Mast Cell Burden Research Articles

Subcellular expression of CD30 in cutaneous mastocytosis-An important factor for targeted treatment.

The subcellular distribution of CD30 on mast cells and the presence of eosinophils in cutaneous mastocytosis require further investigation, especially as the cell surface expression of CD30 is critical for the therapeutic response of systemic mastocytosis to brentuximab vedotin. Investigation of 147 biopsy specimens from 143 patients with cutaneous mastocytosis for mast cell density and distribution, frequency of CD30 expression, CD30 staining patterns, and presence and distribution of eosinophils. Correlation with clinical patterns. Retrospective multicenter immunohistochemical study of CD30 expression, eosinophils and basic clinical data in cutaneous mastocytosis. CD30 expression was found in all samples (cut-off: ≥1%), whereby the staining was predominantly cytoplasmic in 99% of the samples. Additional membrane staining was detected in 62% of the samples. Surface expression of CD30 was more common in biopsy specimens with a high mast cell burden and in biopsy specimens with a higher CD30 expression rate. Eosinophils were admixed in 58% of the samples. Females and older patients showed a trend of a lower mast cell burden. Retrospective study on formalin-fixed and paraffin-embedded tissue without functional analysis. Most cases of cutaneous mastocytosis show cell surface expression of CD30 expression and is, therefore, in principle, accessible for therapy with antibodies against CD30, provided the overall situation of the patient warrants.

Increased expression of formyl peptide receptor-1 by basophils from patients with mastocytosis

BackgroundSymptoms in patients with systemic mastocytosis (SM) are associated with an increase in mast cell burden and release of mast cell-derived mediators. The most frequent presentation of SM is indolent (ISM), with moderate symptoms and prognosis. Basophil numbers in these patients are generally normal. However, when examining basophil activation in patients with ISM, we noted an abnormal response to N-formylmethione-leucyl-phenylalanine (fMLP). ObjectiveTo compare basophil responsiveness to fMLP and anti-IgE in healthy volunteers and patients with ISM and relate findings to fMLP receptor (FPR) expression. MethodsBasophils isolated from peripheral blood of fifteen patients with ISM and fourteen healthy volunteers were stimulated with fMLP or anti-IgE. CD63 expression to assess basophil activation and expression of FPRs were assessed by flow cytometry. ResultsBaseline expression of CD63 on basophils was similar between healthy volunteers and patients with ISM. fMLP induced higher expression of CD63 on basophils from patients with ISM, while responses to anti-IgE were similar between groups. Basophils from patients with ISM also had higher fMLP1 receptor (FPR1) expression, while FPR2 and FPR3 were not detected. fMLP blocked the binding of anti-FPR1 antibody to FPR1, consistent with the conclusion that fMLP signals through FPR1. ConclusionsFMLP-induced basophil activation is higher in patients with ISM which is associated with an increase in FPR1 expression. Further investigation is needed to determine why FPR1 expression is elevated, whether such expression might serve as an additional surrogate marker in the diagnosis of ISM, and whether enhanced responses of basophils to fMPL might have some relationship to unexplained episodes of mediator release.

Autoantibodies to type I interferons in patients with systemic mastocytosis

Autoantibodies to type I interferons have been identified in association with a variety of inflammatory and autoimmune diseases. Type I interferons have demonstrated inhibitory effects on mast cell proliferation and degranulation. Systemic mastocytosis (SM) is a disease characterized by increased mast cell burden and mediator release. Whether autoantibodies to type I interferon are present in the sera of patients with SM, and if so, whether they correlate with characteristics of disease, is unknown. The purpose of this study was to determine whether autoantibodies to type I interferons are observed in the sera of patients with SM, and if so, whether they correlate with biomarkers of disease severity. We analyzed sera from 89 patients with SM for concentrations of autoantibodies to type I interferon by using a multiplex particle-based assay and signal neutralization capacity by using a STAT1 activity assay and then compared these measurements with those in a database of information on 1284 healthy controls. Our cohort was predominantly female (57.3%), with a median age of 56 years. Of the cohort members, 13 produced autoantibodies to IFN-β, 3 to IFN-ω, and 0 to IFN-α. None of the 13 sera demonstrated signal neutralization. Neither autoantibody concentration nor signaling inhibition measurements correlated with tryptase concentrations or D816V allele burden. Although a small subpopulation of patients with SM have autoantibodies to type I interferons, there was no correlation between autoantibody production and signaling inhibition. These data are consistent with the conclusion that autoantibodies to type I interferon do not play a significant role in the pathogenesis or severity of SM.

Histological characterization of liver involvement in systemic mastocytosis.

Systemic mastocytosis (SM) is characterized by the accumulation of atypical mast cells (MCs) in organs. Liver histology of SM has been marginally described and accurate histological classification is critical, given the consequences of aggressive SM diagnosis. We aimed to describe the histological features associated with liver SM using updated tools. Using the database of the French Reference Centre for Mastocytosis, we retrospectively identified patients with a liver biopsy (LB) and a diagnosis of SM. All LB procedures were performed according to the local physician in charge and centrally reviewed by an expert pathologist. A total of 28 patients were included: 6 had indolent SM, 9 had aggressive SM, and 13 had SM with an associated hematologic neoplasm. Twenty-five (89%) patients presented hepatomegaly, and 19 (68%) had portal hypertension. The LB frequently showed slight sinusoid dilatation (82%). Fibrosis was observed in 3/6 indolent SM and in almost all advanced SM cases (21/22), but none of them showed cirrhosis. A high MC burden (>50 MCs/high-power field) was correlated with elevated blood alkaline phosphatase levels (p = .030). The presence of portal hypertension was associated with a higher mean fibrosis grade (1.6 vs. 0.8 in its absence; p = .026). In advanced SM, the presence of nodular regenerative hyperplasia (NRH) was associated with decreased overall survival (9.5 vs. 46.3 months, p = .002). MC infiltration induced polymorphic hepatic lesions and the degree of fibrosis is associated with portal hypertension. NRH identifies a poor prognosis subgroup of patients with advanced SM. Assessing liver histology can aid in SM prognostic evaluation.

