Modified Clotting Factor X Slowed Bleeding in Mouse Model of Hemophilia A, Study Reports

A modified version of the coagulation protein factor X was able to significantly reduce bleeding in a mouse model of hemophilia A, and may offer a way of treating people with this disease due to factor VIII deficiency, a study suggests.

Lab work also showed a potential for treating people with hemophilia B, caused by a lack of clotting factor IX.

This finding was reported in the journal Thrombosis and Haemostasis, in the paper “A Thrombin-Activatable Factor X Variant Corrects Hemostasis in a Mouse Model for Hemophilia A.”

Hemophilia A is caused by a lack of functional factor VIII (FVIII), a protein that is important for blood clotting. It can be treated by replacing FVIII, but because the added back protein is viewed by the body as foreign, up to a third of hemophilia A patients eventually develop antibodies (inhibitors) that bind to FVIII, blocking its activity.

Because of inhibitors, the need exists for treatment strategies that don’t use FVIII itself, particularly for use in controlling acute bleeding episodes.

One such proposed strategy is using factor X (FX), another clotting protein. Broadly speaking, FX is activated by FVIII as part of the chain of events that leads blood to clot. As such, treating a person with hemophilia A with FX itself would be ineffective; without FVIII, the added FX could not become activated.

In a previous study, researchers had created a modified version of FX, termed FX-delAP/FpA. Essentially, the protein was altered so that, rather than being activated by FVIII, it could be activated by another blood clotting protein: thrombin. Notably, thrombin itself is activated by activated FX, thus forming an accelerating feedback loop so that the blood clots rapidly.

The research group behind this study, also in France, found a problem with this modified protein: when injected into lab mice, it was rapidly cleared from the bloodstream — the half-life of FX-delAP/FpA was only about 50 minutes, whereas that of unmodified FX was about five hours. This short half-life makes FX-delAP ill-suited for treatment.

So the researchers created a new modified version of FX, which they called FX/FpA. This modification was similar to that in FX-del/FpA; both are versions of FX that are activated by thrombin. But by tweaking the structure of the protein, the team made a version that was more stable while potentially still being effective. When injected into mice, the half-life of FX/FpA was about four hours — much closer to the unmodified protein, and better suited for therapeutic use.

Researchers then tested FX/FpA in mice that lacked FVIII. They clipped the mice’s tails and measured the amount of blood they lost in 15 minutes. Mice treated with a low or high dose of FX/FpA (0.7 or 3.0 mg/kg of body weight) bled 81 μL and 45 μL, a significant decrease compared to control mice, indicating that FX/FpA was indeed able to control their bleeding.

Placebo-treated mice and mice given unmodified FX bled 236 μL and 424 μL, respectively.

“Our data demonstrate that FX/FpA is efficient in vivo [in living animals], and that the approach of thrombin-activatable FX may be used to bypass FVIII deficiency and restore coagulation in vivo,” the researchers wrote.

They also tested FX/FpA in samples of blood from four people with hemophilia — three with hemophilia A and one with hemophilia B. As a working measure, the researchers looked at the generation of thrombin — which, again, is itself activated by FX.

Relative to human blood from people without hemophilia, blood from all these hemophilia patients had significantly lesser thrombin generation. However, the addition of FX/FpA to their blood significantly increased thrombin levels. Importantly, this was also true in the blood sample from a hemophilia A patient known to have developed antibodies against FVIII.

“Apparently,” the researchers wrote, “FX/FpA is also able to boost thrombin generation in the absence of FVIII or FIX in original patient plasma.”

These data support the use of modified FX as a possible strategy for treating hemophilia, particularly in the presence of inhibitors. But, the researchers acknowledged, more work needs to be done.

“It should be noted that although the molecule corrects thrombin generation in hemophilia A plasma with inhibitors, it has not yet been tested in in vivo models for FIX or FVIII with inhibitors,” they wrote. “More extensive studies should be performed to confirm these observations.”