Warfarin Tablets

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Warfarin


Generic Name: Warfarin sodium
Dosage Form: Tablets

Warning

Warfarin Description

Warfarin sodium (crystalline), is an anticoagulant which acts by inhibiting vitamin K-dependent coagulation factors. Chemically, it is 3-α-Acetonylbenzyl)-4-hydroxycoumarin and is a racemic mixture of the R-and S-enantiomers. Crystalline Warfarin sodium is an isopropanol clathrate. The crystallization of Warfarin sodium virtually eliminates trace impurities present in amorphous Warfarin. Its molecular formula is C19H15NaO4, its molecular weight is 330.31, and its structural formula may be represented by the following:

Crystalline Warfarin sodium occurs as a white, odorless, crystalline powder, is discolored by light and is very soluble in water; freely soluble in alcohol; very slightly soluble in chloroform and in ether.

Warfarin sodium tablets, for oral administration contain either 1 mg, 2 mg, 2.5 mg, 3 mg, 4 mg, 5 mg, 6 mg, 7.5 mg or 10 mg Warfarin sodium. In addition they also contain the following inactive ingredients: corn starch, lactose monohydrate, magnesium stearate, stearic acid and,

1 mg tablets: FD&C Red # 40 Aluminum Lake

2 mg tablets: FD&C Blue # 2 Aluminum Lake and FD&C Red # 40 Aluminum Lake

2.5 mg tablets: D&C Yellow # 10 Aluminum Lake and FD&C Blue # 1 Aluminum Lake

3 mg tablets: FD&C Yellow # 6 Aluminum Lake, FD&C Blue # 2 Aluminum Lake, FD&C Red #40 Aluminum Lake

4 mg tablets: FD&C Blue # 1 Aluminum Lake

5 mg tablets: FD&C Yellow # 6 Aluminum Lake

6 mg tablets: FD&C Blue # 1 Aluminum Lake, FD&C Yellow # 6 Aluminum Lake

7.5 mg tablets: D&C Yellow # 10 Aluminum Lake and FD&C Yellow # 6 Aluminum Lake

10 mg tablets: Dye Free

Warfarin - Clinical Pharmacology

Warfarin sodium and other coumarin anticoagulants act by inhibiting the synthesis of vitamin K dependent clotting factors, which include Factors II, VII, IX and X, and the anticoagulant proteins C and S. Half-lives of these clotting factors are as follows: Factor II - 60 hours, VII - 4 to 6 hours, IX - 24 hours, and X- 48 to 72 hours. The half-lives of proteins C and S are approximately 8 hours and 30 hours, respectively. The resultant in vivo effect is a sequential depression of Factor VII, Protein C, Factor IX, Protein S, and Factor X and II activities. Vitamin K is an essential cofactor for the post ribosomal synthesis of the vitamin K dependent clotting factors. The vitamin promotes the biosynthesis of γ-carboxyglutamic acid residues in the proteins which are essential for biological activity. Warfarin is thought to interfere with clotting factor synthesis by inhibition of the regeneration of vitamin K1 epoxide. The degree of depression is dependent upon the dosage administered. Therapeutic doses of Warfarin decrease the total amount of the active form of each vitamin K dependent clotting factor made by the liver by approximately 30% to 50%.

An anticoagulation effect generally occurs within 24 hours after drug administration. However, peak anticoagulant effect may be delayed 72 to 96 hours. The duration of action of a single dose of racemic Warfarin is 2 to 5 days. The effects of Warfarin sodium may become more pronounced as effects of daily maintenance doses overlap. Anticoagulants have no direct effect on an established thrombus, nor do they reverse ischemic tissue damage. However, once a thrombus has occurred, the goal of anticoagulant treatment is to prevent further extension of the formed clot and prevent secondary thromboembolic complications which may result in serious and possibly fatal sequelae.

Pharmacokinetics

Warfarin sodium is a racemic mixture of the R- and S-enantiomers. The S-enantiomer exhibits 2 to 5 times more anticoagulant activity than the R-enantiomer in humans, but generally has a more rapid clearance.

Absorption

Warfarin sodium is essentially completely absorbed after oral administration with peak concentration generally attained within the first 4 hours.

Distribution

There are no differences in the apparent volumes of distribution after intravenous and oral administration of single doses of Warfarin solution. Warfarin distributes into a relatively small apparent volume of distribution of about 0.14 liter/kg. A distribution phase lasting 6 to 12 hours is distinguishable after rapid intravenous or oral administration of an aqueous solution. Using a one compartment model, and assuming complete bioavailability, estimates of the volumes of distribution of R- and S-Warfarin are similar to each other and to that of the racemate. Concentrations in fetal plasma approach the maternal values, but Warfarin has not been found in human milk (see WARNINGS: Lactation). Approximately 99% of the drug is bound to plasma proteins.