Detecting Changes in Mast Cell Numbers Versus Activation in Human Disease: A Roadblock for Current Biomarkers?

The pathophysiology of mast cell (MC)-driven disorders is diverse, ranging from localized reactions to systemic disorders caused by abnormal accumulation and activation in multi-organ systems. Prompt and accurate diagnosis is critically important, both for informing treatment and objective assessment of treatment outcomes. As new therapeutics are being developed to deplete MCs or silence them (e.g., by engaging inhibitory receptors that block activation), new biomarkers are needed that can distinguish between MC activation vs. burden. Serum tryptase is the gold standard for assessing both mast cell burden and activation; however, commercial tryptase assays have limitations related to timing of release, lack of discernment between inactive (α) and active (β) forms of tryptase, and interpatient variability of baseline levels. Alternative approaches to measuring MC activation include urinary mast cell mediators, flow cytometry-based assays or gene expression profiling. Additional markers of MC activation are needed for use in clinical diagnostics, to help selection of treatment of MC diseases, and for assessing outcomes of therapy. We review the spectrum of disorders with known or suspected MC contribution, describe the utility and limitations of current MC markers and assays, and discuss the need for new markers that can differentiate between MC activation and burden.

Advanced systemic mastocytosis-Revised classification, new drugs and how we treat.

Mastocytosis constitutes the neoplastic proliferation of mast cells and is broadly classified into systemic mastocytosis (SM), cutaneous mastocytosis and mast cell sarcoma. SM is further partitioned into advanced (AdvSM) and non-advanced (SM-non-Adv) subcategories. AdvSM includes aggressive SM (ASM), SM with an associated haematological neoplasm (SM-AHN) and mast cell leukaemia (MCL). In 2022, two separate expert committees representing the 5th edition of the World Health Organization (WHO5) and the International Consensus (ICC) classification systems submitted revised classification criteria for SM, highlighted by the ICC-proposed incorporation of mast cell cytomorphology in the diagnostic criteria for MCL and myeloid-lineage restriction for the AHN component in SM-AHN. Recent developments in SM also include the introduction of KIT-targeting tyrosine kinase inhibitors (KITi), including midostaurin and avapritinib, both drugs have shown potent activity in reducing mast cell and mutant KIT burden and alleviating mast cell-associated organopathy and mediator symptoms; however, their overall impact on survival or superiority over pre-KITi era treatment options (e.g. cladribine) has not been studied in a controlled setting. In the current review, we provide a summary of recent changes in disease classification and an analysis of recent clinical trials and their impact on our current treatment approach in AdvSM.

Performance and Interlaboratory Reproducibility of Droplet Digital Polymerase Chain Reaction (ddPCR) and Real Time PCR for KIT D816V Mutation Detection: A Nationwide Pilot Study By the RIMA (Rete Italiana Mastocitosi) Association

Systemic Mastocytosis (SM) is a rare hematological neoplasm, but its incidence is probably underestimated because of the heterogeneity of clinical symptoms and presentation that may cause diagnostic difficulties especially outside reference centers. Detection of the activating D816V KIT mutation in the bone marrow or peripheral blood is one of the minor criteria for the diagnosis of SM, but it poses some challenges since in the indolent forms (the most frequent) the mast cell burden is very low. For this reason, the National Comprehensive Cancer Network and the European Competence Network on Mastocytosis (ECNM) recommend a high-sensitivity PCR-based assay, such as Allele-Specific Oligonucleotide-Real Time Quantitative PCR (ASO-qPCR) or ddPCR, while Sanger and Next Generation Sequencing are considered not adequate. Accurate quantitation of allele burden (AB) is also desirable since it offers diagnostic (an AB³10% qualifies as a B-finding according to the latest WHO classification) and prognostic information and might be used to monitor response to tyrosine kinase inhibitors. Despite the widespread routine availability of real time and digital PCR technologies, a map of labs across Italy offering KIT D816V testing with adequate sensitivity is lacking, nor have cooperative efforts aimed to check lab performance and reproducibility of diagnostic results ever been performed. Based on these premises, RIMA undertook, with the sponsorship of the GIMEMA Working Party on Chronic Myeloproliferative Neoplasms, a project aimed to: i) build a nationwide network of competent reference laboratories performing KIT D816V mutation testing; ii) promote harmonization of local procedures and ensure adequate proficiency and cross-comparability of results. The pilot phase of this project involved 7 labs (L) spread across the country (Milan, Bologna, Verona, Florence, Rome, Naples, Pavia) serving one or more reference clinical centers for the diagnosis and management of SM. First, a survey aimed to assess the techniques and procedures in use in each participating lab was conducted. Five labs (L2-L6) used a commercial ddPCR assay (KIT p.D816V c.2447A>T, Assay ID dHsaCP2000023; Bio-Rad); one lab (L7) used a home brew ASO-qPCR assay; one lab (L1) used a commercial semi-quantitative real time PCR-based assay (PlentiPlex Mastocytosis D816V kit; Pentabase). All labs performed analyses in duplicate or triplicate using DNA as input material, with amounts variable from 50 to 250ng/replicate. Reported limit of detection (LoD) was 0.0003% for the home brew method and 0.01% for the commercial methods. Subsequently, a round robin test to evaluate and compare performance in terms of sensitivity and inter-lab reproducibility was carried out. Mutated and wild-type DNA isolated from HMC-1.2 and HL-60 cells, respectively, was mixed and diluted to mimic different D816V ABs, from 10% down to 0.01%. Dilutions were externally assessed by the UK Wessex Genomics Laboratory Service, using an accredited ddPCR assay with a limit of detection (LoD) of 0.01%. Of note, the sixth dilution (D6) was scored as slightly below their LoD (0.008%; 3 positive droplets only). Identical batches of blinded vials were then distributed and analyzed in parallel by the 7 participating labs according to their own routine protocols. Positivity/negativity as scored by L1-7 and AB values as scored by L2-7 are detailed in Table 1. All the evaluated methods proved highly accurate in the detection and, for ddPCR and ASO-qPCR, in the quantitation of the KIT D816V (R 2 between 0.988 and 0.998). A very high degree of concordance was achieved, for all dilutions, across different labs and methods (CV between 0.07 and 0.8). D6 was scored borderline by L1 and positive by all other labs. This pilot experience demonstrates that the evaluated PCR-based methods may reliably identify and quantitate the KIT D816V mutation down to an AB of 0.01% with a high degree of cross-comparability of results, thus assisting clinicians in the diagnosis and monitoring of SM patients. To the best of our knowledge, this is the first cooperative effort aimed to assess the performance and reproducibility of KIT D816V mutation testing in SM. Involvement of additional Italian labs is already planned, and further implementation within the ECNM will be proposed. Definition of common SOPs, uniform sample requirements and web-based reporting following the GIMEMA LabNet model will be pursued.

