Coagulation activation in sickle cell trait: an exploratory study.

Recent epidemiologic data suggest that sickle cell trait (HbAS; AS) is a risk factor for venous thromboembolism. We conducted an exploratory study of healthy subjects with AS under baseline conditions to determine whether a chronic basal hyperactivation of coagulation exists, and if so, what mechanism(s) contribute to this state. Eighteen healthy AS individuals were compared to 22 African-American controls with a normal haemoglobin profile (HbAA; AA) and 17 patients with sickle cell disease (HbSS; SS). Plasma thrombin-antithrombin complexes and D-dimer levels were elevated in AS relative to AA patients (P = 0·0385 and P = 0·017, respectively), and as expected, were much higher in SSversusAA (P < 0·0001 for both). Thrombin generation in platelet poor plasma was indistinguishable between AA and AS subjects, whereas a paradoxical decrease in endogenous thrombin potential was observed in SS (P ≤ 0·0001). Whole blood tissue factor was elevated in SS compared to AA (P = 0·005), but did not differ between AA and AS. Plasma microparticle tissue factor activity was non-significantly elevated in AS (P = 0·051), but was clearly elevated in SS patients (P = 0·004) when compared to AA controls. Further studies in larger cohorts of subjects with sickle cell trait are needed to confirm the results of this preliminary investigation.

Studies on the mechanism of action of the aptamer BAX499, an inhibitor of tissue factor pathway inhibitor.

INTRODUCTION: Promoting thrombin generation by inhibiting tissue factor pathway inhibitor (TFPI) is a potentially viable therapeutic approach to the prevention and/or treatment of bleeding in hemophilia. In this report, we studied the interaction between an aptamer (BAX499; formerly ARC19499) and TFPI that resulted in inhibition of TFPI-mediated regulation of the tissue factor pathway. MATERIALS AND METHODS: Enzyme kinetic analyses were performed to study the interaction between BAX499 and recombinant TFPI against factor Xa, the extrinsic Xase and prothrombinase activities. Diluted prothrombin time assay was used to investigate the effects of BAX499 on factor VIII-deficient plasma collected from hemophilia patients. RESULTS: Our results indicate that after binding of BAX499 to TFPI, the TFPI/ BAX499 complex retains factor Xa inhibitory activity, albeit with reduced affinity. When tested in an extrinsic Xase activity assay, BAX499 delayed TFPI-mediated inhibition of extrinsic Xase activity. In addition, BAX499 reversed TFPI inhibition of the prothrombinase complex. BAX499 shortened the dilute prothrombin time in factor VIII-deficient plasma, and when added to freshly drawn hemophilia A blood either with or without a factor VIII inhibitor, the whole blood clotting time was also shortened. These results suggest that BAX499 may be a useful addition to the armamentarium of bypassing agents to control bleeding in hemophilic patients with inhibitors.

Membrane microparticles in VTE and cancer.

Microparticles (MPs) are cellular vesicles produced by all cells in response to apoptosis or cellular activation. In this brief review, the evidence that MPs mediate the thrombotic propensity that characterizes patients with cancer is evaluated. It is concluded that while considerable data exist to support a critical pathophysiologic role of host or tumor cell-derived MPs in cancer-associated thrombosis, causality is not yet firmly established. The results of prospective clinical studies that are currently underway should clarify any causative relationship.

Tissue factor pathway inhibitor reduces experimental lung metastasis of B16 melanoma.

The importance of tissue factor (TF) in tumor biology has been highlighted by studies suggesting its involvement in cell signaling, metastasis and angiogenesis. Since many animal studies have shown that anticoagulant therapy can reduce experimental metastasis, we studied whether the natural inhibitor of TF-mediated blood coagulation, Tissue Factor Pathway Inhibitor (TFPI), might be similarly effective. Using a murine experimental model, we found that intravenous injection of recombinant murine TFPI immediately before introduction of tumor cells reduced metastasis by 83% (P < 0.001). B16 murine melanoma cells stably transfected with a TFPI expression vector exhibited reduced lung seeding following intravenous injection by 81% (P < 0.001) compared with controls. No difference in primary tumor growth was observed between TFPI+ and control cells. Mice receiving intravenous somatic gene transfer of sense TFPI expression vector developed 78% fewer lung nodules than controls (P < 0.05). We conclude that TFPI has significant anti-metastatic activity in this experimental model.