Methyldopa and Chlorothiazide Tablets, chemical structure, molecular formula, Reference Standards

Methyldopa and Chlorothiazide Tablets

»Methyldopa and Chlorothiazide Tablets contain not less than 90.0percent and not more than 110.0percent of the labeled amounts of methyldopa (C10H13NO4)and chlorothiazide (C7H6ClN3O4S2).

Packaging and storage— Preserve in well-closed containers.

USP Reference standards á11ñ— USP Chlorothiazide RS.USP Methyldopa RS.

Identification— Transfer the finely ground contents of 1Tablet to a test tube,add 10mLof dilute alcohol (1in 2),shake for 5minutes,and centrifuge.Use the clear supernatant as the Test solution.Prepare a solution of alcohol and 0.1Nsodium hydroxide (1:1)containing in each mLabout 10mg of USP Methyldopa RSand 10mg of USP Chlorothiazide RS.Apply 20µLof the Test solution on a line parallel to and about 2cm from the bottom edge of a 20-cm ×10-cm thin-layer chromatographic plate (see Chromatography á621ñ)coated with chromatographic silica gel mixture,and apply 20µLof the Standard solution separately on the starting line.Allow the spots to dry,develop the chromatogram in a solvent system consisting of equal volumes of glacial acetic acid,acetone,butyl alcohol,toluene,and water until the solvent front has moved about three-fourths of the length of the plate.Remove the plate from the developing tank,and allow the solvent to evaporate.View the plate under short-wavelength UVlight:the solution under test exhibits two major spots having RFvalues corresponding to those of the two major spots obtained with the Standard solution.

Dissolution á711ñ—

PROCEDURE FOR METHYLDOPA —

Medium: 0.1Nhydrochloric acid;900mL.

Apparatus 2: 75rpm.

Time: 30minutes.

Standard preparation— Dissolve an accurately weighed quantity of USP Methyldopa RSin Dissolution Medium,and dilute quantitatively with the same solvent to obtain a solution having a known concentration of about 275µg of anhydrous methyldopa per mL.

Ferrous tartrate solution— Dissolve 1g of ferrous sulfate,2g of potassium sodium tartrate,and 100mg of sodium bisulfite in water to make 100mL,and mix.Use a freshly prepared solution.

Buffer solution— Dissolve 50g of ammonium acetate in 1000mLof dilute alcohol (1in 5).Adjust with 6Nammonium hydroxide to a pHof 8.5.

Procedure— Filter 35mLof the solution under test,and transfer an aliquot estimated to contain between 2mg and 3mg of methyldopa to a 100-mLvolumetric flask.Adjust the final volume,if necessary,with Dissolution Mediumto 10mL.To a second 100-mLvolumetric flask add 10.0mLof Standard preparation,and to a third 100-mLvolumetric flask add 10.0mLof Dissolution Mediumto provide a blank.Pipet 5.0mLof Ferrous tartrate solutioninto each flask,dilute with Buffer solutionto volume,and mix.Concomitantly determine the absorbances of the treated Standard preparationand test solution at the wavelength of maximum absorbance at about 520nm,with a suitable spectrophotometer,against the reagent blank.Calculate the amount of C10H13NO4dissolved,in mg,taken by the formula:

9(C/V)(AU/AS),

in which Cis the concentration,in µg of anhydrous methyldopa per mL,of USP Methyldopa RSin the Standard preparation;Vis the volume,in mL,of the aliquot of test solution used;and AUand ASare the absorbances of the solutions from the test solution and the Standard preparation,respectively.

Tolerances— Not less than 80%(Q)of the labeled amount of C10H13NO4is dissolved in 30minutes.

PROCEDURE FOR CHLOROTHIAZIDE —

Medium: 0.05M,pH8.0phosphate buffer (see Buffer Solutionsin the section Reagents,Indicators,and Solutions)containing sodium sulfite (1in 5000);900mL.

Apparatus 2: 75rpm.

Time: 60minutes.

Procedure— Determine the amount of C7H6ClN3O4S2dissolved from UVabsorbances of the solution under test,suitably diluted with Dissolution Medium,if necessary,at the wavelength of maximum absorbance at about 317nm in comparison with a Standard solution having a known concentration of USP Chlorothiazide RSin the same Medium.

Tolerances— Not less than 75%(Q)of the labeled amount of C7H6ClN3O4S2is dissolved in 60minutes.

Uniformity of dosage units á905ñ: meet the requirements with respect to methyldopa and to chlorothiazide.

Assay—

Mobile phase— Prepare a filtered and degassed mixture of 0.08Mmonobasic sodium phosphate and methanol (95:5).Adjust by the addition of phosphoric acid to a pHof 2.8.Make adjustments if necessary (see System Suitabilityunder Chromatography á621ñ).

Standard preparation— Transfer to a 100-mLvolumetric flask accurately weighed quantities of USP Methyldopa RSand USP Chlorothiazide RS,equivalent to one-fifth of their labeled amounts,in mg,per Tablet.Add 15mLof water and 5mLof 1Nhydrochloric acid,and sonicate for about 3minutes.Add 10mLof acetonitrile,and sonicate for 2minutes.Dilute with water to volume,and mix.

Assay preparation— Weigh and finely powder not less than 10Tablets.Transfer an accurately weighed portion of the powder,equivalent to the weight of 1Tablet,to a 500-mLvolumetric flask.Add 75mLof water and 25mLof 1Nhydrochloric acid,and sonicate for about 5minutes.Add 50mLof acetonitrile,and sonicate for 10minutes.Dilute with water to volume,and mix.Filter through a 0.45-to 2.0-µm membrane filter,discarding the first 10mL.

Chromatographic system (see Chromatography á621ñ)—The liquid chromatograph is equipped with a 280-nm detector and a 3.9-mm ×30-cm column that contains packing L1.The flow rate is about 2mLper minute.Chromatograph the Standard preparation,and record the peak responses as directed under Procedure:the column efficiency determined from the chlorothiazide peak is not less than 1300theoretical plates,the tailing factor for chlorothiazide peak is not more than 2,the resolution,R,between the chlorothiazide and methyldopa peaks is not less than 7,and the relative standard deviation 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 responses for the major peaks.The relative retention times are about 1.0for methyldopa and 2.5for chlorothiazide.Calculate the quantity,in mg,of chlorothiazide (C6H6ClN3O4S2)in the portion of Tablets taken by the formula:

500C(rU/rS),

in which Cis the concentration,in mg per mL,of USP Chlorothiazide RSin the Standard preparation;and rUand rSare the peak responses of the chlorothiazide peak obtained from the Assay preparationand the Standard preparation,respectively.Calculate the quantity,in mg,of methyldopa (C10H13NO4)taken by the same formula,reading “methyldopa”instead of “chlorothiazide.”

Auxiliary Information— Staff Liaison:Andrzej Wilk,Ph.D.,Senior Scientific Associate

Expert Committee:(PA5)Pharmaceutical Analysis 5

USP28–NF23Page 1260

Phone Number:1-301-816-8305