Leflunomide EP Impurity D, CAS 42831-50-5, corresponds to the small-molecule metabolite 5-methylisoxazole-4-carboxylic acid, formula C5H5NO3, molecular weight 127.10 g/mol. It is a heteroaromatic carboxylic acid associated with leflunomide synthesis and metabolic degradation, reflecting cleavage of the anilide-linked pharmacophore. In a regulatory context, its presence is relevant for impurity profiling and process understanding rather than intrinsic structural alert classes such as nitrosamines. Quantification commonly requires chromatographic resolution by LC-MS/MS or UHPLC coupled with high-resolution MS, with confirmatory characterization by NMR and MS fragmentation analysis when distinguishing it from related isomeric species.
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