Efficacy, safety and tolerability of anagrelide in the treatment of essential thrombocythaemia

Background : Essential thrombocythaemia (ET) has an associated risk of thrombotic and haemorrhagic complications, which can be minimised by control of the platelet count. Anagrelide selectively lowers the platelet count, however, there is little Australasian experience with its use and scant data on symptom control.
Aims : To evaluate the efficacy of anagrelide for platelet reduction and symptom control in a broad cohort of patients with well-defined ET, and to determine the safety and tolerability in such a population.
Methods : Seventeen patients with ET and a platelet count >600X10⁹/L were prospectively enrolled. The evaluable four males and 12 females with a median age of 58 years (range 14–79) included ten patients (63%) previously treated with two or more agents and 12 patients (75%) who had failed other therapies. The median follow-up was seven months (range 15 days to 36 months).
Results : Anagrelide, in an average dose of 1.9 mg/day, reduced the platelet count from a mean of 728X10⁹/L (95% CI 611-845X10⁹/L) to 412X10⁹/L (95% CI 319-504X10⁹/L) ( p <0.001) and maintained it at this level. Fourteen patients (88%) had a platelet reduction to <600X10⁹/L. All symptomatic patients had improvement in symptoms attributable to thrombocythaemia. There were three haemorrhagic and three thrombotic episodes in a total of three patients (19%), including one death from an intracerebral haemorrhage. Six patients (37%) were removed from therapy due to toxicity after a median of 151 days. Side effects included palpitations, abdominal pain and cough.
Conclusions : Anagrelide is efficacious and safe in ET, both for platelet and symptom control. Minor side effects are common, however, tend to occur early and resolve spontaneously in most cases.     

Long-term use of anagrelide in young patients with essential thrombocythemia

Anagrelide is a novel platelet-lowering agent that has recently been approved for use in essential thrombocythemia (ET) and related disorders. Short-term drug efficacy and toxicity data have previously been presented. The purpose of this study was to obtain additional information regarding long-term anagrelide use. This is a retrospective series of 35 young patients (17 to 48 years) with ET who received anagrelide treatment before 1992. Initial drug dosage ranged between 1 and 10 mg/d, and the median maintenance dosage was 2.5 mg/d. The overall initial response rate of 94% included 74% complete remissions and 20% partial remissions. Of the 33 responding patients, 27 (82%) remained on anagrelide therapy for a median of 10.8 years (range, 7 to 15.5). Of these, 66% maintained a complete and 34% a partial remission over the study period. In general, the reporting of somatic side effects decreased over time, and anemia was the only new side effect that emerged after long-term therapy. Eight patients (24%) experienced a more than 3 g/dL decrease in hemoglobin level. Despite active therapy, 20% of the patients experienced a total of 10 thrombotic episodes, and a similar proportion experienced major hemorrhagic events. All thrombohemorrhagic complications occurred at a platelet count of more than 400 x 10(9)/L. It is concluded that long-term treatment of ET with anagrelide is associated with decreased reporting of initial side effects and the development of mild-to-moderate anemia. Complete normalization of platelet counts may be needed to minimize residual thrombohemorrhagic risk during therapy. (Blood. 2001;97:863-866)     

Indications for lowering platelet numbers in essential thrombocythemia

Essential thrombocythemia (ET), one of the chronic myeloproliferative disorders, exposes individuals to significantly increased risk for thrombohemorrhagic complications. Epidemiologic data indicate that the two most prominent risk factors for thrombosis are age greater than 60 years or a history of or presentation with thrombosis at any age. Age is an important factor in selecting among therapeutic options, as the agents used to treat ET may contribute to acute leukemic transformation and other secondary malignancies. Whether or not hydroxyurea (HU) carries these risks is controversial and unresolved, but the uncertainty is a basis for avoiding it in young patients. Alternatives to HU that have established efficacy in lowering platelet counts in ET are interferon and anagrelide. Both are highly effective in reducing platelet numbers, and are apparently not associated with leukemogenicity or mutagenicity. However, approximately 30% of patients find interferon intolerable for long-term therapy. Anagrelide offers the advantage of oral dosing and long-term effectiveness at managing platelet counts. A recent long-term study of young ET patients treated with anagrelide found that all thrombohemorrhagic events occurred in patients with platelet counts greater than 0.4 × 10⁹/L, adding to the evidence that reduction of platelet counts to normal may be required for optimal control of risk. Semin Hematol 40(suppl 1):22-25. © 2003 Elsevier Inc. All rights reserved.     

