Origin and Physicochemical Properties
Low-molecular-weight heparins are produced by depolymerization of conventional (unfractionated) heparin extracted from porcine intestinal mucosa. This process yields shortened polysaccharide chains.
- Chain length: 17 or fewer saccharide units.
- Molecular weight: 1,000 to 10,000 Daltons (average 4,000–5,000 Da).
LMWH preparations are heterogeneous mixtures of various fractions, meaning they differ in composition, molecular weight, and biological activity. For example, the molecular weight of dalteparin sodium is 5,000–6,000 Da, whereas for nadroparin and reviparin, it ranges from 3,900 to 4,500 Da.
Mechanism of Action and Selectivity
Like standard heparin, LMWHs act by activating antithrombin III. However, their shorter chain length fundamentally alters drug specificity.
Unfractionated heparin simultaneously binds both antithrombin III and thrombin (factor IIa). Most LMWH chains are too short to form this ternary complex, but they are long enough to successfully inactivate factor Xa. Consequently, LMWHs inhibit factor Xa 2 to 4 times more strongly than thrombin.
The anti-Xa to anti-thrombin activity ratio varies by specific agent:
- Enoxaparin sodium — 3.8:1.
- Nadroparin and reviparin — 3.2–3.5:1.
- Dalteparin sodium — 2.2–2.7:1.
Advantages Over Standard Heparin
Low-molecular-weight fractions bind significantly less to plasma proteins, vascular endothelium, and macrophages, ensuring a predictable and stable anticoagulant response.
Key pharmacokinetic differences:
- High bioavailability. Subcutaneous bioavailability reaches ≥90%, whereas standard heparin does not exceed 30%.
- Longer duration of action. Drugs act longer, allowing for administration only 1–2 times daily.
- Ease of monitoring. Because the bleeding risk is lower, patients do not require routine laboratory monitoring of coagulation parameters (such as aPTT).
Additionally, unlike unfractionated heparin, LMWHs cause virtually no platelet aggregation inhibition. At the same time, they effectively stimulate fibrinolysis by releasing tissue plasminogen activator from the endothelium.
Clinical Application
LMWH dosage is expressed in International Units (IU) of anti-Xa activity. The primary route of administration is subcutaneous, though intravenous use is possible when a rapid effect is needed (onset within 5 minutes).
Main indications:
- Prevention and treatment of deep vein thrombosis (especially postoperatively).
- Prophylaxis of pulmonary embolism (PE).
- Acute coronary syndrome (myocardial infarction, unstable angina).
LMWHs are particularly important in obstetric practice. They practically do not cross the placental barrier, making them a safe choice for treating and preventing thrombosis in pregnant patients.
Important rule: LMWH agents are not interchangeable. Substituting one agent for another during a treatment course is not recommended.
Safety Profile and Adverse Effects
LMWH therapy has a favorable safety profile but carries inherent risks:
- Bleeding: Occurs significantly less frequently than with unfractionated heparin therapy.
- Thrombocytopenia: Mild form may occur in the first days. Severe immune-mediated heparin-induced thrombocytopenia is rare (less than 1% of cases) because LMWHs form fewer immunogenic complexes with platelet factor 4.
- Other reactions: Allergic manifestations and elevated liver transaminases may occur. The risk of osteoporosis is lower than with standard heparin.
In case of overdose, protamine sulfate is used as an antidote. Note that it is less effective against LMWHs because protamine primarily neutralizes long-chain fragments, which are scarce in low-molecular-weight preparations.