Characterization of impurities in sulfasalazine.

The chemical structures of four impurities isolated from sulfasalazine were determined. Three impurities are the by-products of the reaction process in drug synthesis, i.e., during diazotization of sulfapyridine and during coupling with salicylic acid. Only one contaminant was identified as a starting material, sulfapyridine, in the drug synthesis. The four impurities were characterized as 2-[[p-(2-pyridylsulfamoyl)-phenyl]azo]hydroxybenzene (I), 3-[[P-(2-pyridylsulfamoyl) phenyl]-azo]salicylic acid (II), 5-[[p-[4-(2-pyridylanilino)]-N-phenyl]azo]salicylic acid (III), and sulfapyridine (IV). Compounds I-III are novel molecules, and IV is the precursor of sulfasalazine. The isolation of the impurities was accomplished by TLC and liquid extraction procedures. The methods used to characterize the impurities were a combination of IR, UV, and NMR spectroscopy, mass spectrometry, and TLC. For I and III, comparisons also were made with the synthesized materials to supplement the evaluation.