Coagulation Cascade and Major Classes of Anticoagulants
Anticoagulants, commonly known as blood thinners, are chemical substances that prevent or reduce coagulation of blood, prolonging the clotting time. Some of them occur naturally in blood-eating animals such as leeches and mosquitoes, where they help keep the bite area unclotted long enough for the animal to obtain some blood. As a class of medications, anticoagulants are used in therapy for thrombotic disorders. Oral anticoagulants (OACs) are taken by many people in pill or tablet form, and various intravenous anticoagulant dosage forms are used in hospitals. Some anticoagulants are used in medical equipment, such as sample tubes, blood transfusion bags, heart-lung machines, and dialysis equipment. They can also be used as rodenticides.
Anticoagulants are closely related to antiplatelet drugs and thrombolytic drugs by manipulating the various pathways of blood coagulation. Specifically, antiplatelet drugs inhibit platelet aggregation (clumping together), whereas anticoagulants inhibit the coagulation cascade by clotting factors that happens after the initial platelet aggregation.
- 1 Medical uses
- 2 Adverse effects
- 3 Interactions
- 4 Types
- 4.1 Coumarins (vitamin K antagonists)
- 4.2 Heparin and derivative substances
- 4.3 Synthetic pentasaccharide inhibitors of factor Xa
- 4.4 Directly acting oral anticoagulants
- 4.5 Antithrombin protein therapeutics
- 4.6 Other types of anticoagulants
- 5 Coagulation inhibitor measurement
- 6 Laboratory use
- 7 See also
- 8 References
- 9 External links
The use of anticoagulants is a decision based upon the risks and benefits of anticoagulation. The biggest risk of anticoagulation therapy is the increased risk of bleeding. In otherwise healthy people, the increased risk of bleeding is minimal, but those who have had recent surgery, cerebral aneurysms, and other conditions may have too great of risk of bleeding. Generally, the benefit of anticoagulation is prevention of or reduction of progression of a disease. Some indications for anticoagulant therapy that are known to have benefit from therapy include:
- Atrial fibrillation — commonly forms an atrial appendage clot
- Coronary artery disease
- Deep vein thrombosis — can lead to pulmonary embolism
- Ischemic stroke
- Hypercoagulable states (e.g., Factor V Leiden) — can lead to deep vein thrombosis
- Mechanical Heart Valves
- Myocardial infarction
- Pulmonary embolism
- Restenosis from stents
In these cases, anticoagulation therapy can prevent formation of dangerous clots or prevent growth of clots.
The decision to begin therapeutic anticoagulation often involves the use of multiple bleeding risk predictable outcome tools as non-invasive pre-test stratifications due to the potential for bleeds while on blood thinning agents. Among these tools are HAS-BLED, ATRIA, and CHA2DS2-VASc.
Anticoagulants are also required in cardiopulmonary bypass.
Patients aged 80 years or more may be especially susceptible to bleeding complications, with a rate of 13 bleeds per 100 person-years. Depletion of vitamin K by coumadin therapy increases risk of arterial calcification and heart valve calcification, especially if too much vitamin D is present.
Bleeding risk is especially important to consider in patients with renal impairment and NOAC therapy due to the fact that all NOACs, to some extent, are excreted by the kidneys. Thus, patients with renal impairment may be at higher risk of increased bleeding.
Foods and food supplements with blood-thinning effects include nattokinase, lumbrokinase, beer, bilberry, celery, cranberries, fish oil, garlic, ginger, ginkgo, ginseng, green tea, horse chestnut, licorice, niacin, onion, papaya, pomegranate, red clover, soybean, St. John's wort, turmeric, wheatgrass, and willow bark. Many herbal supplements have blood-thinning properties, such as danshen and feverfew. Multivitamins that do not interact with clotting are available for patients on anticoagulants.
However, some foods and supplements encourage clotting. These include alfalfa, avocado, cat's claw, coenzyme Q10, and dark leafy greens such as spinach. Their intake should be avoided whilst taking anticoagulants or, if coagulability is being monitored, their intake should be kept approximately constant so that anticoagulant dosage can be maintained at a level high enough to counteract this effect without fluctuations in coagulability.
Grapefruit interferes with some anticoagulant drugs, increasing the amount of time it takes for them to be metabolized out of the body, and so should be eaten only with caution when on anticoagulant drugs.
