The management of hemophilia A has profoundly evolved over the past decade. Long based on replacement therapy with factor VIII (FVIII) concentrates, it is now expanding with so-called "non-factor" therapies that are reshaping the therapeutic paradigm. These include emicizumab, a bispecific antibody that binds both FIXa and its substrate FX, thereby mimicking part of the function of FVIIIa, and antibodies directed against tissue factor pathway inhibitor (TFPI), such as marstacimab. One question nonetheless remains debated: what FVIII equivalence should be assigned to these new molecules, given that in vitro assays yield highly variable responses?
To address this question, the authors compared FVIII with emicizumab and with a sequence-identical analogue of marstacimab (SIA-marstacimab) using four in vivo bleeding models in FVIII-deficient mice. These models span a broad spectrum of severity, each requiring a distinct FVIII dose to restore blood loss to the level of wild-type mice: 2.5 IU/kg in the tail vein transection (TVT) model, 5 IU/kg for tail artery transection (TAT), 7.5 IU/kg in the tail clip model, and up to 25 IU/kg in the saphenous vein puncture (SVP) model. An arterial injury thus requires more FVIII than a venous injury performed in an identical manner. Importantly, FVIII treatment produced stable clots, with no spontaneous rebleeding.
At the therapeutic doses selected (55 µg/mL for emicizumab, 16 µg/mL for SIA-marstacimab), both molecules significantly reduced blood loss, but their FVIII equivalence varied across models. For example, emicizumab proved equivalent to 5 IU/kg of FVIII in the tail clip model, and to 10 IU/kg in the SVP model. Strikingly, unlike FVIII, both treatments were associated with spontaneous rebleeding in the TVT, TAT and tail clip models, which the authors attribute to a likely different, more fragile clot morphology.
The proposed interpretation is that these agents are strictly dependent on the local availability of FIXa and TFPI—itself variable according to the location and severity of the injury—whereas FVIII constitutes the rate-limiting factor of coagulation. The authors highlight the limitations of the experimental setup—insufficiently humanized murine models, unmeasured residual TFPI levels, expression of a mouse-specific TFPIγ isoform—and specify that the study was not intended to compare the clinical efficacy of the two antibodies. They conclude that it is unlikely that a single, consistent FVIII equivalence can be assigned to emicizumab, SIA-marstacimab and related molecules.