Validated determination of diacerein and its active metabolite, rhein, by stability indicating constant pattern method as a novel manipulation of zero order spectra

A novel spectrophotometric method named constant absorbance pattern technique was proposed for simple determination of binary mixtures exemplified by diacerein, a treatment of osteoarthritis, and rhein, diacerein main alkaline degradation product and active metabolite. The proposed novel constant absorbance pattern technique was highly accurate, precise, sensitive, specific and stability indicating as well. The proposed technique allowed determination of diacerein and rhein in their binary mixtures manipulating zero order absorption spectra with no need for ratio spectra, derivatization steps, or even preliminary separation. Diacerein was determined at zero absorption spectra depending on the constant absorbance pattern between the two wavelengths 230 and 430 nm while rhein was directly determined at zero order at its λmax, 430 nm. The proposed methods showed obeying for Beer-Lambert's law in the concentration range of 1-16 µg mL−1 for both diacerein and rhein. Validation of the proposed methods was carried out according to ICH guidelines with respect to accuracy and precision. Moreover, the proposed method was applied for determination of Diacerein in different pharmaceutical formulations and results were statistically compared with those of the reported HPLC one where no significant difference was found concerning Student's t-test and F-value.