Betamethasone Sodium Phosphate
(bay'' ta meth' a sone soe' dee um fos' fate).
C22H28FNa2O8P 516.40

Pregna-1,4-diene-3,20-dione, 9-fluoro-11,17-dihydroxy-16-methyl-21-(phosphonooxy)-, disodium salt, (11,16)-.
9-Fluoro-11,17,21-trihydroxy-16-methylpregna-1,4-diene-3,20-dione 21-(disodium phosphate) [151-73-5].
» Betamethasone Sodium Phosphate contains not less than 97.0 percent and not more than 103.0 percent of C22H28FNa2O8P, calculated on the anhydrous basis.
Packaging and storage— Preserve in tight containers.
USP Reference standards 11—
USP Betamethasone Sodium Phosphate RS Click to View Structure
Identification—
B: Thin-Layer Chromatographic Identification Test 201—
Test solution: 1 mg per mL.
Standard solution— Prepare a solution of USP Betamethasone Sodium Phosphate RS in methanol having a concentration of 1 mg per mL.
Developing solvent system— Place 500 mL of butyl alcohol and 200 mL of dilute hydrochloric acid (1 in 12) in a separatory funnel, and mix. Use the organic layer as the developing solvent.
Spray reagent: a mixture of sulfuric acid, methanol, and nitric acid (10:10:1).
Procedure— Proceed as directed in the chapter except to spray the plate with Spray reagent, and heat at 105 for 10 minutes.
C: Ignite it at 800 (see Residue on Ignition 281): the residue responds to the tests for Sodium 191 and for Phosphate 191.
Specific rotation 781S: between +99 and +105.
Test solution: 10 mg per mL, in water.
Water, Method I 921: not more than 10.0%.
Limit of phosphate ions—
Standard Phosphate Solution and Phosphate reagent A— Prepare as directed under Phosphate in reagents (see General Tests for Reagents under Reagents, Indicators, and Solutions).
Phosphate reagent B— Dissolve 350 mg of p-methylaminophenol sulfate in 50 mL of water, add 20 g of sodium metabisulfite, mix to dissolve, and dilute with water to 100 mL.
Procedure— Dissolve about 50 mg of Betamethasone Sodium Phosphate, accurately weighed, in a mixture of 10 mL of water and 5 mL of 2 N sulfuric acid contained in a 25-mL volumetric flask, by warming if necessary. Add 1 mL each of Phosphate reagent A and Phosphate reagent B, dilute with water to 25.0 mL, mix, and allow to stand at room temperature for 30 minutes. Similarly and concomitantly prepare a Standard solution, using 5.0 mL of Standard Phosphate Solution instead of the 50 mg of the substance under test. Concomitantly determine the absorbances of both solutions in 1-cm cells at the wavelength of maximum absorbance at about 730 nm, with a suitable spectrophotometer, using water as the blank. The absorbance of the test solution is not more than that of the Standard solution. The limit is 1.0% of phosphate (PO4).
Limit of free betamethasone—
Test solution— Dissolve 25.0 mg of Betamethasone Sodium Phosphate in water to make 25.0 mL. Transfer 5.0 mL of the solution to a separator, and extract with three 25-mL portions of chloroform. Filter each extract through a chloroform-saturated cotton pledget, combining the filtrates in a conical flask. Evaporate the chloroform on a steam bath with the aid of a current of air to dryness, and dissolve the residue in methanol to make 25.0 mL.
Blank solution— Transfer 5.0 mL of water to a separator, and proceed as directed under Test solution. The methanolic solution so obtained is the Blank solution.
Procedure— Determine the absorbance (A) of the Test solution in a 1-cm cell at the wavelength of maximum absorbance at about 239 nm, with a suitable spectrophotometer, using Blank solution as the blank. Calculate the quantity, in mg, of free betamethasone in the portion of Betamethasone Sodium Phosphate taken by the formula:
3.125A.
The limit is 250 µg (1.0%).
Assay—
Mobile phase— Prepare a filtered and degassed mixture of methanol and 0.07 M anhydrous monobasic potassium phosphate (3:2). Make adjustments if necessary (see System Suitability under Chromatography 621).
Standard preparation— Dissolve an accurately weighed quantity of USP Betamethasone Sodium Phosphate RS in a mixture of methanol and water (3:2), and dilute quantitatively, and stepwise if necessary, with the same mixture of methanol and water to obtain a solution having a known concentration of about 0.17 mg per mL.
Assay preparation— Transfer about 34 mg of Betamethasone Sodium Phosphate, accurately weighed, to a 200-mL volumetric flask, dissolve in and dilute with a mixture of methanol and water (3:2) to volume, and mix.
Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 254-nm detector and a 3.9-mm × 30-cm column that contains packing L1. The flow rate is about 1.5 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the tailing factor is not more than 2, and the relative standard deviation for replicate injections is not more than 3.0%.
Procedure— Separately inject equal volumes (about 20 µL) of the Standard preparation and the Assay preparation into the chromatograph, record the chromatograms, and measure the responses for the major peaks. Calculate the quantity, in mg, of C22H28FNa2O8P in the portion of Betamethasone Sodium Phosphate taken by the formula:
200C(rU / rS)
in which C is the concentration, in mg per mL, of USP Betamethasone Sodium Phosphate RS in the Standard preparation; and rU and rS are the peak responses obtained from the Assay preparation and the Standard preparation, respectively.
Auxiliary Information— Please check for your question in the FAQs before contacting USP.
Topic/Question Contact Expert Committee
Monograph Domenick Vicchio, Ph.D.
Senior Scientific Liaison
1-301-998-6828		 (SM42010) Monographs - Small Molecules 4
Reference Standards RS Technical Services
1-301-816-8129
rstech@usp.org
USP35–NF30 Page 2343
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