Coagulation Factor Xa/IIa — DOACs

The coagulation cascade proceeds via two convergent pathways — the extrinsic (tissue factor-initiated) and intrinsic (contact activation) pathways — both culminating in activation of factor Xa. Factor Xa, in complex with factor Va on phospholipid membranes (the prothrombinase complex), converts prothrombin (factor II) to thrombin (factor IIa) with markedly amplified efficiency. Thrombin then cleaves fibrinogen to fibrin, activates factor XIII for cross-linking, and amplifies its own generation through positive feedback on factors V, VIII, and XI, establishing the central importance of factor Xa and thrombin as therapeutic targets.

Direct oral anticoagulants (DOACs) offer predictable pharmacokinetics, fixed dosing without routine monitoring, and fewer food and drug interactions compared to vitamin K antagonists. Direct factor Xa inhibitors (apixaban, rivaroxaban, edoxaban) bind reversibly to the active site of both free and prothrombinase complex-bound factor Xa, while dabigatran directly inhibits both free and fibrin-bound thrombin. Large trial programmes (RE-LY, ROCKET-AF, ARISTOTLE, ENGAGE AF-TIMI 48) established non-inferiority or superiority to warfarin for stroke prevention in atrial fibrillation with consistent intracranial haemorrhage reduction.

DOAC selection in clinical practice is guided by renal function (all require dose adjustment in moderate-severe CKD; dabigatran is most renally cleared), hepatic function, body weight extremes, indication-specific dosing regimens, and reversal agent availability. Andexanet alfa provides reversal of factor Xa inhibitors, while idarucizumab is specific for dabigatran reversal — both approved in the UK. Extended thromboprophylaxis, cancer-associated thrombosis, and antiphospholipid syndrome management represent areas of evolving evidence for this drug class.