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PRELIMINARY CHARACTERIZATION OF A “FACTOR VIII BYPASSING COAGULANT”

In the last decade there has been growing interest in the use of concentrates of the vitamin K-dependent factors in the treatment of patients who form antibodies against factor V1II.l-6 This “bypassing activity” not only has clinical importance, it also implies a hitherto undescribed mechanism for the activation of the later stages of coagulation. While increased levels of factors VIIa, IXa, and Xa have been found in the “activated” factor IX concentrates,’, there are strong arguments against any of these activated components being the responsible molecular species. Indeed the only formal trial showing efficacy of factor IX complex used nonactivated preparations.6 The in vifro operational definition of the “bypassing activity” is that it shortens the activated partial thromboplastin time ( APTT) of hemophilic plasma but is presumably neither thrombin nor activated factor X. Its presence in factor IX concentrates implies that it is a vitamin K-dependent coagulant containing y-carboxyglutamyl residues. The studies reported here were done in an effort to produce a more characterized standard-reference preparation for the control of commercially produced concentrates recommended for the treatment of patients who acquire an inhibitor to factor VIII. In producing this material, several assumptions were made, including that the “bypassing activity” is adsorbable on barium, activated during clotting, and stable in the presence of plasma protease inhibitors.