Laboratory monitoring of hemophilia A treatments: new challenges

Abstract

Monitoring factor VIII (FVIII) activity has traditionally been complicated by discrepancies between assays for the various sorts of FVIII molecules. The advent of novel nonfactor therapies (emicizumab, fitusiran, and anti-tissue factor pathway inhibitor antibodies) in hemophilia A poses a new level of difficulty on the laboratory monitoring of these patients. To use the correct assays and for a proper interpretation of their results, it is pertinent to understand the mode of action of these nonfactor agents. Furthermore, the biochemical consequences for the different types of activity assays (whether it be specific FVIII activity assays or global coagulation assays) should be taken into account as well. In this review, these aspects will be discussed. In addition, the use of various animal models to estimate FVIII-equivalence of the nonfactor therapies will be presented.

Conflict of interest statement

Conflict-of-interest disclosure: P.J.L. received speaker fees from Biotest, Chugai, Novo Nordisk, Roche, Sanofi, Sobi, and Takeda.

© 2020 by The American Society of Hematology.

Figures

Figure 1.

Comparison of traditional and novel hemophilia therapies. Similarities and differences of hemophilia A treatment between FVIII replacement therapy, emicizumab, fitusiran, and anti-TFPI antibodies with regard to patient target group, mode of administration, and therapeutic levels over time.

Figure 2.

Thrombin generation in FVIII-deficient plasma. Thrombin generation in FVIII-deficient plasma in the presence of emicizumab (A), anti-antithrombin nanobodies (B), or anti-TFPI antibodies (C). In all cases, the addition of the antibodies increases thrombin generation compared with FVIII-deficient plasma alone. (A) Factor XIa-induced thrombin generation in the absence or presence of emicizumab (0-50 μg/mL; P.J.L., unpublished data). (B) Tissue factor-induced thrombin generation in the absence or presence of anti-antithrombin nanobodies. (C) Tissue factor-induced thrombin generation in the absence or presence of polyclonal anti-TFPI antibodies.

Figure 3.

Animal models to estimate FVIII equivalence of nonfactor therapies. Different types of animal models have been used to estimate FVIII equivalence for nonfactor therapies. For emicizumab, a primate acquired hemophilia A model has been used, and a semi-humanized mouse model for FVIII-deficiency. In both models, the use of an FVIII calibrator allowed to estimate the FVIII equivalence to be 10% to 20%. For fitusiran, FVIII-deficient mice displayed a strong reduction of their bleeding tendency. However, no FVIII calibrator curve was used to determine an FVIII equivalence. In an alternative approach using anti-antithrombin nanobodies, inhibition of antithrombin activity appeared to correspond to an FVIII equivalence of ≥20%. Regarding concizumab, a rabbit model for acquired hemophilia has been described. Also in this model, an FVIII calibrator was unavailable. Based on the observed efficiency in the cuticle bleeding model, the FVIII equivalence seems to be ≥20% FVIII.