Warfarin Sodium

WARFARIN SODIUM — warfarin sodium tablet
State of Florida DOH Central Pharmacy

Rx only

Anticoagulant

WARNING: BLEEDING RISK

Warfarin sodium can cause major or fatal bleeding. Bleeding is more likely to occur during the starting period and with a higher dose (resulting in a higher INR). Risk factors for bleeding include high intensity of anticoagulation (INR >4.0), age ≥65, highly variable INRs, history of gastrointestinal bleeding, hypertension, cerebrovascular disease, serious heart disease, anemia, malignancy, trauma, renal insufficiency, concomitant drugs (see PRECAUTIONS), and long duration of warfarin therapy. Regular monitoring of INR should be performed on all treated patients. Those at high risk of bleeding may benefit from more frequent INR monitoring, careful dose adjustment to desired INR, and a shorter duration of therapy. Patients should be instructed about prevention measures to minimize risk of bleeding and to report immediately to physicians signs and symptoms of bleeding (see PRECAUTIONS: Information for Patients).

DESCRIPTION

Crystalline warfarin sodium is an anticoagulant which acts by inhibiting vitamin K-dependent coagulation factors. Chemically, it is 3-(α-acetonylbenzyl)-4-hydroxycoumarin sodium salt 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 empirical formula is C19 H15 NaO4 , and its structural formula may be represented by the following:

Chemical Structure

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, USP for oral use also contain:

All strengths: Anhydrous lactose, corn starch and magnesium stearate
1 mg: D&C red #6 Barium Lake.
2 mg: FD&C blue #2 Aluminum Lake, FD&C red #40 Aluminum Lake.
2.5 mg: FD&C blue #2 Aluminum Lake, D&C yellow #10 Aluminum Lake.
3 mg:FD&C blue #2 Aluminum Lake, FD&C red #40 Aluminum Lake, D&C yellow #10 Aluminum Lake.
4 mg:FD&C blue #1 Aluminum Lake.
5 mg: D&C yellow #10 Aluminum Lake, D&C red #6 Barium Lake.
6 mg: D&C yellow #10 Aluminum Lake, FD&C blue #2 Aluminum Lake.
7.5 mg:D&C yellow #10 Aluminum Lake.
10 mg: Dye Free

CLINICAL PHARMACOLOGY

Warfarin sodium tablets, USP 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 — 4-6 hours, IX — 24 hours, and X — 48-72 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.

Mechanism of Action

Warfarin is thought to interfere with clotting factor synthesis by inhibition of the C1 subunit of the vitamin K epoxide reductase (VKORC1) enzyme complex, thereby reducing the regeneration of vitamin K1 epoxide. The degree of depression is dependent upon the dosage administered and, in part, by the patient’s VKORC1 genotype. 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 tablets, USP 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 tablets, USP is a racemic mixture of the R — and S -enantiomers. The S -enantiomer exhibits 2-5 times more anticoagulant activity than the R -enantiomer in humans, but generally has a more rapid clearance.

Absorption

Warfarin sodium tablets, USP 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 tablets, USP 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.

The S -enantiomer of warfarin is mainly metabolized to 7-hydroxywarfarin by CYP2C9, a polymorphic enzyme. The variant alleles CYP2C9*2 and CYP2C9*3 result in decreased in vitro CYP2C9 enzymatic 7-hydroxylation of S-warfarin. The frequencies of these alleles in Caucasians are approximately 11% and 7% for CYP2C9*2 and CYP2C9*3, respectively1. Patients with one or more of these variant CYP2C9 alleles have decreased S-warfarin clearance (Table 1).2

Table 1. Relationship Between S-Warfarin Clearance and CYP2C9 Genotype in Caucasian Patients
CYP2C9 Genotype N S-Warfarin Clearance/Lean Body Weight (mL/min/kg) Mean (SD)a
a SD=standard deviation.b p<0.001. Pairwise comparisons indicated significant differences among all 3 genotypes.
*1/*1 118 0.065 (0.025)b
*1/*2 or *1/*3 59 0.041 (0.021)b
*2/*2, *2/*3, or *3/*3 11 0.020 (0.011)b
Total 188

Other CYP2C9 alleles associated with reduced enzymatic activity occur at lower frequencies, including *5, *6, and *11 alleles in populations of African ancestry and *5, *9 and *11 alleles in Caucasians.

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