Tretinoin Topical Solution »Tretinoin Topical Solution is a solution of Tretinoin in a suitable nonaqueous,hydrophilic solvent.It contains not less than 90.0percent and not more than 135.0percent of the labeled amount of C20H28O2(w/w). Packaging and storage— Preserve in tight,light-resistant containers. USP Reference standards á11ñ USP Tretinoin RS. Identification— A: Mix 5mLof Topical Solution with 5mLof water:a voluminous yellow precipitate is formed. B: The absorption spectrum,obtained between wavelengths of 300nm and 450nm,of the solution employed for measurement of absorbance in the Assayexhibits maxima and minima at the same wavelengths as that of a similar solution of USP Tretinoin RS,concomitantly measured. Alcohol content á611ñ: between 90.0%and 110.0%of the labeled amount of C2H5OH. Assay— [NOTE—Avoid exposure to strong light,and use low-actinic glassware in the performance of the following procedure.]Transfer to a 100-mLvolumetric flask an accurately weighed quantity of Topical Solution,equivalent to about 375µg of tretinoin,dilute with acidified isopropyl alcohol (prepared by mixing 1mLof 0.01Nhydrochloric acid with isopropyl alcohol to make 1000mLof solution)to volume,and mix.Dissolve an accurately weighed quantity of USP Tretinoin RSin the acidified isopropyl alcohol,and dilute quantitatively and stepwise with the same solvent to obtain a Standard solution having a known concentration of about 3.75µg per mL.Concomitantly determine the absorbances of both solutions in 1-cm cells at the wavelength of maximum absorbance at about 352nm,with a suitable spectrophotometer,using the acidified isopropyl alcohol as the blank.Calculate the quantity,in µg,of C20H28O2in the portion of Topical Solution taken by the formula: 100C(AU/AS), in which Cis the concentration,in µg per mL,of USP Tretinoin RSin the Standard solution,and AUand ASare the absorbances of the solution from the Topical Solution and the Standard solution,respectively. Auxiliary Information— Staff Liaison:Lawrence Evans,III,Ph.D.,Scientist Expert Committee:(PA6)Pharmaceutical Analysis 6 USP28–NF23Page 1957 Phone Number:1-301-816-8389