Medium: 0.1Nhydrochloric acid;900mL.

Apparatus 1: 10-mesh basket;100rpm.

Xanthydrol TS— [NOTEPrepare this solution daily.]To about 150mg of xanthydrol in a 100-mLvolumetric flask,add 10mLof glacial acetic acid,and swirl to dissolve.Dilute with hydrochloric acid to volume,and mix.

Standard solution— Quantitatively dissolve an accurately weighed quantity of USP Dirithromycin RSin 0.1Nhydrochloric acid to obtain a solution having a known concentration of about 0.28mg per mL.

Procedure— After 2hours of operation,remove each Tablet,or the major portion thereof if the Tablet is not intact,from the individual vessel,and subject each Tablet to the test in the Buffer stage.Separately add 0.50mLof acetic anhydride to 0.50mLof the filtered solution under test and to 0.50mLof the Standard solution,and mix.Add 5.0mLof glacial acetic acid,allow to stand for 5minutes,then add 0.50mLof Xanthydrol TS,and allow 30minutes for color development.Determine the amount of C42H78N2O14,including the 16R-and 16S-epimers,dissolved by employing UVabsorption at the wavelength of maximum absorbance at about 540nm.

Medium: pH6.8phosphate buffer;900mL.

Procedure— Proceed as directed for Acid stagebeginning with “Separately add 0.50mLof acetic anhydride”.

Tolerances— Not less than 80%(Q)of the labeled amount of C42H78N2O14,including the 16R-and 16S-epimers,is dissolved in 45minutes.

Potassium phosphate buffer,Mobile phaseand System suitability solution— Proceed as directed in the Assayunder Dirithromycin.

Phosphate buffer— Dissolve 4.35g of dibasic potassium phosphate in 1liter of water,adjust with phosphoric acid to a pHof 8.0,and pass through a filter having a 0.5µm or finer porosity.

Solvent 1— Prepare a mixture of acetonitrile and Phosphate buffer(60:40).

Solvent 2— Prepare a mixture of acetonitrile and Phosphate buffer(98:2).[NOTE—The mixture is cloudy.]

Standard solution— Quantitatively dissolve an accurately weighed quantity of USP Dirithromycin RSin Solvent 1to obtain a solution having a known concentration of about 0.2mg per mL.[NOTE—Inject the Standard solution into the chromatograph immediately after preparation.]

Test solution— Weigh and finely powder not fewer than 20Tablets.Transfer an accurately weighed portion of the powder,equivalent to about 745mg of dirithromycin,to a 50-mLvolumetric flask,dissolve in and dilute with Solvent 2to volume,and mix.Centrifuge a portion of this solution,transfer 6.0mLof the clear supernatant to a 10-mLvolumetric flask,dilute with Phosphate bufferto volume,and mix.[NOTE—Inject the Test solution into the chromatograph immediately after preparation.]

Chromatographic system (see Chromatography á621ñ)—The liquid chromatograph is equipped with a 205-nm detector and a 4.6-mm ×25-cm column that contains 5-µm packing L1and is maintained at a constant temperature of about 40.The flow rate is about 2mLper minute.Chromatograph the System suitability solution,and record the peak responses as directed for Procedure:the relative retention times are about 0.7for 9-(S)-erythromycylamine,1.0for dirithromycin (16R-epimer),and 1.12for dirithromycin 16S-epimer;the resolution,R,between the dirithromycin (16R-epimer)and dirithromycin 16S-epimer is not less than 2.0,and between dirithromycin (16R-epimer)and 9-(S)-erythromycylamine is not less than 5.0;and the tailing factor for the dirithromycin (16R-epimer)peak is not more than 2.0.

Procedure— Separately inject equal volumes (about 20µL)of the Standard solutionand the Test solutioninto the chromatograph,record the chromatograms for a period of time that is not less than three times the retention time of dirithromycin (16R-epimer),and measure the peak areas.Calculate the percentage of each impurity found in the portion of Tablets taken by the formula: in which Cis the concentration,in mg per mL,of USP Dirithromycin RSin the Standard solution;Wis the quantity,in mg,of Tablet powder taken to prepare the Test solution;Lis the labeled amount,in mg,of dirithromycin in each Tablet;Tis the average weight,in mg,of each Tablet;riis the peak response for each impurity obtained from the Test solution;and rSis the peak response of dirithromycin (16R-epimer)obtained from theStandard solution:not more than 1.5%of 9-(S)-erythromycylamine is found;and not more than 5.0%of total impurities is found.[NOTE—Do not regard dirithromycin 16S-epimer as an impurity.]

Potassium phosphate buffer,Mobile phase,System suitability solution,and Solvent— Proceed as directed in the Assayunder Dirithromycin.

Standard preparation— Quantitatively dissolve an accurately weighed quantity of USP Dirithromycin RSin Solventto obtain a solution having a known concentration of about 2.5mg per mL.

Assay preparation— Weigh and finely powder not fewer than 20Tablets.Transfer an accurately weighed portion of the powder,equivalent to about 250mg of dirithromycin,to a 100-mLvolumetric flask,dissolve in and dilute with Solventto volume,and mix.Centrifuge a portion of this solution,and use the clear supernatant as the Assay preparation.

Chromatographic system (see Chromatography á621ñ)—The liquid chromatograph is equipped with a 205-nm detector and a 4.6-mm ×25-cm column that contains 5-µm packing L1and is maintained at a constant temperature of about 40.The flow rate is about 2mLper minute.Chromatograph the System suitability solution,and record the peak responses as directed for Procedure:the relative retention times are about 0.7for 9-(S)-erythromycylamine,1.0for dirithromycin (16R-epimer),and 1.12for dirithromycin 16S-epimer;the resolution,R,between the dirithromycin (16R-epimer)and dirithromycin 16S-epimer is not less than 2.0,and between dirithromycin (16R-epimer)and 9-(S)-erythromycylamine is not less than 5.0;the tailing factor for the dirithromycin (16R-epimer)peak is not more than 2.0;and the relative standard deviation determined from the dirithromycin (16R-epimer)peak for replicate injections is not more than 2.0%.

Procedure— Separately inject equal volumes (about 10µL)of the Standard preparationand the Assay preparationinto the chromatograph,record the chromatograms,and measure the areas for the major peaks.Calculate the quantity,in mg,of dirithromycin (C42H78N2O14),which includes the 16R-and 16S-epimers,in the portion of Tablets taken by the formula: in which Cis the concentration,in mg per mL,of USP Dirithromycin RSin the Standard preparation;rUand rEare the peak responses for dirithromycin (16R-epimer)and dirithromycin 16S-epimer,respectively,obtained from the Assay preparation;and rSis the peak response of dirithromycin (16R-epimer)obtained from the Standard preparation.