Common Pathways for Acute Myeloid Leukemia Progression in Systemic Mastocytosis with Associated Hematologic Neoplasm

Background: Mastocytosis is a heterogenous disease of mast cell (MC) clonal proliferation with pathologic accumulation in organ systems such as but not limited to skin, bone marrow, gastrointestinal tract, liver and spleen. Mast cells are CD34+, CD117+ (stem cell factor, KIT) with greater than 90% of people with mastocytosis harboring KIT D816V activating mutations which allows for increased proliferation and accumulation of neoplastic mast cells leading to downstream mast cell activation and intolerable symptoms. Advanced systemic mastocytosis (advSM) is categorized into three major subtypes, aggressive SM, MC leukemia and SM with an associated hematologic neoplasm (SM-AHN). SM-AHN is the most common advSM subtype accounting for about 75% of advSM cases and typically includes myeloid neoplasms such as MDS, CMML, or MPN. In most pts with SM-AHN, clinical symptoms is driven by the MC component and the goal of treatment with KIT inhibitors is to decrease the MC burden, thus improving quality of life and overall survival (Nat Med refs), However, the median survival of SM-AHN remains poor at 2-3.5 years with many pts transforming to advanced myeloid neoplasms especially an acute myeloid leukemia (AML). The use of next generation sequencing has helped to prognosticate patients with AdvSM. Patients with advSM typically exhibit multiple somatic mutations other than KIT D816V We describe the molecular signature of patients with SM-AHN who transformed to AML. Commonly involved genes are TET2, SRSF2, ASXL1, RUNX1, and CBL with inferior OS and adverse clinical characteristic associated with SRSF2/ ASXL1/ RUNX1 (S/A/R). To better prognosticate, Jawhar and colleagues introduced the MARS score. This incorporated molecular findings in AdvSM to help estimate overall survival which identified 3 risk categories (low, intermediate, and high). The overall survival in low, intermediate and high were not reached, 4.3 years, and 1.9 respectively for patients with AdvSM. While mutational findings in the MARS score can help prognosticate overall survival in these patients, the molecular footprint of progression or transformation to acute myeloid leukemia has not been established. We describe the molecular signature of patients with SM-AHN who transformed to AML. Methods: We analyzed 10 pts with SM-AHN who transformed to AML from 2000-2022. Of those patients, 7 were found to have full next generation sequencing at the time of initial consultation and transformation. These data were collected and descriptive analysis was performed. Results: We identified 7 pts with SM-AHN who progressed to AML. At SM-AHN diagnosis all pts had at least 2 additional co-mutations with a maximum of 7 co-mutations along with KIT D817V. The most common co-mutations were splicing factor mutations ( SRSF2 and SF3B1), TET2, RASEZH2, CUX1 and CBL. Six of the seven received treatment for SM-AHN. These patients received treatment with multikinase inhibitors or hypomethylating agents. The acquired driver mutations were NPM1, FLT3-ITD, RUNX1, and RAS pathway. In patient 1, a new FLT3-ITD and RUNX1 was acquired with an increased VAF of a RAS pathway mutation ( PTPN1) which was originally present in the SM-AHN diagnosis. Patient 3 also developed a new RUNX1 mutation at AML transformation. Patients 4 and 5 had new NPM1 mutations and patients 6 and 7 had a new RAS pathway mutations . Patient 2 acquired new BCOR, RIT1 and CBL mutations. Upon transformation to AML, 5/7 patients had resolution or a decrease of greater than 10% of KIT D816V VAF. One of the seven had no change in KIT D816V VAF at the time of transformation, this was also the only patient to receive no prior therapy. Only 1/7 had a VAF increase of greater than 20% in KIT D817V at transformation. The MARS score was calculated at the time of diagnosis of SM-AHN which showed that only 4 of 7 had a high MARS score. Time from SM-AHN diagnosis to AML progression ranged from 4-31 months. Survival was calculated from time of SM-AHN diagnosis to death which ranged from 6-47 months. Conclusion: Acquisition of known AML driver mutations (RAS, RUNX1, NPM1, FLT3-ITD) are commonly found in SM-AHN patients at time of progression to AML. The MARS score calculated in our patients did not correlate with time to transition to AML.

Elenestinib, an Investigational, Next Generation KIT D816V Inhibitor, Reduces Mast Cell Burden, Improves Symptoms, and Has a Favorable Safety Profile in Patients with Indolent Systemic Mastocytosis: Analysis of the Harbor Trial

