Factor XI

Factor XI (FXI) is produced by the liver and circulates as a homo-dimer in its inactive form. The plasma half-life of FXI is approximately 52 hours. The zymogen factor is activated into factor XIa by factor XIIa (FXIIa), thrombin, and FXIa itself; due to its activation by FXIIa, FXI is a member of the "contact pathway" (which includes HMWK, prekallikrein, factor XII, factor XI, and factor IX). Factor XIa activates factor IX by selectively cleaving arg-ala and arg-val peptide bonds. Factor IXa, in turn, forms a complex with Factor VIIIa (FIXa-FVIIIa) and activates factor X. Physiological inhibitors of factor XIa include protein Z-dependent protease inhibitor (ZPI, a member of the serine protease inhibitor/serpin class of proteins), which is independent of protein Z (its action on factor X, however, is protein Z-dependent, hence its name). == Structure ==

Although synthesized as a single polypeptide chain, FXI circulates as a homodimer. Every chain has a relative molecular mass of approximately 80000. Typical plasma concentrations of FXI are 5 μg/mL, corresponding to a plasma concentration (of FXI dimers) of approximately 30 nM. The FXI gene is 23kb in length, has 15 exons, and is found on chromosome 4q32-35. == Role in disease ==

Deficiency of factor XI causes the rare hemophilia C; this mainly occurs in Ashkenazi Jews and is believed to affect approximately 8% of that population. Less commonly, hemophilia C can be found in Jews of Iraqi ancestry and in Israeli Arabs. The condition has been described in other populations at around 1% of cases. There is little spontaneous bleeding, but surgical procedures may cause excessive blood loss, and prophylaxis is required. Low levels of factor XI also occur in many other disease states, including Noonan syndrome. High levels of factor XI have been implicated in thrombosis, although it is uncertain what determines these levels and how serious the procoagulant state is. Inhibition Pharmacological inhibitors of factor XI that are under clinical development but not yet approved for treatment include the oral factor XIa inhibitors Asundexian (BAY 2433334) and Milvexian as well as the monoclonal anti-factor XI antibody abelacimab (MAA868). The idea behind producing such an inhibitor is that XI is mostly involved in intrinsic/contact activation pathway, which plays a bigger role in thrombosis as opposed to hemostasis, so targeting it may reduce clotting risks without a corresponding increase in bleeding. An abelacimab trial appears to have indeed produced this result. == See also ==