Research Summary

Concizumab Maintains Low Bleeding Rates in Phase 3 Hemophilia Follow-up

Edited by:
Brian McCurdy

Key Highlights

  • Median annualized bleeding rates were 1.7 for hemophilia A and 2.8 for hemophilia B among 42 concizumab recipients.
  • Longer-term efficacy and safety findings were consistent with the earlier 32-week analysis.
  • No thromboembolic events occurred after treatment was restarted with revised dosing and bleed-management guidance.

Once-daily subcutaneous concizumab maintained low bleeding rates through the 56-week cutoff in patients with hemophilia A or hemophilia B without inhibitors, according to phase 3 explorer8 findings published in Blood Advances. Investigators reported stable drug concentrations and no new safety concerns during longer-term follow-up.

The prospective, multicenter, open-label study included males aged ≥12 years. Participants were randomized 1:2 to no prophylaxis or concizumab, or allocated to 2 nonrandomized concizumab groups. The full analysis set comprised 148 patients: 21 assigned to no prophylaxis, 42 randomized to concizumab, and 85 allocated to the nonrandomized treatment groups. Seventeen patients initially receiving no prophylaxis subsequently switched to concizumab after at least 24 weeks.

The cutoff occurred when all participants in the original concizumab groups had completed their 56-week visit or permanently discontinued treatment. Investigators assessed bleeding episodes, pharmacokinetics, pharmacodynamics, and safety.

Study Findings

Median annualized bleeding rates for treated spontaneous and traumatic episodes among concizumab recipients were 1.7 (interquartile range [IQR], 0.0–4.5) for hemophilia A and 2.8 (IQR, 0.0–6.4) for hemophilia B. These findings were consistent with the 32-week results. Concizumab plasma concentrations remained stable. Most breakthrough bleeds were mild or moderate, and most mild or moderate episodes resolved with a single injection of factor concentrate.

The safety analysis included 151 concizumab-exposed patients. Treatment had been paused in 2020 following nonfatal thromboembolic events in 2 explorer8 participants, one participant in explorer7, and one participant in explorer7. After revised dosing and bleed-management guidance were implemented, no further thromboembolic events occurred through the 56-week cutoff. No disseminated intravascular coagulation, thrombotic microangiopathy, or hypersensitivity-type reactions were reported during the study. Injection-site reactions reported between the 32- and 56-week cutoffs were mild.

Clinical Implications

According to the authors, the findings support concizumab as a subcutaneous prophylaxis option, particularly for patients with hemophilia B. They emphasized adherence to the concizumab dosing regimen and guidance for treating breakthrough bleeding when interpreting the longer-term safety findings.

The authors identified the open-label design and nonrandomized intrapatient analysis as limitations. They also advised caution when interpreting exploratory analyses of surgical procedures, target joints, and breakthrough bleeding management.

Expert Commentary

“Overall, the explorer8 56-week cutoff results showed that longer-term daily concizumab prophylaxis was effective in maintaining low bleeding rates and was considered safe and well tolerated in patients with HA/HB without inhibitors,” the researchers concluded.

Reference

Young G, Angchaisuksiri P, Apte S, et al. Concizumab in patients with hemophilia A or B without inhibitors: 56-week cutoff results of the phase 3 explorer8 study. Blood Adv. 2026;10(17):6032-6042. doi:10.1182/bloodadvances.2026019931