Post-Immune Tolerance Induction Prophylactic FVIII Infusions May Not be Necessary for Tolerized Hemophilia a Patients on Emicizumab
Authors: Singh Pragya , Mamauag Erica , Nolfi-Donegan Deirdre , Xavier Frederico 
Published: December 2023

Background:
Hemophilia A is a hereditary bleeding disorder characterized by low factor VIII (FVIII) activity. Patients with severe bleeding phenotypes are at risk for significant bleeding-related morbidity and mortality including the development of hemophilic arthropathy. In recent years, prophylaxis with emicizumab, a bispecific monoclonal antibody, has supplanted prophylactic FVIII infusions due to its ease of self-administration, long half-life, and low annual bleeding rate (ABR), independent of tolerization status to alloantibodies against FVIII (inhibitors). However, it is unclear if emicizumab FVIII-mimic activity is sufficient to achieve hemostasis in moderate-severe bleeds, and therefore, FVIII replacement may still be necessary.
Despite its well-established benefits, the use of emicizumab in previously untreated patients (PUPs), and patients with under 20 exposures to FVIII is still not well understood due to a lack of long-term experience and understanding of its impact on the pathophysiology of FVIII inhibitors. Further investigation is also needed regarding the risk of inhibitor recurrence in patients with previous inhibitors and the concomitant use of emicizumab.
At our institution, patients on emicizumab with a history of inhibitors do not continue post-immune tolerance induction prophylactic FVIII infusions. We also initiate emicizumab before 20 exposures to FVIII, and sometimes before immune tolerance induction (ITI). We explored our patient population to understand inhibitor incidence and recurrence amidst emicizumab prophylaxis.
Methods: We performed a single center retrospective analysis of patients served by the Hemophilia Center of Western Pennsylvania. We identified patients with Hemophilia A on emicizumab prophylaxis. Relevant data was collected including demographics, inhibitor status at time of transition to emicizumab and in the available follow-up period, bleeding events after transition, and number of FVIII exposures before and after transition. Inhibitor recurrence was defined as laboratory evidence of inhibitor using chromogenic Bethesda assay and/or unexplained breakthrough bleeding events despite on-demand FVIII infusions.
Results:
Data was reviewed for 36 patients on emicizumab prophylaxis. Three patients were lost to follow-up. Median age was 15.3 years (range 1.2 - 82). Median follow-up time on emicizumab was 3.5 years (range 0.5 - 5). Two patients had &lt; 20 FVIII exposures prior to emicizumab, and one of those had an inhibitor before emicizumab. In total, 15 patients (45%) were diagnosed with an inhibitor prior to emicizumab. Of these, 8 patients (53%) were tolerized prior to emicizumab, and 7 patients (47%) were not tolerized prior to emicizumab. Median peak inhibitor titer was 4.2 Bethesda units (range 0.7 - 281 Bethesda units). After transition to emicizumab, no tolerized patients for which inhibitor status was tested during the follow-up period had inhibitor recurrence, and 1 non-tolerized patient's inhibitor became undetectable after 4 years of emicizumab therapy (the same patient who had developed an inhibitor with &lt; 20 FVIII exposures prior to emicizumab). Mean ABR was 0.33 events/year for tolerized patients (median 0, range 0 - 3); 0.17 events/year for non-tolerized patients (median 0, range 0 - 1); and 0.60 events/year for patients with no history of inhibitor (median 0, range 0 - 4). Mean number of FVIII exposures per year was 0.09 for tolerized patients (median 0, range 0 - 1); 0.07 for non-tolerized patients (median 0, range 0 - 1); and 1.2 for patients with no history of inhibitor (median 0, range 0 - 20).
Conclusion:
In our small cohort, no patient who was tolerized prior to emicizumab had recurrence of clinical or laboratory evidence of inhibitor while on emicizumab therapy, despite infrequent exposure to FVIII infusion. We did not have enough power to determine whether patients with &lt; 20 FVIII exposures before emicizumab developed inhibitors, though one such patient had inhibitor resolution on emicizumab without ITI. Consistent with other works, overall ABRs in all patients on emicizumab therapy were low. These findings add to other studies on the safety of emicizumab and its impact on the natural history of FVIII inhibitors. Our study also suggests that continuing post-ITI prophylactic FVIII infusions in tolerized patients on emicizumab may not be necessary. Larger-scale studies are required to substantiate our findings.