In vitro effects of Mim8 and combined Mim8-bypassing therapy on thrombin generation, thromboelastography and fibrin clot ultrastructure

Introduction

Mim8, a fully human bispecific IgG4 antibody, FVIIIa mimetic, which emerges as a promising prophylactic treatment option for patients with hemophilia A (HA).

Aim

This study investigates the in vitro hemostatic activity of Mim8 in blood samples from six patients with severe hemophilia A using a thrombin generation assay (TGA) and thromboelastography. The results were compared to those obtained with emicizumab.

Materials and methods

TG was assessed in both platelet-poor (PPP) and platelet-rich plasma (PRP). Fibrin clots formed during TG were collected, and clot ultrastructure was examined using scanning electron microscopy (SEM). Whole blood samples were analyzed using ROTEM-NATEM.

Results

Mim8 significantly improved the TG capacity of severe HA plasma samples. Mim8 6 μg/mL demonstrated higher TG capacity compared to emicizumab 50 μg/mL. In PRP samples fortified with Mim8 6 μg/mL and rFVIIa 90 μg/kg together, ETP levels were fully normalized. No synergistic effect was observed when FVIII concentrate was combined with Mim8 at therapeutic doses due to the higher affinity of FVIII for FIXa and FX compared to Mim8. ROTEM-NATEM in whole blood confirmed the TGA results. SEM showed a more robust fibrin clot structure with thinner fibrin fibers in the presence of Mim8 compared to emicizumab.

Conclusions

Mim8 significantly improves TG in vitro in both PPP and PRP from patients with severe HA, with ETP levels comparable to those of FVIII at 100 IU/dL. The TGA can effectively monitor the combined treatment with Mim8 and either rFVIIa or APCC, which is not possible with currently available routine laboratory tests.