Once-weekly subcutaneous administration of emicizumab markedly decreased the bleeding rate in patients who had hemophilia A with or without factor VIII inhibitors. (Funded by Chugai Pharmaceutical; JapicCTI number, 121934.).

Introduction Emicizumab is a recombinant humanized bispecific monoclonal antibody mimicking the cofactor function of activated factor VIII. Aim In this multicentre, open‐label study (HOHOEMI), we evaluated the efficacy, safety and pharmacokinetics of emicizumab in Japanese paediatric patients aged <12 years with severe haemophilia A without factor VIII (FVIII) inhibitors. Methods Emicizumab was administered subcutaneously, with four loading doses of 3 mg/kg every week followed by maintenance doses of 3 mg/kg every 2 weeks (Q2W) or 6 mg/kg every 4 weeks (Q4W) in 6 and 7 patients, respectively. Results All patients completed at least 24 weeks of treatment. Baseline ages ranged from 4 months to 10 years, and all patients had been treated with FVIII prophylaxis prior to enrolment except a 4‐month‐old patient untreated with FVIII previously. In the respective Q2W and Q4W cohorts, 2/6 and 5/7 patients experienced no treated bleeding events, and annualized bleeding rates for treated bleeding events were 1.3 (95% confidence interval [CI], 0.6‐2.9) and 0.7 (95% CI, 0.2‐2.6). All caregivers preferred emicizumab to the patient's previous treatment. Only one related adverse event (injection site reaction) was observed. There were no thromboembolic events or thrombotic microangiopathy. Individual trough plasma concentrations of emicizumab were within the variability observed in preceding adult/adolescent studies. All patients tested negative for anti‐emicizumab antibodies. Conclusions Emicizumab administered Q2W or Q4W was efficacious and safe in paediatric patients with severe haemophilia A without inhibitors. This study was registered at http://www.clinicaltrials.jp (JapicCTI‐173710).

Summary Emicizumab, a humanised bispecific antibody recognising factors (F) IX/IXa and X/Xa, can accelerate FIXa-catalysed FX activation by bridging FIXa and FX in a manner similar to FVIIIa. However, details of the emicizumab–antigen interactions have not been reported so far. In this study, we first showed by surface plasmon resonance analysis that emicizumab bound FIX, FIXa, FX, and FXa with moderate affinities ( K D = 1.58, 1.52, 1.85, and 0.978 μM, respectively). We next showed by immunoblotting analysis that emicizumab recognised the antigens’ epidermal growth factor (EGF)-like domains. We then performed K D -based simulation of equilibrium states in plasma for quantitatively predicting the ways that emicizumab would interact with the antigens. The simulation predicted that only a small part of plasma FIX, FX, and emicizumab would form antigen-bridging FIX–emicizumab–FX ternary complex, of which concentration would form a bell-shaped relationship with emicizumab concentration. The bell-shaped concentration dependency was reproduced by plasma thrombin generation assays, suggesting that the plasma concentration of the ternary complex would correlate with emicizumab’s cofactor activity. The simulation also predicted that at 10.0–100 μg/ml of emicizumab–levels shown in a previous study to be clinically effective–the majority of plasma FIX, FX, and emicizumab would exist as monomers. In conclusion, emicizumab binds FIX/FIXa and FX/FXa with micromolar affinities at their EGF-like domains. The K D -based simulation predicted that the antigen-bridging ternary complex formed in circulating plasma would correlate with emicizumab’s cofactor activity, and the majority of FIX and FX would be free and available for other coagulation reactions. Institution where the work was carried out: Research Division, Chugai Pharmaceutical Co., Ltd.Supplementary Material to this article is available online at www.thrombosis-online.com.

Key Points Long-term, once-weekly emicizumab was well tolerated with no thromboembolic adverse events in patients with hemophilia A. Long-term, once-weekly emicizumab shows encouraging efficacy irrespective of the presence of FVIII inhibitors in patients with hemophilia A.

Background Emicizumab is an anti-activated factor IX/FX bispecific antibody that mimics activated FVIII cofactor function. Emicizumab does not require activation by thrombin, and its effect on shortening the activated partial thromboplastin time (APTT) is much greater than that of FVIII. Therefore, the APTT has limited utility in hemophilia A (HA) patients treated with emicizumab. Aim To evaluate the global coagulation potential of emicizumab. Methods Clot waveform analysis (CWA) with prothrombin time (PT)/APTT mixed reagents was used to define hemostatic monitoring protocols in HA patients. A modified parameter, adjusted-|min1| (Ad|min1|), was developed. Maximum and minimum percentage transmittance were defined as 100% and 0% in the precoagulation and postcoagulation phases, respectively. Ad|min1| was calculated as an index of the maximum velocity of the coagulation process. Results Ad|min1| obtained with mixed-trigger reagent (PT/APTT/buffer, 1 : 15 : 135) in the presence of emicizumab optimally corresponded to the conversion rate estimated in animals; 0.2-0.4 IU dL equivalent FVIII per 1 μg mL emicizumab). Ex vivo addition of emicizumab to HA plasma with or without inhibitors resulted in concentration-dependent increases in Ad|min1|, with some individual variations. The addition of various concentrations of FVIII to HA plasma mixed with emicizumab resulted in dose-dependent increases in Ad|min1|. Similarly, mixtures of activated prothrombin complex concentrate and emicizumab added to HA plasma resulted in dose-dependent increases in Ad|min1|. In contrast, enhanced coagulation potential appeared to be better defined by the clot time than by Ad|min1| in experiments using recombinant activated FVII. Conclusion The PT/APTT reagent-triggered adjusted CWA could provide a useful means of assessing global coagulation potential in emicizumab-treated HA patients, with enhanced activity neither masking nor being masked by FVIII or bypassing agents.