Open-label, dose-titration and continuation study to assess efficacy, safety, and pharmacokinetics of anagrelide in treatment-naïve Japanese patients with essential thrombocythemia

Although anagrelide is widely used for the treatment of essential thrombocythemia (ET) in the USA and Europe, it is not licensed in Japan. Existing literature has reported differences in polymorphism and activity of CYP1A2 in Japanese and non-Japanese ethnic groups, which may alter anagrelide metabolism. We intended to identify the optimum dosage of anagrelide in treatment-naïve Japanese patients with ET and assess its long-term safety and efficacy. Twelve patients with ET and a platelet count of ≥80 × 10⁴/μL were enrolled. Anagrelide was administered at an initial dose of 0.5 mg/day (weeks 1–4), then increased to 1.0 mg/day (weeks 5–8). During the following maintenance (weeks 9–52) and continuation periods (weeks 53–104), the dose was adjusted according to patient safety data and to maintain target platelet counts (<60 × 10⁴/μL). Increasing the dose led to a decrease in mean platelet count, and target platelet counts were maintained in 11 patients. Adverse events were mild or moderate, and none led to discontinuation. This cohort of Japanese patients exhibited higher pharmacokinetic exposures of anagrelide and its active metabolite than those previously documented in non-Japanese patients. These differences were modest, suggesting specific dosing regimens for Japanese patients are not required.     

Anagrelide,a novel platelet lowering option in essential thrombocythaemia: treatment experience in 48 patients in Germany

Abstract: We report on our treatment experience in Germany with anagrelide, a novel platelet lowering agent, in 48 patients (27 females, 21 males) with essential thrombocythaemia. Their age was between 19 and 79 yr when anagrelide therapy was initiated. Sixteen patients were previously untreated, 15 pretreated with hydroxyurea and 17 had multiple pretreatments. Forty-one of the 48 patients had either microvascular, thromboembolic or bleeding complications. About 50% received low dose acetylsalicylic acid as an adjunct. Their platelet count prior to therapy ranged from 850,000/μl to 3,100,000/μl. Eighty-seven per cent of the patients treated with anagrelide were complete hematological responders, while 13% responded only partially or failed to respond. Twelve of our patients (25%) developed short-term (from a few days to a maximum of 4 wk) side effects including headache (most frequent), palpitations, tachycardia or nausea. Eight patients reported long-term (more than 1 month) adverse effects. However, in only 5 of all 48 patients (10%) were these side effects not acceptable so that treatment had to be discontinued. We have now treated patients for up to 7 yr (median maintenance dose 2.5 mg/d). Preliminary evidence suggests that anagrelide mediated platelet count reduction also prevents recurrence of thromboembolic complications. Hence, anagrelide has the potential to become the first-line platelet-lowering treatment in myeloproliferative disorders with high platelet counts.     

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.     

Anagrelide treatment in 52 patients with chronic myeloproliferative diseases

In this retrospective multi-centre study, we report our experience with anagrelide in the treatment of thrombocytosis in patients with chronic myeloproliferative diseases. Our study included 52 patients (age 20-78 years). The initial anagrelide dose was, in general, 0.5 mg once daily and mean maintenance dosage was 1.7 mg/day. The overall response rate was 79% including 75% complete remission and 4% partial remission. Forty-two patients (81%) had adverse effects and in 29% of the study population, the adverse effects necessitated cessation of anagrelide. The most common adverse effect was moderate anaemia (50%). Two patients experienced erectile dysfunction which has been described only once previously in association with anagrelide treatment. One patient progressed to acute leukaemia. However, this patient had been pre-treated with two potentially leukaemogenic drugs and had only been in short-term treatment with anagrelide. Furthermore, a total of 13 events were recorded. More than 25% of these events occurred in patients with platelet counts between 400 and 600 x 10(9)/l and almost 40% of all events occurred in patients with platelet counts above 400 x 10(9)/l. This observation supports the hypothesis that aggressive control of thrombocytosis to a platelet count <400 x 10(9)/l might reduce the number of thrombohaemorrhagic events. Anagrelide is safe and effective in reducing the platelet counts, but a high proportion of the patients discontinue treatment because of the adverse effects of the drug.     

