The anticoagulant hirudin is a ~7 kDa acidic protein containing 65 amino acid residues. It occurs naturally in leeches (Hirudo medicinalis). It is the most potent natural inhibitor of both soluble and clot-bound thrombin. Hirudin binds thrombin with high affinity and covers more than 20% of the surface area of thrombin, occluding both the active site and exosite I (fibrinogen and PAR recognition site). This coverage blocks thrombus growth and platelet activation. Hirudin is not metabolized in the bloodstream of humans and is eliminated unchanged via kidney filtration. Native hirudin contains a sulfated tyrosyl residue (Tyr63), three disulfide bridges, and a high proportion of dicarboxylic acids. Hirudin is not glycosylated, and lacks tryptophan, arginine, and methionine residues. At least 20 isoforms have been identified and sequenced.

This recombinant hirudin product corresponds to the HV-1 variant sequence, with the exception that the Tyr⁶³ residue is not sulfated.

Antithrombotic efficacy by parenteral route is substantially prolonged by incorporation into cationic liposomes, which act as a sort of sustained-release device.

Hirudin is soluble in water. The literature cites the use of "Dilution Fluid II" (35.7 mM acetic acid, 35.7 mM sodium diethyl barbiturate, 0.85% NaCl, 1% bovine serum albumin, and 0.5% PEG, pH 7.4) to dissolve hirudin (500 ATU/mL) and thrombin. Hirudin is reported to be soluble in pyridine, but practically insoluble in alcohol, ether, acetone, or benzene.

Protein determined by Lowry.

One unit (ATU) will neutralize one NIH unit of thrombin at 37 °C, based on direct comparison to an NIH thrombin reference standard.