Warfarin Tablets prepared from Warfarin Sodium are not necessarily interchangeable with Warfarin Tablets prepared from
Warfarin Sodium Clathrate.
Action and use
Vitamin K epoxide reductase inhibitor; oral anticoagulant (coumarin).
DEFINITION
Warfarin Tablets contain Warfarin Sodium or Warfarin Sodium Clathrate.
The tablets comply with the requirements stated under Tablets and with the following requirements.
Content of warfarin sodium, C19H15NaO4
95.0 to 105.0% of the stated amount.
IDENTIFICATION
Extract a quantity of the powdered tablets containing the equivalent of 0.1 g of warfarin sodium with 30 mL of water, add 0.1 mL of 2M hydrochloric acid, filter, wash the precipitate with water and dry. Warm the residue gently with 3 mL of ethanol (96%), filter and add the filtrate to 25 mL of water containing 0.1 mL of 2M hydrochloric acid. Filter, wash the precipitate with water and dry it at 105°. The infrared absorption spectrum of the residue, Appendix II A, is concordant with the reference spectrum of warfarin (RS 361).
TESTS
Dissolution
Comply with the dissolution test for tablets and capsules, Appendix XII B1.
TEST CONDITIONS
(a) Use Apparatus 1, rotating the basket at 100 revolutions per minute.
(b) Use 900 mL of 0.05M potassium dihydrogen orthophosphate adjusted to pH 6.8 by the addition of 1M sodium hydroxide, at a temperature of 37°, as the medium.
(c) For tablets containing 3 mg or less of warfarin sodium, place a whole number of tablets containing at least 3 mg of warfarin sodium in the basket for each test; for tablets containing more than 3 mg of warfarin sodium, place a single tablet in the basket for each test.
PROCEDURE
After 45 minutes, withdraw a sample of the medium. Measure the absorbance of the filtered sample, suitably diluted with the dissolution medium if necessary, at the maxima at 307 nm and 360 nm, Appendix II B, using the dissolution medium in the reference cell.
DETERMINATION OF CONTENT
Calculate the difference between the two readings (ΔA). Calculate the total content of warfarin sodium, C19H15NaO4, in the medium from the absorbances obtained and taking 428 as the value of ΔA(1%, 1 cm).
LIMITS
The amount of warfarin sodium released is not less than 75% (Q) of the stated amount.
Related substances
Carry out the method for liquid chromatography, Appendix III D, using the following solutions in mixture of 20% v/v methanol.
(1) Shake a quantity of the powdered tablets containing the equivalent of 10 mg of warfarin sodium in 8 mL. Mix with the aid of ultrasound, dilute to produce 10 mL and filter.
(2) Dilute 1 volume of solution (1) to 100 volumes. Dilute 1 volume of the resulting solution to 5 volumes.
(3) 0.002% w/v each of 4-hydroxycoumarin (impurity B), (E)-4-Phenylbut-3-en-2-one (benzalacetone) (impurity C) and (5RS)-3-(2-hydroxyphenyl)-5-phenylcyclohex-2-enone BPCRS (impurity A).
CHROMATOGRAPHIC CONDITIONS
(a) Use a stainless steel column (25 cm × 4.6 mm) packed with cyanosilyl silica gel for chromatography (5 μm) (Waters Spherisorb CN is suitable).
(b) Use isocratic elution and the mobile phase described below.
(c) Use a flow rate of 1.5 mL per minute.
(d) Use a column temperature of 30°.
(e) Use a detection wavelength of 283 nm.
(f) Inject 50 μL of each solution
(g) For solution (1) allow the chromatography to proceed for twice the retention time of warfarin.
MOBILE PHASE
1 volume of glacial acetic acid, 15 volumes of acetonitrile and 85 volumes of water.
When the chromatograms are recorded under the prescribed conditions, the retention times relative to warfarin (retention time about 9 minutes) are: impurity B, about 0.4; impurity C, about 0.6 and impurity A, about 1.2.
SYSTEM SUITABILITY
The test is not valid unless, in the chromatogram obtained with solution (3), the resolution between the peaks due to impurity B and impurity C is at least 2.0.
