Interferon alfa-2b (IFN-a) has been used to treat systemic mastocytosis for over 10 years. The dosage used, frequency of administration, and responses to therapy have varied widely in reported series. Here we report sustained response to high dose IFN-a given for 5 years.

A 44 year-old man with urticaria pigmentosa developed weight loss, anemia and leukocytosis. Bone marrow biopsy was positive for mastocytosis with 50% involvement by mast cells. Treatment was begun with IFN-a 6 million units (MU) subcutaneously 3 times/week and prednisone 10 mg on the alternate days. After 4 weeks the dose of IFN-a was increased to 10 MU 3 times/week and the dose of prednisone was tapered and discontinued. Treatment with IFN-a 10 MU 3x/week continued for the next 5 years.

The patient re-gained his weight loss and the urticaria pigmentosa lesions faded and disappeared over 2-3 years. During 5 years of treatment the following biochemical improvements were observed: normal (initial value–> present value): Tryptase <11.5 ng/ml (>200–>37); Calcitonin 0-15.9 pg/ml (15–>9.3); Urinary prostaglandin F2a </=1000 ng/24 hr (2869–>807); Urinary N-methyl histamine 30-200 ug/g Cr. (1038–>262). Repeat bone marrow biopsy was negative for mastocytosis, and immunophenotyping of bone marrow cells was normal. Side effects included mild depression and biochemical hypothyroidism treated with levothyroxine.

For this patient, chronic treatment with IFN-a 10 MU 3x/wk, a dose higher than commonly used in other series, led to continued clinical and biochemical improvement. High-dose, prolonged treatment with IFN-a should be considered a treatment option for patients with systemic mastocytosis.