NXT007 Reaches Nonhemophilic Factor VIII Levels in Severe Hemophilia A

Advancing Hemostatic Targets in Severe Hemophilia A

The management of severe hemophilia A has evolved from reactive on-demand treatment to prophylaxis aimed at maintaining minimum factor levels to prevent spontaneous joint bleeds [1, 2]. While the introduction of emicizumab provided a subcutaneous alternative to frequent intravenous infusions, some patients continue to experience breakthrough bleeding or require supplemental factor for minor trauma [3, 4]. Current subcutaneous mimetic therapies often achieve steady-state activity levels that remain within the mild hemophilia range rather than reaching physiological, nonhemophilic thresholds [5]. This limitation drives the search for more potent agents that can provide robust hemostatic coverage with extended dosing intervals, potentially improving patient adherence and quality of life. To address this clinical gap, researchers recently evaluated NXT007, a next-generation bispecific antibody designed to reach these higher therapeutic targets.

NXTAGE Trial Design and Dosing Regimen

The NXTAGE trial (jRCT2080224835) is a phase 1/2 study investigating NXT007, an activated factor VIII-mimetic bispecific antibody. This engineered protein is designed to bind to two different targets simultaneously, bridging factor IXa and factor X to mimic the missing cofactor function in the coagulation cascade. The primary analysis focused on the multiple-ascending-dose Part B study, which enrolled men with severe hemophilia A who did not have factor VIII inhibitors (neutralizing antibodies that impede the efficacy of factor replacement therapies). Participants received NXT007 via subcutaneous injection once every four weeks, a schedule designed to assess the feasibility of a less burdensome dosing interval compared to existing prophylactic regimens. The trial utilized four distinct dose cohorts to evaluate the dose-response relationship, with maintenance doses for cohorts B1, B2, B3, and B4 set at 0.072 mg/kg, 0.28 mg/kg, 0.70 mg/kg, and 1.08 mg/kg, respectively. Primary endpoints included safety, tolerability, pharmacokinetics (how the body absorbs and clears the drug), pharmacodynamics (the drug's physiologic effects on coagulation), and clinical efficacy. As a secondary endpoint, the researchers monitored the incidence of anti-drug antibodies, which are immune responses against the therapeutic protein that can potentially lead to a loss of drug exposure. By tracking these metrics, the study aimed to establish the pharmacokinetic and pharmacodynamic profile necessary to reach nonhemophilic coagulation activity.

Pharmacokinetic Profile and Hemostatic Activity

The trial established a substantial duration of exposure across all four study groups, providing a longitudinal view of the drug's behavior. In cohort B1, 10 participants received NXT007 for a median of 114.1 weeks (range 29 to 140 weeks). Cohort B2 included 6 participants with a median exposure of 96.4 weeks (range 88 to 112 weeks). For the higher dose groups, cohort B3 consisted of 6 participants treated for a median of 58.1 weeks (range 52 to 72 weeks), while cohort B4 enrolled 8 participants followed for a median of 22.2 weeks (range 4 to 28 weeks). Analysis of the pharmacokinetic data demonstrated a dose-dependent increase in plasma NXT007 concentration across all cohorts. This predictable rise in drug levels suggests a reliable relationship between the administered subcutaneous dose and systemic exposure, a critical factor for clinicians managing long-term prophylaxis. Furthermore, the researchers measured predicted factor VIII-equivalent activity, a calculation estimating the level of clotting function provided by the mimetic antibody compared to natural factor VIII. The study found that predicted factor VIII-equivalent activity reached nonhemophilic levels of at least 40 IU/dL in cohorts B2, B3, and B4. Achieving this 40 IU/dL threshold is clinically significant because it effectively shifts patients from a state of severe hemophilia into the range of normal physiological hemostasis, potentially allowing for a fully active lifestyle with a minimal risk of spontaneous hemorrhage.

Clinical Efficacy and Bleed Rate Reduction

The primary clinical objective was to evaluate how effectively the subcutaneous administration of NXT007 could prevent hemorrhagic events. The researchers utilized the mean annualized treated bleed rate, which calculates the projected number of bleeding episodes requiring treatment over a one-year period, to quantify efficacy across the four dose cohorts. In the lowest dose group (cohort B1, 0.072 mg/kg), the mean annualized treated bleed rate was 1.48. While this represented a baseline level of control, efficacy improved significantly as the dosage increased. When the maintenance dose was escalated to 0.28 mg/kg in cohort B2, the mean annualized treated bleed rate decreased to 0.28, indicating a substantial reduction in clinical bleeding events. The most robust hemostatic control was observed in the two highest dose groups. In cohort B3 (0.70 mg/kg) and cohort B4 (1.08 mg/kg), the mean annualized treated bleed rate was 0.00 for both groups. This achievement of zero treated bleeds in the higher dose cohorts aligns directly with the pharmacokinetic data showing factor VIII-equivalent activity levels above 40 IU/dL. For practicing hematologists, these findings suggest that maintaining these higher plasma concentrations can effectively eliminate treated bleeding episodes, offering a highly protective prophylactic option.

Safety and Immunogenicity Observations

The safety profile of NXT007 remained favorable throughout the trial, with the majority of adverse events categorized as mild or moderate in severity. While three serious adverse events were reported during the study period, the researchers determined that all 3 serious adverse events were unrelated to NXT007. This safety data is reassuring for clinicians considering long-term prophylaxis, as it indicates the bispecific antibody does not introduce immediate high-grade toxicities even at the highest maintenance dose of 1.08 mg/kg. During the course of the study, two participants discontinued treatment. One participant withdrew due to an adverse event judged to be unrelated to the study drug. The second discontinuation involved a participant in cohort B1 who experienced a complete loss of NXT007 exposure caused by anti-drug antibodies. These neutralizing immune proteins, produced by the body in response to the biologic therapy, can bind to the agent and accelerate its clearance from the system. Immunogenicity monitoring revealed that two participants developed anti-drug antibodies that affected pharmacokinetics, including the participant from cohort B1 who discontinued the trial. For these individuals, the immune response significantly altered the drug's metabolic profile. Despite these isolated cases, the overall incidence of neutralizing immune responses remained low across the four dose cohorts, suggesting that NXT007 could provide a safe, less burdensome, and highly effective monthly dosing regimen for patients with severe hemophilia A.

References

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2. Araujo IDO, Suassuna LF, Santos ILD, Rodrigues DDOW. Efficacy, safety and satisfaction of using emicizumab in hemophilia A patients without factor VIII inhibitors: A systematic review. Hematology Transfusion and Cell Therapy. 2025. doi:10.1016/j.htct.2025.103849

3. Klamroth R, Wojciechowski P, Aballéa S, et al. Efficacy of rFVIIIFc versus Emicizumab for the Treatment of Patients with Hemophilia A without Inhibitors: Matching-Adjusted Indirect Comparison of A-LONG and HAVEN Trials.. Journal of blood medicine. 2021. doi:10.2147/JBM.S288283

4. Warren B. Untreated bleeds: Unveiling the subtleties and challenges of bleeding event counts and patient experience in clinical trials for bleeding disorders. Research and Practice in Thrombosis and Haemostasis. 2022. doi:10.1002/rth2.12832

5. Klamroth R, Wojciechowski P, Aballéa S, et al. Efficacy of rFVIIIFc versus Emicizumab for the Treatment of Patients with Hemophilia A without Inhibitors: Matching-Adjusted Indirect Comparison of A-LONG and HAVEN Trials. Journal of Blood Medicine. 2021. doi:10.2147/jbm.s288283