Ex vivo evaluation of the procoagulant effect of NXT007 prophylaxis in people with Hemophilia A without factor VIII inhibitors: Phase I/II study (NXTAGE) Free

Introduction: NXT007, an emicizumab-based next-generation bispecific antibody, mimics the cofactor function of activated factor VIII (FVIII) and has higher FVIII-mimetic activity and a longer half-life compared to emicizumab. A phase I/II clinical study of NXT007 in healthy volunteers and people with hemophilia A (PwHA) is currently ongoing (NXTAGE; jRCT2080224835). To investigate the pharmacodynamics and coagulation activity of NXT007, activated partial thromboplastin time (aPTT) and thrombin generation (TG) were measured in plasma from PwHA participating in multi-ascending-dose Part B of NXTAGE study.

Methods: Part B of NXTAGE study, consisted of four dosing cohorts, was conducted in PwHA without FVIII inhibitors who have not been treated with emicizumab (Shima et al., Res Pract Thromb Haemost 2025). aPTT was measured using Thrombocheck APTT-SLA and a CS-2400 automated blood coagulation analyzer (Sysmex Corporation). TG assay was performed using clinical repository plasma samples collected from the participants. TG peak height was measured by Medpace Reference Laboratories using the fully automated coagulation analyzer (Ceveron s100, Technoclone) with an activated factor XI trigger, which has been shown to be more robust and sensitive than tissue factor triggers (Waters et al., J Thromb Haemost 2015). Exploratory graphical analyses (for aPTT and TG) and an inhibitory maximum effect model analysis (for aPTT) were performed to investigate the response to different NXT007 plasma concentrations.

Results: aPTT was shortened in a plasma NXT007 concentration-dependent manner up to 5 µg/mL, at which FVIII-equivalent activity estimated based on factor IX-NXT007-factor X ternary complex concentration (Yoneyama et al., Blood 2022) was 28.8 IU/dL. At higher NXT007 concentrations, aPTT shortening was occurred only by a few seconds and appeared to have reached a plateau, where aPTT was below the lower limit of the reference range reported in the manufacturer's kit insert (24–32 seconds). This is likely because FVIIIa-mimetic bispecific antibodies do not require activation for function unlike FVIII, as known with emicizumab (Yoneyama et al., Expert Rev Clin Pharmacol 2023). The model-based analysis showed that NXT007 shortened aPTT to approximately 19 seconds at the maximum effect, and the NXT007 concentration causing 50% of the maximum effect (IC₅₀) was estimated to be 0.190 µg/mL. These values are lower than the mean aPTT at steady state (23.6 seconds) and the IC₅₀ for aPTT shortening (1.1 µg/mL) reported in the Phase III HAVEN 1 study of emicizumab (Schmitt et al., Thromb Haemost 2021). Plasma samples for TG assay were collected from a total of 21 participants in Part B of NXTAGE study who consented to the clinical repository sample collection (n=10, 2, 4, and 5 for Cohorts B-1, B-2, B-3, and B-4, respectively). The TG peak height correlated with plasma NXT007 concentration up to approximately 25 µg/mL. Compared to the reference range established using 120 healthy donor samples, in which the FVIII activity was determined to be 29–133 IU/dL based on a chromogenic assay, the concentration-dependent normalization of TG peak height was observed starting at plasma NXT007 concentrations of ≥ 5 µg/mL. Given that the mean plasma NXT007 concentration was maintained at approximately 7 µg/mL in the maintenance dose period of Cohort B-2, in which the FVIII-equivalent activity of NXT007 was predicted to reach 40 IU/dL, the procoagulant effect of NXT007 was suggested to reach the non-hemophilia level at doses of Cohort B-2 and higher. Notably, the annualized bleed rate for treated bleeds in the maintenance dose period tended to be lower in participants with higher TG peak height. Non-bleeders in the maintenance dose period, consisting mostly of patients in Cohorts B-3 and B-4, tended to have a higher peak height within the reference range compared to bleeders.

Conclusions: These results suggest that NXT007 has the potential to provide a non-hemophilic range of coagulation activity in PwHA.