Molecular formula. C22H28FNa2O8P
Relative molecular mass. 516.4
Graphic formula.
Chemical name. 9-Fluoro-11β,17,21-trihydroxy-16α-methylpregna-1,4-diene-3,20-dione 21-(dihydrogen phosphate) disodium salt; 9-fluoro-11β,17-dihydroxy-16α-methyl-21-(phosphonooxy)pregna-1,4-diene-3,20-dione disodium salt; CAS Reg. No. 2392-39-4.
Description. A white or almost white, crystalline powder; odourless or with a slight odour of ethanol.
Solubility. Freely soluble in water; slightly soluble in ethanol (~750 g/l) TS; practically insoluble in ether R.
Category. Adrenal hormone.
Storage. Dexamethasone sodium phosphate should be kept in a tightly closed container, protected from light.
Additional information. Dexamethasone sodium phosphate is very hygroscopic. Even in the absence of light, it is gradually degraded on exposure to a humid atmosphere, the decomposition being faster at higher temperatures.
Requirements
Definition. Dexamethasone sodium phosphate contains not less than 96.0% and not more than 103.0% of C22H28FNa2O8P, calculated with reference to the anhydrous and ethanol-free substance.
Identity tests
A. Carry out the test as described under 1.14.1 Thin-layer chromatography, using silica gel R1 as the coating substance and a freshly prepared mixture of 3 volumes of 1-butanol R, 1 volume of acetic anhydride R, and 1 volume of water as the mobile phase. Apply separately to the plate 2 μl of each of 4 solutions in methanol R containing (A) 2.5 mg of the test substance per ml, (B) 2.5 mg of dexamethasone sodium phosphate RS per ml, (C) a mixture of equal volumes of solutions A and B, and (D) equal volumes of solution A and a solution of 2.5 mg of prednisolone sodium phosphate RS per ml of methanol R. After removing the plate from the chromatographic chamber, allow it to dry in air until the solvents have evaporated, spray it with a mixture of 10 ml of sulfuric acid (~1760 g/l) TS and 90 ml of ethanol (~750 g/l) TS, heat it at 120°C for 10 minutes, allow it to cool, and examine the chromatogram in ultraviolet light (365 nm). The principal spot obtained with solution A corresponds in position, appearance, and intensity with that obtained with solution B. The principal spot obtained with solution C appears as a single compact spot, whereas the chromatogram of solution D shows 2 closely running spots.
B. Place 0.5 ml of chromic acid TS in a small test-tube and heat in a water-bath for 5 minutes; the solution wets the sides of the tube but there is no greasiness. Add about 3 mg of the test substance and again heat in a water-bath for 5 minutes; the solution no longer wets the sides of the tube.
C. Heat carefully 0.04 g with 2 ml of sulfuric acid (~1760 g/l) TS until white fumes are evolved, add drop by drop nitric acid (~1000 g/l) TS until oxidation is complete, and cool. Add 2 ml of water, heat until white fumes are again evolved, cool, add 10 ml of water, and neutralize with ammonia (~100 g/l) TS, using pH-indicator paper R. Keep half of the solution for test D. The remaining solution yields reaction A described under 2.1 General identification tests as characteristic of orthophosphates.
D. The solution prepared in test C yields reaction B described under 2.1 General identification tests as characteristic of sodium.
Specific optical rotation. Use a 10 mg/ml solution and calculate with reference to the anhydrous and ethanol-free substance; 
Clarity of solution. A solution of 0.10 g in 10 ml of carbon-dioxide-free water R is clear.
Water. Determine as described under 2.8 Determination of water by the Karl Fischer method, method A, using about 0.3 g of the substance. The sum of the contents of water and ethanol (described below), both calculated in mg/g, is not more than 160 mg/g.
Ethanol. Carry out the test as described under 1.14.5 Gas chromatography, using 3 solutions in water containing (1) a mixture of 10 μl of 1-propanol R per ml serving as an internal standard and 10 μl of dehydrated ethanol R per ml, (2) 0.10 g of the test substance per ml, and (3) a mixture of 0.10 g of the test substance and 10 μl of the internal standard per ml. It may be necessary to adjust the content of dehydrated ethanol R in solution (1) to produce a peak of similar height to the corresponding peak in the chromatogram obtained with solution (2).
For the procedure use a column 1.5 m long and 4 mm in internal diameter packed with porous polymer beads (particle size 80-100 μm from a commercial source, is suitable). Maintain the column at 135 °C, use nitrogen R as the carrier gas and a flame ionization detector.
Calculate the content of ethanol in mg/g, assuming the weight per ml at 20 °C to be 0.790 g; not more than 80 mg/g.
pH value. pH of a 10 mg/ml solution in carbon-dioxide-free water R, 7.5-10.5.
Free dexamethasone and other related substances. Carry out the test as described under 1.14.1 Thin-layer chromatography, using silica gel R1 as the coating substance and methanol R as the mobile phase. Apply separately to the plate 2 μl of each of 2 solutions in methanol R containing (A) 10 mg of the test substance per ml, and (B) 0.20 mg of dexamethasone RS per ml. After removing the plate from the chromatographic chamber, allow it to dry in air for 5 minutes, spray it with a solution of 3 g of zinc chloride R in 10 ml of methanol R, heat it at about 125°C for 1 hour, and examine the chromatogram in ultraviolet light (365 nm). Any spot obtained with solution A, other than the principal spot, is not more intense than that obtained with solution B.
Assay. Dissolve about 0.2 g, accurately weighed, in sufficient water to produce 200 ml. Dilute 5 ml to 250 ml with water and measure the absorbance of this solution in a 1-cm layer at the maximum at about 241 nm. Calculate the content of C22H28FNa2O8P, using the absorptivity value of 29.7 
Additional requirements for Dexamethasone sodium phosphate for parenteral use
Complies with the monograph for "Parenteral preparations".
Bacterial endotoxins. Carry out the test as described under 3.4 Test for bacterial endotoxins; contains not more than 31.3 IU of endotoxin RS per mg.
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