Efficacy of emicizumab is maintained throughout dosing intervals for bleed prophylaxis

Background Across the HAVEN clinical trial program, the efficacy of emicizumab has been demonstrated in children, adolescents, and adults with hemophilia A, with or without factor VIII inhibitors. After the 4-week loading dose period, emicizumab concentrations are expected to remain at levels that provide bleed protection throughout the entire dosing interval, regardless of the chosen maintenance dosing regimen, ie, weekly, every 2 weeks, or every 4 weeks. Objectives The objective of this study was to examine the timing of treated bleeds within the dosing intervals for emicizumab administered during the HAVEN 1 to 4 studies. Methods In this post hoc analysis, we pooled data from all the participants of the HAVEN 1 to 4 studies and analyzed the timing of treated bleeds in relation to the emicizumab dose. Results A total of 392 participants were included in this analysis, with a median (range) age of 28.0 years (1.1-77.0 years). Target joints were identified in 237 of 392 (60.5%) participants before the study entry. Overall, 211 of 392 (53.8%) participants experienced 907 treated bleeding events. The total mean (SD) annualized bleeding rate across the 4 studies was 1.6 (5.9). There was no evidence that bleeding events clustered on any 1 particular day in any dosing schedule from HAVEN 1 to 4 (P > .05 for all 3 treatment regimens). Conclusion Data from the HAVEN 1 to 4 trials show consistent bleed prevention within the dosing interval, regardless of the dosing regimen chosen. These findings provide further evidence of the sustained efficacy of emicizumab across all approved dosing regimens to reduce bleeding in people with hemophilia A.


| I N T R O D U C T I O N
People with hemophilia A have an X-linked inherited bleeding disorder characterized by a deficiency in coagulation factor (F)VIII, leading to frequent spontaneous and traumatic bleeding, typically into the joints, muscles, and soft tissues [1]. Recurrent bleeds, particularly into the joints, leads to long-term complications, such as debilitating hemophilic arthropathy and an overall reduced quality of life [1][2][3].
For those with severe hemophilia A (circulating FVIII level of <1 IU/ dL) [4], prophylaxis with clotting factor concentrates or hemostatic products, such as emicizumab, is recommended to prevent bleeding [1].
However, a severe bleeding phenotype rather than factor levels is becoming increasingly recognized as an indication for prophylactic treatment [1] because those with baseline endogenous FVIII levels associated with mild and moderate hemophilia A can still experience joint bleeds and develop significant arthropathy [5]. Prophylaxis initiation before 3 years of age is advised to help prevent life-threatening bleeding, such as intracranial hemorrhage, and long-term musculoskeletal complications that can occur because of recurrent joint and muscle bleeds over time [1,6,7]. However, early prophylaxis with standard FVIII prophylaxis has been found insufficient to fully prevent joint damage through adolescence in severe hemophilia A [7].
Owing to the short half-lives of all replacement therapies (approximately 10-16 hours), traditional methods for prophylaxis require multiple intravenous FVIII infusions per week to replace FVIII at levels high enough to provide sufficient protection against bleeds [8][9][10]. People with hemophilia A are more vulnerable to bleeding when FVIII activity drops below a certain level [11]; historically, trough levels of 1 IU/dL have been targeted [9,11,12]. Increasing amounts of data [13], acknowledged by the recent World Federation of Hemophilia guidelines [1], have indicated that patients remain at risk of occasional clinical and subclinical bleeding with trough levels at 1%, and a trough level of 3% to 5% or higher is more effective in protecting against bleeds and long-term joint damage [1,9,13,14]. Higher trough levels, however, require higher doses or more frequent infusions, which increases costs [1] and can be burdensome to individuals with hemophilia A in terms of psychosocial well-being, physical health, and employment. Therefore, strategies that achieve steady-state hemostasis may be more effective in protecting against bleeds over time [6].
Emicizumab is a subcutaneously administered, bispecific, humanized monoclonal antibody that substitutes for missing or defective FVIII by bridging activated FIX and FX to allow the coagulation cascade to continue in people with hemophilia A [1,15,16]. It can be administered in any of the following 3 dosing regimens: 1.5 mg/kg once weekly, 3 mg/kg every 2 weeks, or 6 mg/kg every 4 weeks. Emicizumab has a long halflife of approximately 4 to 5 weeks [16] and persists in circulation for many months [17]. Plasma emicizumab trough concentrations of approximately 30 μg/mL are maintained whether administered once weekly, every 2 weeks, or every 4 weeks, providing steady-state hemostasis [18][19][20][21][22]. This analysis delves further into the pharmacodynamic efficacy provided by emicizumab to investigate whether there is evidence of waning protection at any given time during the dosing period for any of the 3 approved dosing regimens.  [18][19][20][21]. Participants were defined as having FVIII inhibitors if they had inhibitor titers of ≥0.6 Bethesda units/mL, as measured by the chromogenic Bethesda assay.

