Spectrophotometric stability-indicating methods for the determination of leflunomide in the presence of its degradates

Five simple and sensitive methods were developed for the determination of leflunomide (1) in the presence of its degradates 4-trifluoromethyl aniline (11) and 3-methyl-4-carboxy isoxazole (111). Method A was based on differential derivative spectrophotometry by measuring the Delta D-1 value at 279.5 nm. Beer's law was obeyed in the concentration range of 2.00-20.00 mu g/mL with mean percentage accuracy of 100.07 +/- 1.32. Method B depended on first-derivative spectrophotometry and measuring the amplitude at 253.4 nm. Beer's law was obeyed in the concentration range of 2.00-16.00 mu g/mL with mean percentage accuracy of 98.42 +/- 1.61. Method C was based on the reaction of degradate (11) with 2,6-dichloroquinone-4-chloroimide (Gibbs reagent). The colored product was measured at 469 nm. Method D depended on the reaction of degradate (11) with para-dimethyl aminocinnamaldehyde (p-DAC). The absorbance of the colored product was measured at 533.4 nm. Method E utilized 3-methyl-2-benzothiazolinone hydrazone in the presence of cerric ammonium sulfate with degradate (11). The green colored product was measured at 605.5 nm. The linearity range was 40.00-280.00, 2.40-24.00, and 30-250 mu g/mL with mean percentage accuracy of 100.75 +/- 1.21, 100.13 +/- 1.45, and 99.74 +/- 1.39 for Methods C-E, respectively. All variables were studied to optimize the reaction conditions. The proposed methods have been successfully applied to the analysis of leflunomide in pharmaceutical dosage forms and the results were statistically compared with that previously reported.