Anagrelide Metabolite Induces Thrombocytopenia in Mice by Inhibiting Megakaryocyte Maturation Without Inducing Platelet Aggregation

Objective: The mechanism for anagrelide's potent platelet lowering activity in human subjects is not well defined. Studies related to anagrelide function have been hampered by its lack of activity in nonhuman primates and water insolubility. In an effort to define the mechanism whereby anagrelide exerts its therapeutic effect, we identified a water-soluble metabolite (anagrelide.met). The availability of anagrelide.met allowed, for the first time, parallel in vitro and in vivo animal studies centered on the mechanisms by which anagrelide lowers platelet levels.

Materials And Methods: The effects of anagrelide.met on proliferation and maturation of mega-karyocytes (MKs) as well as platelet production were studied both in vitro and in vivo.

Results: Anagrelide.met is capable of blocking in vitro MK migration by 20% to 40%. At 100 ng/mL, anagrelide.met selectively blocked in vitro MK maturation, resulting in a 50% decrease in the total number of CD41a(+) MKs, corresponding with a 30% decrease in MK ploidy by day 10 and a 60% decrease by day 20. Daily intraperitoneal injections of anagrelide.met 100 microg into BALB/c mice was sufficient to significantly decrease platelet counts within 24 to 48 hours, stabilizing to 40 to 50% of normal levels by day 5. This was associated with a 45% decrease in the number of developing MKs and an increase in thrombopoietin levels. Anagrelide.met did not alter WBC counts, hematocrit, or bleeding time, or lead to any apparent signs of toxicity. Furthermore, unlike the parent anagrelide compound, anagrelide.met did not inhibit ADP-induced platelet aggregation even at high concentrations (10 microg/mL).

Conclusions: We describe a cross-species reactive anagrelide metabolite that selectively inhibits MK maturation and migration, lowering platelet levels without influencing platelet aggregation.