Warfarin is an anticoagulant medicine that is used to forestall thrombosis ( coagulums ) and intercalation in many upsets. Warfarin activity has to be monitored by frequent blood proving for the International Normalized Ratio ( INR ) . Warfarin is a man-made derived function of coumarin, a chemical found of course in many workss — it decreases blood curdling by interfering with vitamin K metamorphosis.
Mechanism of Action of Warfarin
Warfarin is vitamin K adversary that produce it ‘s anticoagulant consequence by interfering with the cyclic interconversion of vitamin K and its 2,3 epoxide ( vitamin K epoxide ) . Vitamin K is a cofactor for the posttranslational carboxylation of glutamate residues to I?-carboxyglutamates on the N-terminal parts of vitamin K-dependent proteins ( Whitlon, et al. , 1978 ; Fasco, et al. , 1982 )
.123456 These curdling factors ( factors II, VII, IX, and X ) require I?-carboxylation for their biological activity. Coumarins produce their anticoagulant consequence by suppressing the vitamin K transition rhythm, thereby doing hepatic production of partly carboxylated and decarboxylated proteins with decreased procoagulant activity.78 In add-on to their anticoagulant consequence, the vitamin K antagonists inhibit carboxylation of the regulative anticoagulant proteins C and S and hence have the possible to exercise a procoagulant consequence.
In the presence of Ca ions, carboxylation causes a conformational alteration in curdling proteins91011 that promotes adhering to cofactors on phospholipid surfaces. The carboxylation reaction requires the decreased signifier of vitamin K ( vitamin KH2 ) , molecular O, and C dioxide, and is linked to the oxidization of vitamin KH2 to vitamin K epoxide. Vitamin K epoxide is so recycled to vitamin KH2 through two reductase stairss. The first, which is sensitive to vitamin K antagonists,123 reduces vitamin K epoxide to vitamin K1 ( the natural nutrient signifier of vitamin K1 ) , while the 2nd, which is comparatively insensitive to vitamin K adversaries, reduces vitamin K1 to vitamin KH2. Treatment with vitamin K antagonists leads to the depletion of vitamin KH2, thereby restricting the I?-carboxylation of the vitamin K-dependent coagulator proteins. The consequence of coumarins can be counteracted by vitamin K1 ( either ingested in nutrient or administered therapeutically ) because the 2nd reductase measure is comparatively insensitive to vitamin K adversaries ( Fig 1 ) . Patients treated with a big dosage of vitamin K1 can besides go warfarin resistant for up to a hebdomad because vitamin K1 accumulates in the liver and is available to the coumarin-insensitive reductase.
Pharmacokineticss and Pharmacodynamics of Warfarin
Warfarin is a racemic mixture of two optically active isomers, the R and S signifiers in approximately equal proportion. It has high bioavailability,1920 is quickly absorbed from the GI piece of land, and reaches maximum blood concentrations in healthy voluntaries in 90 min after unwritten administration.1921 Racemic Coumadin has a half life of 36 to 42 H, circulates bound to plasma proteins ( chiefly albumin ) , and accumulates in the liver where the two isomers are metabolically transformed by different pathways.22 The dose-response relationship of Coumadin is influenced by familial and environmental factors, including a late identified common mutant in the cistron coding for one of the common cytochrome P450 enzymes ( 2C9 ) , the hepatic enzyme responsible for oxidative metamorphosis of the warfarin S-isomer.2324 This mutant probably contributes to the variableness in dose response to warfarin among healthy subjects.25 In add-on to cognize and unknown familial factors, assorted disease provinces, drugs, and dietetic factors can interfere with the response to warfarin.
