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Direct Anticoagulants

For medical students2 min readUpdated 2026-10-10

Direct anticoagulants are a pharmacological class of drugs that suppress the activity of coagulation factors directly in the blood plasma. Their main targets are factor IIa (thrombin) and factor Xa, which are involved in the final stages of fibrin formation.

LocalizationInhibition of coagulation factors directly in blood plasma

Main targetsFactor IIa (thrombin) and factor Xa

MechanismsAntithrombin-dependent and direct active site blockade

Protein CSynthesized in the liver, vitamin K-dependent process

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 cannot block coagulation factors on their own. Their function is to multiply (up to 1,000 times) the activity of endogenous antithrombin III.
Direct active site inhibitors. These drugs bind directly to the active center of the target enzyme—thrombin or factor Xa—completely neutralizing its activity.

Antithrombin-Dependent Anticoagulants and Antithrombin III

Endogenous antithrombin III inactivates thrombin (IIa) in the body, as well as factors Xa, IXa, XIa, and XIIa. However, this protective protein functions effectively only in the presence of cofactors—heparin or heparin-like substances (such as heparan sulfate on the endothelium).

This subgroup includes:

Standard (unfractionated) heparin: heparin sodium, heparin calcium (calciparine).
Low-molecular-weight heparins (LMWHs): enoxaparin sodium, nadroparin calcium, dalteparin sodium, reviparin sodium.
Factor Xa inhibitors: fondaparinux sodium.
Heparinoids: sulodexide, danaparoid sodium.
Antithrombin III concentrates are placed in a separate subgroup for replacement therapy.

Direct Inhibitors and the Protein C System

Direct active site inhibitors interact with enzymes without intermediaries. Prominent representatives include hirudin derivatives, as well as dabigatran (a selective thrombin inhibitor) and rivaroxaban (a factor Xa inhibitor).

Concurrently, the body operates its own protein C system:

Protein C is synthesized in the liver in a vitamin K-dependent process.
Circulating in an inactive form, it is activated by the thrombin–thrombomodulin complex on healthy endothelium (this process is triggered via negative feedback when thrombin is in excess).
Working together with the cofactor protein S (both proteins contain glutamic acid residues for $Ca^{2+}$ calcium binding), activated protein C cleaves and degrades factors VIIIa and Va, ultimately suppressing thrombin generation.

Mnemonic

Direct agents are split in two: some work hand-in-hand with antithrombin III (heparins), while others strike the exact target by binding the enzyme's active center (dabigatran, rivaroxaban).

Frequently asked questions

What are the side effects of direct anticoagulants?

The provided sources list the following side effects for direct anticoagulants:

Heparin sodium — bone demineralization and thrombocytopenia.
Low-molecular-weight heparins — bleeding risk, moderate thrombocytopenia in the first days, immune-mediated thrombocytopenia, elevated liver transaminases, and allergic reactions.
Sulodexide — side effect frequency is comparable to placebo.

What specific antidotes exist for dabigatran and rivaroxaban?

For dabigatran, the specific antidote is idarucizumab. If unavailable, dialysis or hemodialysis can be performed, alongside the administration of combined coagulation factors II, VII, IX, and X (prothrombin complex concentrate). For rivaroxaban, no specific antidote is mentioned in the guidelines; combined 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 LMWHs are the prophylaxis and treatment of thrombosis and embolism in various clinical settings.

Thrombosis and embolism — prevention and treatment of deep vein thrombosis (especially postoperatively), prevention of pulmonary embolism.
Acute coronary syndrome — unstable angina and myocardial infarction.
Obstetrics — 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 requiring prolonged bed rest.

Where exactly do direct anticoagulants exert their pharmacological effect?

The effect is exerted directly in the blood plasma by inhibiting already activated coagulation 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 coagulation 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.

Go deeper

Differences between standard unfractionated heparin and low-molecular-weight heparins (LMWHs)
The role of heparan sulfate on the surface of undamaged endothelium
Participation of calcium ions ($Ca^{2+}$) and glutamic acid residues in the function of proteins C and S
Specifics of replacement therapy with antithrombin III concentrates
Comparative characteristics of direct thrombin inhibitors (dabigatran, hirudin) and factor Xa inhibitors (rivaroxaban, fondaparinux)