| Ethical considerations
All adult participants provided written informed consent before entering the HAVEN studies. Adolescent participants (12-17 years of age) provided written informed assent for the HAVEN studies and children aged 8 to 11 years also provided informed assent for HAVEN 2. Written informed consent was also obtained from parents or legally acceptable representatives in HAVEN 2. All study protocols were approved by the independent ethics committee or institutional review board at each participating institution and were conducted in accordance with the principles of the Declaration of Helsinki and Good Clinical Practice.

| Outcomes and data collection
The primary focus of this analysis was the timing of treated bleeding events within the dosing intervals for emicizumab, which were analyzed as the number and percentage of occurrences during different time

Essentials
• Emicizumab is approved for subcutaneous prophylaxis weekly, every 2 weeks, or every 4 weeks.
• We analyzed the number and timing of treated bleeds in 392 participants from HAVEN 1 to 4.
• Bleeds were not clustered on any 1 particular day in any of the 3 dosing schedules.
• Analysis demonstrated consistent bleed prevention, regardless of the dosing regimen used. intervals since the last dose. A treated bleed was defined as a bleed followed by treatment that promotes hemostasis; bleeds due to surgery or procedures were excluded [23]. Two bleeds of the same type (eg, joint, muscle, or other) and at the same anatomical location were considered 1 bleed if the second occurred within 72 hours of the treatment for the first bleed. The timing of the treated bleeding events that occurred during the 4-week loading dose phase among participants in HAVEN 1 to 4, regardless of the inhibitor status, was also analyzed.
The number of treated bleeds associated with previously missed doses of emicizumab was also captured for all 3 regimens. Missed doses are defined as the following: more than 10 days between 2 doses for the weekly regimen (4-day window), more than 21 days between doses for the "every 2 weeks" regimen (7-day window), and more than 42 days between doses for the "every 4 weeks" regimen (14-day window).
Data were collected by using the same methodology across all 4 studies; bleeds and treatment were self-reported by using the Bleed and Medication Questionnaire. Participants with hemophilia A receiving emicizumab by either treatment assignment or switch to emicizumab following the initial assignment of no prophylaxis were included in the analysis. For participants with changes to their dose, only data from the original dosing regimen were included.

| Data analysis
Bleed analyses were summarized for the entire dosing period, including the loading (3 mg/kg once weekly for 4 weeks) and maintenance phases (1.5 mg/kg once weekly, 3 mg/kg every 2 weeks, or 6 mg/kg every 4 weeks). Annualized bleed rates (ABR; both overall treated and treated traumatic bleed rates) were calculated at the patient level by dividing the individual observed bleeds by the respective exposure time, as a fraction of a year. Taking the average of these numbers across all participants produced the mean ABR and taking the median of these numbers produced the median ABR. Each treated bleed was categorized as having occurred on a given day of the participant's dosing interval from the last emicizumab dose (eg, days 1-7 for those treated weekly, day 1-14 for those treated every 2 weeks, and days 1-28 for those treated every 4 weeks). For any analyses related to the day in which a bleed occurred, including those in the subgroup analyses, the percentage of the bleeds falling on a given day was calculated and summarized.
Sensitivity analyses of the bleeding rates in the individual studies found no discernible differences between the studies; accordingly, adjustments to the pooled analysis on the basis of inter-study differences were not warranted. Similarly, no substantial differences in the patterns of bleeding were observed in a sensitivity analysis between those on emicizumab at the outset and those who switched to emicizumab following an initial assignment of no prophylaxis.
Analysis of the association between the occurrence of bleeds and the day in which a bleed occurred relative to the most recent dose of emicizumab was performed by using a generalized linear model procedure (PROC GENMOD with a repeated statement, negative binomial distribution, and log link), with time as a factor. P values <.05 were considered statistically significant. All analyses were conducted by using SAS v9.4 software.

