Direct Thrombin Inhibitors: Dabigatran
The primary representative of this group is dabigatran. By mechanism of action, it is a direct thrombin inhibitor. The drug binds directly to the catalytic active site of the enzyme. The type of inhibition is reversible and competitive (which distinguishes it from hirudin).
Dabigatran is unique in that it blocks thrombin in two states:
- Free thrombin in blood plasma.
- Fibrin-bound thrombin (this allows it to disrupt the growth of an already formed thrombus).
As a result, the drug prevents the conversion of fibrinogen to fibrin and inhibits thrombin-induced platelet aggregation.
Pharmacokinetics: Administered per os as a prodrug — dabigatran etexilate. Upon entering the bloodstream, it is rapidly hydrolyzed by esterases into its active compound. An acidic environment is required for normal absorption, so tartaric acid is added to the capsule formulation. Bioavailability is very low, approximately 6.5%. It is metabolized via conjugation to form active glucuronides and is excreted primarily unchanged by the kidneys. The elimination half-life is 12–14 hours.
Direct Factor Xa Inhibitors: Rivaroxaban and Apixaban
These drugs act on a different step of the coagulation cascade. Their target is the active site of coagulation factor Xa. Inhibition is reversible and selective.
The site of action for factor Xa inhibitors encompasses the enzyme in two states:
- Free state.
- As part of the prothrombinase complex (where factor Xa acts in conjunction with factor Va and $Ca^{2+}$ ions).
The net effect is the interruption of thrombin generation.
Pharmacokinetics: Unlike dabigatran, these drugs are rapidly absorbed orally and have high bioavailability: approximately 50% for apixaban and up to 80% for rivaroxaban.
Crucial clinical feature: they are substrates for the P-glycoprotein transport protein and cytochrome P-450 isoenzymes. This creates a high risk for drug-drug interactions. Any medications that act as inducers or inhibitors of these systems will alter the plasma concentration of the anticoagulants. The elimination half-life depends on age: in young patients, the half-life of rivaroxaban is 5–9 hours, while in the elderly it prolongs to 12–13 hours.
Clinical Application and Adverse Effects
Direct oral anticoagulants are actively used in two main clinical fields:
- Traumatology and Orthopedics: Prevention of venous thrombosis and pulmonary embolism (PE) following major joint replacement surgery (knee, hip). Here they surpass low-molecular-weight heparins in ease of administration since they do not require injections.
- Cardiology: Prevention of ischemic stroke and systemic thromboembolism in atrial fibrillation. Pathogenesis: in this arrhythmia, thrombi form in the atrial appendages and can migrate into cerebral vessels, causing ischemia.
Dosing Regimen: Rivaroxaban is taken once daily, while apixaban is taken twice daily.
Adverse Effects: The primary risk for the entire class is hemorrhagic complications (bleeding). However, gastrointestinal bleeding occurs less frequently with dabigatran than with warfarin. Specific adverse effects of dabigatran include hepatotoxicity (elevation of liver transaminases) and dyspepsia, which is caused by the irritating effect of tartaric acid in the capsule.