Test specimen— Dry an amount of Injection, equivalent to about 2 mg of alprostadil, on about 500 mg of spectroscopic grade potassium bromide at about 40 to 50 under vacuum. Prepare a pellet from this mixture.

Standard specimen: a similar preparation of USP Alprostadil RS, in dehydrated alcohol.

(Official January 1, 2007)

Mobile phase— Prepare a filtered and degassed mixture of methylene chloride, 1,3-butanediol, and water (1000:6:0.5). Make adjustments if necessary (see System Suitability under Chromatography 621).

Internal standard solution— Dissolve ethylparaben in methylene chloride to obtain a solution containing about 0.05 mg per mL.

Standard stock preparation— Dissolve an accurately weighed quantity of USP Alprostadil RS in dehydrated alcohol, and quantitatively dilute with dehydrated alcohol to obtain a solution having a known concentration of about 0.5 mg per mL.

Standard preparation— Gently evaporate a 0.5-mL portion of the Standard stock preparation to dryness with a stream of nitrogen. Proceed as directed for the Assay preparation beginning with “Add 150 µL of a 1 in 25 freshly prepared solution.”

Assay preparation— Pool the contents of several containers of the Injection, and gently evaporate an accurately measured volume, equivalent to about 0.25 mg of alprostadil, to dryness using a stream of nitrogen. Add 150 µL of a 1 in 25 freshly prepared solution of -bromo-2¢-acetonaphthone in acetonitrile, rinse the inside of the container with this solution, and swirl. Add 150 µL of a freshly prepared 1 in 200 solution of diisopropylethylamine in acetonitrile to the container, rinse the inside of the container with this solution, and swirl. Cap and sonicate to dissolve. Heat the container at 45 for 45 minutes, swirling occasionally. Sonicate again after heating is complete. [NOTE—If the entire sample does not dissolve, the specimen should be discarded.] Evaporate the solution using a stream of nitrogen, add 2.0 mL of Internal standard solution, and mix. Sonicate to dissolve. [NOTE—If incomplete dissolution is still observed, discard the specimen.]

Chromatographic system (see Chromatography 621)— The liquid chromatograph is equipped with a 254-nm detector and a 4.4-mm × 25-cm column that contains packing L18. The flow rate is about 1.5 mL per minute. Chromatograph the Standard preparation, and record the peak responses as directed for Procedure: the relative retention times are about 0.4 and 1.0, for ethylparaben and alprostadil, respectively; the resolution, R, between alprostadil and the internal standard is not less than 9.0; and the relative standard deviation for replicate injections is not more than 2.5%.

Procedure— Separately inject equal volumes 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 alprostadil (C20H34O5) in the volume of Injection taken by the formula: in which C is the concentration, in mg per mL, of USP Alprostadil RS in the Standard stock preparation; and RU and RS are the peak response ratios of alprostadil to the internal standard obtained from the Assay preparation and the Standard preparation, respectively.