The anticoagulant response to warfarin is influenced by pharmacokinetic factors, including drug interactions that affect the soaking up or metabolic clearance of Coumadin, and pharmacodynamic factors that alter the styptic response to given concentrations of the drug. Variability in anticoagulant response besides occurs as a consequence of inaccuracies in research lab testing, patient disobedience, and miscommunication between patient and doctor. Other drugs may act upon the pharmacokinetics of Coumadin by cut downing GI soaking up or by interrupting its metabolic clearance. For illustration, the anticoagulant consequence of Coumadin is reduced by cholestyramine, which impairs its soaking up, and is potentiated by drugs that inhibit warfarin clearance through stereoselective or nonselective pathways.252627 Stereoselective interactions affect oxidative metamorphosis of either the S-isoner or R-isomer of warfarin.2627 Inhibition of S-warfarin metamorphosis is more of import clinically because this isomer is five times more powerful as a vitamin K adversary than the R-isomer.2627 Clearance of S-isomer Coumadin is inhibited by phenylbutazone,2829 sulfinpyrazone,30 metronidazole,31 and trimethoprim- sulfamethoxazole,32 each of which potentiates the consequence of Coumadin on the factor II clip ( PT ) . In contrast, drugs such as Tagamet and Prilosec that inhibit clearance of the R-isomer have merely moderate potentiating effects on the PT in patients treated with warfarin.272833 Amiodarone inhibits the metabolic clearance of both the S-isomer and R-isomer and potentiates the anticoagulant consequence of warfarin.34 The anticoagulant consequence is inhibited by barbiturates,32 rifampicin,34 and carbamazepine,32 which increase its metabolic clearance by bring oning hepatic assorted oxidase activity. Although long-run intoxicant usage has a possible to increase the clearance of Coumadin through a similar mechanism, ingestion of even comparatively big sums of vino was shown in one study29 to hold small influence on PT in topics treated with Coumadin. For a more thorough treatment of the consequence of enzyme initiation on Coumadin therapy, the reader is referred to a critical reappraisal ( Table 2 ) .35
The pharmacodynamics of Coumadin are capable to familial and environmental variableness. Familial opposition to warfarin occurs in rats36 every bit good as in human beings.3738 Patients with familial Coumadins opposition require doses fivefold to 20-fold higher than norm to accomplish an anticoagulant consequence. This upset is attributed to altered affinity of the receptor for Coumadin since the plasma Coumadin degrees required to accomplish an anticoagulant consequence are increased.
Two mis-sense mutants in the factor IX propeptide have been described394041 that cause hemorrhage without inordinate protraction of PT. When affected persons are treated with coumarin drugs, factor IX activity decreases to approximately 1 to 3 % , while degrees of other vitamin K-dependent curdling factors decrease to 30 to 40 % of normal. These mutants are uncommon and have been estimated to happen in & lt ; 1.5 % of the population. A plausible mechanism for the selective addition in coumarin sensitiveness of the mutant factor IX proposed by Chu et al39 reconciles the undermentioned observations: ( 1 ) normal factor IX activity in the absence of coumarin despite decreased binding of the variant propeptide to I?-carboxylase, and ( 2 ) marked suppression of factor IX activity by coumarin despite merely modest suppression of the other three vitamin K-dependent curdling factors.
Subjects having long-run Coumadin therapy are sensitive to fluctuating degrees of dietetic vitamin K,4243 which is provided preponderantly by vitamin K1 in works material.43 The vitamin K1 content of a broad scope of groceries has been listed by Sadowski and associates.44 Phylloquinone acts through the warfarin-insensitive reductase reaction.45 Important fluctuations in vitamin K intake occur in both seemingly healthy and ill subjects.46 Increased consumption of dietetic vitamin K sufficient to cut down the anticoagulant response to warfarin42 occurs in patients on weight-reduction diets devouring green veggies or having vitamin K-containing addendums, and in patients treated with IV addendums incorporating vitamin K. Reduced dietetic vitamin K1 consumption potentiates the consequence of Coumadin in ill patients treated with antibiotics and IV fluids without vitamin K supplementation and in provinces of fat malabsorption. Hepatic disfunction potentiates the response to warfarin through impaired synthesis of curdling factors. Hypermetabolic provinces produced by febrility or thyrotoxicosis addition Coumadin reactivity, likely by increasing the katabolism of vitamin K-dependent curdling factors.4748 Drugs may act upon the pharmacodynamics of Coumadin by suppressing synthesis or increasing clearance of vitamin K-dependent curdling factors or by interfering with other tracts of haemostasis ( Table 3 ) . The anticoagulant consequence of Coumadin is augmented by the second-generation and third-generation Mefoxins, which inhibit the cyclic interconversion of vitamin K,4950 by tetraiodothyronine, which increases the metamorphosis of curdling factors,48 and by Atromid-S, through an unknown mechanism.51 Doses5253 of salicylates & gt ; 1.5 g/d besides augment the anticoagulant consequence of warfarin,54 perchance because these drugs have warfarin-like activity. Acetaminophen has besides been reported to augment the anticoagulant consequence of warfarin,52 although this contention has been challenged ( see below ) . Although Lipo-Hepin potentiates the anticoagulant consequence of Coumadin, in curative doses, it produces merely little protraction of the PT.
