Mechanism of action features
Drugs of this group possess a unique activity profile that favorably distinguishes them from classical anticoagulants. First, they exert their effect independently of antithrombin III. Second, they are capable of inactivating two forms of thrombin simultaneously:
- Freely circulating in blood plasma.
- Bound to fibrin strands within the clot (which prevents further growth of the already formed thrombus).
An important advantage over heparins is a high safety profile regarding blood cells. Direct thrombin inhibitors do not interact with platelet factor 4, and therefore they do not provoke the development of immune thrombocytopenia.
Classification by binding sites
Depending on how the drug molecule interacts with the enzyme's regions, two groups of inhibitors are distinguished:
- Bivalent (blocking two sites). They bind simultaneously to the catalytic center (responsible for proteolysis) and the substrate binding site (anion-binding site, or exosite 1). This group includes hirudin, lepirudin, and bivalirudin.
- Monovalent (blocking only the catalytic center). These are fully synthetic drugs that exclusively block the active center of the enzyme. Representatives: argatroban and dabigatran etexilate.
Hirudin and lepirudin
The prototype of the entire group is hirudin, a 65-amino acid polypeptide (mass 7000 D) originally isolated from the salivary glands of medical leeches (Hirudo medicinalis). It binds to thrombin in a 1:1 ratio, irreversibly blocking both of its sites and preventing the interaction of the enzyme with fibrinogen. Hirudin is highly specific and does not affect other serine proteases.
In clinical practice, lepirudin (trade name Refludan) is used, which is a recombinant version of hirudin, chemically representing [Leu1-Thr2]-63-desulfo-hirudin. It is administered intravenously.
- Indications: Prevention of thromboembolic complications, primarily in patients with heparin-induced thrombocytopenia.
- Features: In approximately 50% of patients, antibodies to the drug are developed. The resulting lepirudin-antibody complex is eliminated from the body slower than the pure drug, which paradoxically prolongs the anticoagulant effect.
- Monitoring and risks: The effect is variable, therefore daily laboratory monitoring of aPTT is required. The main complication is bleeding, while a specific antidote does not exist.
Bivalirudin (Angiox)
Bivalirudin is a synthetic 20-amino acid polypeptide (molecular weight 2180 D). Like hirudin, it is a bivalent inhibitor: its sequence Phe1-Pro2-Arg3-Pro4 blocks the catalytic center, and the hirudin-like part binds to exosite-1.
The main difference from hirudin is the reversibility of action. Thrombin is capable of slowly cleaving the Arg3-Pro4 peptide bond in the bivalirudin molecule, after which the inhibitory effect terminates.
Pharmacodynamics and application:
- Blocks fibrin formation, suppresses the activation of platelets and clotting factors (V, VIII, XIII), as well as protein C.
- Administered intravenously during percutaneous coronary angioplasty and stenting (as an alternative to heparin), as well as in heparin-induced thrombocytopenia.
- Causes less bleeding compared to heparin.
Pharmacokinetics: Begins to act almost instantly. Unlike heparin, it does not bind to plasma proteins, blood cells, and endothelium. It is degraded by proteolysis in plasma (including under the action of thrombin itself). Only 20% is excreted unchanged by the kidneys. The elimination half-life is very short — 25 minutes, but in renal failure, the drug's action is prolonged.
Briefly: answers to topical questions
What are the dosages and administration regimens of dabigatran etexilate?
Dabigatran etexilate is administered orally in various dosages depending on the clinical situation and indications:
- Standard dose (150 mg) — prescribed as 150 mg twice daily in the absence of contraindications and the need for a full dose.
- Reduced dose (110 mg) — prescribed as 110 mg twice daily in case of high bleeding risk or as part of long-term therapy with antiplatelet agents.
- Prophylactic dose — used orally in prophylactic dosages for mild to moderate COVID-19.
What specific antidote exists for dabigatran?
The specific antidote (antagonist) for dabigatran etexilate is idarucizumab (V03AB37).
- Idarucizumab (Idarucizumab) is a specific drug of choice used via intravenous administration to neutralize the action of dabigatran and stop life-threatening bleeding or in case of emergency surgery.
To remember bivalents drugs, use the rule "GIBL": Hirudin (G), And (I), Bivalirudin, Lepirudin. All of them block two sites of thrombin at once.
Class and exam questions
What is the main difference between direct thrombin inhibitors and heparins?
They act completely independently of antithrombin III, are capable of blocking thrombin inside an already formed thrombus, and do not cause immune thrombocytopenia.
Which laboratory parameter needs to be monitored during lepirudin treatment?
Therapy is monitored daily by the level of activated partial thromboplastin time (aPTT).
Is there a specific antidote for bleeding during hirudin or lepirudin therapy?
No, a specific antidote is currently absent for all hirudin-based drugs.
Why is the action of bivalirudin reversible?
Thrombin itself is capable of slowly cleaving the specific peptide bond in the bivalirudin molecule, which leads to the destruction of the drug and the cessation of its inhibitory action.
What else to study on this topic
- Molecular structure of the catalytic center and exosite-1 of thrombin
- Pathogenesis of heparin-induced thrombocytopenia (HIT)
- Pharmacokinetics of lepirudin upon antibody formation
- Synthetic monovalent inhibitors: argatroban and dabigatran etexilate
- The role of thrombin in the activation of clotting factors V, VIII, XIII, and protein C
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