Thrombate III Mechanism of Action

Pharmacology: Pharmacodynamics: Mechanism of Action: Antithrombin, an alpha₂-glycoprotein of molecular weight 58,000, is normally present in human plasma at a concentration of approximately 12.5 mg/dL and is the major plasma inhibitor of thrombin. Inactivation of thrombin by AT occurs by formation of a covalent bond resulting in an inactive 1:1 stoichiometric complex between the two, involving an interaction of the active serine of thrombin and an arginine reactive site on AT. AT is also capable of inactivating other components of the coagulation cascade including factors IXa, Xa, XIa, and XIIa, as well as plasmin. The neutralization rate of serine proteases by AT proceeds slowly in the absence of heparin, but is greatly accelerated in the presence of heparin. As the therapeutic antithrombotic effect of heparin is mediated by AT, heparin in vivo is ineffective in the absence or near absence of AT.
After administration, THROMBATE III temporarily replaces the missing AT in patients with hereditary antithrombin deficiency.
Clinical Studies: In a prospective, open-label clinical trial, 21 subjects were administered THROMBATE III for 16 prophylaxis events (n=13 subjects) and 10 for treatment of thrombosis (n=10 subjects) with 2 subjects receiving THROMBATE III for both prophylaxis and treatment of thrombosis. None of the 13 subjects with hereditary AT deficiency and histories of thromboembolism treated prophylactically on 16 separate occasions with THROMBATE III for high thrombotic risk situations (11 surgical procedures, 5 pregnancies and/or deliveries) developed a thrombotic complication. Heparin was administered in 3 of the 11 surgical procedures. Two of the pregnant subjects received LMW heparin prophylactically during the first trimester, but which was unable to maintain anti-coagulation with increasing dosages. (See Interactions.) They experienced a thrombosis, which subsequently resolved with the addition of THROMBATE III, and were therefore administered THROMBATE III and LMW heparin prophylaxis weekly during the second and third trimesters, and during labor and delivery. These two subjects did not experience a new thrombosis.
Ten subjects with hereditary AT deficiency were treated with THROMBATE III as well as heparin (n=9) for major thrombotic or thromboembolic complications, including 4 subjects with thrombosis during the first trimester of pregnancy. Nine subjects recovered with no additional thromboses or extension of existing thrombosis. The tenth subject died due to complications from the original pulmonary embolism with infarction which preceded treatment with THROMBATE III.
Pharmacokinetics: In a clinical trial of THROMBATE III conducted in asymptomatic subjects with hereditary deficiency of AT, 8 subjects were administered a single dose of THROMBATE III at doses ranging from 25 units/kg to 125 units/kg. Pharmacokinetic parameters were determined using immunologic and functional AT assays (Table 1).

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