Drugs such as aspirin,55 nonsteroidal anti-inflammatory drugs,56 high doses of penicillins,5758 and moxolactam50 addition the hazard of warfarin-associated hemorrhage by suppressing thrombocyte map. Of these, acetylsalicylic acid is the most of import because of its widespread usage and drawn-out effect.59 Aspirin and nonsteroidal anti-inflammatory drugs can besides bring forth stomachic erodings that increase the hazard of upper-GI bleeding.58 The hazard of clinically of import hemorrhage is heightened when high doses of acetylsalicylic acid are taken in combination with high-intensity Coumadin therapy ( INR, 3.0 to 4.5 ) .5560 In two surveies, one study61 in patients with prosthetic bosom valves and the other study62 in symptomless persons at high hazard of coronary arteria disease, low doses of acetylsalicylic acid ( 100 mg/d and 75 mg/d, severally ) were besides associated with increased rates of minor hemorrhage when combined with moderate-intensity and low-intensity Coumadin anticoagulation.
The mechanisms by which erythromycin63 and some anabolic steroids64 potentiate the anticoagulant consequence of Coumadin are unknown. Sulfa drugs and several broad-spectrum antibiotic compounds may augment the anticoagulant consequence of Coumadin by extinguishing bacterial vegetations and worsening vitamin K lack in patients whose diet is deficient of vitamin K.65
Wells and associates66 performed a critical analysis of articles describing possible interaction between drugs or nutrients and Coumadin. Surveies were assigned to one class if the interaction was considered extremely likely, to a 2nd class if interaction was likely, to a 3rd degree if judged possible, and to a 4th degree if dubious. Of 751 commendations retrieved, pertinent consequences from 172 original articles are summarized in Table 3. Strong grounds of interaction was found for 39 of the 81 different drugs and nutrients appraised ; 17 potentiate Coumadin consequence, 10 inhibit, and 12 green goods no consequence. Many other drugs have been reported to either interact with unwritten decoagulants or change the PT response to warfarin,6768 but converting grounds of a causal association is missing. In a case-control study,52 low to chair doses of Datril ( nine or more tablets per hebdomad ) were reported to be associated with overly prolonged INR values. The presence of a causal association between Datril usage and potentiation of a warfarin consequence is unsure. The article52 was supported by an editorial,53 but has been challenged by personal experiences ( instance series ) cited in two letters6970 and by the consequences of a prospective study71 in normal voluntaries. However, until more information is presented, it would be prudent to supervise the INR more often when Datril is used in this measure by patients during warfarin therapy. Indeed, it would be sensible to supervise the PT more often when any drug therapy is added or withdrawn from the regimen of a patient treated with an unwritten decoagulant.
Drug INTERACTIONS: Your physician or druggist may already be cognizant of any possible drug interactions and may be supervising you for them. Make non get down, halt, or alter the dose of any drug, vitamin, or herbal merchandise without look intoing with your physician or druggist first.Warfarin interacts with many prescription, nonprescription, vitamin, and herbal merchandises. This includes medicines that are applied to the tegument or inside the vagina or rectum. The undermentioned interactions listed do non incorporate all possible drug interactions. The interactions with Coumadin normally result in an addition or lessening in the “ blood-thinning ” ( anticoagulant ) consequence. Your physician or other wellness attention professional should closely supervise you to forestall serious hemorrhage or coagulating jobs. While taking Coumadin, it is really of import to state your physician or druggist of any alterations in medicines, vitamins, or herbal merchandises that you are taking.This drug should non be used with the undermentioned medicines because really serious interactions may happen: imatinib, mifepristone.If you are presently utilizing any of these medicines listed supra, state your physician or druggist before get downing warfarin.Aspirin and aspirin-like drugs ( salicylates ) and nonsteroidal anti-inflammatory drugs ( NSAIDs such as isobutylphenyl propionic acid, naproxen, Celebrexs ) may hold effects similar to warfarin. These drugs may increase the hazard of shed blooding jobs if taken during intervention with Coumadin. Carefully look into all prescription/nonprescription merchandise labels ( including drugs applied to the tegument such as pain-relieving pick ) since the merchandises may incorporate NSAIDs or salicylates. Talk to your physician about utilizing a different medicine ( such as Datril ) to handle pain/fever. Low-dose acetylsalicylic acid and related drugs ( such as clopidogrel, ticlopidine ) should be continued if prescribed by your physician for specific medical grounds such as bosom onslaught or shot bar. Consult your physician or druggist for more details.Many herbal medicines have “ blood-thinning ” or “ blood-clotting ” effects, and some may straight impact Coumadin. State your physician before taking any herbal merchandises, particularly bromelains, coenzyme Q10, cranberry, danshen, dong quai, Greek clover, Allium sativum, gingko biloba, ginseng, golden seal, and St. John ‘s wort, among others.This medicine may interfere with a certain laboratory trial to mensurate Elixophyllin degrees, perchance doing false trial consequences. Make certain research lab forces and all your physicians know you use this drug.This papers does non incorporate all possible interactions. Therefore, before utilizing this merchandise, state your physician or druggist of all the merchandises you use. Keep a list of all your medicines with you, and portion the list with your physician and druggist
