Advanced Systemic Mastocytosis Research Articles

Serum chemistry profiling and prognostication in systemic mastocytosis: a registry-based study of the ECNM and GREM

Certain laboratory abnormalities correlate with subvariants of systemic mastocytosis (SM) and are often prognostically relevant. To assess the diagnostic and prognostic value of individual serum chemistry parameters in SM, 2607 patients enrolled within the European Competence Network on Mastocytosis (ECNM) and 575 patients enrolled within the German Registry on Eosinophils and Mast Cells (GREM) were analyzed. For screening and diagnosis of SM, tryptase was identified as the most specific serum parameter. For differentiation between indolent and advanced SM (AdvSM), the following serum parameters were most relevant: tryptase, alkaline phosphatase (AP), ß2-microglobulin, lactate dehydrogenase (LDH), albumin, vitamin B12, and C-reactive protein (P<0.001). With regard to subvariants of AdvSM, an elevated LDH ≥260U/L was associated with multi-lineage expansion (leukocytosis, r=0.37, P<0.001; monocytosis, r=0.26, P<0.001) and the presence of an associated myeloid neoplasm (P<0.001), whereas tryptase levels were highest in mast cell leukemia (MCL vs. non-MCL, 308 µg/L vs. 146 µg/L, P=0.003). Based on multivariable analysis, the hazard-risk weighted assignment of 1 point to lactate dehydrogenase (HR 2.1 [95% CI 1.1-4.0], P=0.018) and 1.5 points each to ß2-microglobulin (HR 2.7 [95% CI 1.4-5.4], P=0.004) and albumin (HR 3.3 [95% CI 1.7-6.5], P=0.001) delineated a highly predictive three-tier risk classification system (0 points, 8.1 years vs. 1 point, 2.5 years, ≥1.5 points, 1.7 years; P<0.001). Moreover, serum chemistry parameters enabled further stratification of IPSM-AdvSM1/2 risk-score classified patients (P=0.027). In conclusion, serum chemistry profiling is a crucial tool in the clinical practice supporting diagnosis and prognostication of SM and its subvariants.

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.

Allogeneic haematopoietic cell transplantation for advanced systemic mastocytosis: Best practice recommendations on behalf of the EBMT Practice Harmonisation and Guidelines Committee.

Systemic Mastocytosis (SM) is a multifaceted clinically heterogeneous disease. Advanced SM (AdvSM) comprises three entities: aggressive SM (ASM), mast cell leukaemia (MCL) and SM with an associated hematologic neoplasm (SM-AHN), the latter accounting for 60-70% of all AdvSM cases. Detection of a disease-triggering mutation in the KIT gene (esp. KIT D816V) in >90% of the patients with ASM or SM-AHN has led to a significant improvement in therapeutic options by the implementation of two KIT-targeting kinase inhibitors: midostaurin and avapritinib. Although complete remissions have been reported, neither of these targeted agents is 'curative' in all patients and the duration of responses varies. The median overall survival, depending on the WHO subtype and scoring result, is approximately 1 to 4 years. Although the European Competence Network on Mastocytosis (ECNM) and American Initiative in Mast Cell Diseases (AIM) consensus groups recommend allogeneic haematopoietic cell transplantation (allo-HCT) in drug-resistant and other high-risk patients, there is a relative lack of information to guide clinicians on which patients with AdvSM should be considered for transplant, and how KIT inhibitors may fit into the transplant algorithm, including their use pre- and post-transplant to optimise outcomes. Following the generation of an expert panel with a specialist interest in allo-HCT and mastocytosis, these best practice recommendations were generated according to the European Society for Blood and Marrow Transplantation (EBMT) Practice Harmonisation and guidelines and ECNM methodology. We aim to provide a practical, clinically relevant and up-to-date framework to guide allo-HCT in AdvsM in 2024 and beyond.

Allogeneic Hematopoietic Cell Transplantation in Advanced Systemic Mastocytosis: A retrospective analysis of the DRST and GREM registries

We identified 71 patients with AdvSM (aggressive SM [ASM], SM with an associated hematologic neoplasm [SM-AHN, e.g., acute myeloid leukemia, SM-AML], mast cell leukemia [MCL]) in two national registries (DRST/GREM) who received an allogeneic hematopoietic cell transplantation (alloHCT) performed in Germany from 1999–2021. Median overall survival (OS) of ASM/SM-AHN (n = 30, 45%), SM-AML (n = 28, 39%) and MCL ± AHN (n = 13, 19%) was 9.0, 3.3 and 0.9 years (P = 0.007). Improved median OS was associated with response of SM (17/41, 41%; HR 0.4 [0.2–0.9], P = 0.035) and/or of AHN (26/43, 60%, HR 0.3 [0.1–0.7], P = 0.004) prior to alloHCT. Adverse predictors for OS included absence of KIT D816V (10/61, 16%, HR 2.9 [1.2–6.5], P < 0.001) and a complex karyotype (9/60, 15%, HR 4.2 [1.8–10.0], P = 0.016). HLA-match, conditioning type or transplantation at centers reporting above-average alloHCTs (≥7) had no impact on OS. Taking into account competing events at years 1, 3 and 5, relapse-related mortality and non-relapse mortality rate were 15%/23%, 20%/30% and 23%/35%, respectively. Irrespective of subtype, subsequent treatment response was achieved in 13/30 (43%) patients and was highest on midostaurin/avapritinib (7/9, 78%). We conclude that outcome of alloHCT in AdvSM is more affected by disease phenotype and treatment response prior to transplant than by transplant characteristics.