Introduction Indolent systemic mastocytosis (ISM) is a rare clonal mast cell disease driven by the KIT D816V mutation in nearly all patients. Patients with ISM can present with debilitating acute and chronic cutaneous, gastrointestinal, neurocognitive, and systemic symptoms caused by the release of inflammatory mediators from abnormal mast cells. In some patients, symptoms are not adequately controlled with best supportive care (BSC) medications. Elenestinib (BLU-263) is a novel, investigational, oral, next-generation tyrosine kinase inhibitor that potently and selectively inhibits KIT D816V with limited central nervous system penetration and pharmacokinetics (PK) that support once daily dosing. HARBOR (NCT04910685) is a randomized, double-blind, placebo-controlled, phase 2/3 study assessing efficacy and safety of elenestinib in patients with ISM. Methods Eligible adult patients with ISM, confirmed following central review of bone marrow (BM) pathology, clinical, and laboratory findings per World Health Organization criteria and a moderate-to-severe symptom score based on minimum mean total symptom score (TSS) of the ISM-Symptom Assessment Form (ISM-SAF©2018), were stratified by baseline serum tryptase (<20 ng/mL vs ≥20 ng/mL) and randomly assigned 3:1 to elenestinib 25 mg, 50 mg, or 100 mg once daily + BSC (“elenestinib”) or placebo + BSC (“placebo”). The primary objective of Part 1 was to determine the recommended dose based on safety, PK, and pharmacodynamics. Secondary endpoints included changes in biomarkers of disease burden (serum tryptase, KIT D816V variant allele fraction [VAF], and BM mast cells) and ISM-SAF TSS. Three additional open-label PK groups enrolled patients at 50 mg, 75 mg, and 100 mg to further characterize PK and safety. Results As of data cutoff on July 3, 2023, a total of 122 patients with ISM have received their assigned treatment in Part 1 of the study; 39 were blinded and randomized to elenestinib or placebo, and 83 were treated with open-label elenestinib in the PK groups (21 patients at 50 mg, 34 patients at 75 mg, and 28 patients at 100 mg). Baseline demographics were similar to those reported for the general ISM population. After 12 weeks of therapy, symptom improvement was observed for all dose cohorts, and symptom reduction by TSS was greater for patients on elenestinib versus placebo in the blinded portion of Part 1. Patients receiving elenestinib at 25 mg, 50 mg, and 100 mg doses showed reduction (mean percent) from baseline for tryptase (-15.4%, -50.9%, and -68.4% vs 3.3%, respectively) and KIT D816VVAF (-37.5%, -70.3%, and -77.0% vs -2.5%, respectively) as compared to placebo. Similar reductions in TSS and disease-related biomarkers were observed in the open-label PK cohorts. After a median treatment duration of 35.3 weeks, elenestinib was well-tolerated at all dose levels. There were no treatment-related serious adverse events and no treatment-related adverse events that led to drug discontinuation. Conclusion Elenestinib at all tested doses demonstrated beneficial effects on disease-related symptoms and biomarkers of mast cell burden in this large and maturing cohort of patients with ISM with a moderate-to-severe symptom burden. Elenestinib was well tolerated across all dose levels with a promising early benefit-risk profile in ISM. These preliminary findings will inform dosing for the planned Part 2 of HARBOR, which will examine elenestinib versus placebo in a randomized, blinded, placebo-controlled setting.

Initial Results from Summit: An Ongoing, 3-Part, Multi-Center, Randomized, Double-Blind, Placebo-Controlled Phase 2 Clinical Study of Bezuclastinib in Adult Patients with Nonadvanced Systemic Mastocytosis (NonAdvSM)

Background: Systemic mastocytosis (SM) is a rare disease characterized by neoplastic mast cell infiltration of extracutaneous tissues and encompasses a spectrum of subtypes that can range from non-advanced to advanced disease. Nonadvanced SM (NonAdvSM) includes two variants of SM: indolent systemic mastocytosis (ISM) and smoldering systemic mastocytosis (SSM). NonAdvSM is often associated with debilitating symptoms which may significantly reduce health-related quality of life (HRQoL). In up to 95% of patients, SM is driven by the somatic KIT D816V gain of function mutation located in exon 17. Bezuclastinib (CGT9486) is an oral, potent, and selective type 1 tyrosine kinase inhibitor (TKI) with activity against KIT D816V. Preclinical studies demonstrate bezuclastinib inhibits KIT D816V with minimal brain penetration. Early results from Part 1 of a Phase 2 trial of patients with AdvSM demonstrate that bezuclastinib is well-tolerated and is associated with encouraging clinical activity. Bezuclastinib resulted in meaningful reductions in markers of disease burden including serum tryptase, bone marrow mast cell burden, and KIT D816V variant allele fraction (DeAngelo et al. [abstract] In: Blood (ASH) 2022; 140 (Supplement 1): 1512-13). These results support the investigation of bezuclastinib as a potential therapy in NonAdvSM. Study Design and Methods: Summit (NCT05186753) is a multi-center, Phase 2, randomized, double-blind, placebo-controlled, 3-part clinical trial of bezuclastinib in patients with NonAdvSM, including ISM and SSM, who have inadequate control of symptoms despite treatment with best supportive care (BSC). Patient-reported outcome measures (PROMs), including mastocytosis activity score (MAS) and mastocytosis quality of life (MC-QoL), are being used to assess treatment impact on symptoms and HRQoL. Approximately 48 patients will be enrolled in Part 1 of the trial with 1:1:1 randomization to one of two doses of bezuclastinib (100 or 200 mg QD for Part 1a; 100 or 150 mg QD for Part 1b) or placebo. Here we report initial data from Part 1a of the Summit trial. Results: Part 1a of Summit enrolled 20 patients; 18 patients had ISM and 2 patients had SSM. In Part 1a, a majority of patients were female (75%) and had an Eastern Cooperative Oncology Group Performance Status of 0-1 (90%) or 2 (10%). Mean age was 54. Patients utilized a median (range) of 3 (2 to 7) BSC medications, with 100% using H1 blockers, 95% using H2 blockers and 40% using leukotriene receptor antagonists. Enrolled patients had a median (range) serum tryptase of 74 (10 to 592) ng/mL and 85% of patients had a serum tryptase ≥20 ng/mL. 75% of patients were KIT D816V positive and median (range) KIT D816V variant allele fraction in whole blood was 0.5% (0 to 32). Median (range) percent bone marrow mast cell burden was 23% (1 to 80%). Utilizing MAS to assess baseline disease severity, a majority of patients self-reported severe disease with a median score of 43 (range 22-79). Patients also self-reported a mean score of 56+/- 19 on the MC-QOL, indicating moderate impact to their HRQoL. Conclusions: The Summit trial evaluating safety and efficacy of bezuclastinib enrolled 20 patients in Part 1a. The patients enrolled in Part 1a of the Summit trial are generally representative of the population of patients with moderate to severe nonadvanced systemic mastocytosis based on number of supportive care medications, symptom severity, and disease impact on HRQoL. Initial safety and efficacy results from Part 1a of the 3-part, randomized, double-blind, placebo-controlled Summit trial will be presented.

