Heparin MOA
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Heparin — Mechanism of Action
Structure
Heparin is a sulfated polysaccharide (heterogeneous mixture of sulfated mucopolysaccharides) isolated from mammalian mast cell-rich tissues — most commercial preparations come from porcine intestinal mucosa. It is a polymer of alternating D-glucuronic acid and N-acetyl-D-glucosamine residues, with a molecular weight range of 5,000–30,000 Da (unfractionated heparin, UFH).
Core MOA — Antithrombin Activation
Heparin's anticoagulant effect is entirely dependent on antithrombin (AT, formerly antithrombin III), an endogenous serine protease inhibitor (serpin) synthesized in the liver.
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Pentasaccharide binding: Heparin binds to antithrombin via a unique pentasaccharide sequence present on only ~1/3 of the chains in commercial UFH preparations. Chains lacking this sequence have little or no anticoagulant activity.
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Conformational change: Binding induces a conformational change in the reactive center loop of antithrombin, making it more accessible to target proteases.
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Accelerated protease inhibition: Antithrombin normally inhibits clotting proteases slowly. Heparin accelerates this reaction ≥1,000-fold.
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Cofactor recycling: Heparin is not consumed — once the antithrombin-protease complex forms, heparin is released intact to activate more antithrombin.
Factors Inhibited
| Factor | Mechanism Required |
|---|---|
| Thrombin (IIa) | Heparin must bind both antithrombin and thrombin simultaneously (bridging) → requires longer chains (≥18 saccharide units) |
| Factor Xa | Heparin needs only to bind antithrombin (conformational change sufficient); shorter chains work |
| Factors IXa, XIa, XIIa | Also inhibited, but less clinically prominent |
This difference explains why LMWH (short chains) inhibits Xa >> IIa, while UFH inhibits both equally.
UFH vs LMWH vs Fondaparinux
| Drug | Chain Length | Anti-Xa : Anti-IIa ratio |
|---|---|---|
| UFH | 5,000–30,000 Da | ~1:1 |
| LMWH (e.g., enoxaparin, dalteparin) | ~4,000–5,000 Da | ~2–4:1 (mostly Xa) |
| Fondaparinux | Synthetic pentasaccharide only | Xa only (no anti-IIa activity) |
Monitoring
UFH is monitored with aPTT (therapeutic range ~60–100 s, or 1.5–2.5× control). Anti-Xa levels can also be used (therapeutic 0.3–0.7 units/mL).
Reversal
Protamine sulfate — a basic polypeptide that binds heparin with high affinity, forming an inactive complex. 1 mg protamine neutralizes ~100 units of heparin.
Key Adverse Effects
- HIT (Heparin-Induced Thrombocytopenia): IgG antibodies form against heparin-PF4 complexes → platelet Fc receptor activation → platelet microparticles → paradoxical thrombosis. Occurs 5–14 days after initiation.
- Bleeding (major dose-dependent risk)
- Elevated liver transaminases (transient, reversible)
— Harrison's Principles of Internal Medicine, 22E; Katzung's Basic and Clinical Pharmacology, 16th Ed.
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