Metabolism

The elimination of Warfarin is almost entirely by metabolism. Warfarin sodium is stereoselectively metabolized by hepatic microsomal enzymes (cytochrome P-450) to inactive hydroxylated metabolites (predominant route) and by reductases to reduced metabolites (Warfarin alcohols). The Warfarin alcohols have minimal anticoagulant activity. The metabolites are principally excreted into the urine; and to a lesser extent into the bile. The metabolites of Warfarin that have been identified include dehydroWarfarin, two diastereoisomer alcohols, 4"-, 6-, 7-, 8- and 10-hydroxyWarfarin. The Cytochrome P-450 isozymes involved in the metabolism of Warfarin include 2C9, 2C19, 2C8, 2C18, 1A2, and 3A4. 2C9 is likely to be the principal form of human liver P-450 which modulates the in vivo anticoagulant activity of Warfarin.

Excretion

The terminal half-life of Warfarin after a single dose is approximately one week; however, the effective half-life ranges from 20 to 60 hours, with a mean of about 40 hours. The clearance of R-Warfarin is generally half that of S-Warfarin, thus as the volumes of distribution are similar, the half-life of R-Warfarin is longer than that of S-Warfarin. The half-life of R-Warfarin ranges from 37 to 89 hours, while that of S-Warfarin ranges from 21 to 43 hours. Studies with radiolabeled drug have demonstrated that up to 92% of the orally administered dose is recovered in urine. Very little Warfarin is excreted unchanged in urine. Urinary excretion is in the form of metabolites.

Elderly

Patients 60 years or older appear to exhibit greater than expected PT/INR response to the anticoagulant effects of Warfarin. The cause of the increased sensitivity to the anticoagulant effects of Warfarin in this age group is unknown. This increased anticoagulant effect from Warfarin may be due to a combination of pharmacokinetic and pharmacodynamic factors. Racemic Warfarin clearance may be unchanged or reduced with increasing age. Limited information suggests there is no difference in the clearance of S-Warfarin in the elderly versus young subjects. However, there may be a slight decrease in the clearance of R-Warfarin in the elderly as compared to the young. Therefore, as patient age increases, a lower dose of Warfarin is usually required to produce a therapeutic level of anticoagulation.

Asians

Asian patients may require lower initiation and maintenance doses of Warfarin. One non-controlled study conducted in 151 Chinese outpatients reported a mean daily Warfarin requirement of 3.3 ± 1.4 mg to achieve an INR of 2 to 2.5. These patients were stabilized on Warfarin for various indications. Patient age was the most important determinant of Warfarin requirement in Chinese patients with a progressively lower Warfarin requirement with increasing age.

Renal Dysfunction

Renal clearance is considered to be a minor determinant of anticoagulant response to Warfarin. No dosage adjustment is necessary for patients with renal failure.

Hepatic Dysfunction

Hepatic dysfunction can potentiate the response to Warfarin through impaired synthesis of clotting factors and decreased metabolism of Warfarin.

The administration of Warfarin sodium via the intravenous (I.V.) route should provide the patient with the same concentration of an equal oral dose, but maximum plasma concentration will be reached earlier. However, the full anticoagulant effect of a dose of Warfarin may not be achieved until 72 to 96 hours after dosing, indicating that the administration of I.V. Warfarin sodium should not provide any increased biological effect or earlier onset of action.

Clinical Trials

ATRIAL FIBRILLATION (AF)

In five prospective randomized controlled clinical trials involving 3711 patients with non-rheumatic AF, Warfarin significantly reduced the risk of systemic thromboembolism including stroke (See Table 1). The risk reduction ranged from 60% to 86% in all except one trial (CAFA: 45%) which stopped early due to published positive results from two of these trials. The incidence of major bleeding in these trials ranged from 0.6 to 2.7% (see Table 1). Meta-analysis findings of these studies revealed that the effects of Warfarin in reducing thromboembolic events including stroke were similar at either moderately high INR (2.0 to 4.5) or low INR (1.4 to 3.0). There was a significant reduction in minor bleeds at the low INR. Similar data from clinical studies in valvular atrial fibrillation patients are not available.