Poor Applicability of Currently Available Prognostic Scoring Systems for Prediction of Outcome in KIT D816V-Negative Advanced Systemic Mastocytosis

Within our nationwide registry, we identified a KIT D816V mutation (KIT D816Vpos.) in 280/299 (94%) patients with advanced systemic mastocytosis (AdvSM). Age, cytopenias and the presence of additional somatic mutations confer inferior overall survival (OS). However, little is known about the characteristics of KIT D816V-negative (D816Vneg.) AdvSM. In 19 D816Vneg. patients, a combination of clinical, morphological and genetic features revealed three subgroups: (a) KIT D816H- or Y-positive SM (KIT D816H/Ypos., n = 7), predominantly presenting as mast cell leukemia (MCL; 6/7 patients), (b) MCL with negative KIT sequencing (KITneg. MCL, n = 7) and (c) KITneg. SM with associated hematologic neoplasm (KITneg. SM-AHN, n = 5). Although &gt;70% of patients in the two MCL cohorts (KIT D816H/Ypos. and KITneg.) were classified as low/intermediate risk according to prognostic scoring systems (PSS), treatment response was poor and median OS was shorter than in a KIT D816Vpos. MCL control cohort (n = 29; 1.7 vs. 0.9 vs. 2.6 years; p &lt; 0.04). The KITneg. SM-AHN phenotype was dominated by the heterogeneous AHN (low mast cell burden, presence of additional mutations) with a better median OS of 4.5 years. We conclude that (i) in MCL, negativity for D816V is a relevant prognostic factor and (ii) PSS fail to correctly classify D816Vneg. patients.

Midostaurin drug interaction profile: a comprehensive assessment of CYP3A, CYP2B6, and CYP2C8 drug substrates, and oral contraceptives in healthy participants.

Midostaurin, approved for treating FLT-3-mutated acute myeloid leukemia and advanced systemic mastocytosis, is metabolized by cytochrome P450 (CYP) 3A4 to two major metabolites, and may inhibit and/or induce CYP3A, CYP2B6, and CYP2C8. Two studies investigated the impact of midostaurin on CYP substrate drugs and oral contraceptives in healthy participants. Using sentinel dosing for participants' safety, the effects of midostaurin at steady state following 25-day (Study 1) or 24-day (Study 2) dosing with 50mg twice daily were evaluated on CYP substrates, midazolam (CYP3A4), bupropion (CYP2B6), and pioglitazone (CYP2C8) in Study 1; and monophasic oral contraceptives (containing ethinylestradiol [EES] and levonorgestrel [LVG]) in Study 2. In Study 1, midostaurin resulted in a 10% increase in midazolam peak plasma concentrations (Cmax), and 3-4% decrease in total exposures (AUC). Bupropion showed a 55% decrease in Cmax and 48-49% decrease in AUCs. Pioglitazone showed a 10% decrease in Cmax and 6% decrease in AUC. In Study 2, midostaurin resulted in a 26% increase in Cmax and 7-10% increase in AUC of EES; and a 19% increase in Cmax and 29-42% increase in AUC of LVG. Midostaurin 50mg twice daily for 28days ensured that steady-state concentrations of midostaurin and the active metabolites were achieved by the time of CYP substrate drugs or oral contraceptive dosing. No safety concerns were reported. Midostaurin neither inhibits nor induces CYP3A4 and CYP2C8, and weakly induces CYP2B6. Midostaurin at steady state has no clinically relevant PK interaction on hormonal contraceptives. All treatments were well tolerated.

A clinical, morphological and molecular study of 70 patients with gastrointestinal involvement in systemic mastocytosis

In 70 patients with KIT D816V positive systemic mastocytosis (SM) including 36 patients with advanced SM (AdvSM), we correlated the extent of reported mucosal mast cell ([m]MC) infiltration of the upper and/or lower gastrointestinal tract (UGIT, n = 63; LGIT, n = 64; both, n = 57) with symptoms and markers of MC burden/subtype. GI symptoms were reported by all patients (mean 2.1 number of symptoms). A strong mMC infiltration was identified in 24 patients (UGIT, 17/63, 27%; LGIT, 19/64, 30%). Concurrent involvement of UGIT and LGIT (n = 12) correlated with female gender (75%) and a higher symptom burden (mean 2.7) but not with MC burden or subtype. Significant differences between non-AdvSM and AdvSM were reported regarding food intolerance (54% vs. 17%), cramping (54% vs. 22%) and weight loss (0% vs. 64%). KIT D816V was identified in 54/56 (96%) available biopsies. In 46 patients, digital PCR revealed a correlation with low albumin levels (r =  − 0.270, P = 0.069) and the KIT D816V VAF in peripheral blood (r = 0.317, P = 0.036) but not with the extent of mMC infiltration or markers of MC burden/subtype. Although MC mediator triggered GI symptoms have a substantial impact on the quality of life, correlation to objective disease parameters is lacking thus making its systematic assessment challenging.

Recent Advances in the Therapeutic Management of Advanced Systemic Mastocytosis.

Advanced systemic mastocytosis (AdvSM) is a rare haematological neoplasm characterised by the accumulation of neoplastic mast cells (MCs) in various organs, resulting in organ dysfunction and reduced life expectancy. The subtypes include aggressive SM (ASM), SM with an associated haematological neoplasm (SM-AHN) and mast cell leukaemia (MCL). The gain of function KIT D816V mutation is present in most cases. The availability of tyrosine kinase inhibitors (TKIs) has revolutionised the treatment landscape for patients with this life-limiting disease. Patients are now able to achieve molecular remission, improved quality of life and improved overall survival. This review focuses on the targeted therapies currently available in clinical practice and within the clinical trial setting for AdvSM. This review also highlights possible future therapeutic targets and discusses therapeutic strategies for this multimutated and clinically heterogeneous disease.

Diagnosis and management of systemic mastocytosis in a community hematology setting.

