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Hirudin is a 65-amino acid peptide and the most potent and specific known inhibitor of thrombin (Ki=0.2 pM). The short elimination half-life of hirudin from the body (1 hour) necessitates the use of a sustained and controlled delivery system.
A proliposome method was used to entrap hirudin in liposomes coated with palmitoyl dextran-coated liposomes and lipid-assemblies. In vitro release studies of hirudin were performed using the lipid systems enclosed in dialysis membranes or deposited in the pores of a vascular graft. The activity of hirudin and released hirudin was measured using a thrombin chromogenic substrate assay.
Entrapment efficiencies of hirudin in lipid-assemblies approached 100%, however, the release of hirudin from these systems was rapid with 90% released in 17 hours. Entrapment efficiencies of hirudin in coated-liposomes ranged from 5% to 55% and were dependent on several variables. Palmitoyl dextran-coated-liposomes showed a burst of 30% hirudin released in 5 hours with an additional 10% to 35% released over the next 600 hours. In all samples, 30–40% of the hirudin remained associated with the lipid-systems even after 600 hours. The released hirudin retained only 33% of its ability to inhibit thrombin when released from uncoated liposomes. However, hirudin retained 95% of its thrombin inhibitory activity when released from palmitoyl dextran-coated liposomes.
Coated liposomes were found to stabilize hirudin and result in greater retention of hirudin's ability to inhibit thrombin's enzymatic activity, although the mechanism is not yet understood.