Safety and Efficacy of Bezuclastinib (CGT9486), a Novel, Highly Selective, Potent KIT D816V Tyrosine Kinase Inhibitor, in Patients with Advanced Systemic Mastocytosis (AdvSM): Results from Part 1 of the Phase 2 Apex Trial

Background: Systemic mastocytosis (SM) is a rare disease characterized by the aberrant proliferation of mast cells (MC). In about 95% of adult patients, SM is driven by a gain-of-function mutation (D816V) in exon 17 of KIT. Advanced SM (AdvSM) is a life-threatening form of SM and includes three subtypes: aggressive SM (ASM), SM with an associated hematologic neoplasm (SM-AHN), and Mast Cell leukemia (MCL). Bezuclastinib is an oral, potent, and selective tyrosine kinase inhibitor (TKI) with activity against KIT D816V. Preliminary results from the on-going Apex study in patients with AdvSM (NCT04996875) were presented previously (DeAngelo et al. [abstract] In: Blood (ASH) 2022; 140 (Supplement 1): 1512-13). Methods: Apex is a randomized Phase 2, open-label, multicenter trial in adult patients diagnosed with AdvSM per 2022 WHO criteria with a baseline serum tryptase of ≥20 ng/mL. In Part 1, patients were randomized 1:1:1:1 to bezuclastinib 50mg BID, 100mg BID, 200mg BID, or 400mg QD. Patients with a history of prior TKI therapy (e.g., avapritinib, midostaurin) are permitted in the trial. Data from Part 1 informed dose selection for Part 2 of the trial. The primary efficacy endpoint for this trial is overall response rate (complete response [CR], CR with incomplete hematologic recovery [CRh], partial response [PR] and clinical improvement [CI]) as assessed by a central review committee based on mIWG-MRT-ECNM response criteria. Results: As of April 2023, Part 1 was fully enrolled with 33 AdvSM patients. Patients were randomized to 50mg BID (n=8), 100mg BID (n=8), 200mg BID (n=8), or 400mg QD (n=9). Disease subtypes included ASM (n=7), MCL (n=2), and SM-AHN (n=24); AHN subtypes included CMML (n=18), MDS (n=3), MPN (n=1), and MDS/MPN-U (n=2). The majority of patients were male (64%), ECOG PS 0-1 (82%) and KITD816V positive at baseline (91%). At baseline, 67% of patients were TKI-naïve; 33% had prior midostaurin, and 15% had prior avapritinib. Median (range) BM MC burden, serum tryptase, and KIT D816V VAF at baseline were 30% (5-90), 163 ng/mL (35-1578), and 7% (0-47), respectively. Of the 33 patients enrolled, 25 patients (76%) had C-findings evaluable per mIWG-MRT-ECNM criteria; 20 patients (61%) were positive for SRSF2/ASXL1/RUNX1 mutation which are associated with poor survival in SM. Conclusion: Enrollment in Part 1 of the Apex trial is complete. Patients enrolled in Part 1 of the Apex trial are generally representative of the population of patients with AdvSM based on patient characteristics and markers of disease. Part 1 includes a small subset of patients with prior use of TKIs. Safety, clinical activity, and clinical objective response for all patients in Part 1 will be presented.

Avapritinib versus Placebo in Indolent Systemic Mastocytosis

BackgroundIndolent systemic mastocytosis (ISM) is a clonal mast-cell disease driven by the KIT D816V mutation. We assessed the efficacy and safety of avapritinib versus placebo, both with best supportive care, in patients with ISM.MethodsWe randomized patients with moderate to severe ISM (total symptom score [TSS] of ≥28; scores range from 0 to 110, with higher numbers indicating more severe symptoms) two to one to avapritinib 25 mg once daily (n=141) or placebo (n=71). The primary end point was mean change in TSS based on the 14-day average of patient-reported severity of 11 symptoms. Secondary end points included reductions in serum tryptase and blood KIT D816V variant allele fraction (≥50%), reductions in TSS (≥50% and ≥30%), reduction in bone marrow mast cells (≥50%), and quality of life measures.ResultsFrom baseline to week 24, avapritinib-treated patients had a decrease of 15.6 points (95% CI, −18.6 to −12.6) in TSS compared to a decrease of 9.2 points (−13.1 to −5.2) in the placebo group; P<0.003. From baseline to Week 24, 76/141 patients (54%; 45% to 62%) in the avapritinib group compared to 0/71 patients in the placebo group achieved a ≥50% reduction in serum tryptase level; P<0.001. Edema and increases in alkaline phosphatase were more common with avapritinib than placebo; there were few treatment discontinuations because of adverse events.ConclusionsIn this trial, avapritinib was superior to placebo in reducing uncontrolled symptoms and mast-cell burden in patients with ISM. The long-term safety and efficacy of this approach for patients with ISM remain the focus of the ongoing trial. (Funded by Blueprint Medicines Corporation; ClinicalTrials.gov number, NCT03731260.)

Skin and bones: systemic mastocytosis and bone.

