Tissue Factor Pathway Inhibitor Causes Unresponsiveness to Activated Prothrombin Complex Concentrates for Hemophilia A Patients with Inhibitors.

Abstract 3663

Bypassing agents such as activated prothrombin complex concentrates (APCC, FEIBA^®) and recombinant activated factor VII (rFVIIa, NovoSeven^®) are effective for most hemophiliac patients with inhibitors. While, some patients exhibit unresponsiveness to the treatment with APCC and/or rFVIIa, but their mechanisms remain unknown. We had a severe hemophilia A patient with inhibitor whose bleeding worsened despite of consecutive infusion of APCC. Switching from APCC to rFVIIa was very effective for his bleeding symptoms, and one-week cessation of bypassing agents had restored good response for APCC. Comprehensive coagulation assay such as thromboelastometry and thrombin generation test (TGT) provided us clear evidence of unresponsiveness to APCC. In particular, tissue factor (TF)-triggered TGT showed two significant features, the prolonged lag time and reduced peak thrombin level even after APCC infusion. Although FII, FVII(a), FIX, FX(a) and protein C contained in APCC were elevated in his plasmas after APCC infusion, increased amounts of these factors did not affect the parameters of TGT described above in vitro. We focused on a natural anticoagulant, tissue factor pathway inhibitor (TFPI), since the prolonged lag time in TF-triggered TGT might result from the impairment of FVII/TF-induced initial reaction of blood coagulation. In vitro experiment on the addition of TFPI to FVIII-deficient plasma with APCC showed similar inhibitory pattern in TGT. TFPI antigen levels (total and free forms) in his plasma actually increased above normal range after APCC infusion, whilst these levels unchanged after rFVIIa infusion and returned to the normal range after one-week cessation, speculating that TFPI might contributes to unresponsiveness to APCC. To confirm this, plasmas from several hemophiliac patients with APCC and/or rFVIIa infusion, including 4 patients with poor response pattern in TGT, were prepared. Among 12 pairs of plasmas (a pair; pre and post bypassing agents), each of 4 pairs were for APCC-poor response (APCC-PR), APCC-good response (APCC-GR), and rFVIIa-good response (FVIIa-GR). Free form TFPI antigen levels (normal; 15–35 ng/ml) increased after infusion in APCC-PR (pre/post; 38±4/51±3 ng/ml, p<0.05) and APCC-GR (28±4/37±4 ng/ml, p<0.05), but not increased in FVIIa-GR (23±3/21±3 ng/ml, p>0.05). Post-infusion levels in APCC-PR were significantly higher than those in APCC-GR (p<0.05). By adding anti-TFPI antibody, plasmas in APCC-PR showed marked increase of peak thrombin levels than those in APCC-GR in TGT, supporting that APCC-PR possessed more TFPI activity. Unexpectedly, ELISAs revealed that total TFPI were contained in APCC at 24±4 ng/unit (corresponded to 25≂f50% of physiological concentration), and 34% of them were free form, speculating that APCC infusion with ≂f90 units/kg appeared to increase free TFPI by ≂f15 ng/ml in plasma. Taken together, our results supported that TFPI contained in APCC attenuated the potentials of thrombin generation in hemophilia A patients with inhibitors, and some patients exhibited APCC-resistance due to TFPI accumulated by the consecutive use of APCC.