (n = 27) prednisone (20-60 mg daily with a slow tapering).42 For 31 patients, IFN-α (0.5-10 × 106 U 3 times per week) represented first-line therapy. The complete, major, and partial remission rates were 3%, 15%, and 35%, respectively, which frequently were accompanied by marked reductions in serum and urine mast cell-derived metabolites. The overall response rates for patients with ISM, ASM, and SM-AHNMD were 60%, 60%, and 45%, respectively. Responses, which lasted for a median of 12 months, did not improve with the concomitant use of prednisone. The presence of mediator symptoms was associated with a favorable response to IFN-α.42
In the largest series of patients with mastocytosis treated with cladribine to date (n = 44: 3 with cutaneous mastocytosis, 19 with ISM, 3 with SSM, 12 with ASM, 6 with SM-AHNMD, and 1 with MCL), major and partial responses were observed in 7 of 12 patients with ASM, in 3 of 3 patients with SSM, in 17 of 19 patients with ISM, in 2 of 3 patients with cutaneous mastocytosis, but no patients with ASM-AHNMD. Responses lasted for a median of 19.5 months.43 In 2 other similar series, responses to cladribine, although they occurred at similar rates, were durable for only 1644 and 11 months,42 respectively. Myelosuppression was the main treatment-related toxicity.44, 45 Circulating immature myeloid cells and myeloproliferative features appeared to predict an inferior outcome with cladribine.42 Although available data support the use of IFN-α or cladribine as frontline therapy for patients with SM, they also highlight the urgent need to improve SM therapy, because they only render responses in a limited number of patients, and the responses generally are short-lived.
The mutant KIT D816V kinase can be detected in 90% of patients with SM.29, 46 Other KIT mutations that map to the tyrosine kinase, juxtamembrane, or transmembrane domains have been described in sporadic cases of SM.47 Several targeted agents with remarkable in vitro activity have been tested in clinical trials to counteract the constitutive kinase activity of KIT mutants. Until recently, results have been largely disappointing. However, preliminary results from a clinical study with midostaurin suggest that this agent has activity in patients with SM.
Imatinib and nilotinib
Imatinib is an effective inhibitor of the c-abl oncogene 1 nonreceptor tyrosine kinase (ABL1) and KIT kinase48 but has negligible activity against KIT D816V.49 Single-arm studies of imatinib50 or imatinib in combination with prednisone51 have failed to produce objective responses. Rare KIT mutations, such as phenylalanine to cysteine at codon 522 (F522C), lysine to isoleucine at codon 509 (K509I), and deletion 419 (del419), as well as the juxtamembrane mutations valine to glycine at codon 560 (V560G) and V559G (but not V559I), are sensitive to imatinib.47, 52 Imatinib 400 mg daily has been approved by the US Food and Drug Administration for adult patients who have ASM either without the KIT D816V mutation or with unknown KIT mutational status. Bone marrow eosinophilia is present in 19% to 33% of patients with SM, and it has been demonstrated that 56% of them carry the FIP1L1-PDGFRα oncogene.53 The majority of these patients respond to imatinib 100 mg daily, whereas patients without FIP1L1-PDGFRα fail to respond irrespective of their KIT D816V mutation status. Imatinib also has been approved by the US Food and Drug Administration for FIP1L1-PDGFRα-positive myeloid malignancies at 100 mg daily with dose escalation to 400 mg daily in patients who have a suboptimal response. In a recent phase 2 study, 186 patients with 40 different malignancies associated with imatinib-sensitive tyrosine kinases received imatinib 400 mg daily (for hematologic malignancies) or 800 mg daily (for solid tumors).54 Five patients with SM were treated (4 with KIT D816V and 1 with KIT D816T). The patient who had KIT D816T achieved a partial response, indicating that this mutation may be imatinib-sensitive; whereas of the 4 patients with KIT D816V, 1 had stable disease.54
Nilotinib is a tyrosine kinase inhibitor (TKI) that is related structurally to imatinib and, at 1 μM, potently inhibits KIT phosphorylation in human mast cell (HMC) 1.1 cells (which carry KIT V560G) but not in HMC 1.2 cells (which carry KIT D816V).55 In a phase 2 study, 23 patients with SM were received nilotinib 400 mg twice daily56 Three of those patients (13%) responded (2 partial responses and 1 minor response) based on serum tryptase, bone marrow mast cell infiltration, and improvement of clinical symptoms. Overall, these data indicate that neither imatinib nor nilotinib is a useful intervention for most patients with SM (who carry KIT D816V).
