Acute thrombocytopenia in patients treated with amiodarone is caused by antibodies specific for platelet membrane glycoproteins.

Amiodarone has been implicated as a cause of thrombocytopenia but the responsible mechanism is unknown. We performed studies in three patients to characterize the pathogenesis of this complication. No amiodarone-dependent, platelet-reactive antibodies were identified using conventional serological techniques. However, water-insoluble amiodarone solubilized in methanol and diluted to 1·0 mg/ml in aqueous buffer reproducibly promoted binding of IgG antibodies in patient serum to platelets. Solid phase assays identified drug-dependent antibodies specific for platelet glycoproteins (GP)Ia/IIa (integrin α2 ß1 ) in each patient and a second antibody specific for GPIIb/IIIa (αII b ß3 integrin) in one patient. When studied by ion mobility analysis and transmission electron microscopy, the serologically active amiodarone preparation, a milky suspension, was found to consist of particles 2-30 nm in diameter, typical of a coacervate, a state characteristic of amiodarone in aqueous medium. The findings provide evidence that thrombocytopenia in the three patients studied was caused by drug-dependent antibodies specific for platelet glycoproteins GPIa/IIa and/or GPIIb/IIIa. We postulate that, in vivo, amiodarone may become incorporated into occult lipophilic domains in platelet glycoproteins, producing structural modifications that are immunogenic in some individuals, and that the resulting antibodies can cause platelet destruction in a person taking this drug.

Thrombopoietin levels in patients with disorders of platelet production: diagnostic potential and utility in predicting response to TPO receptor agonists.

Thrombopoietin (TPO) is the major regulator of megakaryopoiesis. Measurement of serum TPO levels may help distinguish between various causes of thrombocytopenia and predict treatment response to TPO receptor agonists. Serum TPO levels from 118 healthy volunteers and 88 patients with abnormal platelet counts were measured using a quantitative ELISA assay. The mean (range) TPO level in healthy volunteers was 39 (7-99) pg/mL. TPO values were correlated with the patient's diagnosis, platelet count, and response to TPO receptor agonists. 88 patients with history of consumptive thrombocytopenia (39) or hypoproliferative thrombocytopenia (49) were analyzed. Median (interquartile range) TPO level for consumptive thrombocytopenia patients was 63 (48-98) pg/mL with a corresponding median (interquartile range) platelet count of 73 (28-146) × 10(9) /L. In contrast, hypoproliferative thrombocytopenia patients had platelet counts [59 (30-117) × 10(9) /L] comparable with consumptive thrombocytopenia patients, but significantly higher serum TPO levels [706 (358-1546) pg/mL, P < 0.0001]. Analysis of 21 ITP patients treated with TPO receptor agonists demonstrated that a TPO level >95 pg/mL was associated with lack of clinical response (P < 0.002). TPO levels may have diagnostic utility in discriminating between patients with hypoproliferative and consumptive thrombocytopenia. Elevated TPO levels in ITP patients may predict a poor clinical response to treatment with TPO receptor agonists.

Asymptomatic factor VII deficiency: gene analysis and structure-function relationships.

Factor VII Padua is a variant form of factor VII deficiency characterized by a prolongation of the prothrombin time (PT), when the assay is performed using rabbit brain thromboplastin. The PT is normal when performed using either human or ox brain thromboplastin reagents, or a recombinant human tissue factor-based thromboplastin. We report a case of an African-American woman with asymptomatic factor VII deficiency, who had a prolonged PT and factor VII activity levels of 5-8% using rabbit brain thromboplastin, but a normal PT and factor VII activity levels when measured using recombinant human brain thromboplastin or tissue factor. The amino acid substitution (R304Q), which gives rise to factor VII Padua, was found in our patient, making this only the fourth African-American case described to date with this mutation. Our report emphasizes the importance of identifying this benign form of factor VII deficiency in order to avoid unnecessary exposure of patients to treatment with either plasma-derived products or recombinant activated factor VII.

Rosiglitazone-induced immune thrombocytopenia.