LIMITS
Identify any peaks corresponding to impurity B and impurity C in the chromatogram obtained with solution (1), using the chromatogram obtained with solution (3). Multiply the area of any peak corresponding to impurity B by a correction factor of 0.5 and impurity C by a correction factor of 0.4. In the chromatogram obtained with solution (1):
the area of any peak corresponding to impurity A is not greater than 2.5 times the area of the principal peak in the chromatogram obtained with solution (2) (0.5%);
the area of any peak corresponding to impurity B and impurity C is not greater than 0.75 times the area of the principal peak in the chromatogram obtained with solution (2) (0.15% of each);
the area of any other secondary peak is not greater than the area of the principal peak in the chromatogram obtained with solution (2) (0.2%);
the sum of the areas of any other secondary peaks is not greater than 2.5 times the area of the principal peak in the chromatogram obtained with solution (2) (0.5%);
Disregard any peak with an area less than half the area of the principal peak in the chromatogram obtained with solution (2) (0.1%).
Uniformity of content
Tablets containing less than 2 mg and/or less than 2% w/w of warfarin sodium comply with the requirements stated under Tablets using the following method of analysis.
Carry out the method for liquid chromatography, Appendix III D, using the following solutions.
Shake one tablet with 10 mL of 0.01M sodium hydroxide for 15 minutes, add 10 mL of a 2% v/v solution of glacial acetic acid in acetonitrile, centrifuge for 10 minutes. Dilute the clear supernatant liquid, if necessary, with the mobile phase to produce a solution containing 0.0025% w/v of warfarin.
Dilute 1 volume of a 0.010% w/v solution of warfarin BPCRS in 0.01M sodium hydroxide to 4 volumes with the mobile phase.
CHROMATOGRAPHIC CONDITIONS
(a) Use a stainless steel column (10 cm × 4.6 mm) packed with octadecylsilyl silica gel for chromatography (5 μm) (Hypersil ODS is suitable).
(b) Use isocratic elution and the mobile phase described below.
(c) Use a flow rate of 2 mL per minute.
(d) Use an ambient column temperature.
(e) Use a detection wavelength of 283 nm.
(f) Inject 20 μL of each solution.
MOBILE PHASE
1 volume of glacial acetic acid, 45 volumes of water, and 55 volumes of acetonitrile.
DETERMINATION OF CONTENT
Calculate the content of C19H15NaO4 in each tablet using the declared content of C19H16O4 in warfarin BPCRS. Each mg of C19H16O4 is equivalent to 1.071 mg of C19H15NaO4.
ASSAY
For tablets containing 2 mg or less or 2% w/w or less of warfarin sodium
Use the average of the individual results obtained in the test for Uniformity of content.
For tablets containing more than 2 mg and more than 2% w/w of warfarin sodium
Weigh and powder 20 tablets. Carry out the method for liquid chromatography, Appendix III D, using the following solutions.
(1) Shake a quantity of the powdered tablets containing the equivalent of 5 mg of warfarin sodium with 50 mL of 0.01M sodium hydroxide for 15 minutes, add 50 mL of a 2% v/v solution of glacial acetic acid in acetonitrile, centrifuge for 10 minutes and use the clear supernatant liquid.
(2) Dilute 5 mL of a 0.010% w/v solution of warfarin BPCRS in 0.01M sodium hydroxide to 10 mL with the mobile phase.
CHROMATOGRAPHIC CONDITIONS
The chromatographic conditions described under Uniformity of content may be used.
DETERMINATION OF CONTENT
Calculate the content of C19H15NaO4 in the tablets using the declared content of C19H16O4 in warfarin BPCRS. Each mg of C19H16O4 is equivalent to 1.071 mg of C19H15NaO4.
STORAGE
Warfarin Tablets should be protected from light.
LABELLING
When the active ingredient is Warfarin Sodium Clathrate the quantity is stated in terms of the equivalent amount of warfarin sodium.
IMPURITIES
The impurities limited by the requirements of this monograph include those listed under Warfarin Sodium.
