Mechanism of Action
Drugs in this class possess a unique activity profile that distinguishes them from classic anticoagulants. First, they exert their effect independently of antithrombin III. Second, they can inactivate two forms of thrombin simultaneously:
- Free thrombin circulating in the blood plasma.
- Thrombin bound to fibrin fibers within a thrombus (preventing further growth of an existing clot).
A major advantage over heparins is their high safety profile regarding blood cells. Direct inhibitors do not interact with platelet factor 4, and therefore they do not trigger immune-mediated thrombocytopenia.
Classification by Binding Sites
Depending on how the drug molecule interacts with different regions of the enzyme, inhibitors are divided into two groups:
- Bivalent (blocking two sites). They bind simultaneously to the catalytic center (responsible for proteolysis) and the substrate-recognition site (anion-binding exosite 1). This group includes hirudin, lepirudin, and bivalirudin.
- Monovalent (blocking only the catalytic center). These are fully synthetic agents that exclusively block the active center of the enzyme. Examples include argatroban and dabigatran etexilate.
Hirudin and Lepirudin
The prototype of this entire class is hirudin, a 65-amino acid polypeptide (molecular weight 7,000 Da) originally isolated from the salivary glands of medicinal leeches (Hirudo medicinalis). It binds to thrombin in a 1:1 stoichiometry, irreversibly blocking both of its active sites and preventing the enzyme from interacting with fibrinogen. Hirudin is highly specific and does not affect other serine proteases.
In clinical practice, lepirudin (brand name Refludan) is used as a recombinant version of hirudin, chemically corresponding to [Leu1-Thr2]-63-desulfatohirudin. It is administered intravenously.
- Indications: Prevention of thromboembolic complications, primarily in patients with heparin-induced thrombocytopenia.
- Characteristics: Approximately 50% of patients develop antibodies against the drug. The resulting lepirudin-antibody complex is cleared from the body more slowly than the unbound drug, which paradoxically prolongs the anticoagulant effect.
- Monitoring and Risks: The response is variable, requiring daily laboratory monitoring via aPTT. The primary complication is bleeding, and no specific antidote exists.
Bivalirudin
Bivalirudin is a synthetic 20-amino acid polypeptide (molecular weight 2,180 Da). Like hirudin, it is a bivalent inhibitor: its Phe1-Pro2-Arg3-Pro4 sequence blocks the catalytic center, while its hirudin-like tail binds to exosite-1.
The main difference from hirudin is reversibility of action. Thrombin is capable of slowly cleaving the Arg3-Pro4 peptide bond within the bivalirudin molecule, after which the inhibitory effect ceases.
Pharmacodynamics and Clinical Use:
- Blocks fibrin formation, suppresses platelet activation, inhibits clotting factors (V, VIII, XIII), and protein C.
- Administered intravenously during percutaneous coronary intervention (PCI) and stenting (as an alternative to heparin), as well as in heparin-induced thrombocytopenia.
- Causes fewer bleeding complications compared to heparin.
Pharmacokinetics: Onset of action is nearly immediate. Unlike heparin, it does not bind to plasma proteins, blood cells, or the endothelium. It is eliminated via plasma proteolysis (including cleavage by thrombin itself). Only 20% is excreted unchanged by the kidneys. The half-life is very short at 25 minutes, though it is prolonged in renal impairment.