Effect of anagrelide on platelet count and function in patients with thrombocytosis and myeloproliferative disorders.

BACKGROUND: Anagrelide is a quinazolin compound developed initially as an inhibitor of platelet aggregation. Since "in vivo" studies demonstrated that it was responsible for thrombocytopenia in humans, anagrelide has been used recently in a small number of patients with thrombocytosis and myeloproliferative disorders. Platelet count was well controlled in the large majority of patients, and only minimal side effects were observed. PATIENTS: Eight patients (5 with essential thrombocythemia, 2 with chronic granulocytic leukemia, and 1 with idiopathic myelofibrosis) received anagrelide (induction dose 4 mg/die; mean maintenance dose 2 mg/die; mean observation time 26 weeks). Complete blood counts were determined 4 times during the first month, and subsequently every month. "In vivo" and "ex vivo" platelet function was studied before anagrelide and after 4 and 10 days of therapy. RESULTS: Platelet count was reduced and maintained below 500 x 10(9)/L in 5 of 8 patients. Headache, palpitation/tachycardia, gastrointestinal symptoms and a decrease in hemoglobin were the side effects. Anagrelide did not modify the leukocyte count or "in vivo"/"ex vivo" platelet function. CONCLUSIONS: Anagrelide may control thrombocytosis in patients with myeloproliferative disorders, even when traditional drugs have failed. When required, anti-aggregating drugs may be associated with anagrelide, since it has no effect on platelet function.