We report the case of a 69-year-old female with systemic mastocytosis, diagnosed based on widespread pigmented papules and macules, elevated serum tryptase levels and confirmatory skin and bone marrow biopsy, on a background of osteoporosis. A CT demonstrated multiple sclerotic lesions within lumbar vertebral bodies, sacrum and ileum, with surrounding osteolysis but no obvious compression fractures. She was treated with the RANK-L inhibitor denosumab, resulting in significant bone mineral density gain over the following 5 years. However, her serum tryptase levels gradually increased during this period despite treatment with the multikinase inhibitor, midostaurin. It is thus conceivable that her rapid increase in bone mineral density may be partly contributed by a predominance of pro-osteoblastic mediators released by abnormal mast cells, suggestive of more advanced disease. This case highlights the complexities of systemic mastocytosis-related bone disease and the interplay of numerous mediators contributing to a phenotype of both increased bone resorption and formation. Systemic mastocytosis is a neoplastic disease of mast cells characterized by abnormal proliferation and accumulation in the skin and other organs. It is most frequently associated with the somatic gain-of-function KIT D816V mutation. Systemic mastocytosis should be suspected in patients presenting with not only cutaneous symptoms suggestive of mast cell degranulation such as anaphylaxis, flushing or urticaria but also unexplained osteoporosis and gastrointestinal and constitutional symptoms. The prevalence of osteoporosis in systemic mastocytosis is high. Mast cell activation leads to the secretion of numerous chemical mediators which either promote or inhibit osteoclastic and/or osteoblastic activity, with the balance usually in favour of increased bone resorption. However, in advanced diseases with high mast cell burden, mast-cell-derived cytokines and mediators may promote osteoblastic activity, leading to osteosclerosis and apparent increases in bone mineral density. Treatment of osteoporosis in systemic mastocytosis involves antiresorptive therapy with bisphosphonates and more recently, denosumab. There are limited data on the role of osteoanabolic agents.

Evaluation of Salivary Tryptase As A Biomarker For Mast Cell Burden And Activation

Serum tryptase is a surrogate marker of mast cell burden and activation. We hypothesize salivary tryptase may serve as a novel biomarker for mast cell disorders and a diagnostic marker of failed food challenges in food allergic patients.

Avapritinib Improved Skin Findings In Patients With Indolent Systemic Mastocytosis (ISM) In the Registrational, Double-Blind, Placebo Controlled PIONEER Study

ISM, characterized by mast cell (MC) accumulation driven by D816V-mutant KIT, can be associated with debilitating symptoms including skin lesions. Skin findings, including MC burden and immunophenotype, were characterized in patients with ISM treated with avapritinib, a selective KIT D816V inhibitor, vs placebo in PIONEER Part 2 (NCT03731260).

Summit: A 3-Part, Phase 2 Study of Bezuclastinib (CGT9486), an Oral, Selective, and Potent KIT D816V Inhibitor, in Adult Patients with Nonadvanced Systemic Mastocytosis (NonAdvSM)

Background: Systemic mastocytosis (SM) is characterized by neoplastic mast cell infiltration of extracutaneous tissues and encompasses a spectrum of subtypes that can range from non-advanced to advanced disease. NonAdvSM is characterized by the lack of mast cell-related organ damage (e.g., C-findings) and other features of advanced SM such as associated hematologic neoplasms. NonAdvSM includes two variants of SM: indolent systemic mastocytosis (ISM) and smoldering systemic mastocytosis (SSM). ISM affects nearly 85% of the general SM population and is characterized by symptoms related to mast cell degranulation/mediator release. SSM is characterized by higher mast cell burden in the absence of organ damage or an associated hematologic neoplasm. There are currently no approved therapies for the treatment of the underlying causes of ISM or SSM, and anti-mediator therapies are used for symptom control with variable efficacy and tolerability due to disease heterogeneity. In up to 95% of patients, SM is driven by the somatic KIT D816V gain of function mutation located in exon 17; therefore, agents targeting this mutated kinase have been used to treat advanced SM variants. However, off-target activity of these agents may lead to toxicities that limit dosing and thus efficacy in this population. Therefore, an unmet need remains for safe and effective treatments for patients with NonAdvSM. Bezuclastinib inhibits KIT D816V mutations with high potency and specificity while avoiding other kinases with potential off-target liabilities such as PDGFRα, PDGFRβ, VEGFR2, and CSF1R. Bezuclastinib has demonstrated minimal brain penetration and no central nervous system toxicities were observed in preclinical studies. In an ongoing Phase 2 study (Apex) of patients with AdvSM, early data indicate that bezuclastinib was generally well tolerated and is associated with a meaningful reductions in serum tryptase levels, bone marrow mast cell burden, and KIT D816V variant allele fraction. Additionally, bezuclastinib was safely administered in a completed phase 1/2 clinical study in patients with locally advanced, unresectable or metastatic gastrointestinal stromal tumors (GIST), in which activating KIT mutations are found in a majority of patients. In that study, bezuclastinib led to a reduction in KIT exon 17 mutational burden, which was associated with a reduction in tumor burden. These results support the investigation of bezuclastinib as a potential therapy in GIST as well as AdvSM and NonAdvSM. Study Design and Methods: The Summit study (NCT05186753) is a multi-center, Phase 2, double-blind, placebo-controlled, 3-part clinical trial to evaluate the safety, efficacy, and biomarker correlates (e.g., bone marrow mast cell burden, serum tryptase level, and KIT D816V mutation burden) of bezuclastinib in patients with NonAdvSM. Patients diagnosed with ISM or SSM who have inadequate control of their symptoms despite treatment with at least 2 anti-mediator therapies will be included in this trial. Multiple patient-reported outcome measures (PROMs) will be used to follow symptom scores while on treatment. Part 1 of the study will assess the safety profile of bezuclastinib and will determine the recommended dose for Part 2 through the assessment of changes from baseline of a disease specific PROM. Part 2 will evaluate the efficacy of bezuclastinib at the selected dose as compared with placebo through the evaluation of the PROMs and changes in clinical activity markers such as serum tryptase, bone marrow mast cell burden, and KIT D816V mutation allele burden. All patients will continue to receive their baseline anti-mediator treatments throughout the trial. Serial photographic evaluation will assist in the determination of changes of disease in the skin. Part 3 allows patients who have completed treatment in Part 1 or Part 2 of the study, including those initially randomized to placebo, to participate in a long-term extension study and receive open label bezuclastinib. Conclusions: There are currently no approved therapies for patients with NonAdvSM. Data from the Summit study will support further development of bezuclastinib in SM.

