(Official January 1, 2007)

Standard preparation— Dissolve an accurately weighed quantity of USP Methylprednisolone Acetate RS in a mixture of equal volumes of alcohol and chloroform, and dilute quantitatively with the same solvent to obtain a solution having a known concentration of about 500 µg per mL.

Assay preparation— Transfer an accurately weighed quantity of Cream, equivalent to about 5 mg of methylprednisolone acetate, to a 125-mL separator, add 50 mL of solvent hexane, and mix. Extract with three 10-mL portions of acetonitrile, and evaporate the combined extracts on a steam bath with the aid of a current of air nearly to dryness. Transfer the residue to a 10-mL volumetric flask with the aid of one 5-mL portion and two 2-mL portions of a mixture of equal volumes of alcohol and chloroform, dilute with the same solvent to volume, and mix.

Procedure— Divide the area of a suitable thin-layer chromatographic plate (see Chromatography 621) coated with a 0.5-mm layer of chromatographic silica gel mixture, into three equal sections, the left and right sections to be used for the Assay preparation and the Standard preparation, respectively, and the center section for the blank. Apply 250 µL each of the Assay preparation and of the Standard preparation as streaks 2.5 cm from the bottom of the designated section of the plate, and dry the streaks with the aid of a current of air. Develop the chromatogram in a solvent system consisting of a mixture of ethyl acetate and chloroform (7:5) until the solvent front has moved about three-fourths of the length of the plate. Remove the plate from the developing chamber, mark the solvent front, and allow the solvent to evaporate. Locate the principal bands occupied by the Standard preparation and the Assay preparation (see also the Identification test) by viewing under short-wavelength UV light. Mark these bands and the corresponding band in the section of the plate representing the blank. Quantitatively remove the silica gel containing these bands, and transfer to separate glass-stoppered, 50-mL centrifuge tubes. Add 25.0 mL of alcohol to each tube, shake for 2 minutes, and centrifuge at about 1500 rpm for 5 minutes. Transfer 20.0 mL of each supernatant to separate glass-stoppered, 50-mL conical flasks, add 2.0 mL of blue tetrazolium TS to each solution, mix, and to each flask add 2.0 mL of a mixture of 1 volume of tetramethylammonium hydroxide TS and 9 volumes of alcohol. Mix, and allow the solutions to stand in the dark for 90 minutes. Concomitantly determine the absorbances of the solutions in 1-cm cells at the wavelength of maximum absorbance at about 525 nm, with a suitable spectrophotometer, against the blank. Calculate the quantity, in mg, of methylprednisolone acetate (C24H32O6) in the portion of Cream taken by the formula: in which C is the concentration, in µg per mL, of USP Methylprednisolone Acetate RS in the Standard preparation; and AU and AS are the absorbances of the solutions from the Assay preparation and the Standard preparation, respectively.