Key result
At low anti-Xa doses, ATH primarily acts via catalytic inhibition and is less effective than UFH, but at high doses, its direct inhibitory activity is comparable to non-covalent AT+UFH.
Why the study?
Does antithrombin-heparin covalent complex (ATH) inhibit clot formation differently compared to UFH or LMWH in blood and plasma?
Does antithrombin-heparin covalent complex (ATH) inhibit clot formation differently compared to UFH or LMWH in blood and plasma?
The antithrombin-heparin covalent complex exhibits dose-dependent mechanisms of anticoagulation, relying on catalytic inhibition at low doses and direct inhibition at high doses, comparable to UFH.
Answers are grounded in this paper and cite their sources.
ATH dose-dependent mechanisms merit further preclinical testing; leaves open translation to human antithrombotic use versus UFH.
We have developed an antithrombin-heparin covalent complex (ATH) which inhibits coagulation enzymes by two mechanisms: directly, or by catalytic activation of plasma antithrombin (AT). Anticoagulation by ATH was compared to unfractionated heparin (UFH) or low-molecular-weight heparin (LMWH) using a blood-based, tissue factor (TF)-activated thrombelastography (TEG) assay. Simplified TEG assays with plasma or purified plasma components were used to determine the contribution of the direct and catalytic mechanisms to ATH efficacy. Low anti-Xa concentrations of UFH inhibited clot formation significantly more than equivalent concentrations of ATH or LMWH in blood and plasma. ATH had reduced ability to catalyse AT-mediated thrombin (IIa) inhibition compared to UFH. However, at high anti-Xa concentrations, ATH had similar anticoagulant activity to UFH. ATH and non-covalent AT+UFH directly inhibited clotting to a similar degree in AT-deficient plasma. IIa-ATH complexes, which are limited to catalytic inhibition, displayed impaired anticoagulation compared to free ATH, and the magnitude of this effect increased significantly as anticoagulant concentration increased. Kinetic experiments indicated that the rate of reaction of AT with IIa is lower when catalysed by ATH versus UFH. In conclusion, at low anti-Xa doses catalytic inhibition is the primary mechanism of ATH anticoagulation, and the catalytic potential of ATH is reduced relative to UFH. However, the direct inhibitory activity of ATH is comparable to non-covalent AT+UFH, and at high anti-Xa doses the direct inhibitory activity of ATH may play a larger role in anticoagulation.
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Atkinson et al. (2009) studied this question. Antithrombin-heparin covalent complex (ATH) vs. Unfractionated heparin (UFH) or low-molecular-weight heparin (LMWH) was evaluated on Clot formation and anticoagulant activity measured by thrombelastography. At low anti-Xa doses, ATH primarily acts via catalytic inhibition and is less effective than UFH, but at high doses, its direct inhibitory activity is comparable to non-covalent AT+UFH.
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