Hemophilia A (HA) is an X-linked hereditary bleeding disorder caused by deficiency of coagulation factor (F) VIII activity. One of the greatest complications in the treatment of HA is the development of neutralizing alloantibodies, known as FVIII inhibitors. HA patients who develop FVIII inhibitors have limited treatment options available to them and experience greater disease- and treatment-related burdens than HA patients without FVIII inhibitors. Emicizumab, a recently approved bispecific monoclonal antibody, mimics the function of FVIIIa by bridging FIXa and FX to restore effective hemostasis. Although emicizumab and FVIII show some functional similarities, several key differences influence the results of standard laboratory assays when conducted in the presence of emicizumab, and can result in a misleading interpretation of coagulation assays in emicizumab-treated patients. Here, we discuss current laboratory monitoring methods, including activated partial thromboplastin time, FVIII one-stage clotting assays, FVIII chromogenic assays, and global coagulations assays; address why these conventional methods may be inappropriate for monitoring of HA patients receiving emicizumab; and suggest alternative methods applicable to monitoring HA treatment in an evolving treatment landscape.

The advent of extended half‐life (EHL) recombinant clotting factors and innovative non‐factor replacement therapeutics, such as emicizumab, offers several advantages over existing products for the prophylactic treatment of people living with hemophilia (PwH). These include low annual bleeding rates with less frequent dosing, higher trough plasma concentrations, and a more convenient route of administration. However, increasing use of these therapies poses challenges to clinicians and coagulation laboratories due to the lack of standardized assays for monitoring of hemostatic parameters, and the potential for misinterpretation of test results, which may jeopardize patient safety. Definitive diagnosis of hemophilia and treatment monitoring is reliant on demonstrating factor VIII (FVIII; hemophilia A) or factor IX (FIX; hemophilia B) deficiency using a functional coagulation assay. The most frequently used assays are based on activated partial thromboplastin time, using a one‐stage or two‐stage process. While one‐stage and chromogenic assays have performed well with human‐derived FVIII and FIX and full‐length recombinant products, EHL recombinant factors are heterogeneous in structure and mode of action and therefore show wide variation in activity levels between different one‐stage assays, and between one‐stage and chromogenic assays. In the context of the recommended stepwise approach for laboratory diagnosis of hemophilia, we examine the diagnostic challenges associated with the use of EHL factors and novel non‐factor therapeutics and consider the optimal diagnostic approach in PwH who are receiving these treatments. Ultimately, accurate diagnostic solutions are a prerequisite for personalized therapy to minimize treatment burden and improve quality of life in PwH.

Ibrutinib and acalabrutinib are approved for B cell malignancies and novel Bruton's tyrosine kinase (Btk) inhibitors undergo clinical testing also in B cell-driven autoimmune disorders. Btk in platelets mediates platelet activation via glycoprotein (GP) VI, which is crucial for atherosclerotic plaque-induced platelet thrombus formation. This can be selectively inhibited by Btk inhibitors. Since patients on second-generation Btk inhibitors apparently show less bleeding than patients on ibrutinib, we compared the effects of ibrutinib and four novel irreversible Btk inhibitors on GPVI-dependent platelet aggregation in blood and in vitro bleeding time. Low concentrations of collagen which induced the same low degree of GPVI-mediated platelet aggregation as atherosclerotic plaque material were applied. IC50 values for collagen (0.2–0.5 µg/mL)-induced platelet aggregation after 15-minute pre-incubation were: ibrutinib 0.12 µM, BGB-3111 0.51 µM, acalabrutinib 1.21 µM, ONO/GS-4059 1.20 µM and evobrutinib 5.84 µM. Peak venous plasma concentrations of ibrutinib (0.5 µM), acalabrutinib (2 µM) and ONO/GS-4059 (2 µM) measured after anti-proliferative dosage inhibited collagen-induced platelet aggregation, but did not increase PFA-200 closure time on collagen/epinephrine. Closure times were moderately increased by 2- to 2.5-fold higher concentrations of these inhibitors, but not by BGB-3111 (1 µM) and evobrutinib (10 µM). Prolonging platelet drug exposure to 60 minutes lowered IC50 values of any Btk inhibitor for GPVI-mediated aggregation by several fold, and 5- to 10-fold below anti-proliferative therapeutic drug plasma levels. In conclusion, low blood concentrations of ibrutinib and the novel Btk inhibitors suffice for GPVI selective platelet inhibition relevant for atherothrombosis but do not impair primary haemostasis.

The prognosis of acquired haemophilia A (AHA) is severe and treatment options are limited. Emicizumab is a novel bispecific humanized monoclonal antibody in the treatment of inherited AHA with inhibitors. An 83-year-old AHA patient with congestive heart failure and a high risk for thromboembolic and cardiac events who had initially been treated successfully with steroids and substitution of recombinant B-domain-deleted porcine FVIII developed severe bleeding complications and a secondary increase in inhibitor titres after 4 weeks of treatment. Conventional therapeutic strategies failed, and the patient was subsequently treated with emicizumab on off-label and named patient use premises. After the application of emicizumab, the clinical conditions stabilized and no further substitution of coagulation factors was needed. The patient could be discharged and survived 36 days in a cardiac rehabilitation centre without indications for spontaneous bleeding or thromboembolic events. We suggest that the effects of emicizumab in acquired haemophilia should be evaluated in clinical trials.

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