MONTELUKAST, (S)- is a Montelukast impurity A. Montelukast is a leukotriene receptor antagonist (LTRA) used for the maintenance treatment of asthma and to relieve symptoms of seasonal allergies. Montelukast sodium possesses one chiral carbon center, and a racemic form of montelukast sodium contains equal amounts of the two enantiomeric forms, i.e., the S and R optical isomers. The biologically active form of montelukast sodium is the R stereoisomer. Since the biological activity of the two enantiomers can be different, it is imperative that the amount of the S stereoisomer in the commercial product be controlled to within acceptable limits. The amount of the S optical isomer of montelukast should be lower than 0.15%