TABLE 1
CLINICAL STUDIES OF Warfarin IN NON-RHEUMATIC AF PATIENTS*
Study N PT Ratio INR Thromboembolism % Major Bleeding
Warfarin- Control % Risk p- value Warfarin- Control
Treated Patients Reduction Treated Patients
Patients Patients
AFASAK 335 336 1.5-2.0 2.8-4.2 60 0.027 0.6 0.0
SPAF 210 211 1.3-1.8 2.0-4.5 67 0.01 1.9 1.9
BAATAF 212 208 1.2-1.5 1.5-2.7 86 <0.05 0.9 0.5
CAFA 187 191 1.3-1.6 2.0-3.0 45 0.25 2.7 0.5
SPINAF 260 265 1.2-1.5 1.4-2.8 79 0.001 2.3 1.5

*All study results of Warfarin vs. control are based on intention-to-treat analysis and include ischemic stroke and systemic thromboembolism, excluding hemorrhage and transient ischemic attacks.

MYOCARDIAL INFARCTION

WARIS (The Warfarin Re-Infarction Study) was a double-blind, randomized study of 1214 patients 2 to 4 weeks postinfarction treated with Warfarin to a target INR of 2.8 to 4.8. [But note that a lower INR was achieved and increased bleeding was associated with INR"s above 4.0; (see DOSAGE AND ADMINISTRATION)]. The primary endpoint was a combination of total mortality and recurrent infarction. A secondary endpoint of cerebrovascular events was assessed. Mean follow-up of the patients was 37 months. The results for each endpoint separately, including an analysis of vascular death, are provided in the following table:

TABLE 2
%Risk
Event Warfarin Placebo Reduction
(N=607) (N=607) RR(95%CI) (p-value)
Total Patient Years of Follow-up 2018 1944
Total Mortality 94 (4.7/100 py) 123 (6.3/100 py) 0.76 (0.60, 0.97) 24 (p=0.030)
Vascular Death 82 (4.1/100 py) 105 (5.4/100 py) 0.78 (0.60, 1.02) 22 (p=0.068)
Recurrent MI 82 (4.1/100 py) 124 (6.4/100 py) 0.66 (0.51, 0.85) 34 (p=0.001)
Cerebrovascular Event 20 (1.0/100 py) 44 (2.3/100 py) 0.46 (0.28, 0.75) 54 (p=0.002)

RR=Relative risk; Risk reduction=(I-RR); CI=Confidence interval; MI=Myocardial infarction; py=patient years

WARIS II (The Warfarin, Aspirin, Re-Infarction Study) was an open-label randomized study of 3630 patients hospitalized for acute myocardial infarction treated with Warfarin target INR 2.8 to 4.2, aspirin 160 mg/day, or Warfarin target INR 2.0 to 2.5 plus aspirin 75 mg/day prior to hospital discharge. There were approximately four times as many major bleeding episodes in the two groups receiving Warfarin than in the group receiving aspirin alone. Major bleeding episodes were not more frequent among patients receiving aspirin plus Warfarin than among those receiving Warfarin alone, but the incidence of minor bleeding episodes was higher in the combined therapy group. The primary endpoint was a composite of death, nonfatal reinfarction, or thromboembolic stroke. The mean duration of observation was approximately 4 years. The results for WARIS II are provided in the following table1:

Table 3: WARIS II - Distribution of Separate Events According to Treatment Group*
Event Aspirin Warfarin Aspirin Plus Warfarin Rate Ratio p-value
(N=1206) (N=1216) (N=1208) (95% CI)
No.of Events
Reinfarction 117 90 69 0.56 (0.41-0.78)a <0.001
0.74 (0.55-0.98)b 0.03
Thromboembolic stroke 32 17 17 0.52 (0.28-0.98)a 0.03
0.52 (0.28-0.97)b 0.03
Major Bleeding c 8 33 28 3.35a (ND) ND
4.00b (ND) ND
Minor Bleeding d 39 103 133 3.21a (ND) ND
2.55b (ND) ND
Death 92 96 95 0.82

* CI denotes confidence interval.

a The rate ratio is for aspirin plus Warfarin as compared with aspirin.

b The rate ratio is for Warfarin as compared with aspirin.

cMajor bleeding episodes were defined as nonfatal cerebral hemorrhage or bleeding necessitating

surgical intervention.

d Minor bleeding episodes were defined as non-cerebral hemorrhage not necessitating surgical intervention of blood transfusion.

ND =not determined.

MECHANICAL AND BIOPROSTHETIC HEART VALVES

In a prospective, randomized, open label, positive-controlled study2 in 254 patients, the thromboembolic-free interval was found to be significantly greater in patients with mechanical prosthetic heart valves treated with Warfarin alone compared with dipyridamole-aspirin (p<0.005) and pentoxifylline-aspirin (p<0.05) treated patients. Rates of hromboembolic events in these groups were 2.2, 8.6, and 7.9/100 patient years, respectively. Major bleeding rates were 2.5, 0.0, and 0.9/100 patient years, respectively.

