Two main subgroups by mechanism of action
The pharmacological effect of these drugs is achieved through two fundamentally different pathways:
- Antithrombin-dependent agents (heparin and analogues). They are unable to block clotting factors on their own. Their function is to manifoldly enhance (up to 1,000-fold) the activity of endogenous antithrombin III.
- Direct active-site inhibitors. These drugs bind directly to the active site of the target enzyme—thrombin or factor Xa—completely neutralizing its activity.
Antithrombin-dependent anticoagulants and antithrombin III
Endogenous antithrombin III in the body inactivates thrombin (IIa), as well as factors Xa, IXa, XIa, and XIIa. However, the effective functioning of this protective protein is only possible in the presence of cofactors: heparin or heparin-like substances (e.g., heparan sulfate on the endothelium).
This subgroup includes:
- Standard (unfractionated) heparin: heparin sodium, heparin calcium (calciparin).
- Low-molecular-weight heparins (LMWHs): enoxaparin sodium, nadroparin calcium, dalteparin sodium, reviparin sodium.
- Factor Xa inhibitors: fondaparinux sodium.
- Heparinoids: sulodexide, danaparoid sodium.
- Antithrombin III preparations are classified into a separate subgroup for replacement therapy.
Direct inhibitors and the protein C system
Direct active-site inhibitors interact with enzymes without intermediaries. Prominent examples include hirudin preparations, as well as dabigatran (a selective thrombin inhibitor) and rivaroxaban (a factor Xa inhibitor).
Concurrently, the body's own protein C system functions as follows:
- The synthesis of protein C occurs in the liver and is dependent on vitamin K.
- Circulating in an inactive form, it is activated by the “thrombin–thrombomodulin” complex on healthy endothelium (this process is triggered via a negative feedback mechanism in the presence of excess thrombin).
- Working together with cofactor protein S (both proteins contain glutamic acid residues for $Ca^{2+}$ calcium binding), activated protein C cleaves and degrades factors VIIIa and Va, which ultimately inhibits thrombin generation.
Briefly: answers to topical questions
What are the side effects of direct anticoagulants?
According to the cited sources, direct anticoagulants have the following side effects:
- Heparin sodium — bone demineralization and thrombocytopenia.
- Low-molecular-weight heparins — risk of bleeding, moderate thrombocytopenia in the first days, immune-mediated thrombocytopenia, elevated liver transaminases, and allergic reactions.
- Sulodexide — the frequency of side effects is comparable to placebo.
What antidotes exist for dabigatran and rivaroxaban?
For dabigatran, there is a specific antidote: idarucizumab. If unavailable, dialysis or hemodialysis may be performed, along with the administration of combined blood coagulation factors II, VII, IX, and X (prothrombin complex concentrate). For rivaroxaban, no specific antidote is specified in the cited guidelines; combined blood coagulation factors II, VII, IX, and X (prothrombin complex concentrate) are used.
What are the main indications for prescribing low-molecular-weight heparins?
The main indications for prescribing low-molecular-weight heparins are the prevention and treatment of thrombosis and embolism in various clinical situations.
- Thrombosis and embolism — prevention and treatment of deep vein thrombosis (especially in the postoperative period), prevention of pulmonary embolism.
- Acute coronary syndrome — unstable angina and myocardial infarction.
- Obstetric practice — used for the prevention and treatment of thrombosis during pregnancy (they do not cross the placenta).
- Specific conditions — prevention of thrombosis in hospitalized patients with ulcerative colitis and in patients with tick-borne encephalitis on prolonged bed rest.
Direct agents are divided into two types: some work hand-in-hand with antithrombin III (heparins), while others strike precisely at the target by binding the active site of the enzyme (dabigatran, rivaroxaban).
Class and exam questions
Where exactly is the pharmacological effect of direct anticoagulants exerted?
The effect is exerted directly in blood plasma by inhibiting already activated clotting factors.
How do antithrombin-dependent anticoagulants work?
They do not block factors on their own. They act only in a complex with endogenous antithrombin III, accelerating the inactivation of clotting factors by 1,000-fold.
Which factors are degraded by the activated protein C system?
Activated protein C, together with protein S, causes the degradation of factors Va and VIIIa, leading to the suppression of thrombin synthesis.
What else to study on this topic
- Differences between standard unfractionated heparin and low-molecular-weight heparins (LMWHs)
- The role of heparan sulfate on the surface of intact endothelium
- The involvement of calcium ions ($Ca^{2+}$) and glutamic acid residues in the function of proteins C and S
- Features of replacement therapy with antithrombin III preparations
- Comparative characteristics of direct thrombin inhibitors (dabigatran, hirudin) and factor Xa inhibitors (rivaroxaban, fondaparinux)
Master the entire topic in the «Pharmacology» course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Direct-Acting Anticoagulants" and 5,304 other pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 116 video lessons on the subject
Other topics in the section "Drugs affecting hemostasis and thrombosis"
Free video lessons: pharmacologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.