Margareth C. Ozelo, MD, PhD:
To better understand the rationale of novel rebalancing agents for the treatment of bleeding disorders, one must first understand the importance of maintaining a perfect balance of procoagulant proteins and anticoagulant proteins—the principle of hemostasis.

People with hemophilia A do not produce enough FVIII, and those with hemophilia B do not produce enough FIX. Both experience bleeding problems due to the lack of perfect clot formation.

For decades, hemophilia treatment was centered on using factor replacement therapy. We would replace the deficient protein using FVIII or FIX concentrates, which would restore the balance between bleeding and clotting, thus reestablishing hemostasis. Now, novel agents are challenging this standard approach to care by using anticoagulant inhibition as a counterbalance for low factor levels to rebalance homeostasis.^37,38

Margareth C. Ozelo, MD, PhD:
First, I will discuss the TFPI, which is a Kunitz-type serine protease inhibitor that maintains tissue factor‒induced clotting via FXa-dependent inhibition of FVIIa. By using an anti-TFPI molecule, it is possible to decrease the natural inhibition of the coagulation initiation phase, resulting in restored thrombin generation.^38,39

Margareth C. Ozelo, MD, PhD:
Different principles are used with regard to TFPI inhibition, including blood protein‒targeting aptamers, which are peptides that bind to specific target proteins. BAX499 was developed with this goal.⁴⁰ However, in 2012, the trial evaluating BAX499 was closed because of the increase of bleeding events and the lack of efficacy.⁴¹ Currently, the most promising approach is the use of monoclonal antibodies targeting TFPI.

There are currently 3 monoclonal antibodies in development that bind to the TFPI Kunitz-2 domain, including concizumab, marstacimab, and MG113.^38,42

Befovacimab is another anti-TFPI monoclonal antibody that is different from the others mentioned because it binds to both Kunitz-1 and Kunitz-2 domains. However, during a dose-escalating phase II trial of this agent, 3 participants developed thrombosis, leading to the early closing of this study.⁴³

Margareth C. Ozelo, MD, PhD:
The series of clinical trials evaluating concizumab in patients with hemophilia is known as “explorer.” Both the explorer4 and explorer5 studies evaluated the efficacy and safety of daily SC concizumab. In the phase II explorer4 trial, SC concizumab was dose escalated from 0.15 mg/kg to 0.2 or 0.25 mg/kg, according to participants’ bleeding event history, in patients with hemophilia A or B with inhibitors. Then, explorer5 evaluated the same dose escalation rate in people with severe hemophilia A without inhibitors. Those previously treated with prophylaxis or on-demand agents were eligible to participate in either trial.

In explorer4, concizumab prophylaxis significantly reduced participants’ estimated ABR compared with on-demand treatment. In addition, explorer5 resulted in a reduced ABR with concizumab vs participants’ historical ABRs.

Regarding safety data in these trials, concizumab was well tolerated, with no deaths, adverse events leading to withdrawals, or thromboembolic events observed. Of note, 25% of participants developed antidrug antibodies (ADAs) without any observed clinical effects.

In 2020, explorer5 was halted because of safety events in explorer7 and explorer8, which are phase III concizumab studies of patients 12 years of age and older with hemophilia A and B with inhibitors and without inhibitors, respectively. In these trials, 3 patients presented with 5 nonfatal thromboembolic events. All patients received concomitant factor-containing medication on the day of or days preceding event onset.^44,45

Margareth C. Ozelo, MD, PhD:
To mitigate the risk of thrombotic events when using concizumab, researchers developed a new protocol that outlined the concomitant use of hemostatic agents to manage bleeding episodes, as well as guidance on an updated concizumab dosing regimen.⁴⁶

The updated concizumab regimen is as follows: After a loading dose of 1.0 mg/kg, participants started with a daily SC dose of 0.2 mg/kg of concizumab, with the option of changing the dose to 0.15 or 0.25 mg/kg at Week 4. If participants’ concizumab plasma concentration was <200 ng/mL, the dose was increased to 0.25 mg/kg. However, if one’s concizumab plasma concentration was >4000 ng/mL, the dose was decreased to 0.15 mg/kg.⁴⁷

Margareth C. Ozelo, MD, PhD:
As presented in the primary analysis of the phase III explorer7 study, 80 participants with hemophilia A and 53 with hemophilia B with inhibitors were enrolled (N = 133). Furthermore, 52 underwent randomization, 21 transferred from explorer4, and 50 previously used on-demand or prophylaxis therapy and were assigned to receive prophylaxis concizumab. In total, 114 participants were assigned to be in groups 2-4 to receive prophylaxis concizumab.

The concizumab protocol here was updated after a treatment pause to follow the risk mitigation strategy previously discussed.

