Anagrelide is a new therapeutic compound recently demonstrated to have a rapid and selective thrombocytopenic effect in humans. The effects of anagrelide were evaluated in plasma clot and liquid suspension cultures of optimally stimulated normal human peripheral blood megakaryocyte progenitors in order to determine the mechanism of its thrombocytopenic activity. In plasma clot cultures, at clinically relevant, therapeutic concentrations (5 to 50 ng/mL), anagrelide exerted no significant inhibitory effect on megakaryocyte colony numbers or colony size. Only at anagrelide concentrations of 10 to 500 times therapeutic doses did anagrelide inhibit megakaryocyte colony development: an anagrelide concentration of 5 micrograms/mL reduced colony numbers by 57% and colony size by 31%. In contrast, lower, therapeutic anagrelide concentrations exerted profound effects in liquid culture on megakaryocyte cytoplasmic maturation, size, and DNA content. When present for the entire 12-day culture duration, anagrelide induced left- shifted megakaryocyte maturation and reduced both megakaryocyte ploidy and megakaryocyte diameter. Anagrelide, at concentrations of 5 to 50 ng/mL, shifted the modal cultured megakaryocyte morphologic stage from III to II, reduced the model ploidy value from 16N to 8N, and decreased the mean megakaryocyte diameter by up to 22%, from 27.6 microns to 21.6 microns. Megakaryocyte diameter was significantly reduced in most instances, even when analyzed as a function of morphologic stage. When anagrelide was added to the cultures after 6- and 9-day delays (during the final 6 and 3 days, respectively, of culture), similar inhibitory effects on megakaryocyte maturation stage and ploidy distribution were observed. However, the magnitude of the left-shift in ploidy appeared to be less as the duration of anagrelide exposure was reduced. Conversely, megakaryocyte diameter was not significantly affected by the shorter 3- and 6-day anagrelide exposures. These data indicate that therapeutic concentrations of anagrelide influence primarily the postmitotic phase of megakaryocyte development, decreasing platelet production by reducing megakaryocyte size and ploidy, as well as by disrupting full megakaryocyte maturation. Inhibition of megakaryocyte diameter appears to require more prolonged anagrelide exposure than inhibition of maturation stage and ploidy. The molecular mechanisms responsible for the inhibitory effects of anagrelide on megakaryocytopoiesis remain to be defined.

Investigations linking human megakaryocyte development and cell biology have been hindered by an inability to obtain large, relatively pure megakaryocyte cell preparations from in vitro stem cell cultures. We report here that such preparations can be generated from liquid cultures of normal human peripheral blood mononuclear cells stimulated by a serum source of megakaryocyte colony stimulating activity (Meg- CSA, the 0% to 60% ammonium sulfate protein fraction of aplastic canine serum). Adherent-depleted peripheral blood mononuclear cells are suspended at 5 x 10(5) to 10(6) cells/mL in supplemented liquid culture medium, platelet-poor human plasma 20% (vol/vol) and 1 to 2 mg/mL serum Meg-CSA protein. After 12 to 14 days of incubation, megakaryocytes constitute 3.0 +/- 2.9% (mean +/- SD, n = 8) of the unseparated cultured cell population. Megakaryocytes can be enriched by counterflow centrifugal elutriation to a purity of 58 +/- 14% (+/- SD) with a recovery of 13 +/- 7% and a viability of 67 +/- 19%. This algorithm results in the average isolation of approximately 3 x 10(5) enriched megakaryocytes from a 100-mL starting volume of peripheral blood. Cultured megakaryocytes exhibit normal light and ultrastructural morphology by Wright-Giemsa staining and electron microscopic analysis. After a 12-day culture interval, enriched megakaryocyte preparations exhibit morphologic stage distributions that are similar to normal human marrow. Stage distributions move rightward with culture duration indicating partial synchrony of megakaryocyte maturation. On cytospin preparations, megakaryocyte diameter averages 30.2 +/- 1.5 microns and increases with maturation stage. Flow cytometric analyses demonstrate the expression of platelet glycoproteins (GP) Ib and IIb/IIIa by the cultured megakaryocytes. The modal ploidy of the enriched cells at day 12 of culture is 16N and most remaining megakaryocytes are 8N or 32N. Liquid culture of serum Meg-CSA-stimulated human peripheral blood mononuclear cells represents a valuable investigative tool that should permit studies of human megakaryocyte biology that have not been possible in the past.