A: Transfer a volume of Injection, equivalent to 125 mg of hydroxyprogesterone caproate, to a 60-mL separator containing 10 mL of solvent hexane, 8 mL of methanol, and 2 mL of water. Insert the stopper, shake for 2 minutes, and allow the phases to separate. To 3 mL of the lower layer add sulfuric acid dropwise until a color develops, then add 3 mL of methanol: a purple color develops, and the solution, when viewed under long-wavelength UV light, exhibits a pale yellow fluorescence.

B: Evaporate 4 mL of the Assay preparation, obtained as directed in the Assay, on a water bath to dryness, and dissolve the residue in 0.5 mL of chloroform. Apply 10 µL of this solution and 10 µL of a solution of USP Hydroxyprogesterone Caproate RS in chloroform, containing 400 µg per mL, to a thin-layer chromatographic plate (see Chromatography 621) coated with a 0.25-mm layer of chromatographic silica gel mixture, on a line about 2.5 cm from the bottom edge and about 2 cm apart. Place the plate in a developing chamber that contains and that has been equilibrated with a mixture of 3 volumes of chloroform and 1 volume of ethyl acetate. Develop the plate until the solvent front has moved to about 10 cm above the points of application. Remove the plate, mark the solvent front, and dry. Spray the plate with a mixture of 1 volume of sulfuric acid and 3 volumes of alcohol, and heat in an oven at 105 for 5 minutes: the RF value of the principal yellowish green spot obtained from the solution under test corresponds to that obtained from the Standard solution.

Isoniazid reagent— Dissolve 375 mg of isoniazid and 0.47 mL of hydrochloric acid in 500 mL of methanol.

Standard preparation— Dissolve a suitable quantity of USP Hydroxyprogesterone Caproate RS, accurately weighed, in methanol, and dilute quantitatively and stepwise with methanol to obtain a solution having a known concentration of about 50 µg per mL.

Assay preparation— Transfer to a 250-mL volumetric flask an accurately measured volume of Injection, equivalent to about 250 mg of hydroxyprogesterone caproate, add methanol to volume, and mix. Pipet 5 mL of this solution into a 100-mL volumetric flask, add methanol to volume, and mix.

Procedure— Pipet 5 mL of Assay preparation into a glass-stoppered, 50-mL conical flask. Pipet 5 mL of Standard preparation into a similar flask. To each flask, add 10.0 mL of Isoniazid reagent, mix, and allow to stand in a water bath at 30 for about 45 minutes. Concomitantly determine the absorbances of both solutions at the wavelength of maximum absorbance at about 380 nm, with a suitable spectrophotometer, using as a blank a mixture of 5 mL of methanol and 10 mL of Isoniazid reagent. Calculate the quantity, in mg, of hydroxyprogesterone caproate (C27H40O4) in each mL of the Injection taken by the formula: in which C is the concentration, in µg per mL, of USP Hydroxyprogesterone Caproate RS in the Standard preparation; V is the volume, in mL, of Injection taken; and AU and AS are the absorbances of the solutions from the Assay preparation and the Standard preparation, respectively.