Allogeneic haematopoietic cell transplantation in advanced systemic mastocytosis in the new era: A <scp>CIBMTR</scp> study

We investigated the outcomes of patients with advanced systemic mastocytosis (AdvSM) undergoing allogeneic haematopoietic cell therapy (alloHCT) in the US. Twenty-seven adult (median age, 58 years) patients with AdvSM received alloHCT from 2014 to 2021. Most patients were white (93%), male (63%) and had systemic mastocytosis (SM) with associated haematological neoplasm (SM-AHN, 70%), received peripheral blood grafts (89%) and non-myeloablative regimens. KIT mutation was detected in 14 of the reported 16 patients (87.5%). Post-transplantation cyclophosphamide (PTCy) was used in approximately 50% of the cohort. Approximately 1 year after alloHCT, median bone marrow mast cell burden decreased from 15% pre-alloHCT to 1.5% (range 0-8) at 1 year post-alloHCT. The median serum tryptase decreased from 55 ng/mL (range 3-400 ng/mL) pre-alloHCT to 18 ng/mL (range 0-481 ng/mL) at 1 year post-alloHCT. At 1 year, progression-free survival (PFS) and overall survival (OS) were 59% (95% confidence interval [CI], 41%-77%) and 74% (95% CI, 56%-89%) respectively. In the subset of patients with SM-AHN, 1-year PFS and OS were 74% (52%-91%) and 79% (95% CI, 58%-94%) respectively. The cumulative incidence of relapse/progression for the entire population of patients with SM (n = 27) was 4% (95% CI, 0-14) at 100 days and 15% (95% CI, 4-31) at 1 year. For the subset of patients with AHN, the cumulative incidence was 11% (95% CI, 1-28) at 100 days and 21% (95% CI, 6-42) at 1 year. Relapse/progression was the cause of death in six of 12 patients (5 died of AHN and 1 died of SM progression). Eighteen and nine patients received a KIT inhibitor before alloHCT and after alloHCT respectively. In addition, six patients received a KIT inhibitor for the treatment of relapse/progression of SM after alloHCT. Although it is difficult to delineate the exact effect of alloHCT and KIT inhibitors because of the size of the study in this era, the use of alloHCT and KIT inhibitors pre- and post- alloHCT is increasing. In the near future, larger studies are required to provide further insights into this subject.