Contact activation system (Intrinsic pathway of blood coagulation) (CAS)

The intrinsic protease system, commonly referred to as the contact activation system or the intrinsic pathway of blood coagulation, is a biochemical cascade involving several plasma-derived serine proteases that initiate blood clotting and inflammatory responses. Its core components include Coagulation Factor XII (Hageman factor), Factor XI, and Plasma Prekallikrein, which are activated in a sequential manner upon contact with anionic surfaces or inflammatory polyanions. Unlike the tissue factor-mediated extrinsic pathway, the intrinsic protease system is largely dispensable for physiological hemostasis, which has positioned it as a high-value therapeutic target for 'hemostasis-sparing' anticoagulants that prevent pathological thrombosis without the high bleeding risks of traditional agents. Beyond coagulation, the system is the primary driver of the kallikrein-kinin pathway, leading to the production of bradykinin; consequently, inhibitors of this system are standard treatments for hereditary angioedema (HAE) to prevent life-threatening swelling and vascular permeability. Current biotech development is heavily focused on next-generation Factor XI and XII inhibitors for indications such as stroke prevention, atrial fibrillation, and secondary prevention of myocardial infarction.

Other names

Intrinsic protease systemIntrinsic coagulation pathwayIntrinsic coagulation protease systemPlasma contact systemKallikrein-kinin systemContact activation pathway

Selective inhibition of specific serine proteases within the cascade—primarily Factor XIa, Factor XIIa, or Plasma Kallikrein—to prevent the propagation of thrombin generation or the excessive release of the inflammatory mediator bradykinin.