Coagulant potentials of emicizumab in the plasmas from infant and toddler patients with hemophilia A

Background Emicizumab significantly reduces bleedings in patients with hemophilia A (PwHA). A clinical study (HAVEN 7; NCT04431726) for PwHA aged less than or equal to 12 months is ongoing, but emicizumab‐driven coagulation potential in PwHA in early childhood remains to be clarified. Aim To investigate the in vitro or in vivo coagulation potential of emicizumab in plasmas obtained from infant and toddler PwHA. Methods Twenty‐seven plasma samples from 14 infant/toddler PwHA (aged 0–42 months, median 19 months) who received emicizumab (n = 9), factor (F)VIII products (n = 8), or no treatment (n = 10) were obtained. FVIII activity in FVIII‐treated plasmas was cancelled by the addition of anti‐FVIII monoclonal antibody (mAb). Emicizumab‐treated plasmas (in vivo) and emicizumab‐spiked plasmas (in vitro) were analyzed. Emicizumab‐untreated plasma or emicizumab‐treated plasma supplemented with two anti‐emicizumab mAbs were used as references. Adjusted maximum coagulation velocity (Ad|min1|) by clot waveform analysis and peak thrombin (Peak‐Th) by thrombin generation assay was assessed. Results Ad|min1| values in 24 samples were improved by the presence of emicizumab. Values did not improve in the three remaining samples (aged 1, 23, and 31 months). Although the presence of emicizumab showed an age‐dependent increase in Peak‐Th in 20 samples, this increase was not observed in seven samples (aged 0, 1, 1, 2, 8, 19, and 36 months). Emicizumab‐dependent increases in both Ad|min1| and Peak‐Th were shown in 18 samples, and increases in either parameter were shown in eight samples. One sample (from patient aged 1 month) showed no increase in both, however. Conclusion Emicizumab could improve coagulant potential in plasmas from infant/toddler patients with hemophilia A.