In a prospective, open label, clinical trial comparing moderate (INR 2.65) vs. high intensity (INR 9.0) Warfarin therapies in 258 patients with mechanical prosthetic heart valves, thromboembolism occurred with similar frequency in the two groups (4.0 and 3.7 events/100 patient years, respectively). Major bleeding was more common in the high intensity group (2.1 events/100 patient years) vs. 0.95 events/100 patient years in the moderate intensity group3.

In a randomized trial in 210 patients comparing two intensities of Warfarin therapy (INR 2.0 to 2.25 vs. INR 2.5 to 4.0) for a three month period following tissue heart valve replacement, thromboembolism occurred with similar frequency in the two groups (major embolic events 2.0% vs. 1.9%, respectively and minor embolic events 10.8% vs. 10.2%, respectively). Major bleeding complications were more frequent with the higher intensity (major hemorrhages 4.6%) vs. none in the lower intensity4.

Indications and Usage for Warfarin

Warfarin sodium tablets are indicated for the prophylaxis and/or treatment of venous thrombosis and its extension, and pulmonary embolism.

Warfarin sodium tablets are indicated for the prophylaxis and/or treatment of the thromboembolic complications associated with atrial fibrillation and/or cardiac valve replacement.

Warfarin sodium tablets are indicated to reduce the risk of death, recurrent myocardial infarction, and thromboembolic events such as stroke or systemic embolization after myocardial infarction.

Contraindications

Anticoagulation is contraindicated in any localized or general physical condition or personal circumstance in which the hazard of hemorrhage might be greater than the potential clinical benefits of anticoagulation, such as:

Pregnancy:Warfarin sodium tablets are contraindicated in women who are or may become pregnant because the drug passes through the placental barrier and may cause fatal hemorrhage to the fetus in utero. Furthermore, there have been reports of birth malformations in children born to mothers who have been treated with Warfarin during pregnancy.

Embryopathy characterized by nasal hypoplasia with or without stippled epiphyses (chondrodysplasia punctata) has been reported in pregnant women exposed to Warfarin during the first trimester. Central nervous system abnormalities also have been reported, including dorsal midline dysplasia characterized by agenesis of the corpus callosum, Dandy-Walker malformation, and midline cerebellar atrophy. Ventral midline dysplasia, characterized by optic atrophy, and eye abnormalities have been observed. Mental retardation, blindness, and other central nervous system abnormalities have been reported in association with second and third trimester exposure. Although rare, teratogenic reports following in utero exposure to Warfarin include urinary tract anomalies such as single kidney, asplenia, anencephaly, spina bifida, cranial nerve palsy, hydrocephalus, cardiac defects and congenital heart disease, polydactyly, deformities of toes, diaphragmatic hernia, corneal leukoma, cleft palate, cleft lip, schizencephaly, and microcephaly.

Spontaneous abortion and still birth are known to occur and a higher risk of fetal mortality is associated with the use of Warfarin. Low birth weight and growth retardation have also been reported.

Women of childbearing potential who are candidates for anticoagulant therapy should be carefully evaluated and the indications critically reviewed with the patient. If the patient becomes pregnant while taking this drug, she should be apprised of the potential risks to the fetus, and the possibility of termination of the pregnancy should be discussed in light of those risks.

Hemorrhagic tendencies or blood dyscrasias.

Recent or contemplated surgery of: (1) central nervous system; (2) eye; (3) traumatic surgery resulting in large open surfaces.

Bleeding tendencies associated with active ulceration or overt bleeding of: (1) gastrointestinal, genitourinary or respiratory tracts; (2) cerebrovascular hemorrhage; (3) aneurysms-cerebral, dissecting aorta; (4) pericarditis and pericardial effusions; (5) bacterial endocarditis.

Threatened abortion, eclampsia and preeclampsia.

Inadequate laboratory facilities.

Unsupervised patients with senility, alcoholism, or psychosis or other lack of patient cooperation.

Spinal puncture and other diagnostic or therapeutic procedures with potential for uncontrollable bleeding.

Miscellaneous: major regional, lumbar block anesthesia, malignant hypertension and known hypersensitivity to Warfarin or to any other components of this product.

