Long-lasting complete response to imatinib in a patient with systemic mastocytosis exhibiting wild type KIT

Systemic mastocytosis (SM) is a hematopoietic disorder characterized by abnormal expansion of mast cells (MCs) in visceral organs. The skin is involved in most cases. In adult patients the transforming KIT mutation D816V is usually present and confers resistance against imatinib. Therefore, imatinib is not recommended for patients with KIT D816V+ SM. Nonetheless, imatinib may work in patients with SM lacking KIT D816V. However, little is known about long-term efficacy and safety of this drug in SM. We report on a 62-year-old female patient with indolent SM (ISM) who suffered from severe debilitating skin involvement despite therapy with anti-mediator-type drugs, psoralen and ultraviolet-A-radiation. Although multifocal MC infiltrates were detected in the bone marrow by immunohistochemistry, no KIT mutation was found by sequencing analysis. In 2003, treatment with imatinib (induction, 400 mg/day; maintenance, 200 mg/day) was initiated. During therapy, skin lesions and tryptase levels decreased. Treatment was well tolerated without any side effects. After 10 years, skin lesions have disappeared and the tryptase level is within normal range. This case-study confirms the long-term efficacy and safety of imatinib in patients with SM lacking activating KIT mutations. Imatinib should be considered in select cases of SM in whom MCs exhibit wild-type KIT.