Systemic mastocytosis (SM) is a rare and potentially severe hematologic disorder characterized by the clonal proliferation of mast cells (MCs) into various organs. The clinical manifestations of advanced SM are caused by the uncontrolled release of cytokines and vasoactive amines from MC and disease-induced organ dysfunction. Patients with SM typically present with symptoms such as flushing, pruritus, diarrhea, and headaches, but outcomes following active treatment have not been well characterized. In this study, the clinical characteristics, treatment patterns, and natural history of an SM patient cohort diagnosed and treated within a community hematology network in the United States is described. We identified 105 patients who were diagnosed and managed in one of 19 community hematology clinics up to an index date of 1 October 2022. Data collection included patient and disease characteristics, baseline biochemistry and hematology, SM diagnostic criteria being met, biomarkers tested, CD2 and/or CD25 expression in MCs as well as serum tryptase levels at presentation. Data abstraction also included supportive care drugs and anticancer therapy used, treatment response, reason for discontinuation, and overall survival by disease subtype. A total of 105 SM patients were identified who met the inclusion criteria. The specific SM subtypes were indolent (47.6%), aggressive (9.5%), SM with an associated hematological neoplasm (19.0%), MC leukemia (1.9%), and subtype not documented (21.9%). Regardless of subtype, approximately 62% of patients did not receive SM-directed active therapy. Only 26% of patients with indolent systemic mastocytosis (ISM) received treatment compared to 65.6% with advanced subtypes. Relative to ISM cohort, the relative risk of death in patients with the advanced SM subtypes was approximately 15 times greater (hazard ratio = 15.0; 95% confidence interval: 3.3 to 66.5). SM patients present with multiple underlying symptoms, within various disease subtypes that are difficult to diagnose in a timely manner. As a result, many patients do not receive active drug therapy for their disease. Therefore, greater disease awareness is required as well as new tools for earlier disease detection.

Current Challenges in the Diagnosis of Pediatric Cutaneous Mastocytosis

Pediatric mastocytosis is mostly a cutaneous disease classified as cutaneous mastocytosis (CM), which is characterized by mast cell (MCs) accumulation in the skin and the absence of extracutaneous involvement. Based on the morphology of skin lesions, CM can be divided into three major forms: maculopapular CM (MPCM), diffuse CM (DCM) and mastocytoma of the skin. A positive Darier’s sign is pathognomonic for all forms of CM. MPCM is the most common form, presenting with red-brown macules or slightly raised papules. Mastocytoma is characterized by solitary or a maximum of three nodular or plaque lesions. DCM is a rare, severe form which presents as erythroderma, pachydermia and blistering in the infantile period of the disease. CM is associated with MC mediator-related symptoms, most commonly including pruritus, flushing, blistering, diarrhea and cramping. Anaphylactic shock occurs rarely, mainly in patients with extensive skin lesions and a significantly elevated serum tryptase level. Childhood-onset MPCM and mastocytoma are usually benign diseases, associated with a tendency for spontaneous regression, while DCM is associated with severe mediator-related symptoms, an increased risk of anaphylaxis and, in some cases, underlying systemic mastocytosis (SM). In contrast to adults, SM is a rare finding in children, most commonly presenting as indolent SM. However, advanced SM sporadically occurs.

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.

A Phase 1 Study of Avapritinib in Combination with Decitabine in Patients with Systemic Mastocytosis with an Associated Hematologic Neoplasm

Background and Rationale: Systemic mastocytosis (SM) is a clonal hematologic neoplasm characterized by abnormal accumulation of mast cells which can lead to symptoms related to mast cell degranulation and/or, in the case of advanced systemic mastocytosis (AdvSM), organ dysfunction due to mast cell infiltration. AdvSM is a category of SM comprised of aggressive systemic mastocytosis (ASM), mast cell leukemia (MCL), and SM with an associated hematologic neoplasm (SM-AHN). Among these, SM-AHN is the most common subtype, accounting for approximately 70% of AdvSM. A KIT D816V mutation found in up to 95% of SM cases is the primary oncogenic driver, making it a key therapeutic target. Avapritinib, a highly potent and selective inhibitor of D816V mutant KIT, has demonstrated high rates of rapid, deep, and durable responses in all AdvSM subtypes, including SM-AHN. However, a recent analysis of the EXPLORER study showed that avapritinib-treated patients were still at risk for progression of the AHN component, despite continued control of mast cell disease burden, indicating that combination therapy is likely to be required for optimal control of SM-AHN due to its inherent molecular complexity. Hypomethylating agents (HMA) form the backbone of therapy for advanced myelodysplastic syndrome (MDS) and myelodysplastic/myeloproliferative neoplasm (MDS/MPN), hematologic neoplasms most likely to co-occur with SM in patients (pts) with SM-AHN. To date, the combination of HMA and KIT inhibitors has not been clinically evaluated. In this study, we aim to assess the safety and tolerability of the combination of avapritinib and decitabine in patients with SM-AHN. Study design: The primary objective of the study is to establish the recommended dose (RP2D) of avapritinib in combination with decitabine based on dose-limiting toxicity, overall safety, and clinical activity data. A dose evaluation steering committee (DESC) comprised of participating investigators will review the totality of safety and efficacy data to determine RP2D. Secondary endpoints include SM overall response rate by the end of 6 cycles by the m-IWG consensus criteria and proposed ECNM-AIM criteria ( Gotlib et al., J Allergy Clin Immunol Pract, 2022), as well as incidence and frequency of adverse events. Exploratory endpoints will consist of patient reported outcomes, progression free survival, overall survival, and correlative biomarkers linked to mast cell disease or AHN. Adult pts with a diagnosis of SM-AHN by the WHO 2022 classification criteria will be eligible for inclusion. Eligible pts must have any of the following myeloid AHN per WHO 2022 criteria: MDS, chronic myelomonocytic leukemia, MDS/MPN with neutrophilia, MDS/MPN with SF3B1 mutation and thrombocytosis, or MDS/MPN- not otherwise specified. Patients with MDS must have intermediate, high- or very-high risk by IPSS-R. Patients with very-low or low-risk MDS by IPSS-R must be intolerant/refractory to ESA or have serum EPO &amp;gt; 200 U/L. A baseline platelet count of ≥ 75 x 10 9/L and ≥ 25 x 10 9/L will be required for inclusion in the dose-finding and the dose-expansion phases, respectively (Figure). Key exclusion criteria include a diagnosis of acute myeloid leukemia, history of intracranial hemorrhage or risk of major hemorrhage, prior treatment with avapritinib or decitabine with documented progression in SM or AHN component, respectively, or history of treatment with alternative KIT inhibitor or azacitidine within 4 weeks of study treatment initiation. In the dose finding part of the study, a Bayesian adaptive design will be employed to assess different doses of avapritinib (50 to 200 mg PO daily) in combination with fixed doses of decitabine (oral decitabine/cedazuridine: 35/100 mg/day or intravenous decitabine: 20 mg/m 2/day on days 1-5) in a 28-day treatment cycle. Close attention will be paid to treatment-emergent thrombocytopenia with protocol-driven treatment holds and dose modification strategies employed when platelet count drops below 50 x 10 9/L. After RP2D is determined, an additional 10 patients will be enrolled in the dose expansion phase. Patients with platelet count ≥ 25 x 10 9/L and &amp;lt; 75 x 10 9/L, will receive lead-in dosing with decitabine or decitabine/cedazuridine with the ability to add avapritinib if sufficient platelet thresholds are achieved. This study will open at 7 sites in the United States and is anticipated to open in January 2024.