Rosiglitazone is one of the members in the thiazolidinedione (TZD) class of anti-diabetic agents that have proven efficacy in the treatment of patients with type 2 diabetes. We studied serum from a patient who developed acute, severe thrombocytopenia after exposure to rosiglitazone maleate (Avandia) and proposed the mechanisms for rosiglitazone-induced thrombocytopenia. Tested by flow cytometry, the patient's serum was positive for rosiglitazone-induced antibody with the binding ratio of 5.93 (mean fluorescence intensity, MFI) in the presence of the patient's serum and rosiglitazone in a final concentration of 0.53 mmol/l. The antibody was found to bind both glycoprotein (GP) IIb-IIIa complex and GP Ib/IX complex by MAIPA assay using five different monoclonal antibodies (mAbs) against GP complexes Ib/IX, GPIIb/IIIa or GPIa/IIa. Immunoprecipitation studies showed that both GPIIb/IIIa and GP Ib/IX complex were precipitated by antibody in the presence, but not in the absence of rosiglitazone. These findings provide evidence that immune thrombocytopenia can be caused by sensitivity to the antidiabetic agent rosiglitazone maleate. This report documents the first case of rosiglitazone-induced immune thrombocytopenia.

Glycoprotein IIb/IIIa complex is the target in mirtazapine-induced immune thrombocytopenia.

Mirtazapine (MW 265.36), a tetracyclic antidepressant of the piperazine-azapine group which augments central noradrenergic and serotonergic activity, is currently used as an oral antidepressant. We report a case of severe thrombocytopenia in a 66-year-old patient occurring after mirtazapine administration, suggesting an immune mechanism. This report documents the first case of mirtazapine-induced immune thrombocytopenia. The patient's serum was screened for drug-induced anti-platelet antibody with the chromium(51) (Cr(51)) platelet lysis technique. The drug-dependent antibody was characterized using flow cytometry, the monoclonal antibody immobilization of platelet antigens assay (MAIPA assay), and immunoprecipitation. By the Cr(51) platelet lysis technique, we obtained an equivocal result for the detection of mirtazapine-induced antibody. However, the patient's serum tested positive for mirtazapine-induced antibody by flow cytometry. The results showed that the binding ratio of 5.7 (mean fluorescence intensity) in the presence of the patient's serum and mirtazapine in a final concentration of 1.0 mmol/L was strongly positive. The antibody was found to bind the glycoprotein (GP) IIb/IIIa complex by MAIPA assay by using five different monoclonal antibodies against GP complexes Ib/IX, GPIIb/IIIa, or GPIa/IIa. Immunoprecipitation studies showed that the GPIIb/IIIa complex was precipitated by antibody in the presence, but not in the absence, of mirtazapine. These findings provide evidence that immune thrombocytopenia can be caused by sensitivity to the antidepressant mirtazapine. This is the first well-documented case of mirtazapine-induced immune thrombocytopenia.

Association of C807T, Pl(A), and -5 C/T Kozak genotypes with density of glycoprotein receptors on platelet surface.

This study investigates whether three platelet glycoprotein (GP) polymorphisms, C807T in GP Ia, Pl(A1/A2) in GP IIIa, and -5 T/C Kozak in GP Ibalpha gene, influence the density of the three important adhesion and activation receptors on the platelet surface. Fifty-four healthy donors were genotyped according to the three polymorphisms, and densities of the corresponding GPs were measured by flow cytometry. Our study confirmed the association between C807T polymorphism and platelet surface expression of GP Ia-IIa and GP Ia and demonstrated that the density of GP Ibalpha or GP IX is not associated with the Kozak polymorphism. Although the Pl(A1/A2) polymorphism did not affect the expression of GP IIb-IIIa and GP IIIa on the platelet surface, flow-cytometric analysis employing murine monoclonal antibody SZ21 against GP IIIa can be applied to distinguish Pl(A1/A1) and Pl(A1/A2) polymorphism.

von Willebrand factor-cleaving protease inhibitor in a patient with human immunodeficiency syndrome-associated thrombotic thrombocytopenic purpura.

Antibodies that inhibit von Willebrand Factor (VWF)-cleaving protease activity occur in patients with acute thrombotic thrombocytopenic purpura (TTP) and often persist in the chronic phase. A deficiency of this protease is likely to be responsible for the generation of ultrahigh VWF multimers and influence the formation of intra-arterial platelet aggregates that result in microangiopathic haemolytic anaemia, thrombocytopenia and end in organ failure. This report demonstrates complete deficiency of VWF-cleaving protease and the presence of a concentration-dependent IgG1 inhibitor in the plasma of a patient with acquired immunodeficiency syndrome (AIDS). These data may contribute to understanding the pathophysiology of human immunodeficiency syndrome (HIV)-related TTP.