Nonfatal thromboembolic events occurred in 3 patients with risk factors, and treatment was discontinued. However, no thromboembolic events were reported after concizumab therapy was restarted. In comparing the efficacy of participants who were not being treated with prophylaxis concizumab (group 1) vs those who were randomized to receive treatment (group 2) during the observational phase, researchers saw a significant decrease in estimated mean ABR—11.8 vs 1.7, respectively (P <.001). In addition, 64% of participants in group 2 vs 11% of participants in group 1 had no bleed episodes after 24 weeks on the study.⁴⁷

Recently, Canada approved concizumab for the treatment of people 12 years of age and older with hemophilia B with FIX inhibitors, and Japan approved concizumab for the treatment of hemophilia A and B with inhibitors.^48,49

Margareth C. Ozelo, MD, PhD:
Another monoclonal antibody that targets TFPI is marstacimab. In a phase Ib/II clinical trial, 26 participants with hemophilia A or B with or without inhibitors received SC prophylaxis with marstacimab. Participants were divided into 4 cohorts with predetermined weekly fixed doses (300-450 mg) of treatment. Those without inhibitors received either a 300-mg loading dose followed by a 150-mg fixed dose (cohort 2), a 300-mg fixed dose (cohort 1), or a 450-mg fixed dose (cohort 3). Then, in cohort 4, participants with inhibitors received a 300-mg fixed dose.⁵⁰

Margareth C. Ozelo, MD, PhD:
Cohorts 1 and 4 each included 7 total participants, and cohorts 2 and 3 each included 6 total participants. The majority had hemophilia A and were 18-44 years of age. BMI ranged from 21.0-26.1 mg/kg across the 4 cohorts, and mean ABR ranged from 14.7-23.0, with the highest among participants in cohort 1.⁵⁰

Margareth C. Ozelo, MD, PhD:
In comparing mean ABRs across cohorts, marstacimab prophylaxis was associated with significantly lower rates with all dosages compared with pretreatment data and an external on-demand control group.

No thromboembolic or serious treatment-related adverse events were observed. Although no one at baseline had positive ADA samples, 3 participants (11.5%) developed ADAs. However, none had neutralizing antibodies, and detected ADAs were determined to have no impact on the safety, pharmacokinetics, or pharmacodynamics of marstacimab.⁵⁰

Margareth C. Ozelo, MD, PhD:
The phase III BASIS trial evaluated the use of marstacimab prophylaxis among patients 12-75 years of age with severe hemophilia A or moderate to severe hemophilia B with or without inhibitors. Patients were treated with marstacimab during a 12-month active treatment period (ATP) and compared with routine prophylaxis or on-demand factor replacement in a 6-month observational period before the start of the ATP. Following the 12-month ATP, patients had the option to continue receiving marstacimab in a long-term extension study.

The preliminary results of this trial recently were presented during the 2023 American Society of Hematology Annual Meeting and Exposition. First, 128 patients were enrolled into the 6-month observational phase. Of these, 116 participants continued into the treatment phase, receiving a 300-mg loading dose, followed by 300-mg weekly dose of SC marstacimab.

According to mean ABR data, marstacimab demonstrated superiority compared with on-demand treatment (P <.001) and routine prophylaxis (noninferiority; P = .0376). The mean ABR among patients previously receiving routine prophylaxis decreased by 35% during the ATP with marstacimab. All reductions in ABR with marstacimab were consistent across disease types and age groups for on-demand therapy and generally consistent across disease types and age groups for routine prophylaxis (NCT03938792).⁵¹

Margareth C. Ozelo, MD, PhD:
Researchers reported no deaths or thrombotic events, including among participants who enrolled on the long-term extension study.⁵¹ Approximately 20% (23 of 116) of the participants tested positive for ADA with no impact on marstacimab safety.

In total, 6 of these patients actually tested positive for neutralizing antibodies, which were treatment induced and transient. Neutralizing antibodies resolved by the end of the study.

Margareth C. Ozelo, MD, PhD:
Now, I am going to move on to the propagation phase of the rebalancing hemostasis pathway. The target here is ATIII, as it inhibits thrombin, FXa, and FIXa.³⁹

Margareth C. Ozelo, MD, PhD:
Fitusiran is an siRNA that lowers AT production in the liver by targeting AT messenger RNA. As a result, it promotes homeostasis with a <1-mL fixed dose administered subcutaneously. The goal here is to improve thrombin generation in patients with hemophilia A or B with or without inhibitors, therefore leading to consistent homeostasis over time.

Previous studies have demonstrated that SC fitusiran lowers AT levels, is well tolerated, and suggests a potential associated decrease in bleeding episodes.⁵²

Margareth C. Ozelo, MD, PhD:
In an open-label extension phase II study, researchers evaluated monthly fixed-dose fitusiran (50 mg and 80 mg) among people with hemophilia A or B with or without inhibitors. In total, 33 participants were enrolled, including 14 with and 19 without inhibitors.

