2-[2-Methoxy-4-[[[(2-methylphenyl)sulfonyl]amino]carbonyl]phenyl]methyl] Zafirlukast-d3 is the isotope labelled analog of 2-[2-Methoxy-4-[[[(2-methylphenyl)sulfonyl]amino]carbonyl]phenyl]methyl] Zafirlukast (M265710); an impurity of Zafirlukast (Z125000) which is a potent, selective and orally active cysteinyl leukotriene type 1 receptor antagonist. Also used as an antiasthmatic.