Anti-emicizumab antibodies and their relevance in clinical practice.

Background: Emicizumab is licensed for treatment of people with hemophilia A (HA) of all ages, with and without factor (F)VIII inhibitors. It is well tolerated, and most of the treatment-related adverse events are of mild intensity and transient. As for other therapeutic proteins, the potential of emicizumab to induce anti-drug antibodies (ADAs) should be considered when a decrease in treatment efficacy is observed. Data from 7 phase 3/3b pivotal studies showed that 5.1% of treated patients developed ADAs, <1% being activity neutralizing. Among them, 1 case required discontinuation of emicizumab due to loss of treatment efficacy. To date, among several thousands of patients treated with emicizumab, 5 cases of ADAs requiring treatment discontinuation have been reported. Objectives: Monitoring anti-emicizumab antibodies in 67 subjects with congenital HA who switched to emicizumab prophylaxis from FVIII products or bypassing agents. Methods: The anti-emicizumab antibodies were tested, depending on patient availability, at baseline and longitudinally at 5, 10, 20, and 50 weeks after the first dose, as well as when clinically required. Results: ADAs were detected in 4 of 67 cases (5.9%) on at least 2 occasions and were not necessarily associated to significant decreased emicizumab concentration, activated partial thromboplastin time prolongation or bleeding episodes. Only 1 of 4 ADA-positive patients required emicizumab discontinuation due to treatment failure. Conclusion: The present findings confirm that the development of anti-emicizumab antibodies is a rare event, particularly those with neutralizing activity. Routine monitoring should be reserved only for patients with clinical manifestations of bleeding when therapy failure is suspected.