Anticoagulants are often used to treat acute deep vein thrombosis. People using anticoagulants to treat this condition should avoid using bed rest as a complementary treatment because there are clinical benefits to continuing to walk and remaining mobile while using anticoagulants in this way. Bed rest while using anticoagulants can harm patients in circumstances in which it is not medically necessary.
A number of anticoagulants are available. The traditional ones (warfarin, other coumarins and heparins) are in widespread use. Since the 2000s a number of agents have been introduced that are collectively referred to as directly acting oral anticoagulants (DOACs), novel oral anticoagulants (NOACs), or non-vitamin K antagonist oral anticoagulants. These agents include direct thrombin inhibitor (dabigatran) and factor Xa inhibitor (rivaroxaban, apixaban, betrixaban and edoxaban) and they have been shown to be as good or possibly better than the coumarins with less serious side effects. The newer anticoagulants (NOACs/DOACs), are more expensive than the traditional ones and should be used with care in patients with kidney problems. There is an antidote for the factor Xa inhibitors ��� andexanet alfa. Also, idarucizumab was FDA approved for the reversal of dabigatran in 2015.
Coumarins (vitamin K antagonists)
These oral anticoagulants are derived from coumarin, which is found in many plants. A prominent member of this class is warfarin (Coumadin) and was found to be the dominant anticoagulant prescribed in a large multispecialty practice. It takes at least 48 to 72 hours for the anticoagulant effect to develop. Where an immediate effect is required, heparin must be given concomitantly. These anticoagulants are used to treat patients with deep-vein thrombosis (DVT), pulmonary embolism (PE) and to prevent emboli in patients with atrial fibrillation (AF), and mechanical prosthetic heart valves. Other examples are acenocoumarol, phenprocoumon, atromentin, and phenindione.
Heparin and derivative substances
Heparin is the most widely used clinical anticoagulant worldwide. Heparin is a naturally occurring glycosaminoglycan. There are three major categories of heparin: unfractionated heparin (UFH), low molecular weight heparin (LMWH), and ultra-low-molecular weight heparin (ULMWH). Unfractionated heparin is usually derived from pig intestines and bovine lungs. UFH binds to the enzyme inhibitor antithrombin III (AT), causing a conformational change that results in its activation. The activated AT then inactivates factor Xa, thrombin, and other coagulation factors. Heparin can be used in vivo (by injection), and also in vitro to prevent blood or plasma clotting in or on medical devices. In venipuncture, Vacutainer brand blood collecting tubes containing heparin usually have a green cap.
Low molecular weight heparin (LMWH)
Low molecular weight heparin (LMWH), is produced through a controlled depolymerization of unfractionated heparin. LMWH exhibits higher anti-Xa/anti-IIa activity ratio and is useful as it does not require monitoring of the APTT coagulation parameter and has fewer side effects.
Synthetic pentasaccharide inhibitors of factor Xa
- Fondaparinux is a synthetic sugar composed of the five sugars (pentasaccharide) in heparin that bind to antithrombin. It is a smaller molecule than low molecular weight heparin.
Directly acting oral anticoagulants
The directly acting oral anticoagulants (DOACs) were introduced on and after 2008. There are five DOACs currently on the market: dabigatran, rivaroxaban, apixaban, edoxaban and betrixaban. They were also previously referred to as "new/novel" and "non-vitamin K antagonist" oral anticoagulants (NOACs).
Compared to warfarin, NOACs have a rapid onset action and relatively short half-lives; hence, they carry out their function more rapidly and effectively, and allow for drugs to quickly reduce their anticoagulation effects. Routine monitoring and dose adjustments of NOACs is less important than for warfarin, as they have better predictable anticoagulation activity.
Both NOACs and warfarin are equivalently effective, but NOACs are less influenced by diet and medications compared to warfarin. However, there is presently no countermeasure for most NOACs unlike in warfarin; nonetheless, the short half-lives of NOACs will result in its effects to swiftly recede. A reversal agent for dabigatran, idarucizumab, is currently available and approved for use by the FDA. Rates of adherence to NOACs are only modestly higher than adherence to warfarin among patients prescribed these drugs, and thus adherence to anticoagulation is universally poor, despite hopes that NOACs would lead to higher adherence rates.
NOACs are a lot more expensive than warfarin, after having taken into consideration the cost of frequent blood testing associated with warfarin.