Dasatinib is a multikinase inhibitor that is 300-fold more potent than imatinib against the breakpoint cluster region-(BCR)-ABL1 kinase and is highly active against KIT (50% inhibitory concentration [IC50], 5 nM) and PDGFRβ (IC50, 28 nM).57 Notably, dasatinib inhibits the kinase activity of KIT D816V with comparable efficacy to that of wild-type KIT (IC50, 37 nM and 79 nM, respectively) and, at a concentration of 0.1 μM, displayed differential cytotoxicity against neoplastic mast cells in bone marrow mononuclear cell cultures from patients with SM.58 KIT D816Y was 10-fold more sensitive to dasatinib than KIT D816V/F, suggesting that the conformational changes of the KIT activation loop caused by different mutants greatly influence the inhibitory activity of dasatinib in vitro.59 Disappointingly, in a phase 2 trial of dasatinib, only 2 of 33 patients with SM (18 with ISH, 9 with ASH, and 6 with SM-AHNMD) achieved complete remission that lasted for 8 months and 18 months.60 The marginal activity of dasatinib in SM has been corroborated in other small series.61, 62
Midostaurin is a potent multikinase inhibitor that is active against protein kinase C (PKC), fms-related tyrosine kinase 3 (FLT3), PDGFRA, vascular endothelial growth factor receptor 2 (VEGFR-2), and KIT.63 Midostaurin 1 μM abrogates the phosphorylation of KIT kinase in HMC-1.1 cells and in HMC-1.2 cells in a time-dependent and dose-dependent fashion (IC50, 50-250 nM). With similar efficacy, midostaurin suppressed the growth of human mast cells bearing either KIT D816V or wild-type KIT and synergized with nilotinib against both HMC-1 subclones, with cladribine against HMC-1.1 cells,55 and with dasatinib.64 In a phase 2 trial, midostaurin was administered orally at 100 mg twice daily to 26 patients (4 with ASM, 14 with SM-CMML, 3 with SM-MDS, 1 with SM-MDS/MPN-unspecified, and 4 with MCL).65 KIT D816V was detected in 18 of 26 patients (69%). Eighteen patients (69%) responded, including 10 (38%) with a major response (6 incomplete remissions and 4 pure clinical responses), 5 (19%) with a good partial response, and 3 with a minor partial response. Four patients had stable disease, and 4 progressed. A positive correlation was observed between KIT D816V and major response (P = .0095). High-quality responses included normalization of albumin and liver function tests, improvement in platelet count and hemoglobin level, complete resolution of ascites and pleural effusion, normalization of weight, reductions of hepatosplenomegaly, and improvement of cutaneous lesions, mediator symptoms, and performance status.65 These results with midostaurin suggest a role for this agent in the future management of patients with SM. However, more rigorous response criteria are warranted to properly assess the clinical benefit of midostaurin.66
Masitinib (AB1010) is a TKI that is active against KIT, PDGFR, and LYN kinases.67 Masitinib inhibits SCF-induced proliferation and KIT tyrosine phosphorylation with an IC50 of 150 ± 80 nM in Ba/F3 cells expressing human or mouse wild-type KIT. However, the activity of masitinib against KIT D816V was 1000-fold lower compared with that exhibited against cells carrying mutations in the juxtamembrane domain of KIT. A study of masitinib in 202 dogs with mast cell tumors proved that this TKI was safe and significantly active compared with placebo.68 It must be noted, however, that the effect of masitinib in dogs was observed regardless of KIT mutational status.68 This may indicate that some other mechanism also is active other than KIT inhibition. In a phase 2 study, 25 patients (1 with smoldering mastocytosis, 17 with ISM, and 7 with cutaneous mastocytosis) were randomized to receive masitinib at either 3 mg/kg daily (n = 13) or 6 mg/kg daily (n = 12) for 12 weeks.69 Symptomatic improvement was observed in 14 of 25 patients (56%). However, only 1 of 15 assessable patients experienced a reduction in bone marrow mast cell burden, and 7 of 14 patients had partial improvements in skin lesions. A dose of 6 mg/kg daily provided the best benefit:risk ratio and was recommended for future studies. However, only 24% of patients in that series carried the KIT D816V mutation, and no patients with ASM or MCL were included.
