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

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 x 10(9) platelets/L (mean +/- 1 SD) with markedly increased production (237 +/- 74 x 10(9) platelets/L per day versus 43.1 +/- 8.4 x 10(9) 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 microm (versus 37 microm in controls; range, 21 microm for 2N to 56 microm for 64N cells). The volume increased to 48 x 10(3) microm(3) versus 26 x 10(3) microm(3) in controls, and the number increased to 14 x 10(6)/kg (versus 7 x 10(6)/kg in controls), resulting in 3.7-fold increase in megakaryocyte mass (66 x 10(10) microm(3)/kg versus 18 x 10(10) microm(3)/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 x 10(9) platelets/L and the turnover rate to 81 x 10(9) platelets/L per day. The platelet survival was unchanged. Following therapy, megakaryocyte number decreased to 8 x 10(6)/kg, diameter to 40 microm, and volume to 34 x 10(3) microm(3) with a normalized modal ploidy of 16N, resulting in a megakaryocyte mass reduced by 60% (28 x 10(10) microm(3)/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.