Interferon α therapy for patients with essential thrombocythemia: Final results of a Phase II study initiated in 1986

BACKGROUND. In 1986, a Phase II trial of recombinant interferon-α (IFN-α) was initiated as therapy for patients with essential thrombocythemia (ET). METHODS. Patients were treated with subcutaneous IFN-α at a dose of 5 × 10⁶ units/m² daily. In responding patients, the therapy lasted at least 3 years. RESULTS. Twenty-three patients (14 females and 9 males; median age, 41 years; age range, 20-63 years) with a median platelet count of 1350 × 10⁹/L were treated. After a median follow-up of 174 months (14.5 years), 15 of 20 evaluable patients (75%) responded, including 14 patients who achieved a complete hematologic response (CHR) (6 of them with bone marrow remission) and 1 patient who demonstrated a partial response. The median time to response was 6 months (range, 0.5-36 months), and the median response duration was 48 months (range, 5-114 months). Seven patients who achieved a CHR and were taken off therapy after they completed 3 years of maintenance therapy sustained their response for a median of 28 months. No symptoms or signs of thrombosis or hemorrhage were observed in responding patients. Eleven of 14 patients (78%) who achieved a CHR developed a recurrence, and 2 of 5 patients with recurrences who were rechallenged with IFN-α achieved a second response. The treatment was tolerated relatively well. CONCLUSIONS. IFN-α was safe and effective therapy for patients with ET, and the ability of IFN-α to reverse disease pathology and possibly modify the clinical course of patients with ET warrants its investigation in larger, prospective trials.