Decreased survival and increased rate of fibrotic progression in essential thrombocythemia chronicled after the FDA approval date of anagrelide

First-line cytoreductive drug of choice in high risk essential thrombocythemia (ET) is currently hydroxyurea, a practice based on the results of a randomized study; second-line drugs of choice include pegylated interferon-α, busulfan and anagrelide. Anagrelide clinical trials were pioneered by the late Murray N. Silverstein (1928-1998) of the Mayo Clinic whose studies led to FDA approval in March 1997. The current study represents a retrospective examination of the potential impact of anagrelide therapy on survival and disease complications in ET. 1076 patients with ET were considered (median age 58 years; females 63%); risk distribution, according to the international prognostic score for ET (IPSET), was 28% high, 42% intermediate, and 30% low. Overall (OS), myelofibrosis-free (MFFS) and thrombosis-free survival data were compared for ET patients diagnosed before and after the 1997 FDA approval date for anagrelide; a significant difference was apparent in OS (P = .006; HR 1.4, 95% CI 1.1-1.7) and MFFS (P < .001; HR 4.2, 95% CI 2.7-6.5), in favor of patients diagnosed prior to 1997; the difference was sustained during multivariable analysis that included IPSET. Similarly stratified survival data in polycythemia vera (n = 665) and primary myelofibrosis (n = 1282) showed no similar impact on survival (P = .3 and .17, respectively). The current study represents a retrospective analysis and suggests significantly decreased OS and MFFS in ET patients diagnosed after the FDA approval date of anagrelide. Whether or not anagrelide therapy was to blame for the worsening of OS and MFFS over time cannot be assumed and requires validation in a prospective study.     

Essential thrombocytemia and acute coronary syndrome: clinical profile and association with other thromboembolic events

BACKGROUND: The existence of a relationship between essential thrombocytemia (ET) and acute coronary syndromes (ACS) has been suggested. METHOD: Data from eleven consecutive patients admitted with ET in the cardiology department were reviewed. RESULTS: Nine patients (82%) presented with ACS and two with bradycardia. Patients with ACS had a mean age of 67+/-11 years. Risk factors, especially hypertension (6, 66.7%), and smoking (6, 66.7%) were frequent. Average platelet count was 509 778+/-282 126/mm3. Significant coronary lesions were found in five of six patients studied with coronary angiography. During hospitalization, a patient suffered a thrombotic stroke, a massive pulmonary embolism, and partial aortic thrombosis. Another patient had a transient ischemic attack. Discharge treatments were aspirin (78%), clopidogrel (56%), acenocumarol (33%), hydroxyurea (56%) and anagrelide (44%). Three patients (27%) had thromboembolic events during follow-up (median 1.6 years), 2 patients had coronary events and 1 patient had venous thrombosis. There were neither significant haemorrhages nor deaths. CONCLUSIONS: Patients with ET and ACS have similar profiles to those of traditional ACS with frequent risk factors and significant coronary artery disease. Association with other thrombotic events can be seen during admission and follow-up while haemorrhagic complications seem to be rare.     

Studies of platelet volume, chemistry and function in patients with essential thrombocythaemia treated with Anagrelide

Anagrelide (imidazoquinazolin derivative) is a new compound proposed for the treatment of myeloproliferative disorders. In this study, Anagrelide was given to patients with essential thrombocythaemia (ET) in a compassionate-use protocol. The aim of this study was to test the effect of this drug not only on the platelet count but also on platelet volume, chemistry and function, which has not previously been reported. Thus, in ET, different functional or structural platelet abnormalities were reported: a shortening of the bleeding time, hypoaggregation to several agonists, and in particular a lack of response to adrenalin, an increase in the amount of total platelet glycoprotein IV (or CD36), and an abnormal migration of thrombospondin on electrophoresis. These different parameters were studied before and during therapy with Anagrelide. Although the platelet count was corrected, no functional or chemical abnormality was improved. Furthermore, platelet volume was shown to be constantly increased under Anagrelide. Thus, Anagrelide, in reducing the platelet count, may possibly decrease the risk of thrombosis and haemorrhage. Nevertheless, if the risk of thromboses and/or myelofibrosis is related not only to the platelet count but also to the platelet abnormalities, the persistence of a thrombocytopathy in patients treated with Anagrelide must be taken in consideration. Our data suggest that thromboses and myelofibrosis are clinical end-points which should be included in future large-scale use of Anagrelide.     

