3-Methoxy-4-[[5-[(methoxycarbonyl)amino]-1-methyl-d3-1H-indol-3-yl]methyl]benzoic Acid is the labeled analogue of 3-Methoxy-4-[[5-[(methoxycarbonyl)amino]-1-methyl-1H-indol-3-yl]methyl]benzoic Acid (M263190), an impurity of Zafirlukast (Z125000), a potent, selective and orally active cysteinyl leukotriene type 1 receptor antagonist. Used as an antiasthmatic.