Drugs That Promote Fibrinolysis

An important application of fibrinolytic drugs has been to dissolve thrombi in acutely occluded coronary arteries, thereby to restore blood supply to ischaemic myocardium, to limit necrosis and to improve prognosis. The approach is to give a plasminogen activator intravenously by infusion or by bolus injection in order to increase the formation of the fibrinolytic enzyme plasmin. Those currently available include:

Streptokinase is a protein derived from p-haemolytic streptococci: it forms a complex with plasminogen (bound loosely to fibrin) where it converts plasminogen to plasmin. Too rapid administration causes abrupt fall in blood pressure. The tV2 is 20 min.

Anistreplase (anisoylated plasminogen streptokinase activator complex, APSAC), is the plasminogen-streptokinase complex (above) in which the enzyme centre that converts plasminogen to plasmin is protected from deactivation, so prolonging its action.

Fig. 28.2 Blood fibrinolytic system

The t1/, is 70 min. It is not available in some countries.

Urokinase made from human fetal kidney cells in tissue culture, is a direct activator of plasminogen. The tl/2 is 15 min.

Streptokinase, anistreplase and urokinase are not well absorbed by fibrin thrombi and are called non-fibrin-selective. They convert plasminogen to plasmin in the circulation, which depletes plasma fibrinogen and induces a general hypocoagulant state. This does not reduce their local thrombolytic potential but increases the risk of bleeding.

Recombinant prourokinase, as the name suggests, is produced by recombinant DNA technology; on binding to fibrin it converts to urokinase. The t'/2 is 7 min.

Alteplase (rt-PA) (tV2 5 min) is tissue type plasminogen activator produced by recombinant

DNA technology. Reteplase (tV2 15 min) is another recombinant human protein.

Recombinant prourokinase and alteplase are termed fibrin-selective, for they bind strongly to fibrin, and are capable of dissolving aging or lysis-resistant thrombi better than nonfibrin-selective agents. These drugs are less likely to produce a coagulation disturbance in the plasma, i.e. they are selective for thrombi.

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