Factor VIII Inhibitor-Bypassing Agents Act by Inducing Thrombin Generation and Can Be Monitored by a Thrombin Generation Assay

Abstract

Factor VIII (FVIII)-bypassing agents have complex modes of action but all control bleeding in inhibitor patients by triggering the generation of thrombin. No routine test is available for monitoring this therapy in patients with inhibitors against FVIII. We present an assay that records FEIBA- or FVIIa-mediated changes in thrombin generation (TG) in FVIII inhibitor plasma samples. In plasma samples spiked with FEIBA TG was normalized above 0.4 U/ml, while for recombinant FVIIa (rFVIIa) more than 12.5 µg/ml were required to induce TG in the absence of tissue factor (TF). Addition of TF increased the TG potential of rFVIIa in vitro. This assay seems suitable for monitoring the pharmacokinetics of inhibitor bypassing agents during treatment and possibly for predicting responses to treatment.

Tests of global haemostasis and their applications in bleeding disorders

“…The process of thrombin generation and fibrin clot formation can be captured with greater sensitivity and completeness by tests that measure global haemostasis. These include the thrombin generation tests/assay (TGT/ TGA) [5,7], thromboelastography(TEG) [8] and the activated partial thromboplastin time (APTT) waveform analysis (WA) [9] using different instrument systems. These tests have not only helped in more complete assessment of the process of normal haemostasis but have also provided newer insights into the evaluation of disorders of haemostasis.…”

Recollections on thrombin generation

“…Several studies suggest that bleeding tendency in hemophilia and other congenital clotting factor deficiencies is better indicated by TG than by factor levels [49,50]; and possibly better by TG in PRP than in PPP [51]. TG reflects the effect of inhibitor bypassing therapy in hemophiliacs [52,53] and has been used as a guidance for prophylaxis during operations [54]. As mentioned above, the main clinical application probably will be the control of antithrombotic therapy, both for new and existing drugs and maybe also in preoperative screening.…”

The use of thromboelastography, thrombin generation test and clotting tests to evaluate the efficacy of hemostatic therapy with recombinant activated factor VII in hemophilia patients with inhibitor

“…Стандартные клоттинговые тесты изменяются после введения rFVIIa. Активиро-ванное частичное тромбопластиновое время (АЧТВ) у больных с ингибиторной формой гемофилии укора-чивается под действием rFVIIa, однако, как правило, не нормализуется, укорочение протромбинового вре-мени (ПВ) отражает активацию внешнего пути сверты-вания, которая может не коррелировать с гемостатиче-ским эффектом rFVIIa [3][4][5][6][7].…”

“…В этой связи для оценки эффективности препа-рата используются интегральные тесты, в частности тромбоэластография (ТЭГ), тромбоэластометрия (РОТЭМ), тест генерации тромбина, которые позво-ляют даже титровать дозу rFVIIa во время оператив-ных вмешательств у больных с ингибиторной формой гемофилии [3,5,8,9].…”