Aspirin in polycythemia vera and essential thrombocythemia: current facts and perspectives

Polycythemia vera presenting as complex regional pain syndrome of the lower limbs

CASE REPORT

A 73-year-old woman with bilateral severe pain in her legs and feet was admitted to the hospital. The pain had started spontaneously without any known trauma a year earlier and had progressively worsened to the point that she was unable to walk. The patient complained of pain, discoloration, and sweating of the lower extremities. Physical examination revealed swollen, cold, wet, and blue extremities, with both allodynia and hyperalgesia, fulfilling criteria for a diagnosis of complex regional pain syndrome (CRPS) type 1. Laboratory findings demonstrated a hemoglobin level of 180 g/L(-1) and a hematocrit of 47%, leading to a diagnosis of polycythemia vera (PCV). Her condition slowly improved after repeated venesection, mild analgesics, and amitriptyline.

CONCLUSIONS

Polycythemia vera has not been described previously as a contributing factor in CRPS. In the present case, the diagnosis of PCV was important, because its treatment also improved the CRPS.

Aspirin and platelet-lowering agents for the prevention of vascular complications in essential thrombocythemia

Low-risk essential thrombocythemia patients include patients aged 18 to < 80 years with no vascular risk factor or previous thrombosis, no associated disease, a normal life expectancy, and a platelet count between 400 and 1,000 x 10(9)/L up to 1,500 x 10(9)/L. Asymptomatic essential thrombocythemia patients may be at risk for microvascular circulation disturbances. The indication for low-dose aspirin in asymptomatic essential thrombocythemia patients is uncertain, therefore randomization for aspirin 50 mg versus placebo is recommended. Symptomatic essential thrombocythemia patients with erythromelalgia and its ischemic complications, atypical transient ischemic attacks, minor stroke, visual disturbances and "superficial thrombophlebitis" in the absence of bleeding, vascular risk factors, or vascular disease have a clear indication for aspirin in a regular dose. To determine whether 50 mg/day is as effective as 100 mg/day for the prophylaxis of microvascular circulation disturbances in essential thrombocythemia, a randomized trial comparing low-dose aspirin 50 mg versus 100 mg at platelet counts between 400 and 1,000 up to 1,500 x 10(9)/L is recommended. To address the question whether reduction of the platelet count to normal (< 350 x 10(9)/L) is as effective as low-dose aspirin for the long-term relief of microvascular circulation disturbances, a randomized study comparing low-dose aspirin with the correction of platelet count to normal by anagrelide is recommended. High-risk essential thrombocythemia patients have a clear indication for platelet reductive therapy, including: (a) platelets > 1,500 x 10(9)/L, history of major thrombosis (myocardial infarction, stroke, peripheral occlusive vascular disease), or presence of vascular disease (e.g., arteriosclerosis); (b) history or presence of spontaneous or major bleedings, bleedings elicited by low-dose aspirin for the secondary prevention of vascular complications in essential thrombocythemia at platelet counts < 1500 x 10(9)/L, and side effects of long-term aspirin treatment such as gastritis; and progression from low- to high-risk essential thrombocythemia patients during follow-up or progressive myeloproliferative disease such as significant splenomegaly, myelofibrosis, leukocytosis, etc. To address the question of optimal treatment of high-risk essential thrombocythemia patients, randomization for anagrelide versus interferon at < 65 years of age and anagrelide versus hydroxyurea at an age > 65 years is recommended.

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 > 600 x 10(9)/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 728 x 10(9)/L (95% CI 611-845 x 10(9)/L) to 412 x 10(9)/L (95% CI 319-504 x 10(9)/L) (p < 0.001) and maintained it at this level. Fourteen patients (88%) had a platelet reduction to < 600 x 10(9)/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.

