Targeting tissue factor as an antithrombotic strategy.
It is generally accepted that the initial event in coagulation and intravascular thrombus formation is the exposure of cell-surface protein, such as tissue factor (TF). TF is exposed to the flowing blood as a consequence of vascular injury induced, for instance, by PTCA, or by spontaneous rupture of an atherosclerotic plaque. Expression of TF may also be induced in monocytes and endothelial cells in several conditions such as sepsis and cancer, causing a more generalized activation of clotting. In addition to its essential role in hemostasis, TF may be also implicated in several pathophysiological processes, such as intracellular signaling, cell proliferation, and inflammation. For all these reasons, TF has been the subject of intense research focus. Many experimental studies have demonstrated that inhibition of TF:factor VIIa procoagulant activity is a powerful inhibitor of in vivo thrombosis and that this approach usually results in a less-pronounced bleeding tendency compared with other "more classical" antithrombotic interventions. Alternative approaches may be represented by antibodies directed against TF, by transfection of the arterial wall with natural inhibitors of the TF:factor VIIa complex, such as the TF pathway inhibitor, or with catalytic RNA (ribozyme), which could inhibit the expression of the TF protein by the disruption of cellular TF mRNA. All these approaches seem particularly attractive because they may result in complete inhibition of local thrombosis without incurring potentially harmful systemic effects. Further studies are warranted to determine the efficacy and safety of such approaches in patients.