Abstract
Background: NXT007 is a bispecific, monoclonal antibody that mimics the function of activated factor (F)VIII. NXT007 was engineered and optimized based on emicizumab. The Phase I/II multiple ascending dose (MAD) study of NXT007 (NCT05987449) is an open-label, non-randomized, multicenter trial, testing increasing subcutaneous doses of NXT007 in people with hemophilia A (PwHA). NXT007 demonstrated comparable thrombin generation (TG) to FVIII levels in the non-hemophilia range in in vitro spiking experiments (Teranishi-Ikawa et al. J Thromb Haemost 2024). Here, we present the pharmacodynamic (PD) and exploratory biomarker data, as well as pharmacokinetic–PD relationships, from this NXT007 study.
Methods: Prior to starting NXT007, participants received one dose of 40 IU/kg of their standard-of-care FVIII treatment during a pre-treatment visit. This dose, based on an average in vivo recovery (IVR) of 2 IU/dL per IU/kg, was expected to achieve plasma FVIII activity of approximately 80 IU/dL. Plasma samples were collected prior to FVIII infusion and 15 minutes (min), 1 hour (h) and 3 h post infusion. During the NXT007 treatment period, samples were collected prior to loading doses of NXT007 at Days 1 and 15, and at trough concentrations during maintenance treatment every 4 weeks from Day 29. PD biomarkers included activated partial thromboplastin time (aPTT), FVIII-like activity (human chromogenic FVIII activity assay, Hyphen BioMed) and FXIa-triggered TG (Ceveron s100, Technoclone), and were measured in plasma samples collected during the pre-treatment and NXT007 treatment visits. To compare the PD effects of FVIII and NXT007, samples collected at Day 29 during NXT007 treatment, when loading doses were completed and maintenance dosing started, were compared with the samples taken 15 min after FVIII administration. In addition, FIX and FX concentrations, prothrombin time (PT), D-dimer, prothrombin fragment 1+2 (PF1+2) and fibrinogen were measured during NXT007 treatment. NXT007 plasma concentrations were measured using a validated immunoassay.
Results: Large variability was observed in the measured plasma FVIII activity after FVIII administration during the pre-treatment visit. The 15 min post-FVIII administration time point was used for comparing PD effects during NXT007 treatment, as that is when the highest FVIII activity was generally observed. After the first NXT007 dose in all cohorts, aPTT shortened and remained below normal range throughout the study. FVIII-like activity and TG peak height increased during NXT007 loading doses and were sustained from Day 29 onwards. PD effects increased with increasing NXT007 doses. The maximum effect on aPTT was reached at low, sub-therapeutic NXT007 concentrations. FVIII-like activity increased linearly with increasing NXT007 plasma concentrations and TG peak height increased following an Emax model approaching maximum effect at the highest concentrations.
In the lowest dose NXT007 cohort (Cohort 1), mean TG peak height at Day 29 was lower than the mean TG peak height 15 min post FVIII administration (pre-treatment visit). Mean TG peak heights in Cohorts 2 and 3 at Day 29 were close to the mean post-FVIII-infusion TG peak height at the pre-treatment visit, which was expected to achieve FVIII-like activity within the non-hemophilia range. No changes in FIX and FX concentrations, PT, D-dimer or fibrinogen were observed during NXT007 treatment, and increasing NXT007 concentrations had no impact on these markers. A slight increase in PF1+2 was seen, but was not considered to be of clinical significance.
Conclusions: PD markers were well correlated with NXT007 concentrations and demonstrated stable activity throughout the maintenance-dosing period, while safety biomarkers remained unaffected. In PwHA receiving prophylaxis with NXT007, the TG peak height during the steady state was in the non-hemophilia range and comparable to that obtained in participants with FVIII levels in the normal range.
