In most patients with systemic mastocytosis (SM), including aggressive SM and mast cell leukemia (MCL), neoplastic cells express the oncogenic KIT mutation D816V. KIT D816V is associated with constitutive tyrosine kinase (TK) activity and thus represents an attractive drug target.

Acquired hemophilia A (AHA) is a severe bleeding disorder caused by inhibiting autoantibodies to coagulation factor VIII (FVIII). For hemostatic treatment, bypassing agents, human or porcine FVIII are currently standard of care. Emicizumab is a bispecific, FVIII-mimetic therapeutic antibody, that reduced the annualized bleeding rates in congenital hemophiliacs. Here we report on 12 patients with AHA, 6 male, 6 female, age 74 yrs (64/80) (all data medians and IQR), treated with emicizumab. Initial FVIII was <1%, inhibitor 22.3BU/mL (range 3-2000). Eight patients had severe bleeding. Emicizumab was started with 3mg/kg sc. weekly for 2-3 doses, followed by 1.5mg/kg every 3 weeks to keep the lowest effective FVIII levels. For FVIII monitoring, chromogenic assays with human and bovine reagents were used. All patients received immunosuppression with steroids and/or rituximab. After the first dose of emicizumab, APTT normalized in 1-3 days, FVIII (human reagents) exceeded 10% after 11 (7.5/12) days. Hemostatic efficacy was obtained and bypassing therapy stopped after 1.5 (1/4) days. FVIII (bovine reagents) exceeded 50%, indicating complete remission, after 115 (67/185), and emicizumab was stopped after 31 days (15/79), in median 5 injections (range 3-9) were given. No patient died from bleeding or thromboembolism, and no breakthrough bleeding was observed after the first dose of emicizumab. In conclusion, emicizumab seems to be an effective hemostatic therapy for AHA, with the advantages of sc. therapy (every 1-3 weeks), good hemostatic efficacy, early discharge, reduction of immunosuppression and adverse events.

Mastocytosis is a hematopoietic neoplasm characterized by expansion of KIT D816V-mutated clonal mast cells in various organs and severe or even life-threatening anaphylactic reactions. Recently, hereditary α-tryptasemia (HαT) has been described as a common genetic trait with increased copy numbers of the α-tryptase encoding gene, TPSAB1, and associated with an increased basal serum tryptase level and a risk of mast cell activation. The purpose of our study was to elucidate the clinical relevance of HαT in patients with mastocytosis. TPSAB1 germline copy number variants were assessed by digital PCR in 180 mastocytosis patients, 180 sex-matched control subjects, 720 patients with other myeloid neoplasms, and 61 additional mastocytosis patients of an independent validation cohort. α-tryptase encoding TPSAB1 copy number gains, compatible with HαT, were identified in 17.2% of mastocytosis patients and 4.4% of the control population (p<0.001). Patients with HαT exhibited higher tryptase levels than patients without HαT (median tryptase in HαT+ cases: 49.6 ng/mL vs. HαT- cases: 34.5 ng/mL, p=0.004) independent of the mast cell burden. Hymenoptera venom hypersensitivity reactions and severe cardiovascular mediator-related symptoms/anaphylaxis were by far more frequently observed in mastocytosis patients with HαT than in those without HαT. Results were confirmed in an independent validation cohort. The high prevalence of HαT in mastocytosis hints at a potential pathogenic role of germline α-tryptase encoding TPSAB1 copy number gains in disease evolution. Together, our data suggest that HαT is a novel emerging robust biomarker in mastocytosis that is useful for determining the individual patient´s risk of developing severe anaphylaxis.