Warnings

The most serious risks associated with anticoagulant therapy with Warfarin sodium are hemorrhage in any tissue or organ5 (see  BLACK BOX WARNING) and, less frequently (<0.1%), necrosis and/or gangrene of skin and other tissues. Hemorrhage and necrosis have in some cases been reported to result in death or permanent disability. Necrosis appears to be associated with local thrombosis and usually appears within a few days of the start of anticoagulant therapy. In severe cases of necrosis, treatment through debridement or amputation of the affected tissue, limb, breast or penis has been reported. Careful diagnosis is required to determine whether necrosis is caused by an underlying disease. Warfarin therapy should be discontinued when Warfarin is suspected to be the cause of developing necrosis and heparin therapy may be considered for anticoagulation. Although various treatments have been attempted, no treatment for necrosis has been considered uniformly effective. See below for information on predisposing conditions. These and other risks associated with anticoagulant therapy must be weighed against the risk of thrombosis or embolization in untreated cases.

It cannot be emphasized too strongly that treatment of each patient is a highly individualized matter. Warfarin sodium, a narrow therapeutic range (index) drug, may be affected by factors such as other drugs and dietary Vitamin K. Dosage should be controlled by periodic determinations of prothrombin time (PT)/lnternational Normalized Ratio (INR) or other suitable coagulation tests. Determinations of whole blood clotting and bleeding times are not effective measures for control of therapy. Heparin prolongs the one-stage PT. When heparin and Warfarin are administered concomitantly, refer below to CONVERSION FROM HEPARIN THERAPY for recommendations.

Caution should be observed when Warfarin sodium is administered in any situation or in the presence of any predisposing condition where added risk of hemorrhage, necrosis, and/or gangrene is present.

Anticoagulation therapy with Warfarin sodium may enhance the release of atheromatous plaque emboli, thereby increasing the risk of complications from systemic cholesterol microembolization, including the "purple toes syndrome". Discontinuation of Warfarin sodium therapy is recommended when such phenomena are observed. Systemic atheroemboli and cholesterol microemboli can present with a variety of signs and symptoms including purple toes syndrome, livedo reticularis, rash, gangrene, abrupt and intense pain in the leg, foot, or toes, foot ulcers, myalgia, penile gangrene, abdominal pain, flank or back pain, hematuria, renal insufficiency, hypertension, cerebral ischemia, spinal cord infarction, pancreatitis, symptoms simulating polyarteritis, or any other sequelae of vascular compromise due to embolic occlusion. The most commonly involved visceral organs are the kidneys followed by the pancreas, spleen, and liver. Some cases have progressed to necrosis or death.

Purple toes syndrome is a complication of oral anticoagulation characterized by a dark, purplish or mottled color of the toes, usually occurring between 3 to 10 weeks, or later, after the initiation of therapy with Warfarin or related compounds. Major features of this syndrome include purple color of plantar surfaces and sides of the toes that blanches on moderate pressure and fades with elevation of the legs; pain and tenderness of the toes; waxing and waning of the color over time. While the purple toes syndrome is reported to be reversible, some cases progress to gangrene or necrosis which may require debridement of the affected area, or may lead to amputation.

Heparin-induced thrombocytopenia

Warfarin sodium should be used with caution in patients with heparin-induced thrombocytopenia and deep venous thrombosis. Cases of venous limb ischemia, necrosis, and gangrene have occurred in patients with heparin-induced thrombocytopenia and deep venous thrombosis when heparin treatment was discontinued and Warfarin therapy was started or continued. In some patients sequelae have included amputation of the involved area and/or death6.

A severe elevation (>50 seconds) in activated partial thromboplastin time (aPTT) with a PT/INR in the desired range as been identified as an indication of increased risk of postoperative hemorrhage.

The decision to administer anticoagulants in the following conditions must be based upon clinical judgment in which the risks of anticoagulant therapy are weighed against the benefits:

Lactation: Based on very limited published data, Warfarin has not been detected in the breast milk of mothers treated with Warfarin. The same limited published data reports that some breast-fed infants, whose mothers were treated with Warfarin, had prolonged prothrombin times, although not as prolonged as those of the mothers. The decision to breastfeed should be undertaken only after careful consideration of the available alternatives.Women who are breastfeeding and anticoagulated with Warfarin should be very carefully monitored so that recommended PT/INR values are not exceeded. It is prudent to perform coagulation tests and to evaluate Vitamin K status in infants at risk for bleeding tendencies before advising women taking Warfarin to breast-feed. Effects in premature infants have not been evaluated.

Severe to moderate hepatic or renal insufficiency.

Infectious diseases or disturbances of intestinal flora: sprue, antibiotic therapy.

