# Clinical and Biological Characteristics of Four Patients with Aggressive Systemic Mastocytosis Treated with Midostaurin

## TL;DR

This paper reports on four patients with aggressive systemic mastocytosis who showed clinical improvement after treatment with midostaurin, a tyrosine kinase inhibitor targeting the KIT mutation.

## Contribution

The study provides real-world evidence of midostaurin's effectiveness in managing aggressive systemic mastocytosis with KIT D816V mutations.

## Key findings

- All four patients showed clinical and biological improvement after starting midostaurin therapy.
- Midostaurin was associated with improved organ function and symptom control in patients with aggressive systemic mastocytosis.
- KIT inhibition with midostaurin offers meaningful clinical benefit for patients with inadequate responses to traditional therapies.

## Abstract

Systemic mastocytosis (SM) is a rare and heterogeneous disorder characterized by clonal proliferation and accumulation of neoplastic mast cells in one or more organs, most commonly the bone marrow, liver, spleen, and skin. Among its clinical variants, aggressive SM (ASM) presents organ damage and debilitating symptoms due to extensive mast cell infiltration. The management of ASM remains challenging, primarily because treatment must address both symptom control and disease progression. Background/Objectives: Recent therapeutic approaches have focused on tyrosine kinase inhibitors (TKIs) that target the oncogenic KIT driver mutation, predominantly the D816V mutation, which is implicated in mast cell proliferation. We report a case series of four patients diagnosed with ASM to highlight the real-world experience in the management of ASM. All patients had confirmed KIT D816V mutations and presented with signs of advanced organ dysfunction, such as marked hepatosplenomegaly, cytopenia, and significant bone marrow infiltration. First-line therapies, including cytoreductive agents or other TKIs were used. Responses varied in these patients, and ultimately, they were initiated on or transitioned to midostaurin, a multikinase TKI. Results: All four patients, after the initiation of midostaurin, presented clinical and biological improvement—at least a clinical improvement response according to the International Working Group-Myeloproliferative Neoplasms Research and Treatment & European Competence Network on Mastocytosis (IWG-MRT-ECNM) criteria. These findings highlight the benefits of KIT inhibition in managing ASM, especially for patients with inadequate responses to traditional therapies. The impact of midostaurin on organ function, mast cell burden, and symptom control emphasizes the importance of the timely integration of TKIs into therapeutic protocols. However, optimal treatment duration, long-term safety, and the development of acquired resistance remain critical questions that warrant further studies. Larger prospective trials are needed to better delineate the prognostic factors associated with sustained response, refine patient selection, and explore combination strategies that may enhance therapeutic efficacy. Conclusions: The patients presented in this case series benefited from midostaurin therapy, showing either a clinical improvement or partial response according to the IWG-MRT-ECNM criteria. Our case series illustrates that KIT inhibitors can offer meaningful clinical benefit in ASM, reinforcing their position as an emerging cornerstone option in ASM management.