Category:Anticoagulants

An anticoagulant is a medicine that interferes with the coagulation cascade of the blood, slowing or preventing the formation of fibrin clot. The category is divided clinically by route (parenteral and oral) and pharmacologically by target: the indirect inhibitors of factor Xa and thrombin that work through antithrombin (the heparins and fondaparinux), the vitamin K antagonists that reduce the hepatic synthesis of factors II, VII, IX, and X (warfarin and its congeners), and the direct oral anticoagulants that bind their target enzyme directly (the direct factor Xa inhibitors and the direct thrombin inhibitors). Each class has distinct kinetics, distinct monitoring requirements, and distinct reversal strategies.

The first anticoagulant medicine was an accident in a Johns Hopkins laboratory. In 1916 a Baltimore medical student named Jay McLean, working in William Howell's physiology laboratory on a project to identify a thromboplastic substance from dog liver, isolated a fat-soluble preparation that produced the opposite effect: when added to blood, it prevented clotting.^[1] Howell and his student Holt subsequently extracted a water-soluble polysaccharide from dog liver and named it heparin for hepar, the liver. Clinical use waited fifteen years; in 1935 the Toronto physician Donald Murray, working with the Connaught Laboratories, introduced purified heparin into postoperative venous thromboembolism prevention, and by 1940 the medicine had a routine clinical place.

The first oral anticoagulant was an agricultural mystery. Throughout the 1920s, North American cattle ranchers reported a fatal haemorrhagic disorder in cattle that ate spoiled sweet clover (Melilotus alba) hay. The Canadian veterinarian Frank Schofield in 1922 linked the condition to the spoilage, and in 1933 Karl Paul Link at the University of Wisconsin identified the causal compound as dicoumarol, a coumarin derivative formed by fungal oxidation of natural coumarins in the wet hay. Link's group developed a series of synthetic dicoumarol analogues; the most useful was numbered compound 42, named warfarin (an acronym for the Wisconsin Alumni Research Foundation that funded the work, suffixed with the -arin ending of coumarin).^[2] Warfarin was originally introduced in 1948 as a rodenticide, on the reasoning that any compound that bled cattle would bleed rats. Its clinical introduction came after a 1951 case in which a U.S. Army inductee attempted suicide by ingesting a large dose of warfarin-based rat poison and was successfully reversed with vitamin K; the safety margin proved adequate for human use, and warfarin was approved for clinical anticoagulation in 1954.

The molecular basis of warfarin's action was worked out two decades later. Warfarin and its congeners inhibit vitamin K epoxide reductase, the hepatic enzyme that regenerates reduced vitamin K from its epoxide form. Vitamin K-dependent γ-carboxylation of glutamate residues on factors II, VII, IX, and X (and on proteins C and S) is required for the calcium-binding and membrane-attachment properties of these factors; without reduced vitamin K, these factors are synthesised but functionally inert. The inter-individual variability in warfarin dose requirement (a five-fold range) was traced in the 2000s to genetic polymorphisms in the CYP2C9 enzyme that metabolises warfarin and in VKORC1 itself, and pharmacogenomic dose-prediction algorithms (incorporating CYP2C9*2, *3 and VKORC1 -1639G>A genotypes alongside age, weight, and concurrent medications) reduce time-to-stable-INR in observational studies but have not been shown to reduce bleeding in randomised trials.^[3]

The development of the low-molecular-weight heparins in the 1980s (enoxaparin, dalteparin, tinzaparin), the synthetic pentasaccharide fondaparinux in 2001, and the direct thrombin inhibitors hirudin and lepirudin (from leech-saliva sequence, recombinant) extended the parenteral end of the category. The transformative event of the 2010s was the introduction of the direct oral anticoagulants. The direct thrombin inhibitor dabigatran (Boehringer Ingelheim, 2010) and the direct factor Xa inhibitors rivaroxaban (Bayer, 2011), apixaban (Bristol-Myers Squibb/Pfizer, 2012), and edoxaban (Daiichi Sankyo, 2015) demonstrated non-inferior or superior efficacy compared to warfarin for atrial fibrillation stroke prevention and for venous thromboembolism, with substantially lower rates of intracranial haemorrhage and without the requirement for routine INR monitoring or dose-titration.^[4] The DOACs have largely displaced warfarin in atrial fibrillation, deep venous thrombosis, and pulmonary embolism in patients without specific contraindications (mechanical heart valves, antiphospholipid syndrome, severe renal impairment, hepatic insufficiency).

Reversal of anticoagulation, when bleeding occurs or when emergency surgery is required, is a parallel chapter of the same pharmacology. Vitamin K reverses warfarin over hours; four-factor prothrombin complex concentrate reverses warfarin within minutes by infusing the carboxylated factors that warfarin's mechanism prevents the liver from making. Protamine sulfate reverses unfractionated heparin essentially completely and low-molecular-weight heparin partially. The direct-thrombin inhibitor dabigatran has a specific reversal agent, the humanised Fab fragment idarucizumab (2015); the direct factor Xa inhibitors are reversed by recombinant inactivated factor Xa decoy andexanet alfa (2018), or, with less specific efficacy and lower cost, by four-factor PCC.

By target:

• Indirect inhibitors (antithrombin-mediated):
  • Unfractionated heparin (parenteral, monitored by activated partial thromboplastin time)
  • Low-molecular-weight heparins: enoxaparin, dalteparin, tinzaparin (subcutaneous, weight-based dosing)
  • Synthetic pentasaccharide: fondaparinux (selective anti-Xa via antithrombin)
• Vitamin K antagonists (oral, hepatic synthesis inhibition):
  • Warfarin (the standard agent in North America)
  • Acenocoumarol, phenprocoumon (European agents, similar mechanism, different half-life)
• Direct oral anticoagulants:
  • Direct thrombin inhibitors: dabigatran
  • Direct factor Xa inhibitors: rivaroxaban, apixaban, edoxaban, betrixaban
• Direct thrombin inhibitors, parenteral (now niche, used in heparin-induced thrombocytopenia): argatroban, bivalirudin, the recombinant hirudins lepirudin (discontinued) and desirudin
• Reversal agents (not anticoagulants but listed for reference):
  • Vitamin K (phytonadione) for warfarin
  • Four-factor prothrombin complex concentrate for warfarin and (off-label) for factor Xa inhibitors
  • Protamine sulfate for heparin
  • Idarucizumab (Fab fragment) for dabigatran
  • Andexanet alfa (decoy factor Xa) for rivaroxaban, apixaban

The boundary of this category is "medicine that inhibits the formation of fibrin in the coagulation cascade." The antiplatelet agents (aspirin, clopidogrel, ticagrelor, prasugrel, glycoprotein IIb/IIIa inhibitors) act on platelet function rather than on the coagulation cascade and are collected separately, even though the clinical indications overlap (atrial fibrillation, acute coronary syndromes, vascular intervention). The fibrinolytic medicines (tissue plasminogen activator, urokinase, streptokinase), which lyse already-formed fibrin rather than preventing its formation, are listed under a separate category. The procoagulant medicines (tranexamic acid, factor VIIa, fibrinogen concentrate, the haemostatic agents used in haemophilia) are the opposite of anticoagulants and belong to their own category. Heparin is sometimes used as an antithrombotic in renal-replacement-therapy and extracorporeal-circuit anticoagulation rather than as a clinical anticoagulant in the conventional sense; that usage is listed on the heparin page but is not the indication that defines this category.

This category page is an encyclopedia article about its subject. The actual index of medicines belonging to the category is generated automatically by the wiki engine, from category-membership declarations on the individual medicine pages, and appears at the foot of the page below the references.