Trauma which may result in internal bleeding.

Surgery or trauma resulting in large exposed raw surfaces.

Indwelling catheters.

Severe to moderate hypertension.

Known or suspected deficiency in protein C mediated anticoagulant response: Hereditary or acquired deficiencies of protein C or its cofactor, protein S, have been associated with tissue necrosis following Warfarin administration. Not all patients with these conditions develop necrosis, and tissue necrosis occurs in patients without these deficiencies. Inherited resistance to activated protein C has been described in many patients with venous thromboembolic disorders but has not yet been evaluated as a risk factor for tissue necrosis. The risk associated with these conditions, both for recurrent thrombosis and for adverse reactions, is difficult to evaluate since it does not appear to be the same for everyone. Decisions about testing and therapy must be made on an individual basis. It has been reported that concomitant anticoagulation therapy with heparin for 5 to 7 days during initiation of therapy with Warfarin sodium may minimize the incidence of tissue necrosis. Warfarin therapy should be discontinued when Warfarin is suspected to be the cause of developing necrosis and heparin therapy may be considered for anticoagulation.

Miscellaneous: polycythemia vera, vasculitis, and severe diabetes.

Minor and severe allergic/hypersensitivity reactions and anaphylactic reactions have been reported.

In patients with acquired or inherited Warfarin resistance, decreased therapeutic responses to Warfarin sodium have been reported. Exaggerated therapeutic responses have been reported in other patients.

Patients with congestive heart failure may exhibit greater than expected PT/INR response to Warfarin sodium, thereby requiring more frequent laboratory monitoring, and reduced doses of Warfarin sodium.

Concomitant use of anticoagulants with streptokinase or urokinase is not recommended and may be hazardous. Please note recommendations accompanying these preparations.)

Precautions

Periodic determination of PT/INR or other suitable coagulation test is essential (see DOSAGE ANDADMINISTRATION: LABORATORY CONTROL).

Drug-Drug and Drug-Disease Interactions

Numerous factors, alone or in combination, including changes in diet, and medications, includingbotanicals, may influence response of the patient to anticoagulants. It is generally good practice tomonitor the patient’s response with additional PT/INR determinations in the period immediately afterdischarge from the hospital, and whenever other medications, including botanicals, are initiated,discontinued or taken irregularly. The following factors are listed for reference; however, other factors mayalso affect the anticoagulant response.

Drugs may interact with Warfarin sodium through pharmacodynamic or pharmacokinetic mechanisms.Pharmacodynamic mechanisms for drug interactions with Warfarin sodium are synergism (impaired hemostasis,reduced clotting factor synthesis), competitive antagonism (vitamin K), and altered physiologic controlloop for vitamin K metabolism (hereditary resistance). Pharmacokinetic mechanisms for drug interactionswith Warfarin sodium are mainly enzyme induction, enzyme inhibition, and reduced plasma protein binding. It isimportant to note that some drugs may interact by more than one mechanism.

The following factors, alone or in combination, may be responsible for INCREASED PT/INR response:

ENDOGENOUS FACTORS:

blood dyscrasias- hepatic disorders
See CONTRAINDICATIONS infectious hepatitis
cancer jaundice
collagen vascular disease hyperthyroidism
congestive heart failure poor nutritional state
diarrhea steatorrhea
elevated temperature vitamin K deficiency

EXOGENOUS FACTORS:

Potential drug interactions with Warfarin sodium are listed below by drug class and by specific drugs.

