title: The stabilization and release of hirudin from liposomes or lipid-assemblies coated with hydrophobically modified dextran.
abstract: Hirudin is a 65-amino acid peptide and the most potent and specific known inhibitor of thrombin (K(i) = 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.
authors.affiliation: Center for Pharmaceutical Science & Technology, College of Pharmacy, University of Kentucky, Lexington, Kentucky 40536-0082, USA. rjmump2@pop.uky.edu
authors.forename: R J
authors.initials: RJ
authors.lastname: Mumper
authors.forename: A S
authors.initials: AS
authors.lastname: Hoffman
mesh_terms: D003911:Dextrans / Q000737:chemistry* / Q000378:metabolism*; D004337:Drug Carriers / Q000737:chemistry / Q000378:metabolism; D004355:Drug Stability; D006629:Hirudins / Q000737:chemistry* / Q000378:metabolism* / Q000494:pharmacology; D057927:Hydrophobic and Hydrophilic Interactions; D050356:Lipid Metabolism; D008055:Lipids / Q000737:chemistry; D008081:Liposomes / Q000737:chemistry / Q000378:metabolism*; D008567:Membranes, Artificial; D008970:Molecular Weight; D010169:Palmitic Acids / Q000737:chemistry / Q000378:metabolism; D013917:Thrombin / Q000037:antagonists & inhibitors
chemical_list: D003911:Dextrans; D004337:Drug Carriers; D006629:Hirudins; D008055:Lipids; D008081:Liposomes; D008567:Membranes, Artificial; D010169:Palmitic Acids; C010911:dextran phosphate; D013917:Thrombin