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.     

The platelet thrombopoietin receptor number and function are markedly decreased in patients with essential thrombocythaemia

Essential thrombocythaemia (ET) is a relatively common myeloproliferative disorder characterized by an elevated platelet count. As thrombopoietin (TPO) and the TPO receptor (c-mpl) regulate platelet production in normal physiology, their role in ET was investigated. A well-characterized cohort of 23 ET patients was evaluated and followed for 3 years. The TPO levels in these ET patients (189 +/- 131 pg/ml) were the same as in normal subjects (179 +/- 112 pg/ml) and TPO was not produced by ET platelets. There were 5.6 +/- 5.5 TPO binding sites/ET platelet vs. 56 +/- 17 TPO binding sites/normal platelet and this was associated in ET patients with normal-sized platelet c-mpl protein and mRNA, but a 10-fold reduction in platelet c-mpl mRNA. The K(d) for the TPO receptor on ET platelets was 66 +/- 30 pmol/l vs. 163 +/- 31 pmol/l on normal platelets, but the c-mpl cDNA had a normal nucleic acid sequence. The decreased number of ET platelet TPO receptors resulted in a fourfold decrease in the platelet-dependent TPO clearance (0.30 +/- 0.14 ml/h/10(9) ET platelets vs. 1.24 +/- 0.38 ml/h/10(9) normal platelets) at a time when the platelet count in ET patients was 2.7-fold above normal. The fourfold decrease in the TPO clearance, elevated platelet mass and resulting normal total TPO clearance explain the normal TPO levels. These results also suggest that the thrombocytosis in ET may be attributed to an alteration of the normal feedback interaction between TPO and its receptor and not as a result of any defect in the structure of TPO or c-mpl.     

A phase III randomized,multicentre, double blind,active controlled trial to compare the efficacy and safety of two different anagrelide formulations in patients with essential thrombocythaemia – the TEAM-ET 2·0 trial

Anagrelide is an established treatment option for essential thrombocythaemia (ET). A prolonged release formulation was developed with the aim of reducing dosing frequency and improving tolerability, without diminishing efficacy. This multicentre, randomized, double blind, active-controlled, non-inferiority trial investigated the efficacy, safety and tolerability of anagrelide prolonged release (A-PR) over a reference product in high-risk ET patients, either anagrelide-naïve or -experienced. In a 6 to 12-week titration period the individual dose for the consecutive 4-week maintenance period was identified. The primary endpoint was the mean platelet count during the maintenance period (3 consecutive measurements, day 0, 14, 28). Of 112 included patients 106 were randomized. The mean screening platelet counts were 822 × 10⁹/l (95% confidence interval (CI) 707–936 × 10⁹/l) and 797 × 10⁹/l (95% CI 708–883 × 10⁹/l) for A-PR and the reference product, respectively. Both treatments effectively reduced platelet counts, to mean 281 × 10⁹/l for A-PR (95% CI 254–311) and 305 × 10⁹/l (95% CI 276–337) for the reference product (P < 0·0001, for non-inferiority). Safety and tolerability were comparable between both drugs. The novel prolonged-release formulation was equally effective and well tolerated compared to the reference product. A-PR provides a more convenient dosing schedule and will offer an alternative to licensed immediate-release anagrelide formulations.     

Treatment indications and choice of a platelet-lowering agent in essential thrombocythemia

The therapeutic strategy in patients with essential thrombocythemia (ET) is a difficult balance between the prevention of bleeding and thrombotic complications and the risks of drug side effects and toxicity. Major bleeding is rare and seems to be related to higher platelet counts. Therefore, a platelet count greater than 1500 x 10(9)/L is generally regarded as an indication for cytoreduction. Thrombotic complications include microvascular occlusive symptoms, which are usually reversible with low-dose aspirin, and large vessel thrombosis. The risk of major thrombosis is higher in patients with ET who are older than 60 years and who have had a previous occlusive event. In this high-risk group, the nonalkylating agent hydroxyurea significantly reduces the rate of vascular complications and has emerged as the treatment of choice. However, the long-term risk:benefit ratio of hydroxyurea remains disputed because its leukemogenic potential has not been ruled out. This holds also for other myelosuppressive agents such as busulfan and pipobroman. Other drugs of particular interest for young patients include recombinant interferon-alpha and anagrelide. Both of these drugs are effective in lowering platelet counts, but their efficacy in reducing clinical complications remains to be demonstrated. Furthermore, interferon and anagrelide have frequent and clinically important side effects. Thus, further clinical studies are required to establish their role in the management of patients with ET.     

