Comment on Huang et al, page 980
When the knife slips, sometimes the bleeding does not stop. A novel type of tissue factor has been engineered to home to the injury and stop the bleeding.
Some patients have life-threatening bleeding caused by blood coagulation abnormalities that cannot be treated by replacement of a missing protein. Examples include hemophilia patients who have developed antibodies to factor VIII, patients with massive hemorrhage, and patients in whom anticoagulants complicate hemostasis. At this time, recombinant factor VIIa is the most efficacious therapy available. However, it carries a significant risk of thrombosis, particularly stroke.
Tissue factor is the primary physiologic instigator of normal blood coagulation. Tissue factor forms a complex with activated factor VII (factor VIIa). This complex (see figure) activates factor X, and also factor IX, driving the common pathway of blood coagulation. In theory, delivery of active tissue factor to the site of bleeding could be a potent and specific mechanism for staunching hemorrhagic blood loss.
Tissue factor is a transmembrane protein that is primarily expressed on extravascular cells, exposed to liquid blood only after it flows from a ruptured blood vessel. However, a small amount of tissue factor circulates in blood,1 probably located primarily on microvesicles derived from the plasma membranes of activated monocytes.2 This circulating tissue factor is apparently a driving force in thrombus formation within the lumen of minimally injured blood vessels.3
The activity of tissue factor that is exposed to blood is tightly controlled both because its activity is encrypted on a quiescent cell and also because the active form is quickly inhibited. The de-encryption mechanism includes exposure of plasma membrane phosphatidylserine that enhances binding of factor VIIa to tissue factor and activity of the assembled factor VIIa-tissue factor complex toward factor X.4,5 Thus, phosphatidylserine exposure regulates tissue factor activity, and data suggest that it is exposed at sites of vascular injury.
In this issue, Huang and colleagues report that they have joined the procoagulant extramembrane fragment of tissue factor to the phosphatidylserine-binding protein, annexin V. The strategy is based upon the assumptions that cells at the site of vascular injury will expose a phosphatidylserine target for annexin V and that the novel, membrane-bound protein will have clot-instigating potency similar to native, transmembrane tissue factor.
The results in their report must have gratified Huang et al, indicating that the chimeric molecule retains the phosphatidylserine-binding properties of annexin V and the procoagulant properties of the extramembrane fragment of tissue factor. While in solution, the tissue factor-annexin V exhibits very little procoagulant activity. However, upon binding to a membrane with exposed phosphatidylserine, the chimeric protein has procoagulant activity equivalent to intact tissue factor (see figure). In vivo experiments indicate that the tissue factor-annexin V chimera shortens the bleeding time following tail transection in mice. Following heparin treatment, administration of the chimeric protein shortens the prolonged bleeding time toward the normal range. Thus, it appears that this novel therapeutic agent is able to target procoagulant activity to the site of vascular injury.
As always, important questions remain. How safe is the tissue factor-annexin V chimera with regard to thrombosis, particularly stroke? On which cells does the tissue factor-annexin V chimera actually function? Would the pharmacokinetics and the safety profile make this a reagent that might require only a single injection near the time of hemorrhagic risk? The promising information in the current report provides ample motivation to vigorously pursue answers to the remaining questions. ▪