High and low affinity heparin compared with unfractionated heparin as antithrombotic drugs.

A preparation of heparin was separated by affinity chromatography into two fractions: one of high ( HAH ) and the other of low (LAH) affinity to antithrombin III. These two fractions were compared with unfractionated heparin ( UFH ) by in vitro assay and their ability to impair experimental stasis thrombosis was also examined. Although the in vitro activity of HAH was double that of UFH , HAH was less effective than UFH as an antithrombotic drug; LAH was virtually inactive, both in vitro and in vivo. A mixture of 30 micrograms/kg of HAH and 50 micrograms/kg of LAH was as effective in preventing thrombosis as 80 micrograms/kg of UFH , and was more effective than 40 micrograms/kg of HAH alone, demonstrating that LAH potentiates the action of HAH in vivo.