Fondaparinux is a synthetic pentasaccharide anticoagulant. Unlike unfractionated heparin, it cannot be reversed by protamine. What structural-functional reason explains protamine's inability to neutralize fondaparinux?

• A Fondaparinux is too small to form a stable electrostatic complex with protamine's positively charged arginine residues ✓
• B Fondaparinux lacks the 18-saccharide chain necessary for simultaneous thrombin binding, so protamine has no binding target
• C Fondaparinux is renally excreted before protamine can bind it in the circulation
• D Fondaparinux directly inhibits thrombin and protamine reverses only Xa inhibitors

Explanation

Protamine is a basic protein that neutralizes heparin via electrostatic binding — its positively charged arginine residues bind to the negatively charged sulfate and carboxylate groups of heparin chains, forming stable complexes. Unfractionated heparin (MW ~15,000 Da, ~50 saccharide units) and LMWH (MW ~4,500–5,000 Da, ~13–22 saccharide units) are large enough to form stable complexes with protamine. Fondaparinux is a synthetic pentasaccharide of only 5 monosaccharide units (MW ~1,728 Da) — too small to form a stable, neutralizing complex with protamine sulfate. This is a pure molecular size/structural reason. Andexanet alfa (anti-Xa reversal agent) can partially neutralize fondaparinux but is not the specific reversal agent of choice.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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