Treatment of polycythaemia vera and essential thrombocythaemia

The clinical course in both polycythaemia vera (PV) and essential thrombocythaemia (ET) is characterized by significant thrombohaemorrhagic complications and variable risk of disease transformation into myeloid metaplasia with myelofibrosis or acute myeloid leukaemia. Randomized studies have shown that the risk of thrombosis was significantly reduced in ET with the use of hydroxyurea (HU) and in PV with the use of chlorambucil or 32P. However, the use of chlorambucil or 32P has been associated with an increased risk of leukaemic transformation. Subsequently, other studies have suggested that both HU and pipobroman may be less leukaemogenic and as effective as chlorambucil and 32P for preventing thrombosis in PV. However, the results from these prospective studies have raised concern that even HU and pipobroman may be associated with excess leukaemic events in both ET and PV. The recent introduction of anagrelide as a specific platelet-lowering agent, the demonstration of treatment efficacy with interferon-alpha, and the revived interest in using low-dose acetylsalicylic acid provide the opportunity to initiate prospective randomized studies incorporating these treatments.

The use of aspirin in polycythaemia vera and primary thrombocythaemia

In polycythaemia vera (PV; polycythaemia rubra vera, primary proliferative polycythaemia) and primary thrombocythaemia (PT; essential thrombocythaemia), occlusive complications in the microvasculature and larger vessels are a significant cause of morbidity and mortality. Central to the pathogenesis of these complications are the quantitative and qualitative platelet changes present in these myeloproliferative disorders. Aspirin irreversibly inactivates cyclo-oxygenase in platelets. This leads to a reduced production of platelet thromboxane A2 which has vasoconstricting and platelet aggregatory properties. In haematologically normal individuals, aspirin has been shown to reduce thrombo-embolic complications in populations at risk of these events. In PV and PT, aspirin has been shown to specifically eliminate the micro-circulatory and vasomotor manifestations and there is some evidence of a reduction in larger vessel occlusion. Low-dose aspirin has been shown to substantially reduce the raised thromboxane A2 production of platelets in PV and PT patients. The incidence of haemorrhagic side-effects of aspirin are minimized by the use of low doses. Haemorrhagic events are particularly found in patients with platelet counts > 1000 x 10(9)/l and these events are enhanced by aspirin therapy in these patients. Aspirin should be used with caution in patients with dyspeptic symptoms or a history of peptic ulceration or bronchospasm. Precise PCV control (< 0.45) and cytoreduction (platelets < 400 x 10(9)/l) should be used in patients with PV to minimize the vascular occlusion risk but routine cytoreduction is proposed only for those at particular risk of vascular occlusion in PT. In the acute presentation of patients with vascular occlusion, cytoreduction and an aspirin dose of 300 mg a day is proposed, reducing to 75 mg a day with the control of symptoms and signs, while 75 mg a day may play a role as prophylactic therapy in the prevention of thrombosis. However, there are no prospective studies in PT to demonstrate the benefit/risk profile and to confirm these recommendations, while a randomized prospective placebo-controlled study of low-dose aspirin in PV has only recently been initiated.

New drugs in essential thrombocythemia and polycythemia vera

Among the chronic myeloproliferative disorders, polycythemia vera and essential thrombocythemia are unique because of their association with thrombohemorrhagic manifestations and their relatively indolent clinical course. Patients with essential thrombocythemia may not have a significant shortening of life-expectancy and most may not require specific therapy. However, patients with polycythemia vera have a significant risk of transformation of polycythemia vera into acute leukemia or postpolycythemic myelofibrosis (or both). 'High-risk-for-thrombosis' patients with either polycythemia vera or essential thrombocythemia require specific therapy with a platelet-lowering agent to prevent thrombotic complications. Currently, the standard agent used for this is hydroxyurea. However, its tetratogenic and leukemogenic potential has been of concern. As a result, new platelet-lowering agents are being evaluated in the treatment of polycythemia vera and essential thrombocythemia. Anagrelide and interferon alfa are two such agents and have been shown to be effective in reducing platelet counts in patients with chronic myeloproliferative disorders. The putative mechanism of action of these drugs, their specific activity in polycythemia vera and essential thrombocythemia, side-effect profile, and current indications are discussed herein.