Predicting the Clinical Course of Treatment with Midostaurin in Patients with Advanced Systemic Mastocytosis

Introduction: Advanced systemic mastocytosis (AdvSM) is characterized by presence of the KIT D816V mutation and variably other somatic mutations in the mast cell but also non-mast cell lineages, e.g. monocytes or eosinophils, in ≥80-90% of patients. Midostaurin is an approved multikinase/KIT inhibitor for treatment of AdvSM. The intricate landscape in assessment of response, resistance or progression, the occurrence of adverse events with its impact on dose modification and the recent approval of the specific KIT D816V inhibitor avapritinib contribute to complex decision-making. We here sought to characterize the most valuable baseline and on-treatment parameters for prediction of response and survival in midostaurin-treated patients with AdvSM. Methods: On basis of the ´German Registry on Disorders of Eosinophils and Mast Cells' (GREM), this retrospective-prospective analysis included 79 patients with AdvSM. Treatment was initiated between 2009 and 2022 accumulating into a 239 patient-years overall follow-up period. In addition to common serological, morphological, and molecular response parameters, we specifically examined the recurrent observation of an early flare-up in alkaline phosphatase (AP) levels, which was defined as a ≥25% AP increase within three months after treatment initiation, followed by a subsequent decline below baseline within six months. Results: Midostaurin was used in first-line (1L) or second and further lines (2L+; median 1 prior treatment line, range 0-4) in 63 (80%) and 16 (20%) patients, respectively. In 1L/2L+, median overall survival (OS) was 2.6/2.4 years. At baseline, the presence of additional somatic mutations in SRSF2 (n=36, 46%, HR 2.0 [1.2-3.6], P=0.014), ASXL1 (n=15, 19%, HR 2.0 [1.0-3.9], P=0.043), and EZH2 (n=6, 8%, HR 4.3 [1.7-11.3], P=0.003) significantly stratified patients according to their hazard ratio-weighted risk (median OS, 5.8 vs. 2.9 vs. 1.0 years; P&amp;lt;0.001). In 25 evaluable patients, the variant allele frequency in 72 additional somatic mutations remained stable (mean change 8%, absolute change &amp;gt;5% in 16/72 (22%) mutations). New mutations were identified in 9/25 (36%) patients during follow-up (Figure). Durable responses (complete, partial, clinical improvement) according to modified Valent criteria, IWG-MRT-ECNM criteria or Pure Pathological Response (PPR) criteria over a median duration between 12 and 24 months were achieved in 60%, 29% and 13% of patients, respectively. Within the first 12 months, duration of response was associated with an OS benefit ( P&amp;lt;0.001). An AP flare-up was observed in 22/79 (28%) patients, was paralleled by contemporaneous decrease of serum tryptase (median -35%, range -82 - 191%) and resulted into improved OS (median 5.8 vs. 2.2 years, P=0.007). Multivariable analysis identified the AP flare-up ( P=0.029), a ≥25% reduction of bone marrow (BM) mast cell (MC) infiltration ( P=0.046) and a ≥25% reduction of the KIT D816V expressed allele burden (EAB; P=0.034) at any time within the first 6 months as independent markers for improved OS (Table). Dynamic risk assessment of the independent parameters during follow-up allowed to abrogate a negative baseline risk-categorization. The initial dose was 200 mg/d (100 mg BID, including patients with a prespecified rapid dose escalation), 150 mg/d or 100 mg/d in 63/79 (80%), 2/79 (3%) and 14/79 (18%) patients, respectively. In the combined 150 mg and 100 mg cohorts, a dose increase to 200 mg/day was achieved in 9/16 (56%) patients. The 200 mg could be maintained at month 3, 6, 12, 24, and 36 in 50/63 (79%), 30/48 (63%), 23/34 (68%), 15/22 (68%) and 9/9 (100%) patients, respectively. Overall, 96 adverse events (AE) led to dose-adjustment (including temporary or permanent discontinuation). Hematologic and nonhematologic toxicities accounted for 24 (25%) and 36 (38%) of AEs with neutropenia (n=8, 8%) and nausea/emesis (n=22, 22%) being the most common AEs leading to dose-adjustment. Of note, the various dose levels had no impact on response or survival. Conclusions: Baseline mutations in SRSF2, ASXL1 and EZH2 conferred resistance to midostaurin. The various dose levels had no impact on response or survival. The on-treatment parameters AP flare-up, and a ≥25% reduction of BM MC infiltration and of KIT D816V EAB abrogated a negative baseline risk-categorization. No new safety signals occurred.

