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Pharmacology · Heparins

Fondaparinux !

A synthetic five-sugar molecule, injected under the skin, that makes antithrombin block factor Xa. It has no effect on thrombin.

Updated 2026-11-01

  • Class: Heparins
  • Target: Antithrombin (and through it, factor Xa only)
  • ATC code: B01AX05

The figure shows antithrombin bound to a heparin pentasaccharide, the five-sugar sequence that fondaparinux reproduces synthetically.

Target in 3D: Antithrombin (and through it, factor Xa only). PDB structure 1AZX (RCSB PDB, public domain). View the structure at RCSB PDB

Mechanism of action

Fondaparinux is a fully synthetic copy of the pentasaccharide, the five-sugar sequence through which heparin binds antithrombin. It binds antithrombin specifically and makes it neutralise factor Xa about 300 times faster. Because it is too short to bridge antithrombin to thrombin, it does not inactivate thrombin and has no known effect on platelet function. It does not bind platelet factor 4 to a significant degree.

  1. Fondaparinux binds antithrombin through its pentasaccharide
  2. Antithrombin neutralises factor Xa much faster
  3. Thrombin itself is not inhibited; less thrombin is formed

Pharmacokinetics

Absorption
Rapid and complete under the skin; the anti-Xa effect peaks in about three hours.
Distribution
Mainly in the blood, bound specifically to antithrombin.
Elimination
Mainly unchanged in the urine, so it accumulates when kidney function falls and in people over 75.
Half-life
Long: about 17 to 21 hours.

Indications

  • Prevention of deep vein thrombosis after hip fracture, hip or knee replacement, or abdominal surgery. Deep vein thrombosis →
  • Treatment of deep vein thrombosis or acute pulmonary embolism, together with warfarin. Pulmonary embolism →

Adverse effects

Common

  • Bleeding
  • Rash, itching or bruising at the injection site
  • Low platelet count
  • Dizziness, trouble sleeping

Contraindications

  • Severe kidney impairment
  • Active major bleeding
  • Bacterial endocarditis
  • Thrombocytopenia with a positive anti-platelet antibody test in the presence of fondaparinux
  • Low body weight, for prophylaxis in adults

Key interactions

Other anticoagulants, antiplatelets, NSAIDs
Raise the risk of bleeding; such drugs are stopped beforehand when possible.
Neuraxial anaesthesia or epidural catheters
Risk of spinal haematoma.

What is monitored

  • Kidney function before and during treatment
  • Platelet count and haemoglobin
  • Signs of bleeding and, after spinal procedures, neurological checks

Administration and safety

  • Subcutaneous only; not mixed with other injections or infusions
  • The air bubble in the prefilled syringe is not expelled
  • The needle guard contains natural latex rubber: latex allergy is asked about
  • There is no known antidote; protamine does not reverse it

Nursing care

  • Check kidney function and body weight before the first dose
  • Rotate injection sites and watch for bruising
  • Watch for bleeding, back pain or leg weakness after spinal or epidural procedures
  • Teach self-injection when treatment continues at home

Pregnancy and breastfeeding

Published reports and postmarketing data have not shown a clear link with harm to the fetus; it is used when clearly needed.

Test yourself

Think of the answer, then tap the question to see it.

Why does fondaparinux not inhibit thrombin?

It is only the five-sugar sequence: it activates antithrombin against factor Xa but is too short to bridge antithrombin and thrombin.

Which kidney situation contraindicates fondaparinux?

Severe kidney impairment, because it is eliminated unchanged in the urine and its half-life is already long.

Can protamine reverse fondaparinux?

No. There is no known antidote for fondaparinux.

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Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: DailyMed - FONDAPARINUX SODIUM injection, solution (DailyMed (U.S. National Library of Medicine), 2026); Fondaparinux Injection: MedlinePlus Drug Information (MedlinePlus (U.S. National Library of Medicine), 2025); 1AZX: ANTITHROMBIN/PENTASACCHARIDE COMPLEX (RCSB Protein Data Bank, 1999).

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Representative model, for educational purposes only.