The effects of anagrelide on human megakaryocytopoiesis

Anagrelide, an inhibitor of platelet aggregation, decreases the number of platelets in normal subjects and in patients with myeloproliferative disorders. We describe studies aimed at discovering the general mechanism(s) by which anagrelide acts. We examined three hypotheses: (1) anagrelide shortens platelet survival, (2) anagrelide inhibits the proliferation of megakaryocytic-committed progenitor cells (CFU-M), and (3) anagrelide inhibits maturation of megakaryocytes. We observed that anagrelide did not shorten platelet survival. Proliferation of CFU-M in vivo was not affected by anagrelide, although high concentrations of anagrelide inhibited CFU-M in vitro . In-vivo and in-vitro anagrelide altered the maturation of megakaryocytes, causing a decrease in their size and changing other morphometric features. We conclude that anagrelide decreases the number of platelets primarily by interfering with the maturation of megakaryocytes.     

Essential Thrombocythemia (ET): Moving from Palliation to Cure

Essential thrombocythemia (ET) is a clonal hematopoietic stem cell myeloproliferative disorder characterized by megakaryocytic hyperplasia and persistent thrombocytosis. The clinical presentation and evolution of ET are heterogeneous. This review highlights the current treatment options in the management of ET, including hydroxyurea, anagrelide and both regular and pegylated interferons. Anagrelide, while very effective at controlling counts and symptoms in most patients, may not consistently reduce the bone marrow megakaryocyte mass. Interferon is very effective and not associated with leukemogenesis, but has not been proven to restore polyclonal hematopoiesis and has significant dose-related adverse events. Pegylated interferon represents a significant improvement over the unmodified interferon preparations. Novel therapeutic options directed towards eradication of the malignant ET clone are required.     

Essential thrombocythemia (ET): moving from palliation to cure

Essential thrombocythemia (ET) is a clonal hematopoietic stem cell myeloproliferative disorder characterized by megakaryocytic hyperplasia and persistent thrombocytosis. The clinical presentation and evolution of ET are heterogeneous. This review highlights the current treatment options in the management of ET, including hydroxyurea, anagrelide and both regular and pegylated interferons. Anagrelide, while very effective at controlling counts and symptoms in most patients, may not consistently reduce the bone marrow megakaryocyte mass. Interferon is very effective and not associated with leukemogenesis, but has not been proven to restore polyclonal hematopoiesis and has significant dose-related adverse events. Pegylated interferon represents a significant improvement over the unmodified interferon preparations. Novel therapeutic options directed towards eradication of the malignant ET clone are required.     

Phase III,single-arm study investigating the efficacy,safety, and tolerability of anagrelide as a second-line treatment in high-risk Japanese patients with essential thrombocythemia

Essential thrombocythemia (ET) is usually managed by anti-platelet therapy. European guidelines recommend that patients with ET at high risk of developing thrombohemorrhagic events should be placed on cytoreductive therapy (CRT). In Japan, hydroxycarbamide (HC) is the most widely used CRT; however, treatment options for patients who become intolerant or refractory to initial treatment are limited. This study sought to determine the efficacy, safety, and tolerability of anagrelide in high-risk Japanese adults with ET who were intolerant or refractory to their first-line CRT. Fifty-three patients were enrolled in the study. Of those, 67.9 % had a platelet response (<60 × 10⁴/µL) and 45.3 % achieved normalization of platelet counts (≤40 × 10⁴/µL) on anagrelide therapy. The median time to platelet count response was 98.5 days and the median time to platelet count normalization was 274.0 days. The median daily dose administered was 1.9 mg/day. The most common adverse events observed during anagrelide treatment were anemia, headache, palpitations, and diarrhea. The majority of these were either mild or moderate in severity. Overall, the safety profile of anagrelide in high-risk Japanese patients with ET was consistent with the European Summary of Product Characteristics.     

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.