Fondaparinux Sodium

Mechanism

Mechanism of Action

Fondaparinux is a synthetic, selective inhibitor of Factor Xa. Chemically, it is a synthetic pentasaccharide identical to the minimal sequence of heparin that has binding affinity for AT-III.

Molecular Mechanism: Exclusive Neutralization of Factor Xa

1. Fondaparinux selectively binds to AT-III with extremely high affinity (K_d ≈ 7.4 nM).
2. This binding induces an irreversible conformational change in AT-III that multiplies the rate of neutralization of plasma Factor Xa by AT-III.
3. Since the synthetic chain consists only of the essential pentasaccharide (without additional extension), it does not exert a bridging effect on thrombin (Factor IIa), having zero anti-IIa activity.
4. Being a totally synthetic polymer, it does not interact with Platelet Factor 4 (FP4), virtually eliminating the risk of developing type II heparin-induced thrombocytopenia (HIT).

Pharmacokinetics

Pharmacokinetics

• Route of administration: Deep subcutaneous prefilled injection; intravenous bolus only in specific indication of STEMI.
• Bioavailability (F): 100% subcutaneously. Maximum plasma concentration (C_max) achieved approximately 2 hours after the dose.
• Distribution: Very small volume of distribution (7 to 11 L), limited mainly to the blood space. Plasma protein binding > 94% (specifically to AT-III).
• Metabolism and Excretion: It is not actively metabolized. It is excreted unchanged through urine through glomerular filtration.
• Half-life (t_1/2): Prolonged, approximately 17 to 21 hours, allowing a single daily dosage. In patients with moderate renal failure (Cl_Cr 30 - 50 mL/min) the half-life increases to about 30 hours.

Indicators and dose

Indications

• Prevention of VTE in patients undergoing major orthopedic surgery of the lower extremities or high-risk abdominal surgery.
• Treatment of acute DVT and PE (in combination with warfarin or another oral anticoagulant).
• Treatment of unstable angina or non-ST segment elevation AMI (NSTEMI) where urgent percutaneous coronary intervention is not planned (< 120 min).
• Treatment of choice in patients with suspicion or history of HIT requiring acute parenteral anticoagulation.

Dosage and Settings

• TEV prophylaxis: 2.5 mg SC once a day started between 6 and 8 hours after surgery. Contraindicated in Cl_Cr < 30 mL/min.
• EVTE Treatment: Adjusted according to body weight:
  - Weight < 50 kg: 5 mg SC every 24 hours.
  - Weight 50 - 100 kg: 7.5 mg SC every 24 hours.
  - Weight > 100 kg: 10 mg SC every 24 hours.
• Renal adjustment: Reduce the dose by half or monitor with specific anti-Xa levels if CrCl is between 30 and 50 mL/min. Totally contraindicated if it is less than 30 mL/min.

Pregnancy and Breastfeeding

FDA Category B. Data is limited. Minimally crosses the placenta. The use of enoxaparin is preferred during pregnancy and fondaparinux is only recommended if there is a strong indication due to the patient's previous HIT. Compatible with breastfeeding (undetectable excretion in preclinical models).

Security

Contraindications

Absolute

• Hypersensitivity to the active ingredient or excipients.
• Clinically significant severe active bleeding.
• Severe renal failure (Cl_Cr < 30 mL/min) due to unacceptable risk of drug accumulation and massive hemorrhage.
• Active acute bacterial endocarditis.

Relative

• Planned epidural or spinal anesthesia.
• Low body weight (< 50 kg for prophylactic indications, increased risk of bleeding).
• Severe liver failure with alteration of coagulation factors.

Adverse Effects (ADR)

• Hemorrhage: Primary complication, manifested as gastrointestinal or urinary bleeding or bruising at the injection site.
• Anemia: Reduction in hemoglobin concentration secondary to hidden microbleeds.
• Thrombocytosis and minor thrombocytopenia: Mild self-limiting platelet fluctuations. Virtually free of autoimmune HIT induction (anecdotal reported cases).
• Hypokalemia: Less common than with common heparin but possible.

Interactions

Does not inhibit or induce cytochrome P450 isoenzymes, reducing metabolic interactions. The hemorrhagic effect increases if it is associated with potent hemostasis inhibitors. There is no specific antidote or reversal approved for clinical use for Fondaparinux. Activated prothrombin complex concentrate (FEIBA) or recombinant factor VIIa have been used in cases of life-threatening hemorrhage on a compassionate basis.