Heparin is a widely used anticoagulant. Because of its negative charge, it forms complexes with positively charged platelet factor 4 (PF4). This can induce anti-PF4/heparin IgG Abs. Resulting immune complexes activate platelets, leading to the prothrombotic adverse drug reaction heparin-induced thrombocytopenia (HIT). HIT requires treatment with alternative anticoagulants. Approved for HIT are 2 direct thrombin inhibitors (DTI; lepirudin, argatroban) and danaparoid. They are niche products with limitations. We assessed the effects of the DTI dabigatran, the direct factor Xa-inhibitor rivaroxaban, and of 2-O, 3-O desulfated heparin (ODSH; a partially desulfated heparin with minimal anticoagulant effects) on PF4/heparin complexes and the interaction of anti-PF4/heparin Abs with platelets. Neither dabigatran nor rivaroxaban had any effect on the interaction of PF4 or anti-PF4/heparin Abs with platelets. In contrast, ODSH inhibited PF4 binding to gel-filtered platelets, displaced PF4 from a PF4-transfected cell line, displaced PF4/heparin complexes from platelet surfaces, and inhibited anti-PF4/heparin Ab binding to PF4/heparin complexes and subsequent platelet activation. Dabigatran and rivaroxaban seem to be options for alternative anticoagulation in patients with a history of HIT. ODSH prevents formation of immunogenic PF4/heparin complexes, and, when given together with heparin, may have the potential to reduce the risk for HIT during treatment with heparin.
"ODSH can compete with immobilized heparin for PF4 binding and can displace PF4 from cell surfaces [156,157]. When combined with heparin, ODSH reduces immunogenicity in vivo  and ameliorates HIT antibody mediated platelet activation in vitro [155,156]. When used together, the capacity of ODSH to sequester a proportion of available PF4 without generating immunogenic complexes may be an effective way to shift the PF4/heparin ratio toward less antigenic complexes. "
[Show abstract][Hide abstract] ABSTRACT: Heparin Induced Thrombocytopenia (HIT) is caused by antibodies that recognize platelet factor 4 (PF4) associated with polyanionic glycosaminoglycan drugs or displayed on vascular cell membranes. These antibodies are elicited by multimolecular complexes that can occur when heparin is administered in clinical settings associated with abundant PF4. Heparin binding alters native PF4 and elicits immune recognition and response. While the presence of heparin is integral to immunogenesis, the HIT antibody binding site is within PF4. Thus HIT antibodies develop and function to cause thrombocytopenia and/or thrombosis only in the presence of PF4. Future emphasis on understanding the biology, turnover and regulation of PF4 may lead to insights into the prevention and treatment of HIT.
"All three drugs are given orally and have been shown to be effective anticoagulants in various indications such as thrombosis prophylaxis after major surgery , deep vein thrombosis , and atrial fibrillation . Owing to their structure they do not interact with PF4 , which has been systematically assessed for rivaroxaban and dabigatran [ Krauel et al . 2011a ] . Once they are approved for certain indications , these drugs are ideal replacements for heparin in patients with a history of HIT . They may also be appropriate for treatment of patients with acute HIT based on theoretical considerations . However , the lack of a validated tool for monitor - ing of these new drugs may cause problems"
[Show abstract][Hide abstract] ABSTRACT: Heparin-induced thrombocytopenia (HIT) is a drug-mediated, prothrombotic disorder caused by immunization against platelet factor 4 (PF4) after complex formation with heparin or other polyanions. After their binding to PF4/heparin complexes on the platelet surface, HIT antibodies are capable of intravascular platelet activation by cross-linking Fcγ receptor IIA leading to a platelet count decrease and/or thrombosis. Diagnosis of HIT is often difficult. This, and the low specificity of the commercially available immunoassays, leads currently to substantial overdiagnosis of HIT. Timing of onset, the moderate nature of thrombocytopenia, and the common concurrence of thrombosis are very important factors, which help to differentiate HIT from other potential causes of thrombocytopenia. A combination of a clinical pretest scoring system and laboratory investigation is usually necessary to diagnose HIT. Although HIT is considered to be a rare complication of heparin treatment, the very high number of hospital inpatients, and increasingly also hospital outpatients receiving heparin, still result in a considerable number of patients developing HIT. If HIT occurs, potentially devastating complications such as life-threatening thrombosis make it one of the most serious adverse drug reactions. If HIT is strongly suspected, all heparin must be stopped and an alternative nonheparin anticoagulant started at a therapeutic dose to prevent thromboembolic complications. However, the nonheparin alternative anticoagulants bear a considerable bleeding risk, especially if given to patients with thrombocytopenia due to other reasons than HIT. While established drugs for HIT are disappearing from the market (lepirudin, danaparoid), bivalirudin, fondaparinux and potentially the new anticoagulants such as dabigatran, rivaroxaban and apixaban provide new treatment options.
[Show abstract][Hide abstract] ABSTRACT: Heparin has remained the most commonly used anticoagulant in hemodialysis patients (HD). Its use is usually safe but, in some cases, important adverse effects can occur. Heparin-induced thrombocytopenia (HIT) is an immuno-mediated condition due to the formation of PF4/heparin/IgG complex leading to the activation of platelets and coagulative cascade. The consequent prothrombotic hypercoagulable state may cause venous or arterial thrombosis, skin gangrene and acute platelet activation syndrome. Clinical and laboratory findings may be suggestive for HIT, but formal diagnosis requires the demonstration of the presence of circulating antibodies. Clinical management is complex including the withdrawal of any form of heparin and the administration of anticoagulants. In addition, since anticoagulation is routinely required to prevent clotting of the dialysis lines and membranes, in HD patients presenting HIT it is mandatory to establish heparin-free anticoagulation strategies. Thus, the use of citrate, direct thrombin inhibitors or eparinods have been proposed as alternative anticoagulation approaches in HIT. Here, we review the most important pathogenic factors and clinical features of HIT occurring in HD patients.
Giornale italiano di nefrologia: organo ufficiale della Societa italiana di nefrologia 07/2014; 31(3).
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