Decreased Survival Among Patients with Indolent Systemic Mastocytosis: A Population-Level Retrospective Cohort Analysis Using Healthcare Claims Dataset

Background Systemic mastocytosis (SM) is a rare, clonal mast cell disease driven by the KIT D816Vmutation in ~95% of cases and can be subdivided into two predominant subtypes - advanced SM (AdvSM) and indolent SM (ISM). AdvSM comprises ~15% of all patients and is characterized by mast cell proliferation and infiltration causing subsequent dysfunction of various organ systems, leading to reduced overall survival. Indolent systemic mastocytosis (ISM) comprises the bulk of the remaining ~85% of patients and is characterized by a less severe clinical course that is nevertheless associated with a significant symptom burden that may worsen over time, poor quality of life, and the potential for life-threatening anaphylaxis. Recent epidemiologic data indicate ISM is a heterogenous disease. A meaningful subset of patients with disease risk factors demonstrates high symptom burden, which may be associated with higher rates of disease progression and increased risk of death (Sperr, et al. Lancet Hematol. 2019). Building upon this work, we conducted an analysis to evaluate overall survival (OS) in ISM patients. Methods Using a nationally representative health claims dataset provided by Komodo Health covering 230 million US patients (2015 to 2022), we identified 8,332 patients with ISM using a claims-based algorithm. This algorithm leverages combinations of ICD-10 dx codes for SM and organ dysfunction, procedure codes (bone marrow biopsy), and use of SM-directed therapies (Mukherjee et al ASH 2022). Patients were included if full medical and pharmacy claim information was available for &amp;gt;12 consecutive months post-diagnosis, and median follow-up was 4.0 years. ISM was inferred as a dx if only the broad systemic mastocytosis ICD-10 code was used (D47.02) and there was no preceding evidence of AdvSM or an associated hematologic neoplasm. Disease progression to AdvSM was inferred if there was subsequent evidence of mast cell leukemia, aggressive SM, or an associated hematologic neoplasm OS was then calculated, anchored to the first observed ICD-10 dx code for SM. Mortality information was available within the claims dataset and was additionally inferred if a patient did not generate any medical or pharmacy claims for 12 or more consecutive months. Three, independent non-SM control cohorts (N=2000 each) were established for comparison, based on a stratified sampling strategy matched by gender, age, Charlson Comorbidity Index, payer status, and race/ethnicity of the ISM population. Multiple cohorts were plotted to illustrate consistency in survival across independently-sampled control cohorts. As positive control, we compared OS with a cohort of patients with ASM, a disease cohort with known increased mortality. Results We observed a statistically significant difference in OS among patients with ISM versus the matched non-SM control cohorts ( p&amp;lt;0.0001, with a log-rank hazard ratio of 1.64; Figure 1A). Excess mortality was noted among ISM patients with or without evidence of disease progression ( p&amp;lt;0.0001, with a hazard ratio of 1.44) compared to matched non-SM cohorts ( Figure 1B). As expected, OS was worst in patients with AdvSM ( p&amp;lt;0.0001, with a hazard ratio of 6.64). The ISM cohort presented to the emergency room (ER) 0.70 times per year, compared to 0.31 times per year for the combined non-ISM control cohorts, based on the sum of ER visits observed across the cumulative cohort observation period. Patients with ISM who died during the study interval had the highest frequency of ER visits, at a rate of 1.09 visits per year. Conclusions This is the first and the largest population-level analysis of OS in the ISM population in the US. Results demonstrate a modest but statistically significant decrease in survival among these patients even early in their disease course (first 4 years). This increase in mortality was observed even among patients with no evidence of progression to advanced SM and was additionally associated with &amp;gt;2x higher rates of ER visits over the observation period. These results suggest that acute, severe symptoms of SM may contribute to the excess mortality seen among patients with ISM and warrant focused research to identify specific patient characteristics or disease features in ISM that confer higher risk of mortality.

A Model of Cumulative Risk of Disease Progression Among Patients with Indolent Systemic Mastocytosis

Introduction Indolent systemic mastocytosis (ISM), a clonal mast cell disease driven by the KIT D816V mutation in ~95% of cases, can cause lifelong debilitating cutaneous, gastrointestinal, and neurocognitive symptoms with some cases progressing to advanced systemic mastocytosis (AdvSM). While several studies have quantified variable rates of disease progression from ISM to AdvSM based on the absolute frequency of these events, these studies were limited by short median follow-up time considering the long clinical course of this condition sometimes in decades, thereby leading to underestimation of the cumulate rates of progression from ISM to AdvSM. We conducted a Markov analysis using population level data in the US to estimate the lifetime risk of progression for ISM patients. Methods We modeled the total course of SM as a discrete-time Markov chain, enabling us to estimate cumulative rates of disease progression based on fixed-interval empirical observations from the literature and, where there is no literature available, expert opinion. The Markov model assumes that patients enter the model as at-risk ISM, not-at-risk ISM, or de novo AdvSM, based on the assumed relative incidence of each subtype. Not-at-risk patients will then move to death after a period of time, with an annualized risk of death from disease. At-risk patients with ISM will remain in that state, progress to AdvSM (which may be tyrosine kinase inhibitor treated or not) or die. Finally, those who progress to AdvSM will either remain in their state or die. See Figure1 for details, where each model parameter is reflected as a line between states and modeled based on an annualized rate of state transition. See Figure 2 for a full list of model parameters. Two hundred trials were performed, in which the model parameters were statistically sampled from a range of likely values, based on a triangular distribution between minimum, mode, and maximum. For each trial, the disease course for 5000 patients with SM was then simulated from initial SM diagnosis until death. Results Based on the rates of disease incidence, progression, treatment, and survival documented within various studies, we estimate the cumulative risk (± standard deviation) of disease progression from ISM to AdvSM to be 18.0% ± 3.1%, based on a probabilistic sampling of the underlying assumptions. Conclusions This study suggests that the cumulative rate of disease progression among patients with ISM may be higher than previously recognized. In our earlier work, we showed a higher rate of disease progression in ISM patients with high symptom burden (Mukherjee S, et al. ASH 2022). These data highlight the importance of close monitoring for signs of progression and warrant further research to identify subsets of ISM patients who may have a higher underlying risk of progression.