After the first dose of fitusiran and at a median of 13 months, all participants experienced an almost 80% reduction in AT levels regardless of their assigned fixed dose. Furthermore, an exploratory analysis of bleeding events found a median ABR of 1 and 0 among those without and with inhibitors, respectively. No thromboembolic events were observed.

The ATLAS series of clinical trials evaluated the efficacy and safety of fitusiran 80 mg monthly. The 3 studies in this series include:

• ATLAS-INH, which enrolled participants with hemophilia A or B with inhibitors and previously treated with on-demand therapy
• ATLAS-A/B, which enrolled participants with hemophilia A or B without inhibitors and previously treated with on-demand therapy
• ATLAS-PPX, which enrolled participants with hemophilia A or B with or without inhibitors and previously treated with factor concentrate or BPA prophylaxis

With the report of a series of thromboembolic events occurring in September 2017, all fitusiran trials were paused. This safety concern occurred in a patient with hemophilia A without inhibitors who was participating in the open-label extension phase II study. The patient later died of cerebral edema after concomitant treatment with FVIII concentrates and resulting sinus vein thrombosis. By December, researchers adopted a thrombotic risk mitigation strategy, and trials resumed with particular attention paid to any symptoms that suggested thrombosis.^53-55

Margareth C. Ozelo, MD, PhD:
One important step that was taken by the manufacturer and researchers regarding thrombotic risk with fitusiran was the adoption of revised breakthrough bleeding management guidelines. Dosing and/or frequency of the concomitant use of factor concentrates or BPA to manage bleeding events were reduced for those being treated with fitusiran in clinical trials. The guidelines recommended single-dose amounts for each therapy and defined maximum dosages to not exceed. If more frequent or higher dosages of any were needed, researchers would need to discuss this with the medical monitor of the study and clinical advisor.^55,56

Margareth C. Ozelo, MD, PhD:
As mentioned before, the phase III ATLAS A/B study enrolled people 12 years of age and older with hemophilia A or B without inhibitors and previously treated with on-demand therapy. In total, 120 participants were randomized to receive either fitusiran prophylaxis (n = 80) or on-demand factor treatments (n = 40). On the treatment arm, participants received 80 mg of SC fitusiran once monthly.

Median ABR for the treatment arm was 0 vs 21.8 for the on-demand arm. Furthermore, the estimated mean ABR was significantly lower among those treated with fitusiran (3.1) vs on-demand therapy (31.0; P <.0001). Lastly, no thrombotic events or deaths were observed in this study.⁵⁷

Margareth C. Ozelo, MD, PhD:
Next is the phase III ATLAS-INH study, which enrolled people 12 years of age and older with hemophilia A or B with inhibitors and previous treatment with on-demand therapy. In total, 57 participants were randomized to receive either fitusiran prophylaxis (n = 38) or on-demand BPA therapy (n = 19). On the treatment arm, participants received 80 mg of SC fitusiran once monthly.

Median ABR for the treatment arm was 0.0 vs 16.8 for the on-demand arm. Furthermore, the estimated mean ABR was significantly lower among those treated with fitusiran (1.7) vs on-demand BPA therapy (18.1; P <.0001). Two participants on the treatment arm experienced a suspected or confirmed thrombotic event, and researchers reported no deaths.⁵⁸

Margareth C. Ozelo, MD, PhD:
In analyzing the safety data in both ATLAS-INH and ATLAS-A/B, fitusiran appears to present with a considerable safety profile. Among those on the treatment arm of ATLAS-INH, 2 participants experienced a total of 4 thrombotic events, with 3 of these events occurring in the same individual. One participant experienced deep vein thrombosis, superficial thrombophlebitis, and subclavian vein thrombosis, which led to a treatment interruption. Researchers suspect the second participant experienced spinal vessel thrombosis related to fitusiran use, ultimately resulting in treatment discontinuation. Of note, this study included people with hemophilia A or B with inhibitors.

On the other hand, no thrombotic events were reported in ATLAS A/B, which evaluated people with hemophilia A or B without inhibitors. However, researchers in both trials observed several key treatment-emergent adverse events (eg, increases in alanine aminotransferase and aspartate aminotransferase levels, cholecystitis, and symptomatic cholelithiasis) among those treated with fitusiran.^57,58

Margareth C. Ozelo, MD, PhD:
In the ATLAS-PPX trial, fitusiran was evaluated in people 12 years of age and older with severe hemophilia A or B who were previously treated with factor concentrate or BPA prophylaxis. In total, researchers enrolled 80 participants: 19 with inhibitors and 46 without inhibitors.

