5-?(1,?1-?Dimethylethyl)?-?2-?hydroxy-?α,?α-?dimethyl-?4-?nitro-benzeneacetic Acid Methyl Ester is an intermediate in synthesizing Ivacaftor Carboxylic Acid (I940605). It is an impurity of Ivacaftor (I940600); a drug used in the treatment of cystic fibrosis.