Packaging and storage— Preserve in collapsible tubes,or in well-closed,light-resistant containers.

USP Reference standards á11ñ— USP Oxytetracycline RS.USP Polymyxin B Sulfate RS.

Minimum fill á755ñ: meets the requirements.

Water,Method Iá921ñ: not more than 1.0%,10mLof a mixture of toluene and methanol (7:3)being used in place of methanol in the titration vessel.

Assay for oxytetracycline— Proceed with Ointment as directed in the Assay for oxytetracyclineunder Oxytetracycline Hydrochloride and Hydrocortisone Ointment.

Assay for polymyxin B— Transfer an accurately weighed quantity of Ointment,equivalent to about 10,000USP Polymyxin B Units,to a 15-mLcentrifuge tube,add 10mLof ether,stir,and centrifuge for 10minutes.Decant and discard the clear ether.Wash the residue with 10mLof ether,centrifuge for 10minutes,decanting and discarding the clear ether.Wash the residue with several 10-mLportions of acetone,centrifuging,decanting,and discarding each washing until the yellow color is removed from the residue.[NOTE—Take care not to remove any of the residue with the washings.]Add 0.2mLof polysorbate 80to the residue,and mix.Transfer the mixture to a 100-mLvolumetric flask with the aid of Buffer No.6,dilute with the same solvent to volume,and mix.Proceed as directed for polymyxin Bunder Antibiotics—Microbial Assays á81ñ,using an accurately measured volume of this solution diluted quantitatively with Buffer No.6to yield a Test Dilutionhaving a concentration of polymyxin Bassumed to be equal to the median dose level of the Standard.

Expert Committee:(PA7)Pharmaceutical Analysis 7

USP28–NF23Page 1453

Phone Number:1-301-816-8335