Researchers observed a significant decrease in estimated ABR when patients began fitusiran (2.9 with fitusiran vs 7.5 in the previous 6 months with factor/BPA prophylaxis; P = .0008). Furthermore, observed ABR also was reduced during treatment with fitusiran (0.0) vs the previous 6 months of factor/BPA prophylaxis (4.4). Finally, fitusiran significantly improved participants’ health-related quality of life compared with Haem-A-QoL scores the previous 6 months with factor/BPA prophylaxis (P <.01).

In terms of safety, 2 participants receiving fitusiran prophylaxis each experienced a suspected or confirmed thrombotic event. Both resulted in the discontinuation of treatment. No deaths due to a treatment-related adverse event were observed.⁵⁹

Margareth C. Ozelo, MD, PhD:
In October 2020, the dosing in fitusiran clinical trials was voluntarily paused because of reports of nonfatal thrombotic events. It was determined that the risk of serious vascular thromboembolic events may be greater when AT levels are <10% among those being treated with fitusiran. In addition, those being treated and who maintained AT levels <25% were less likely to see fitusiran prevent bleeding events. Therefore, a revised dosing regimen was adopted to allow the trials to resume in December 2020.

In this new dosing regimen, patients’ AT levels are evaluated to ensure that they are meeting a target of 15% to 35%. The initial fitusiran dose is 50 mg every other month, and patients’ AT levels are evaluated. For those with AT levels >35% in 2 consecutive labs, the fitusiran dose is escalated to 50 mg once monthly. Then, if their AT levels continue to be >35% in later consecutive labs, it is recommended to dose escalate to the maximum dose of 80 mg once monthly.

However, if after the starting dose of fitusiran, patients’ AT levels are <15% in 1 test, the fitusiran dose should be decreased to 20 mg every other month. Then, if their AT levels continue to be <15% in later consecutive labs, patients should discontinue fitusiran altogether. On the other hand, if their AT levels increase to >35% after 2 consecutive labs and after the first dosage decrease, it is recommended to increase the dosage frequency to 20 mg once monthly.

The ongoing phase III ATLAS-NEO study, which is evaluating fitusiran in people 12 years of age and older with hemophilia A and B with or without inhibitors, is using this new revised dose regimen.^55,60

As we learn more about optimal dosing of siRNA agents such as fitusiran leading to enhanced hemostasis, we may be able to expand treatment options for patients with hemophilia and improve their quality of life while reducing their overall treatment burden.

Margareth C. Ozelo, MD, PhD:
A final target for rebalancing hemostasis is the protein C/S complex. With this objective, SerpinPC—a serine protease inhibitor that targets APC—was developed and is being evaluated in clinical trials.^55,61

Margareth C. Ozelo, MD, PhD:
Data on SerpinPC recently were presented at the 2024 Annual Congress of the European Association for Haemophilia and Allied Disorders.  

Results from part 5 (Weeks 97-148) of the phase IIa PRESent-2 trial were discussed. In this 6-part trial, 23 participants 18-60 years of age with severe hemophilia A or B with or without inhibitors were initially enrolled in part 2 (Weeks 1-24) of the trial to evaluate 0.3, 0.6, and 1.2 mg/kg of SerpinPC, administered by SC dosing every 4 weeks for a 24 weeks. Then, parts 3, 4, and 5 were sequential extensions of part 2, where part 5 looked exclusively at administering 1.2 mg/kg of SerpinPC every 2 weeks for 52 weeks. Of note, by the time participants started part 5 of this study, they already had received SerpinPC for approximately 2 years.⁶²

Margareth C. Ozelo, MD, PhD:
After continuous treatment with SerpinPC at 1.2 mg/kg every 2 weeks (part 5), there was a 96% median decrease in ABR from baseline. In addition, a 94% decrease in target joints was seen across all participants.

Furthermore, in terms of safety, transient ADAs were detected in 1 participant, who was ultimately determined to have no neutralizing ADAs. No serious adverse events related to SerpinPC were observed in parts 2-5.⁶²

Margareth C. Ozelo, MD, PhD:
In summary, jump-starting thrombin generation with rebalancing agents such as concizumab, marstacimab, or fitusiran is a promising strategy for treating patients with hemophilia A and B with or without inhibitors. Regarding SerpinPC, further data are necessary to determine its long-term efficacy and safety in this patient population. In addition, because all these agents are administered subcutaneously, patients’ treatment burden is effectively reduced.

However, some considerations need to be addressed. The use of rebalancing agents has the potential to increase patients’ risk of thrombotic events because they interfere with the balance of hemostasis. Furthermore, there is a lack of a clear monitoring strategy—particularly when using these various rebalancing agents.

Finally, there appears to be opportunity for the expansion of indications for these agents not only to treat patients with hemophilia, but also for anyone with a bleeding disorder.