Interferon-alpha in the treatment of essential thrombocythemia.

Interferon alpha (IFN) inhibits the growth of megakaryocytic progenitors in normal hematopoiesis and in patients with essential thrombocythemia (ET) leading to a reduction of peripheral platelet counts. The effectiveness in the induction therapy of patients with ET is demonstrated in 11 international studies including 212 patients. With an average dose of about 3 mill IU IFN daily, the response rate was about 90%. Further studies investigated the practicability and the success of IFN maintenance therapy. The results show that IFN can effectively control platelet counts over a period of several years. During maintenance the IFN dose could be reduced in the majority of patients. However, sustained unmaintained complete remissions were obtained in only 12% of the patients. Side effects were frequently the limiting factors in treatment with IFN especially in older patients. Analyzing a total of 273 patients, IFN therapy was terminated in 25% against the primary treatment plan. Of the currently effective drugs in controlling the platelet counts in ET, IFN is the only antiproliferative agent with immuno-modulating properties. Thus far, no leukemogenic or significant gonadotoxic effects have been observed. In a subset of the patients off all therapy, the sustained remissions support a long-term tumor load reduction effect by IFN. Thus, IFN is a promising agent in cytoreductive treatment of ET especially in younger patients.