Mechanism of action
By its origin, streptokinase is a foreign protein synthesized by $\beta$-hemolytic Streptococcus.
The most important pharmacological feature of the drug is the indirect activation of fibrinolysis. Streptokinase itself is not an enzyme and does not possess proteolytic activity. Upon entering the systemic circulation, it binds to a plasminogen molecule, forming an equimolar "streptokinase-plasminogen" complex.
It is this newly formed complex that acquires catalytic properties. It begins to affect the remaining (free) plasminogen molecules, converting them into an active proteolytic enzyme — plasmin.
Pharmacological effects
The primary goal of thrombolytic therapy is the dissolution (lysis) of already formed thrombi and reperfusion of ischemic tissues.
The action of streptokinase lacks high selectivity for the thrombus. The drug exerts a systemic effect, activating both the plasminogen localized on fibrin strands within the thrombus and the plasminogen freely circulating in blood plasma.
Excessive plasmin generation in the bloodstream leads to systemic fibrinogenolysis — the destruction of circulating fibrinogen. The fibrinogen degradation products resulting from this process have an additional anticoagulant effect by reducing platelet aggregation.
Indications for use
The drug is prescribed exclusively for acute thromboses. The highest clinical efficacy is achieved when administered within the first hours following a vascular catastrophe.
- ST-elevation acute coronary syndrome (ACS) (coronary thrombosis, myocardial infarction). The optimal "therapeutic window" is 3–6 hours.
- Pulmonary embolism (PE).
- Deep vein thrombosis (DVT).
- Acute peripheral arterial thrombosis.
- Retinal vessel thrombosis.
Side effects and limitations
- Bleeding: The main and most dangerous complication of therapy. It is a direct consequence of systemic fibrinogenolysis and depletion of clotting factors.
- Immunogenicity (antigenic properties): Being a bacterial protein, the drug induces antibody production. There is a high risk of allergic reactions up to anaphylactic shock. The probability of severe immune complications increases critically upon repeat administration.
- Resistance: Circulating antibodies in the blood (following a streptococcal infection or previous administration of a fibrinolytic) can bind and inactivate the drug, nullifying the efficacy of therapy.
- General reactions: Arterial hypotension (hemodynamic disturbances) and dyspeptic disorders (nausea, vomiting) may develop.
Formulations and prescription guidelines
The drug is dosed in international units (IU). It is administered via intravenous drip infusion. Available in ampoules of 250,000 and 500,000 IU. The infusion solution is prepared immediately before use (ex tempore).
Prescription example: Rp.: Streptokinasi 250 000 IU D.t.d. N. 1 S. Dissolve the contents of the ampoule in 50 ml of sterile 0.9% sodium chloride solution (prepare ex tempore). Administer intravenously by drip at a rate of 25–30 drops per minute.
Briefly: answers to topical questions
Which drug is the specific antidote for bleeding caused by streptokinase?
Specific agents for the inactivation of fibrinolytics in case of bleeding:
- Aminocaproic acid — binds to plasminogen, blocks its conversion to plasmin, and prevents the action of already formed plasmin on fibrin.
- Aprotinin — a fibrinolysis inhibitor.
Which anticoagulants are usually combined with streptokinase to prevent rethrombosis?
Sources mention heparin-group anticoagulants for combination with streptokinase:
- Sodium heparin — indicated along with streptokinase in the treatment of embolism.
- Enoxaparin sodium — there is evidence of its utility when using streptokinase in patients with acute myocardial infarction.
Enoxaparin sodium surpasses sodium heparin in efficacy, but offers no advantages over it in terms of hemorrhagic safety: the risk of bleeding is comparable.
Streptokinase acts as an "indirect systemic cracker": it does not cut the thrombus itself, but recruits plasminogen, forcing it to dismantle all fibrin and fibrinogen in the blood, leaving behind "evidence" (antibodies and allergy risk).
Class and exam questions
Why is it dangerous to administer streptokinase to a patient repeatedly?
The drug is of bacterial origin and possesses high immunogenicity. Upon re-administration, the risk of severe allergic reactions (including anaphylactic shock) increases sharply, and accumulated antibodies can inactivate the drug.
What is the fundamental difference in the mechanism of action between streptokinase and urokinase?
Streptokinase is an indirect activator; it must first form a complex with plasminogen. Urokinase is a direct activator; it independently catalyzes the conversion of plasminogen to plasmin.
What is anistreplase and why is it needed?
This is a modified drug (prodrug) consisting of a complex of streptokinase and acylated plasminogen. The acyl group temporarily blocks the catalytic center, preventing premature activation and prolonging the drug's duration of action.
Why does platelet aggregation decrease during streptokinase therapy?
Due to its systemic effect, a large amount of plasmin is formed, which degrades circulating fibrinogen. Fibrinogen degradation products exhibit antiplatelet activity.
What else to study on this topic
- Comparative pharmacodynamics of systemic (streptokinase, urokinase) and fibrin-specific (alteplase, reteplase/tenecteplase) thrombolytics.
- Pathogenesis of systemic fibrinogenolysis and methods of drug-induced cessation of bleeding in fibrinolytic overdose.
- Pharmacokinetics of anistreplase: dependence of the deacylation rate on the type of acyl group.
- Endogenous fibrinolysis system: physiological role of tissue plasminogen activator and prourokinase.
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