Other targeted small-molecule inhibitors
Many other small-molecule multikinase inhibitors have been identified as active preclinically against mutant KIT isoforms. The latter include OSI-930, which also is a potent inhibitor of kinase insert domain receptor (KDR) and PDGFRβ70; tandutinib, an FLT3 inhibitor71; PD180970 and PD173955, which have activity against ABL1 and the v-src sarcoma viral oncogene homolog (SRC)72, 73; AP23464 and AP2384874; and the indolinone-based compounds SU11652, SU11654, and SU1165575; AGL2043, which reportedly also inhibits FLT3 and PDGFR76; and EXEL-086277 (Table 3). To date, none of these agents have been tested in clinical trials.
|Study||Compound||KIT Mutation Tested||Cell Proliferation Inhibition, IC50|
|Dubreuil 200967||Masitinib||Wild-type||150±80 nM|
|Del 547-555||5.0±0.3 nM|
|Garton 200670||OSI-930||Wild-type||<100 nM|
|Corbin 200478||Tandutinib||V560G||40 nM|
|Corbin 200478||PD180970||V560G||40 nM|
|Corbin 200574||AP23464||V560G||100 nM|
|Liao 200275||SU11652, SU11654, and SU11655||Wild type||10 nM|
|Juxtamembrane domain||10-100 nM|
|Catalytic domain||250-500 nM|
|Pan 200777||EXEL-0862||V560G||510 nM|
Biologic Agents for the Treatment of SM
Thalidomide, an immunomodulatory drug, was given at doses of 100 to 300 mg daily to 2 patients with advanced SM.79 After a follow-up of 12 months, thalidomide rendered a partial remission in both patients with significant improvement of symptoms, peripheral blood counts, and bone marrow mast cell burden and reductions in hepatosplenomegaly. By contrast, lenalidomide, which is a more potent immunomodulatory drug, did not have any activity in 2 patients who had SM associated with multiple myeloma.80 Neoplastic mast cells express the low-affinity interleukin-2 receptor (IL-2R) CD25,81 which lends itself as a therapeutic target. A potential approach to target CD25 is that of using denileukin diftitox (DAB389IL-2 [ONTAK]), a DNA-derived cytotoxic protein composed of the amino acid sequences of diphtheria toxin fragments A and B followed by the sequences for IL-2. On binding to CD25, denileukin diftitox is internalized and cleaved, thus liberating the diphtheria toxin, which induces cell death. In a pilot study, denileukin diftitox was given to 8 symptomatic patients with SM (despite optimal supportive care) for a median of 6 cycles, but none of the patients responded.82
Daclizumab is a monoclonal humanized antibody that targets CD25, the IL-2Rα subunit aberrantly expressed on the surface of neoplastic mast cells. Four patients with symptomatic KIT D816V-positive ASM received daclizumab at a dose of 1 mg/kg intravenously over 30 minutes on Days 1, 4, 8, 15, and 22 of each cycle.83 One of those patients, who had previously failed therapy with imatinib and denileukin diftitox, experienced massive fragmentation of mast cells in the bone marrow after the first cycle of treatment, suggesting extensive daclizumab-induced mast cell destruction with remarkable improvement of SM-associated symptoms.83
Novel Experimental Therapies
Although mutant KIT constitutively activates the cell growth regulator mammalian target of rapamycin (mTOR), results with the mTOR inhibitor everolimus in 10 patients enrolled on a phase 2 trial have been dissapointing.84 Similarly, JAK2 kinase has been proposed as an integral part of the SCF/KIT signaling pathway.85 Treatment of HMC-1.1 or HMC-1.2 cells with the selective JAK2 inhibitor TG101348 resulted in marked inhibition of cell proliferation (IC50, 740 and 407 nM, respectively).86 It is noteworthy that the combination of TG101348 with dasatinib at sub-IC50 concentrations resulted in synergistic inhibition of HMC-1.2 cell proliferation.86