| Baseline participant and treatment characteristics
In total, 401 participants were enrolled in HAVEN 1 to 4 (Supplementary Clinical trial name and population information and is therefore only included in the loading dose analysis. One participant in HAVEN 3 was assigned to no prophylaxis but was lost to follow up and received no emicizumab treatment; this participant was excluded from all analyses on this basis. Only the HAVEN 4 expansion cohort (n = 41) was included in this analysis; the run-in cohort (n = 7) was excluded because of the use of a different dosing regimen (6 mg/kg every 4 weeks for 24 weeks without a loading dose).
Overall, 392 participants across HAVEN 1 to 4 were included in the present analysis (

| Timing of treated bleeds
In the HAVEN 1-4 pooled analysis, 211 of 392 participants (53.8%) experienced a total of 907 treated bleeding events across the loading and maintenance dose periods (Table 3 and Supplementary Table). The      FVIII, factor VIII; IQR, interquartile range; NE, not evaluable. a One participant in HAVEN 1 discontinued prior to emicizumab treatment and was excluded; another participant discontinued on day 24 of emicizumab due to the physician's decision and is therefore only included in the loading dose analysis. One participant in HAVEN 3 was assigned to no prophylaxis but was lost to follow up and was therefore excluded from all analyses on this basis. Only the HAVEN 4 expansion cohort (n = 41) was included in this analysis; the run-in cohort (n = 7) was excluded due to the use of a different dosing regimen (6 mg/kg once weekly for 24 weeks with no loading dose). b Data represent emicizumab exposure during the efficacy period only. c Two participants included in HAVEN 3 and noted as without FVIII inhibitors in this analysis had negative results on the inhibitor assay conducted locally to determine eligibility, but inhibitors were detected in centrally tested baseline samples (3.7 and 3.1 Bethesda units/mL, respectively). Titers declined spontaneously during the trial for both participants.
T A B L E 3 Number of treated bleeds across the whole treatment period.  The whole treatment period includes the loading phase, which was the same for all regimens (3 mg/kg once weekly for 4 weeks) and the maintenance phases (1.5 mg/kg once weekly, 3 mg/kg every 2 weeks, or 6 mg/kg every 4 weeks).

PIPE ET AL.
F I G U R E 1 Proportion of treated bleeds occurring at different times since the latest dose of emicizumab in the participants of HAVEN 1 to 4 during the maintenance treatment phase. The total percentage of bleeds by days from the most recent dose was calculated within each study (HAVEN 1-4). Bleeds that occurred past the intended dosing interval window are not shown on the bar chart; this includes 14 bleeds for the weekly regimen, 5 bleeds for the "every 2 weeks" regimen, and 2 bleeds for the "every 4 weeks" regimen. A sensitivity analysis identified no significant association between the 3 emicizumab regimens and bleeding patterns observed (P = .8624).
regimens of emicizumab [26]. In the HAVEN program, the relationship between plasma emicizumab concentration levels and ABR almost reaches a plateau above 30 μg/mL, which may suggest that this concentration achieves the optimal effect in terms of providing protection against bleeds [26].
Across HAVEN 1 to 4, 79.6% of participants did not miss a dose of emicizumab, indicating high overall adherence with the study protocols. Across all dosing regimens, only a minority of treated bleeding events occurred in participants who had a previously missed dose. A previously published descriptive assessment of the relationship between emicizumab exposure and the rate of bleeding events among the clinical trial participants identified a minor trend toward a reduction in the ABR with increasing median emicizumab concentration, suggesting that reducing emicizumab exposure to lower levels may potentially increase the risk of bleeding [22]. Because there was only a small percentage of missed emicizumab doses in this analysis (0.4%, n = 159/40,307), we did not identify any trends between missing a dose and subsequent bleeding events requiring treatment with FVIII or bypassing agents.
The "every 4 weeks" regimen has demonstrated slightly lower mean steady-state trough plasma concentrations in the HAVEN 4 study (40 μg/mL) compared with dosing once weekly or every 2 weeks in the HAVEN 1 to 3 studies (52 μg/mL or 48 μg/mL, respectively); however, emicizumab concentrations were maintained at levels associated with efficacy [20,27]. The choice of regimen can therefore be dependent on individual patient preference, considering desired lifestyle, without the need for pharmacokinetic monitoring [20].  [19][20][21]. Furthermore, data from the Phase III STASEY trial were not included in this analysis, and the occurrence and timing of untreated bleeds from the HAVEN 1 to 4 studies were also not considered.