Allogeneic Hematopoietic Stem Cell Transplantation in Advanced Systemic Mastocytosis

Purpose Advanced systemic mastocytosis (AdvSM) is driven by the KIT D816V mutation in &amp;gt;90% of patients (pts.). Depending on subtype, median survival is between 1.5 and 4 years. Despite recent advances on responses and survival due to the availability of effective KIT inhibitors, allogeneic hematopoietic stem cell transplantation (alloHSCT) remains the only curative treatment option. Data on optimal timing, conditioning regimens, and pre- as well as post-alloHSCT treatments for maintenance or relapse are however scarce. Current recommendations mainly refer to a retrospective analysis published almost 10 years ago (Ustun et al., JCO 32, 2014). With the advent of new therapies and improved transplant procedures, more recently collected data on alloHSCT in AdvSM are needed for actual assessment of optimal treatment strategies. Patients &amp; Methods This retrospective analysis identified 71 pts. with AdvSM who underwent alloHSCT at 20 German centers between 1999 and 2021. Data were collected via the 'German Stem Cell Transplantation Registry (DRST)' and the 'German Registry on Disorders of Eosinophils and Mast Cells (GREM)'. Additional baseline characteristics and response status prior to, during and post alloHSCT were provided by the participating centers. Results At time of alloHSCT, median age was 59 years (range 21-84); 55/71 (77%) pts. were male. Median time between diagnosis of AdvSM and alloHSCT was 1.2 years (range 0.1-16.7). Pts. were grouped into (i) aggressive systemic mastocytosis (ASM, n=6) and SM with an associated hematological neoplasm (SM-AHN, n=24), totaling 30/71 (42%) ASM/SM-AHN pts., (ii) SM with an associated acute myeloid leukemia (SM-AML; 28/71, 39%) and (iii) mast cell leukemia ± AHN (MCL±AHN; 13/71, 18%). The median follow-up time after alloHSCT was 1.4 years (range 0-20.4). PFS and OS were 52% (SD ± 6.1%) and 62% (SD ± 5.9%) at 1 year, and 39% (SD ± 6.3%) and 50% (SD ± 6.2%) at 3 years. Discrimination between the three cohorts allowed a three-tier risk stratification (median PFS, 4.5 vs. 0.7 vs. 0.3 years, P&amp;lt;0.001; median OS, 9.0 vs. 3.3 vs. 0.9 years, P=0.007, Figure 1). Overall non-relapse mortality rate was 30% (21/71) including infectious complications in 8/21 (38%), GvHD in 1/21 (5%), cardiotoxicity in 1/21 (5%) and ‘combinations/not further specified’ in 11/21 (52%) pts. The 100-day non-relapse mortality was 8% (6/71; ASM/SM-AHN, 0/30; SM-AML, 4/28, 14%; MCL±AHN, 2/13, 15%). OS was associated with response status of SM (17/41, 41%; median OS 4.6 vs. 1.1 years, P=0.029) or AHN (26/43, 60%, median OS not reached vs. 0.4 years, P=0.003) prior to alloHSCT ( Figure 2). Univariable analyses revealed absence of a KIT D816V mutation (HR 2.9 [1.2-6.5], P&amp;lt;0.001) and a complex karyotype (HR 4.2 [1.8-10.0], P=0.016) as adverse prognostic factors regarding OS. PFS was significantly impacted by diagnosis of MCL±AHN (HR 3.5 [1.7-7.5], P&amp;lt;0.001), absence of a KIT D816V mutation (HR 2.5 [1.2-5.4], presence of a complex karyotype (HR 4.2 [1.8-10.0], P=0.016), P=0.021) and absence of the use of tyrosine kinase inhibitors prior to alloHSCT (HR 0.461 [0.249-0.854], P=0.014). In contrast, HLA match (complete vs. incomplete), type of conditioning (myeloablative vs. dose-reduced intensity), use of total body irradiation (≥8 Gy yes vs. no) or the transplantation at high volume centers (≥7 patients yes vs. no) were not associated with significant differences in PFS or OS. During or after alloHSCT, 30/71 (42%) pts. showed either refractory (9/71, 13%) or relapsed (21/71, 29%) disease (R/R), being highest among MCL±AHN pts. (11/13, 85%; ASM/SM-AHN, 9/30, 30%; SM-AML, 10/28, 36%; P=0.002). R/R SM was predominantly observed in MCL±AHN (10/11, 91%) while R/R AHN was higher in SM-AML patients (6/10, 60%) and ASM/SM-AHN (5/9, 56%). The median time to relapse was 0.6 years (range 0.1-4.5). Post alloHSCT, a response to various treatment regimens was achieved in 13/30 (43%) pts. (ASM/SM-AHN, 3/9, 33%; SM-AML, 5/10, 50%; MCL±AHN, 5/11, 45%) with highest response rates in 9 pts. receiving midostaurin and/or avapritinib (7/9, 78%). Conclusions In pts. with AdvSM eligible for alloHSCT, i) baseline characteristics affect outcome of alloHSCT more than specific transplant procedures, ii) alloHSCT should not be performed in pts. without response prior to alloHSCT and iii) post-alloHSCT strategies such as preemptive or residual disease guided maintenance therapy should be considered to improve post-alloHSCT outcome.

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.