Recently, the proapoptotic Bcl-2 family member Bim has been identified as a tumor suppressor in neoplastic mast cells87 that is down-regulated by SCF stimulation and by the oncogenic KIT D816V kinase.87, 88 Accordingly, it has been demonstrated that the KIT inhibitor midostaurin and also the proteasome inhibitor bortezomib up-regulate Bim in HMC-1.1 cells and HMC-1.2 cells. More important, both agents cooperated with the BH3 mimetic obatoclax to re-express Bim and abrogate the growth of neoplastic mast cells,87 potentially opening a new avenue for the treatment of SM.
The inhibition of KIT mRNAs with short hairpin RNAs (shRNA) decreases the proliferation of both HMC-1.1 cells and HMC-1.2 cells followed by a recovery period of 5 days.89 Anti-KIT shRNA combined with α-tocopherol sensitizes cells to a caspase-dependent apoptosis that results in a higher percentage of cell death compared with anti-KIT shRNA therapy alone.90 These results were replicated in a nude mice injected with HMC-1.1 cells.91 It also has been demonstrated that triptolide inhibits the growth of human mast cells that harbor D816V and D816Y KIT mutations by inhibiting KIT mRNA levels, inducing apoptosis through the down-regulation of Mcl-1 and X-linked inhibitor of apoptosis (XIAP), and reducing phosphorylated and total levels of the downstream KIT targets STAT3, serine/threonine proteinase kinase Akt, and extracellular regulated kinases 1 and 2 (Erk1/2).91 Similar activity has been described for the inhibitor of Mcl-1 and the protein synthesis inhibitor homoharringtonine. Treatment with this agent induced marked inhibition of the growth of mast cells that carried both V560G and D816V or D814Y KIT isoforms, which were associated with the inhibition of KIT-dependent phosphorylation of STAT3/5 and Akt.92
An alternative experimental approach is that of using heat shock protein 90 (HSP90) inhibitors. HSP90 is a cellular chaperone responsible for several client proteins, such as KIT, p53, and AKT. Treatment of wild-type and mutant KIT canine bone marrow-derived, cultured mast cells with the novel HSP90 inhibitor STA-9090 resulted in growth inhibition, caspase-3/7-dependent apoptosis, and decreased phosphorylated KIT and phosphorylated AKT, but not ERK, both in vitro and in a canine mastocytoma xenograft model.93
Conclusion and Future Directions
Treatment of SM is indicated in symptomatic patients, for whom both IFN and cladribine are reasonable options. Despite the availability of several agents that have activity against KIT D816V, the therapeutic paradigm of KIT inhibition in patients with SM has not translated into tangible clinical benefits for patients with SM. A prime example supporting this contention is the poor clinical activity reported with the use of potent in vitro inhibitors of mutant KIT, such as dasatinib. These data suggest that, despite the importance of KIT for the homeostasis of mast cells, the constitutive kinase activity of mutant KIT may be dispensable, at least in part, for maintenance of the malignant SM clone. It is conceivable that approaches using combinations of KIT D816V inhibitors with agents that abrogate the activity of signaling molecules downstream from KIT kinase (eg, STAT5, JAK2) or other pathways involved in mast cell biology (eg, bcl-2) may render better results.
No specific funding was disclosed.
CONFLICT OF INTEREST DISCLOSURES
The authors made no disclosures.