Effects of anagrelide on in vivo megakaryocyte proliferation and maturation in essential thrombocythemia

Abstract

To define the mechanism by which anagrelide normalizes the platelet count in essential thrombocythemia, we studied in vivo megakaryocytopoiesis in 10 newly diagnosed patients prior to and while on anagrelide therapy. Using flow cytometric analysis of aspirated marrow, megakaryocytopoiesis was quantified and correlated with the autologous platelet production rate. Megakaryocytes were identified by CD41a expression and enumerated in relation to the nucleated marrow erythroid precursors. Megakaryocyte diameters were directly measured by time-of-flight technique, and cell ploidy was measured by DNA staining. Two to 3 thousand megakaryocytes were analyzed in each sample. In the 10 patients, the platelet count was 1063 ± 419 × 109platelets/L (mean ± 1 SD) with markedly increased production (237 ± 74 × 109 platelets/L per day versus 43.1 ± 8.4 × 109 platelets/L per day in healthy individuals). The platelet survival was 8.2 ± 1.1 days versus 9.0 ± 0.5 days in healthy controls (P > .05). Megakaryocyte diameter was increased to 46 μm (versus 37 μm in controls; range, 21 μm for 2N to 56 μm for 64N cells). The volume increased to 48 × 103 μm3 versus 26 × 103 μm3 in controls, and the number increased to 14 × 106/kg (versus 7 × 106/kg in controls), resulting in 3.7-fold increase in megakaryocyte mass (66 × 1010 μm3/kg versus 18 × 1010 μm3/kg). Cell ploidy was enhanced showing a modal ploidy of 32N (versus 16N in healthy controls) with marked increase in 64N and 128N cells (P < .05). Anagrelide therapy reduced the platelet counts to 361 ± 53 × 109 platelets/L and the turnover rate to 81 × 109 platelets/L per day. The platelet survival was unchanged. Following therapy, megakaryocyte number decreased to 8 × 106/kg, diameter to 40 μm, and volume to 34 × 103 μm3 with a normalized modal ploidy of 16N, resulting in a megakaryocyte mass reduced by 60% (28 × 1010 μm3/kg;P < .05). This reduction in cell mass closely correlated with the reduction in platelet count and production rate by 66% (r = 0.96). The present data indicate that in essential thrombocythemia anagrelide therapy decreases circulating platelets by reducing both megakaryocyte hyperproliferation and differentiation.