Direct factor Xa inhibitors
Drugs such as rivaroxaban, apixaban and edoxaban work by inhibiting factor Xa directly (unlike the heparins and fondaparinux, which work via antithrombin activation). Also betrixaban from Portola Pharmaceuticals, darexaban (YM150) from Astellas, and more recently letaxaban (TAK-442) from Takeda and eribaxaban (PD0348292) from Pfizer. Betrixaban is significant as it is the only oral factor Xa inhibitor approved by the FDA for use in acutely medically ill patients. The development of darexaban was discontinued in September 2011: in a trial for prevention of recurrences of myocardial infarction in top of dual antiplatelet therapy (DAPT), the drug did not demonstrate effectiveness and the risk of bleeding was increased by approximately 300%. The development of letaxaban was discontinued for acute coronary syndrome in May 2011 following negative results from a Phase II study.
Direct thrombin inhibitors
Another type of anticoagulant is the direct thrombin inhibitor. Current members of this class include the bivalent drugs hirudin, lepirudin, and bivalirudin; and the monovalent drugs argatroban and dabigatran. An oral direct thrombin inhibitor, ximelagatran (Exanta) was denied approval by the Food and Drug Administration (FDA) in September 2004 and was pulled from the market entirely in February 2006 after reports of severe liver damage and heart attacks. In November 2010, dabigatran etexilate was approved by the FDA to treat atrial fibrillation.
NOAC relevance to dental treatments
As in any invasive procedures, patients on anticoagulation therapy have increased risk for bleeding and caution should be used along with local hemostasis methods to minimize bleeding risk during the operation as well as post-operatively. However, with regards to NOACs and invasive dental treatments, there has not been enough clinical evidence and experience to prove any reliable adverse-effects, relevance or interaction between these two. Further clinical prospective studies on NOACs are required to investigate the bleeding risk and haemostasis associated to surgical dental procedures.
Recommendations of modifications to usage/dosage of NOACs prior to dental treatments are made based on the balance of severity of each procedure and also the individual's bleeding risks and renal functionality. With low bleeding risk of dental procedures, it is recommended that NOAC medicine still be taken by the patient as per normal, so as to avoid increase in the risk of thromboembolic event. For dental procedures with a higher risk of bleeding complications (i.e. complex extractions, adjacent extractions leading to large wound or more than three extractions), the recommended practice is for patient to miss or delay a dose of their NOAC before such procedures so as to minimize the effect on bleeding risk.
Antithrombin protein therapeutics
The antithrombin protein itself is used as a protein therapeutic that can be purified from human plasma or produced recombinantly (for example, Atryn, which is produced in the milk of genetically modified goats.)
Other types of anticoagulants
- Batroxobin, a toxin from a snake venom, clots platelet-rich plasma without affecting platelet functions (lyses fibrinogen).
- Hementin is an anticoagulant protease from the salivary glands of the giant Amazon leech, Haementeria ghilianii.
- Vitamin E
Coagulation inhibitor measurement
A Bethesda unit (BU) is a measure of blood coagulation inhibitor activity. It is the amount of inhibitor that will inactivate half of a coagulant during the incubation period. It is the standard measure used in the United States, and is so named because it was adopted as a standard at a conference in Bethesda, Maryland.
Laboratory instruments, blood transfusion bags, and medical and surgical equipment will get clogged up and become non-operational if blood is allowed to clot. In addition, test tubes used for laboratory blood tests will have chemicals added to stop blood clotting. Apart from heparin, most of these chemicals work by binding calcium ions, preventing the coagulation proteins from using them.
- Ethylenediaminetetraacetic acid (EDTA) strongly and irreversibly chelates (binds) calcium ions, preventing blood from clotting.
- Citrate is in liquid form in the tube and is used for coagulation tests, as well as in blood transfusion bags. It binds the calcium, but not as strongly as EDTA. Correct proportion of this anticoagulant to blood is crucial because of the dilution, and it can be reversed with the addition of calcium. It can be in the form of sodium citrate or acid-citrate-dextrose.
- Oxalate has a mechanism similar to that of citrate. It is the anticoagulant used in fluoride oxalate tubes used to determine glucose and lactate levels.
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...in a market now dominated by one of the oldest mainstay pills in medicine: the blood thinner warfarin. At least five next-generation blood thinners are in advanced testing to treat or prevent potentially debilitating or life-threatening blood clots in surgery and heart patients. First candidates could reach the market in 2009.
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Things have changed dramatically with the introduction of the new oral anticoagulants (NOACs) — dabigatran, a factor IIa (thrombin) inhibitor, and the factor Xa inhibitors rivaroxaban and apixaban. Clinical trials have shown them therapeutically superior, or at least non-inferior, to VKAs, with less serious side effects.
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