Reappraisal of Mast Cell Leukemia Based on a Single Institution Review of 16 Cases: Role of Mast Cell Morphology in Determining Clinical Outcome

Background: Mast cell leukemia (MCL) is the leukemic variant of systemic mastocytosis (SM); in addition to meeting conventional diagnostic criteria for SM, it is defined by presence of ≥20% mast cells (MC) in the bone marrow (BM) aspirate smear. Presence of circulating MC is not mandatory; MCL is subclassified into leukemic and aleukemic variants based on whether MC comprise ≥10% or <10% of circulating leukocytes, respectively. Given the rarity of MCL, important disease aspects remain incompletely defined, including frequency of evolution from antecedent SM, frequency of associated hematological neoplasm (AHN), prevalence of KITD816V/other mutations, and the prognostic relevance/reproducibility of MC morphology. Furthermore, MCL (as currently defined) has a variable clinical course, with acute and chronic disease variants being recognized; one pitfall is that the latter group may be clinically indistinguishable from other SM subtypes with high BM MC burden. The aim of the current project is to identify clinical, laboratory and pathology features that determine prognosis/survival outcome in MCL patients. Methods: The Mayo Clinic clinical and hematopathology databases were interrogated to identify MCL cases. Patients were retrospectively recruited after confirming research authorization and institutional review board approval. The diagnosis of MCL was per conventional diagnostic criteria. Clinical and laboratory data were collected at the time of MCL diagnosis. BM slides were reviewed by 3 hematopathologists (KKR, KK, and DC), with each slide set being independently reviewed by at least 2 pathologists, using prespecified criteria. MC were classified as either mature (atypical MC type I) or immature (atypical MC type II [promastocytes] and metachromatic blasts) based on the predominant MC population identified. (Valent et al., Ann Oncol. 2014 Sep;25(9):1691-1700). Patient follow up was updated in July 2022; data were censored at last follow up or death. Results: A total of 16 MCL patients were identified (8 females, median age 65 years, range 41-75). Six patients (37%) had de novo MCL; the remaining had antecedent mast cell disease (smoldering SM=4, aggressive SM=3, SM+AHN=2, cutaneous mastocytosis=1). The AHN in 2 patients was CMML and MDS/MPN-unclassifiable, respectively. Four patients (25%) had biopsy confirmed cutaneous involvement. Ten (63%) and 9 (64%) patients had hepatomegaly and splenomegaly, respectively. Median (range) hemoglobin was 9.7 g/dL (7.2-11.9), leukocytes 8.7 x 109/L (0.7-43.9), platelets 123 x 109/L (14-633), serum tryptase 614 ng/mL (69-1684), and serum albumin 3.7 g/dL (2.6-4.7). B- and C-findings were present in 15 (94%) and 12 (75%) patients, respectively. Seven (44%) and 5 (31%) patients had circulating MC and blood eosinophil count >500/µL, respectively. Nine of 13 (69%) and 3 of 14 (19%) evaluable patients harbored KITD816V and abnormal karyotype, respectively. Seven (44%) patients displayed a predominantly 'immature’ MC morphology. After a median follow up of 3.9 months, 12 deaths were recorded. The median OS was 15 months; 2 patients underwent allogeneic stem cell transplant (ASCT). Factors associated with significantly inferior overall survival (OS) on univariate analysis were: immature MC morphology (median OS 0.5 vs 20 months; p<0.01), peripheral blood eosinophilia (p<0.01), and absence of KITD816V (p=0.03). No survival difference was seen in 4 patients treated with midostaurin and/or avapritinib. On multivariable analysis, immature MC morphology retained significance for OS (Figure); among 7 patients with immature MC morphology, 5 had died within a month of their diagnosis, while 2 remain alive at 0.2- and 1-months follow-up. By contrast, among 9 patients with mature MC morphology, 7 had died after a follow-up period of 2.5-59 months, including one receiving ASCT and died at 16 months follow-up, while 2 remain alive at 5.6- and 164.2-months follow-up, the latter post-ASCT. Treatment details and NGS data (7 patients) will be presented at the meeting. Conclusions: The current study identifies immature MC morphology as the most important predictor of inferior survival in MCL. Our observations suggest the need to refine current diagnostic criteria for MCL, in order to distinguish a biologically distinct patient population with a uniformly poor clinical outcome (i.e., 'true’ MCL) from those with more favorable outcome, despite high MC burden. Figure 1View largeDownload PPTFigure 1View largeDownload PPT Close modal

Preliminary Safety and Efficacy from Apex, a Phase 2 Study of Bezuclastinib (CGT9486), a Novel, Highly Selective, Potent KIT D816V Tyrosine Kinase Inhibitor, in Adults with Advanced Systemic Mastocytosis (AdvSM)

