Identification and Structural Characterization of Novel Process‐Related Impurities Present in Oseltamivir Phosphate Drug Substance and Its Precursor

ABSTRACT This article describes the identification, synthesis, and characterization of unknown impurities present in the oseltamivir phosphate drug substance and its precursor. Four unknown impurities were identified in the drug substance using high‐performance liquid chromatography analysis: Relative retention times of 1.05 and 2.50 azide impurities were found in the drug substance's precursor, and their corresponding impurities were found in the drug substance in amine form. Initially, the LC‐MS method was used to screen these impurities by identifying the mass of the impurities. Based on the impurities' mass and the drug substance's synthesis route, possible ways to form impurities were predicted. All these impurities were synthesized as predicted and characterized through infrared spectroscopy, nuclear magnetic resonance, and high‐resolution mass spectrometry techniques. It was found that these impurities are new and the synthesis method for these impurities is novel and has not been documented in the literature. The azido‐impurities found in this study are categorized as genotoxic impurities by Class 3 of the International Council for Harmonisation M7(R2) criteria, and these impurities must be regulated below certain thresholds.