These bind to the catalytic site of thrombin, bind to thrombin in clot, and are resistant to agents that degrade heparin. Hirudin, a 65-amino-acid peptide, is derived from the medicinal leech and is one of the most potent naturally occurring anticoagulants. A recent large clinical trial was terminated because hirudin resulted in an increased incidence of intracranial bleeding relative to heparin (1.3 vs 0.Z percent). 29 Bivalirudin (Hirulog) reduced the short-term risk of postischemic complications relative to high-dose unfractionated heparin in patients who underwent percutaneous interventions for unstable or postinfarction angina, but benefits did not persist long term.30 Also, a low-molecular-weight thrombin inhibitor, inogatran, does not appear to offer benefits over unfractionated heparin for the treatment of patients with ACS.31 Thus, at this time, there is no clearly defined role for direct thrombin inhibitors in the treatment of ACS.
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