Background: Systemic mastocytosis (SM) is a rare disease characterized by the proliferation of mast cells (MC). In about 95% of adult patients, SM is driven by a gain-of-function mutation (D816V) in exon 17 of the KIT gene. Advanced SM (AdvSM) is an aggressive, life-threatening form of SM and includes three subtypes; aggressive SM (ASM), SM with an associated hematologic neoplasm (SM-AHN), and MC leukemia (MCL). Bezuclastinib is an oral, highly selective, and potent tyrosine kinase inhibitor (TKI) with specificity for mutations in KIT exons 9, 11, 17, and 18, including D816V, while sparing closely related kinases (e.g., PDGFRα, PDGFRβ, and CSF1R). Results from a phase 1/2 study in patients with advanced solid tumors coupled with preclinical data have led to the on-going evaluation of bezuclastinib in patients with AdvSM, the Apex study (NCT04996875). The primary objective of this study is to evaluate the safety, efficacy, pharmacokinetic (PK), and pharmacodynamic (PD) profile of bezuclastinib in all AdvSM subtypes. Methods: Apex is a Phase 2, open-label, 2-part, multicenter study in adult patients with AdvSM per WHO diagnostic criteria, who have measurable SM-related organ damage according to modified IWG-MRT-ECNM consensus eligibility and response criteria (mIWG), and have a baseline serum tryptase of ≥20 ng/mL. Patients with a history of prior TKI therapy (e.g., avapritinib, midostaurin) are permitted in the study. In Part 1, patients are randomized 1:1:1:1 to bezuclastinib 50, 100, or 200 mg BID, or 400 mg QD. Data from Part 1 will inform dose selection for Part 2. The primary efficacy endpoint for this study is overall response rate (complete response [CR], CR with incomplete hematologic recovery [CRh], partial response [PR] and clinical improvement [CI]) as assessed by a Central Response Review Committee and based on mIWG response criteria. Results: As of May 24, 2022, 11 mIWG evaluable patients with AdvSM (ASM: n=2, SM-AHN: n=8, and MCL: n=1) were randomized in Part 1 to 50mg BID (n=3), 100mg BID (n=3), 200mg BID (n=3), and 400mg QD (n=2). The majority of patients were male (82%) and treatment naïve (82%) with ECOG PS 0-1 (82%) and positive for KIT D816V (91%). Median (range) BM MC burden and serum tryptase at baseline were 30% (7-80%) and 182 ng/mL (68-1578 ng/mL), respectively. Bezuclastinib was well-tolerated across all dose levels. Treatment Emergent Adverse Events (TEAEs) reported in >1 patient included anemia (n=3), neutropenia (n=2), and thrombocytopenia (n=2), a majority of which were low-grade. One serious AE of hypersensitivity (mediator flare) was reported. Two patients had an AE-related dose reduction with one patient subsequently re-escalating to the randomized dose. Preliminary clinical activity data showed that all 11 patients experienced a >50% reduction in serum tryptase from baseline, 10/11 (91%) within the first week of treatment; 6 (55%) patients achieved a serum tryptase level <20 ng/mL, 4/6 within the first cycle of treatment. Among patients with available data after ≥2 cycles of treatment, all 8 achieved a median reduction of 68% in KIT D816V variant allele fraction (VAF) and ≥50% reduction in BM MC, with 6/8 (75%) achieving complete clearance of MC aggregates. There were no reported instances of cognitive effects or intracranial bleeding. Conclusion: Early data suggest that bezuclastinib is well-tolerated and is associated with encouraging early signs of clinical activity as demonstrated by meaningful reductions in serum tryptase, MC burden, and KIT D816V VAF. Enrollment is currently ongoing. Updated safety and clinical activity as well as initial clinical objective response data will be presented.

MPN-352 Clinicopathologic and Molecular Correlates of Organ Damage Across the Spectrum of Advanced Systemic Mastocytosis.

Advanced SM (AdvSM) comprises 3 subtypes: aggressive SM (ASM), SM with an associated hematologic neoplasm (SM-AHN), and mast cell leukemia (MCL). The patterns of organ damage, which have been defined by WHO, IWG, and modified IWG (mIWG) criteria, across the spectrum of AdvSM have not been well characterized. 1) To describe the distribution of organ damage in AdvSM patients (pts) at initial presentation or at the time of trial enrollment; 2) To evaluate clinicopathologic and molecular correlates of organ damage. Observational study of adults with AdvSM. Tertiary referral center. Our cohort included 240 AdvSM pts: 66 from the Stanford MPN registry and 174 from the phase I EXPLORER and phase II PATHFINDER studies of avapritinib in AdvSM. None. Hematologic and non-hematologic organ damage as defined by WHO, IWG, and mIWG criteria. Thirty-seven pts (15%) had ASM, 165 (69%) had SM-AHN, and 38 (16%) had MCL. Pts with MCL had the highest transfusion requirement and the largest liver and spleen volumes by imaging. In multivariate analyses comparing SM-AHN to ASM/MCL, SM-AHN pts had higher absolute neutrophil and monocyte counts and presence of SRSF2, ASXL1, or RUNX1 (S/A/R) mutations. ASM/MCL pts had higher tryptase levels and bone marrow mast cell burden. Compared to midostaurin-naïve pts, those who had received midostaurin had a greater bone marrow mast cell burden (P=0.06) but less frequently met WHO liver (P<0.001) and WHO malabsorption (P<0.001) criteria. Compared to pts with KIT D816V only, pts with ≥1 S/A/R mutations had lower median hemoglobin (P=0.02) and platelet count (P=0.02) and higher direct bilirubin (P=0.02), alkaline phosphatase (P=0.03), and spleen volume (P=0.06). They exhibited more WHO-defined cytopenias (P=0.007) and were more likely to meet WHO hypersplenism criteria (P=0.03). In AdvSM, patterns of organ damage reflect disease subtype, prior therapy, and mutation profile. These data can help inform enrollment of AdvSM pts into clinical trials that currently require ≥1 organ damage findings. Grant Acknowledgments: ASH HONORS Award, NIH grant P30CA124435.

Functional imaging with dual-energy computed tomography for supplementary non-invasive assessment of mast cell burden in systemic mastocytosis

Systemic mastocytosis (SM) is characterized by multifocal accumulation of neoplastic mast cells (MCs), predominately affecting the bone marrow (BM). Imaging with computed tomography (CT) is used for assessment of bone mineral density and structure. However, the value of functional imaging with dual-energy CT (DECT) and the assessment of virtual-non-calcium attenuation values (VNCa-AV) for visualization of BM disease burden in SM has not yet been assessed. DECT of the axial skeleton was performed in 18 patients with SM (indolent SM [ISM], n = 6; smoldering SM [SSM]/advanced SM [AdvSM], n = 12) and 18 control subjects. VNCa-AV were obtained in 5 representative vertebraes per patient and correlated with laboratory, morphologic and molecular parameters. VNCa-AV strongly correlated with quantitative BM MC infiltration (r = 0.7, R2 = 0.49, P = 0.001) and serum tryptase levels (r = 0.7, R2 = 0.54, P < 0.001). Mean VNCa-AV were significantly higher in SSM/AdvSM as compared to ISM (− 9HU vs. − 54HU, P < 0.005) and controls (− 38HU, P < 0.005). Nine of 10 (90%) patients with a VNCa-AV > − 30HU and 7/7 (100%) patients with a VNCa-AV > − 10HU had SSM or AdVSM. BM VNCa-AV provide information about the MC burden of SM patients and correlate with SM subtypes. DECT may therefore serve as a supplementary tool for SM diagnosis, subclassification and monitoring in a one-stop-shop session.