Prognostic Characterization of Advanced Mastocytosis Patients Treated with Midostaurin According to Diagnosis and Mutation-Adjusted Risk Score: A Nation-Wide Ceremast Study of 170 Patients

Introduction: Mastocytosis is a spectrum of diseases characterized by the accumulation of atypical mast cells (MC) in tissues. The most frequent form in adult patients is systemic mastocytosis (SM) including indolent SM, bone marrow mastocytosis, smoldering SM and advanced SM (adv-SM). Adv-SM encompasses aggressive SM (ASM), MC leukemia (MCL), SM associated with hematological neoplasms (SM-AHN) and is associated with a poor outcome. Prior the use of tyrosine kinase inhibitors (TKIs), therapeutic options were limited in adv-SM patients. The first TKI developed for adv-SM with KIT D816V mutation was midostaurin, which greatly improved outcomes for patients. However, most patients are primary refractory or relapse early (&amp;lt;12 months) after midostaurin onset. To identify high risk adv-SM patients, a prognostic score the so-called Mutation-Adjusted Risk Score (MARS) have been developed. However, given the heterogeneity of outcomes according to subtype of adv-SM, the identification of high-risk patients within each subtype is critical to optimize therapy. To address this question, we investigated predictive value on outcome of MARS score risk in midostaurin treated Adv-SM patients in a larger group of patients, according to each subtype of adv-SM. Methods: We conducted a nationwide retrospective study of all patients with Adv-SM patients included in the French national mastocytosis reference center registry (CEREMAST). Main inclusion criteria were: (i) adults patients treated with midostaurin in France since 2009 (ii) diagnosis of adv-SM according to WHO 2016 classification (iii) presence of C findings. Patients that have received combination therapy (i.e. azacytidine) or with chronic type of MCL without C finding or lymphoid neoplasm AHN or AML prior midostaurin onset were excluded. Response to midostaurin was assessed according to Valent criteria and MARS score was used to stratify patients into 3 risk groups (high risk, intermediate and low). Results: Overall, 170 patients treated with midostaurin were identified, including 46 ASM patients, 11 MCL patients and 113 SM-AHN patients. Among SM-AHN patients, chronic myelomonocytic leukemia was the most frequent neoplasm (52%). Response to midostaurin was significantly different regarding subtype of adv-SM (73% in ASM vs 27% in MCL vs 50% in SM-AHN, p=0.006). With a median follow-up of 19 months since midostaurin start, median overall survival (OS) was 69, 9.9 and 32 months (p=0.0013) and time to treatment failure (TTF) was 30, and 3.6 and 9 months in ASM, MCL and SM-AHN (p&amp;lt;0.0001), respectively. MARS risk group distribution was heterogeneous between ASM, MCL and SM-AHN (p=0.026). In MARS low risk group, median OS was not reached (NR) and 58 months in ASM and SM-AHN respectively (p=0.034, Figure 1). Median OS for both low risk ASM and low risk SM-AHN were significantly different compared with intermediate/high-risk (int/high-risk) groups (NR vs 33 months for ASM, p=0.003 and 58 months vs 24 months, p=0.017). Int/high-risk SM-AHN was the only group with a median OS not significantly different compared with MCL (24 months vs 9 months p=0.266) Median TTF was 60 months for low risk ASM, 26 months for low risk SM-AHN and only 6.7 months for high/int-risk ASM, 8.4 months for high/int-risk SM-AHN and 3.6 months for MCL. For the 19 patients who underwent hematopoietic stem cell transplantation (HSCT), median OS was 88 months but did not reach statistical significance compared to non-transplanted patients (47 months, p=0.15). In multivariate analysis, MARS and WHO mastocytosis subtype were the only variables independently associated with OS ( figure 2). MARS and midostaurin response were not predictive for both AHN progression and AML transformation in SM-AHN, which occurred in 31% and, 22% of patients respectively. In contrast, presence of portal hypertension/ascites (68% vs 29%, p&amp;lt;0.001) and abnormal karyotype (40 vs 11%, p=0.007) were predictive of AML transformation. Conclusions: We report herein the largest cohort of adv-SM patients treated with midostaurin. We identified portal hypertension/ascites and karyotype abnormalities as risk factors predictive of AML evolution. Subtype of adv-SM are heterogeneous and, together with MARS, are predictive for both OS and TTF. According to these variables, 5 sub-groups of adv-SM have been identified with different outcomes prompting for specific management.

Development and Validation of a Diagnostic Tool for the Timely Diagnosis of Patients with Systemic Mastocytosis

Background: Systemic mastocytosis (SM), a rare, clonal mast cell disease associated with heterogenous clinical presentation, is characterized by severe, debilitating and often unpredictable skin, gastrointestinal, neurocognitive, and systemic symptoms (including life-threatening anaphylaxis), long-term co-morbidities, and in Advanced SM (AdvSM), organ damage due to mast cell infiltration. SM consists of 2 distinct groups of variants, advanced and non-advanced: AdvSM includes aggressive SM (ASM), SM with an associated hematologic neoplasm (SM-AHN), and mast cell leukemia (MCL); non-AdvSM includes indolent SM (ISM) and smoldering SM (SSM). Arriving at a definitive diagnosis may take years and is one of the challenges of SM management. To aid clinicians in the identification of SM, we developed and validated a diagnostic algorithm using data from select community hematology practices in the United States. Methods: A sample of 209 patients (SM and control) was obtained from the Quality Cancer Care Alliance (QCCA), a network of 19 community oncology and hematology practices. Data collection consisted of patient characteristics, laboratory parameters, and signs and symptoms at presentation. General linear models (GLM) with a logit link function were used in a backwards elimination process with the p value set at &amp;lt; 0.05 to identify patient factors at presentation that were associated with a diagnosis of SM. A Dx scoring algorithm (range: 0-26) was then derived from the final model coefficients, with the intent of correctly differentiating between SM and non-SM patients. A receiver operating characteristic (ROC) curve analysis was then done to measure the Dx accuracy of the algorithm. Results: Data from 105 SM and 104 non-SM control (diagnosed with hematological cancers) patients were collected from QCCA. The SM cohort included patients with different SM subtypes, including: ISM (47.6%); ASM (9.5%); SM-AHN (19.0%); MCL (1.9%); and subtype not documented (21.9%). The factors identified as being predictive ofa correct diagnosis of SM were 1) patient age, 2) lymph node status, 3) absolute neutrophil count, and 4-7) the following symptoms within 30 days of presentation: diarrhea, rash, skin lesions, and unintended weight loss. In the algorithm, patients were assigned scores for each diagnostic factor they possessed [Figure 1]. The final score was then associated with an overall percent likelihood of a positive SM diagnosis. The area under the ROC curve was 0.89 (95%CI 0.84-0.93), indicating good predictive accuracy. Patients with a total score of ≥ 9 units were found to be at highest risk for a positive SM diagnosis with the associated sensitivity, specificity, and positive predictive values of 84.8%, 76.0%, and 82.7%, respectively. Conclusions: This model indicates that the presence or absence of 7 characteristics may help determine whether patients are at increased risk of having SM. Use of this diagnostic algorithm may help facilitate earlier detection of SM by raising clinical suspicion to facilitate appropriate clinical workup, which would allow the timely initiation of effective targeted therapies for this rare disease. Additional application and external validation of the algorithm in a new sample of 162 patients managed through another community oncology network is ongoing.