Classes of Drugs
5-lipoxygenase Inhibitor Antimalarial Agents Hypnotics† Thyroid Drugs
Adrenergic Stimulants, Central Antineoplastics† Hypolipidemics† Tuberculosis Agents†
Alcohol Abuse Reduction Antiparasitic/Antimicrobials Bile Acid-Binding Resins† Uricosuric Agents
Preparations Antiplatelet Drugs/Effects Fibric Acid Derivatives Vaccines
Analgesics Antithyroid Drugs† HMG-CoA Reductase Vitamins†
Anesthetics, Inhalation Beta-Adrenergic Blockers Inhibitors†
Antiandrogen Cholelitholytic Agents Leukotriene Receptor Antagonist
Antiarrhythmics† Diabetes Agents, Oral Monoamine Oxidase Inhibitors
Antibiotics† Diuretics† Narcotics, prolonged
Aminoglycosides (oral) Fungal Medications, Intravaginal, Nonsteroidal Anti-Inflammatory
Cephalosporins, parenteral Systemic† Agents
Macrolides Gastric Acidity and Peptic Ulcer Proton Pump Inhibitors
Miscellaneous Agents† Psychostimulants
Penicillins, intravenous, high Gastrointestinal Pyrazolones
dose Prokinetic Agents Salicylates
Quinolones (fluoroquinolones) Ulcerative Colitis Agents Selective Serotonin Reuptake
Sulfonamides, long acting Gout Treatment Agents Inhibitors
Tetracyclines Hemorrheologic Agents Steroids, Adrenocortical†
Anticoagulants Hepatotoxic Drugs Steroids, Anabolic (17-Alkyl
Anticonvulsants† Hyperglycemic Agents Testosterone Derivatives)
Antidepressants† Hypertensive Emergency Agents Thrombolytics
Specific Drugs Reported
acetaminophen dextrothyroxine levamisole prednisone†
alcohol† diazoxide levofloxacin propafenone
allopurinol diclofenac levothyroxine propoxyphene
aminosalicylic acid dicumarol liothyronine propranolol
amiodarone HCl diflunisal lovastatin propylthiouracil†
argatroban disulfiram mefenamic acid quinidine
aspirin doxycycline methimazole† quinine
atenolol erythromycin methyldopa rabeprazole
atorvastatin† esomeprazole methylphenidate ranitidine†
azithromycin ethacrynic acid methylsalicylate ointment (topical) rofecoxib
bivalirudin ezetimibe metronidazole sertraline
capecitabine fenofibrate miconazole, (intravaginal, oral, simvastatin
cefamandole fenoprofen systemic) stanozolol
cefazolin fluconazole moricizine hydrochloride† streptokinase
cefoperazone fluorouracil nalidixic acid sulfamethizole
cefotetan fluoxetine naproxen sulfamethoxazole
cefoxitin flutamide neomycin sulfinpyrazone
ceftriaxone fluvastatin norfloxacin sulfisoxazole
celecoxib fluvoxamine ofloxacin sulindac
cerivastatin gefitinib olsalazine tamoxifen
chenodiol gemfibrozil omeprazole tetracycline
chloramphenicol glucagon oxandrolone thyroid
chloral hydrate† halothane oxaprozin ticarcillin
chlorpropamide heparin oxymetholone ticlopidine
cholestyramine† ibuprofen pantoprazole tissue plasminogen activator (t-PA)
cimetidine ifosfamide paroxetine tolbutamide
ciprofloxacin indomethacin penicillin G, intravenous tramadol
cisapride influenza virus vaccine pentoxifylline trimethoprim/sulfamethoxazole
clarithromycin itraconazole phenylbutazone urokinase
clofibrate ketoprofen phenytoin† valdecoxib
cyclophosphamide† ketorolac piperacillin valproate
danazol lansoprazole piroxicam vitamin E
dextran lepirudin pravastatin† Warfarin overdose
zafirlukast
zileuton

also: other medications affecting blood elements which may modify hemostasis

dietary deficiencies

prolonged hot weather

unreliable PT/INR determinations

†lncreased and decreased PT/INR responses have been reported.

 The following factors, alone or in combination, may be responsible for DECREASED PT/INR response:

 

ENDOGENOUS FACTORS:
edema hypothyroidism
hereditary coumarin resistance nephrotic syndrome
hyperlipemia

EXOGENOUS FACTORS:

Potential drug interactions with Warfarin sodium are listed below by drug class and by specific drugs.

 

Classes of Drugs
Adrenal Cortical Steroid Antihistamines Gastric Acidity and Peptic Ulcer Oral Contraceptives, Estrogen
Inhibitors Antineoplastics† Agents† Containing
Antacids Antipsychotic Medications Hypnotics† Selective Estrogen Receptor
Antianxiety Agents Antithyroid Drugs† Hypolipidemics† Modulators
Antiarrhythmics† Barbiturates Bile Acid-Binding Resins† Steroids, Adrenocortical†
Antibiotics† Diuretics† HMG-CoA Reductase Tuberculosis Agents†
Anticonvulsants† Enteral Nutritional Supplements Inhibitors† Vitamins†
Antidepressants† Fungal Medications, Systemic† Immunosuppressives

 

Specific Drugs Reported
alcohol† cholestyramine† 6-mercaptopurine propylthiouracil†
aminoglutethimide clozapine methimazole† raloxifene
amobarbital corticotropin moricizine hydrochloride† ranitidine†
atorvastatin† cortisone nafcillin rifampin
azathioprine cyclophosphamide† paraldehyde secobarbital
butabarbital dicloxacillin pentobarbital spironolactone
butalbital ethchlorvynol phenobarbital sucralfate
carbamazepine glutethimide phenytoin† trazodone
chloral hydrate† griseofulvin pravastatin† vitamin C (high dose)
chlordiazepoxide haloperidol prednisone† vitamin K
chlorthalidone meprobamate primidone Warfarin underdosage

 also: diet high in vitamin K

unreliable PT/INR determinations

†Increased and decreased PT/INR responses have been reported.

