Heparin

Mechanism of anticoagulant action

Heparin does not directly affect clotting factors. It works as a cofactor for an endogenous inhibitor—antithrombin III (ATIII). For successful binding, the drug molecule must have a unique pentasaccharide sequence, which is present in only 30% of the molecules in the standard mixture.

By binding to ATIII, heparin alters its spatial structure, which accelerates the inactivation of clotting factors (especially thrombin and factor Xa) by approximately 1,000 times. The efficiency of this process directly depends on the length of the polysaccharide chain:

• Long chains (more than 18 monosaccharides, mass > 5400 Da) act as a "bridge." They are capable of simultaneously holding the ATIII molecule and thrombin (factor IIa), providing antithrombin activity.
• Short chains (less than 18 monosaccharides) cannot hold both molecules. They lack antithrombin activity, but successfully activate ATIII to block factor Xa.

Standard unfractionated heparin contains predominantly long chains, thus inhibiting thrombin and factor Xa equally (1:1).

Pleiotropic (non-anticoagulant) effects

In addition to interfering with the coagulation cascade, the drug possesses several additional pharmacological properties:

1. Anti-atherosclerotic action: stimulates the release of lipoprotein lipase, accelerating the hydrolysis of triglycerides, and inhibits the proliferation of vascular smooth muscle cells.
2. Anti-inflammatory effect: inhibits the enzyme hyaluronidase and reduces vascular wall permeability.
3. Fibrinolytic activity: promotes the release of tissue plasminogen activator (tPA) from the endothelium.
4. Effect on bone tissue and electrolytes: with prolonged use, it increases parathyroid hormone activity (increasing the risk of osteoporosis) and suppresses aldosterone synthesis (risk of hyperkalemia).

Pharmacokinetics and routes of administration

The drug is obtained by extraction from the mucous membrane of pig intestines or cattle lungs. It is not used orally (per os) due to a lack of absorption.

The drug does not cross the placental barrier, so it is approved for use in pregnant women. It is metabolized in the liver with the participation of the enzyme heparanase, and breakdown products are excreted by the kidneys. With prolonged therapy, tolerance may develop due to the depletion of endogenous antithrombin III reserves.

Side effects and overdose

The most common complication of therapy is bleeding. In case of a mild overdose, discontinuing the drug is sufficient. In life-threatening hemorrhages, a specific antagonist—protamine sulfate—is administered intravenously, which chemically binds heparin into an insoluble complex.

Of particular danger is heparin-induced thrombocytopenia (HIT). It occurs in two types:

• Early (benign): occurs on days 2–4, is mild, and does not require discontinuation of therapy.
• Late (immune-mediated): develops on days 5–12. The body produces IgG antibodies against the complex of heparin and platelet factor 4. These antibodies damage the endothelium and cause massive platelet activation. A paradoxical reaction occurs: against the background of a drop in platelet count, severe thromboses develop (infarctions, strokes, gangrene). The condition requires immediate discontinuation of the drug and switching to alternative anticoagulants (e.g., lepirudin).

Topical application

The drug is widely used in the form of ointments and gels for local action. As part of combination products (e.g., with benzocaine and nicotinic acid benzyl ester), it provides antithrombotic, anti-edematous, and analgesic effects. The main indications for external use are subcutaneous hematomas, varicose veins, and superficial thrombophlebitis.

Class and exam questions

What else to study on this topic

• Biochemical structure of the pentasaccharide sequence and its role in antithrombin III activation.
• Differences in the pharmacokinetics of unfractionated heparin versus low-molecular-weight heparins (LMWHs).
• Mechanisms of tolerance development to anticoagulants during long-term therapy.
• Pathogenesis of immune heparin-induced thrombocytopenia and the role of platelet factor 4.
• Specifics of protamine sulfate administration and risks associated with its overdose.

Other topics in the section "Drugs affecting hemostasis and thrombosis"

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