Interferon alpha-2b in the long-term treatment of essential thrombocythemia.

We treated 35 patients affected with essential thrombocythemia (ET) with interferon (IFN) alpha-2b. Our treatment scheme consisted of (a) a 6-month induction phase and (b) a continuous maintenance phase. During the induction phase, our results showed that using 21 million units (mu) of IFN weekly platelet counts fell below 600 x 10(9)/1 in about 90% of patients. These data demonstrate that well-tolerated doses of IFN can rapidly correct excessive thrombocytosis. During the continuous maintenance phase, 61% of patients required 3 mu three times a week, 15% once a week, and 24% daily. Thus the minimal IFN doses able to maintain platelet count below 600 x 10(9)/l varied between 3 and 21 mu per week. During long-term treatment, subjective side effects were tolerable, especially using 3 mu three times a week. We conclude that IFN alpha-2b is an effective drug in the long-term treatment of ET.