Liquid Biopsy Enhances Diagnostic Workup of Hereditary Alpha-Tryptasemia (HaT) and Systemic Mastocytosis (SM) in Patients with Mast Cell-Mediated Symptoms and Elevated Basal Tryptase Levels

Background and Objectives Hereditary alpha-Tryptasemia (HaT) and Systemic Mastocytosis (SM) both display elevated basal serum tryptase (BST) levels and share mast cell-mediated digestive, neuropsychiatric, cardiovascular, cutaneous and musculoskeletal symptoms. Lack of awareness and numerous differential diagnoses complicate and often delay diagnosis, while patients suffer from impaired quality of life and do not receive suitable treatment. HaT is defined by an autosomal dominant inherited amplification of an alpha-Tryptase encoding TPSAB1 gene and is found in 5-6% of the general population. SM is characterized by an activating cKIT D816V mutation that can be detected in 90% of patients. Currently, there is no test available to discriminate between HaT and SM. We aimed to establish a non-invasive blood test supporting the diagnostic workup of these still unrecognized entities. Methods We established a customized digital droplet (dd)PCR triplex assay to detect HaT and a highly sensitive ddPCR singleplex assay for cKIT D816V mutation. We designed a clinical assessment form to record the symptoms. We identified the frequency of HaT and cKIT D816V mutation in genomic DNA isolated from peripheral blood buffy coat samples in 112 patients presenting to our institution with suspected mast cell-mediated symptoms and highly normal (&amp;gt;8-11.4ng/ml) or elevated (&amp;gt;11.4 ng/ml) BST levels (prevalence cohort, Fig. 1). For symptom assessment, we performed blood testing and symptom assessment in these 112 patients and 34 additional patients presenting with suspected SM (n=146, symptom cohort, Fig. 2). As a control cohort we tested 62 age- and sex-matched patients presenting with BST levels &amp;lt; 6 ng/ml (thus not suspected to have HaT or SM). Results HaT accounted for 82 out of 112 cases (73.2%) of BST levels of 8 ng/ml or higher, while cKIT D816V indicating SM was identified in only 5 cases. In 28 patients (24,1%) no HaT genotype or cKIT D816V mutation was detected. One patient with negative cKIT D816V testing but clinical signs of SM was confirmed by bone marrow biopsy, increasing SM prevalence to 6/112 (5.4%). Interestingly the HaT and SM proportions increased to 91% (HaT) and 7.5% (SM) when higher BST cut-off level of 11.4 ng/ml and to 92,9% (HaT) and 28.6% (SM) when higher BST cut-off level of 20 ng/ml were used (Fig. 1). Of note, 21.4% of patients with BST &amp;gt; 20 ng/ml had both, HaT and SM, and no patient with BST &amp;gt;20 ng/ml was tested negative for HaT and SM. In the symptom cohort, we found significant differences in type and severity of symptoms between HaT, SM and the control group. The occurrence of digestive symptoms was up to twice as high in HaT or SM compared to the control group, for flatulence (HaT 83.5%; control group 53.2%) and diarrhea (SM 68.8%; control group 25.8%) or heartburn (SM 56.3%; control group 25.8%). Nausea was significantly more common in HaT (51.7%) than in SM patients (21.9%). HaT patients more frequently reported neuropsychiatric symptoms compared to control group and SM patients. This included memory impairment (HaT 45.9%; SM 23.3%; control group 16.1%) sleep disturbances (HaT 55.8%; SM 43.8%; control group 33.3%) and fatigue (HaT 67.4%; SM 50.0%; control group 37.7%). Conclusions Simultaneous, non-invasive testing for cKIT D816V and HaT in patients with suspected mast cell mediator symptoms allows discrimination of HaT from SM. Adding BST &amp;gt; 8 ng/ml and &amp;gt;11,4ng/ml to the diagnostic workup in symptomatic individuals increases the pretest probability to detect HaT to 73.8% and 91.0%. Diagnosed patients experience mental relieve through personal and medical acknowledgement of their symptoms and suitable treatment can be initiated. According to our results, HaT testing should be initiated in all individuals presenting with mast cell mediator symptoms and BST &amp;gt; 8 ng/ml, and cKIT testing should be initiated in patients with BST &amp;gt;11.4 ng/ml. In patients with BST &amp;gt;20 ng/ml, cKIT D816V positive blood testing or signs of advanced SM, bone marrow based diagnostic workup is still mandatory to cover diagnostic criteria for SM according to WHO.