Because a patient may be exposed to a combination of the above factors, the net effect of Warfarin sodium on PT/INR response may be unpredictable. More frequent PT/ INR monitoring is therefore advisable. Medications of unknown interaction with coumarins are best regarded with caution. When these medications are started or stopped, more frequent PT/INR monitoring is advisable.

It has been reported that concomitant administration of Warfarin and ticlopidine may be associated with cholestatic hepatitis.

Botanical (Herbal) Medicines: Caution should be exercised when botanical medicines (botanicals) are taken concomitantly with Warfarin sodium. Few adequate, well-controlled studies exist evaluating the potential for metabolic and/or pharmacologic interactions between botanicals and Warfarin sodium. Due to a lack of manufacturing standardization with botanical medicinal preparations, the amount of active ingredients may vary. This could further confound the ability to assess potential interactions and effects on anticoagulation. It is good practice to monitor the patient’s response with additional PT/INR determinations when initiating or discontinuing botanicals.

Specific botanicals reported to affect Warfarin sodium therapy include the following:

• Bromelains, danshen, dong quai (Angelica sinensis), garlic, Ginkgo biloba, ginseng, and cranberry products are associated most often with an INCREASE in the effects of Warfarin sodium.

• Coenzyme Q10 (ubidecarenone) and St. John’s wort are associated most often with a DECREASE in the effects of Warfarin sodium.

Some botanicals may cause bleeding events when taken alone (e.g., garlic and Ginkgo biloba) and may have anticoagulant, antiplatelet, and/or fibrinolytic properties. These effects would be expected to be additive to the anticoagulant effects of Warfarin sodium. Conversely, other botanicals may have coagulant properties when taken alone or may decrease the effects of Warfarin sodium.

Some botanicals that may affect coagulation are listed below for reference; however, this list should not be considered all-inclusive. Many botanicals have several common names and scientific names. The most widely recoginized common botanical names are listed.

Botanicals that contain coumarins with potential anticoagulant effects:

Alfalfa

Angelica (Dong Quai)

Aniseed

Arnica

Asa Foetida

Bogbean1

Boldo

Buchu

Capsicum2

Cassia3

Celery

Chamomile

(German and Roman)

Dandelion3

Fenugreek

Horse Chestnut

Horseradish

Licorice3

Meadowsweet1

Nettle

Parsley

Passion Flower

Prickly Ash (Northern)

Quassia

Red Clover

Sweet Clover

Sweet Woodruff

Tonka Beans

Wild Carrot

Wild Lettuce

 

Miscellaneous botanicals with anticoagulant properties:
Bladder Wrack (Fucus) Pau d’arco

 

Botanicals that contain salicylate and/or have antiplatelet properties

Agrimony4

Aloe Gel

Aspen

Black Cohosh

Black Haw

Bogbean1

Cassia3

Clove

Dandelion3

Feverfew

Garlic5

German Sarsaparilla

Ginger

Ginkgo Biloba

Ginseng (Panax)5

Licorice3

Meadowsweet1

Onion5

Policosanol

Poplar

Senega

Tamarind

Willow

Wintergreen
Botanicals with fibrinolytic proper

Bromeleins

Capsicum2

Garlic5

Ginseng (Panax)5

Inositol Nicotinate

Onion5
Botanicals with coagultant properties

Agrimony4

Goldenseal

Mistletoe 

Yarrow

1Contains coumarins and salicylate.

2Contains coumarins and has fibrinolytic properties.

3Contains coumarins and has antiplatelet properties.

4Contains salicylate and has coagulant properties.

5Has antiplatelet and fibrinolytic properties.

Effect on Other Drugs

Coumarins may also affect the action of other drugs. Hypoglycemic agents (chlorpropamide and tolbutamide) and anticonvulsants (phenytoin and phenobarbital) may accumulate in the body as a result of interference with either their metabolism or excretion.

Special Risk Patients

Warfarin sodium is a narrow therapeutic range (index) drug, and caution should be observed when Warfarin sodium is administered to certain patients such as the elderly or debilitated or when administered in any situation or physical condition where added risk of hemorrhage is present.

Intramuscular (I.M.) injections of concomitant medications should be confined to the upper extremities which permits easy access for manual compression, inspec




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