pH 6.0 sodium phosphate buffer— Prepare 6 L of 0.1 M dibasic sodium phosphate, adjust with about 40 mL of hydrochloric acid to a pH of 6.0 ± 0.05, add 600 mg of trypsin, and mix.

Medium: pH 6.0 sodium phosphate buffer; 900 mL.

Apparatus 2: 100 rpm.

Time: 45 minutes.

Standard solution— Transfer about 15 mg of USP Azithromycin RS, accurately weighed, to a 50-mL volumetric flask. Add 25 mL of Medium, and sonicate briefly to dissolve. Dilute with Medium to volume, and mix. Transfer 2.0 mL of this solution to a 25-mL volumetric flask, dilute with Mobile phase (prepared as directed in the Assay) to volume, and mix. Transfer 4.0 mL of this solution to a second 25-mL volumetric flask, dilute with Mobile phase to volume, and mix.

Test solution— Filter a portion of the solution under test through a filter having a porosity of 0.5 µm or less. Transfer 2.0 mL of the filtrate to a 25-mL volumetric flask, dilute with Mobile phase to volume, and mix. Transfer 4.0 mL of this solution to a second 25-mL volumetric flask, dilute with Mobile phase to volume, and mix.

Procedure— Determine the amount of azithromycin (C38H72N2O12) dissolved, using filtered portions of the solution under test, and employing the procedure set forth in the Assay, making any necessary modifications. Calculate the quantity, in mg, of azithromycin (C38H72N2O12) dissolved by the formula: in which C is the concentration, in mg per mL, of USP Azithromycin RS in the Standard solution; P is the potency, in µg, of azithromycin (C38H72N2O12) per mg of USP Azithromycin RS; and rU and rS are the azithromycin peak responses obtained from the Test solution and the Standard solution, respectively.

Tolerances— Not less than 75% (Q) of the labeled amount of azithromycin (C38H72N2O12) is dissolved in 45 minutes.

Mobile phase, Standard preparation, Resolution solution, and Chromatographic system— Proceed as directed in the Assay under Azithromycin.

Assay preparation— Remove, as completely as possible, the contents of not fewer than 20 Capsules, and weigh accurately. Mix the combined contents, and transfer an accurately weighed quantity of the powder, equivalent to about 250 mg of anhydrous azithromycin, to a 250-mL volumetric flask. Add about 175 mL of acetonitrile, and shake by mechanical means for 30 minutes. Dilute with acetonitrile to volume, and mix. Place about 40 mL of the resulting suspension in a centrifuge tube, and centrifuge. Transfer 2.0 mL of the clear supernatant to a 50-mL volumetric flask, dilute with Mobile phase to volume, and mix. Transfer 2.0 mL of this solution to a 25-mL volumetric flask, dilute with Mobile phase to volume, and mix.

Procedure— Proceed as directed for Procedure in the Assay under Azithromycin. Calculate the quantity, in mg, of azithromycin (C38H72N2O12) in the portion of Capsules taken by the formula: in which C is the concentration, in mg per mL, of USP Azithromycin RS in the Standard preparation; P is the potency, in µg of azithromycin (C38H72N2O12) per mg, of USP Azithromycin RS; and rU and rS are the azithromycin peak responses obtained from the Assay preparation and the Standard preparation, respectively.