Synthesis and characterization of critical process related impurities of an asthma drug – Zafirlukast

Zafirlukast is an oral leukotriene receptor antagonist (LTRA) for the treatment of pulmonary disorders such as asthma. During the process development of zafirlukast, eight process related impurities were observed at a level of 0.1–0.15 area percent. Synthesis and characterization of these impurities and investigation of the root cause of their formation is described.