• Hematology Clinical Trials
• Thrombophilia
• Thrombotic Disorders

• Hematology
• Hematology Clinical Trials
• Thrombophilia
• Thrombotic Disorders

The trade-off between antithrombotic effect and bleeding hinders the efficacy of current anticoagulant therapy. An antibody in development has demonstrated dissociation of the antithrombotic effect from the bleeding effect in preclinical studies, showing promise as a revolutionary drug for the treatment of thrombosis, said Trevor Baglin, MB, ChB, PhD, Addenbrookes Hospital, Cambridge, United Kingdom.

He described a naturally occurring antibody discovered in a 54-year-old woman with a chronic subdural hematoma due to a head injury. The patient had a degree of anticoagulation consistent with hemophilia. In vitro coagulation assays showed a prothrombin time of 50 seconds, an activated partial thromboplastin time of 189 seconds, and a thrombin time >60 seconds, which normally would cause a life-threatening bleeding disorder. Yet, the patient recovered fully (no neurologic deficit) with conservative treatment without further intervention. Her only other relevant history was having undergone knee surgery 5 months prior with no bleeding episode; a preoperative clotting screen was not performed at that time.

The patient was found to have an acquired inhibitor of coagulation in the form of a monoclonal paraprotein immunoglobulin (Ig)A, present in her natural serum at a level of >5 g/L⁻¹. The antibody was purified by Jacalin agarose and when added back to plasma, was a potent inhibitor of thrombin, binding with a Kd of 1 nM, with a fast on-rate and slow off-rate. Exosite 1 binding was suggested by inhibition of fibrinogen cleavage but with retained cleavage of a small reporter molecule (a fluororphore). Competitive binding with fluorescently labeled hirugen confirmed exosite 1 as the binding site on thrombin. A Fab fragment was prepared from the antibody and cocrystallized with human PPACK-thrombin, and a 1.9Å structure was solved showing an interaction between the antibody and exosite 1 of thrombin, the main point of contact being CDRH3 of the antibody.

The purified antibody was tested in two animal models of arterial thrombosis. At a maximum concentration of 4 nM, which completely abolished thrombosis, tail bleeding times were not prolonged. At 40nM and higher concentrations of the antibody, fibrin deposition was reduced both early and late. In a murine carotid artery occlusion-model, with a 5% ferric chloride injury, the antibody prevented arterial occlusion and maintained blood flow in 4 of 7 animals. At the maximum concentration studied (400 nM), blood loss from a tail clip was not increased compared with saline controls.

By comparison, at 5% ferric chloride, a dose of heparin (200 U/kg) that causes bleeding in all animals in this thrombosis/bleeding model has no effect on arterial thrombosis, said Dr. Baglin. At 1000 U/kg, heparin reduces occlusion but universally causes fatal bleeding.

He concluded that an antibody that appears to prevent thrombosis without causing bleeding has been identified. A therapeutic derivative of this IgA may potentially permit unlimited dose escalation of antithrombotic therapy without increasing bleeding, given the dissociation of the antithrombotic effect from the bleeding effect.

• © 2013 MD Conference Express®