Anticoagulants inhibit the formation or action of one or more of the clotting factors involved in the sequence of reactions
that causes blood to coagulate. Most
anticoagulants are essentially prophylactic, which means that they are used in the treatment of existing
thrombosis (clot, or thrombus, formation) only to inhibit further clotting. Thrombolytic drugs relieve clots that have already formed. Heparin and the oral anticoagulants warfarin and dicumarol are equally effective in treating venous thromboembolisms, but only heparin is useful in treating arterial thrombosis. Thrombolytics developed in the 1980s include streptokinase, an enzyme that can be injected into the coronary artery to stop a heart attack; and tissue plasminogen activator (TPA), a blood protein now available in large quantities through recombinant-DNA techniques. The ongoing development of such drugs has been aided by the 1987 isolation of the body protein that initiates the blood-clotting process.