Research Summary

Phase 3 Concizumab Maintains Low Bleed Rates in Hemophilia

Anthony Calabro, MA

Key Highlights

  • Median annualized bleeding rates remained low through the 56-week cutoff with once-daily subcutaneous concizumab.
  • Median rates for treated spontaneous and traumatic bleeds were 1.7 for hemophilia A and 2.8 for hemophilia B.
  • Overall, 86.3% of target joints present at baseline were considered resolved at the 56-week cutoff.
  • No thromboembolic events occurred after treatment restarted under the revised dosing and bleed-management guidance.

Once-daily subcutaneous concizumab maintained low bleeding rates through the 56-week cutoff among patients with hemophilia A or B without inhibitors, according to phase 3 explorer8 findings published in Blood Advances. The longer-term efficacy and safety findings were consistent with results from the study’s 32-week analysis.

Explorer8 was a prospective, multicenter, open-label phase 3a study of male patients aged ≥12 years and weighing >25 kg with severe hemophilia A or moderate to severe hemophilia B without inhibitors. Patients receiving on-demand treatment were randomized 1:2 to continue without prophylaxis or receive concizumab. Additional participants were assigned to nonrandomized concizumab groups.

The full analysis set included 148 patients—82 with hemophilia A and 66 with hemophilia B. Of these, 21 were randomized to no prophylaxis, 42 were randomized to concizumab, and 85 entered the nonrandomized concizumab groups. After ≥24 weeks, 17 patients initially assigned to no prophylaxis switched to concizumab. Efficacy, pharmacokinetic, pharmacodynamic, and safety outcomes at the 56-week cutoff were assessed descriptively without statistical hypothesis testing.

Study Findings

Among 144 patients exposed to concizumab under the revised dosing regimen, the median annualized bleeding rate for treated spontaneous and traumatic bleeding episodes was 1.7 (interquartile range [IQR], 0.0-4.5) for hemophilia A and 2.8 (IQR, 0.0-6.4) for hemophilia B. Zero treated bleeding episodes were reported for 28.8% of patients with hemophilia A and 21.9% of those with hemophilia B.

Among 63 patients with at least 1 target joint at baseline, 69 of 80 target joints (86.3%) were considered resolved at the 56-week cutoff. Concizumab concentrations, free tissue factor pathway inhibitor suppression, and thrombin generation remained stable.

The safety analysis included 151 concizumab-exposed patients. No thromboembolic events occurred after treatment restarted following the implementation of a revised dosing regimen and bleed-management guidance. Between the 32- and 56-week cutoffs, 8 serious adverse events occurred; investigators considered all unlikely to be related to concizumab, and all patients recovered.

Clinical Implications

According to the authors, the findings support concizumab as a subcutaneous prophylactic option for patients with hemophilia A or B without inhibitors, particularly those with hemophilia B, for whom approved subcutaneous nonfactor treatment options are limited. Most mild or moderate breakthrough bleeds were managed successfully with a single factor injection administered according to labeling and study guidance.

The authors identified the open-label design and nonrandomized intrapatient comparison as limitations. They also cautioned that analyses of target joints, breakthrough bleeding, surgical outcomes, and bleeding rates according to baseline target-joint status were exploratory.

Expert Commentary

“Longer-term daily concizumab prophylaxis was effective in maintaining low bleeding rates and was considered safe and well tolerated,” 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.202601993