The subject invention provides a pharmaceutical composition comprising N-ethyl-N-phenyl-1,2,-dihydro-4-hydroxy-5-chloro-1-methyl-2-oxoquinoline-3-carboxamide or the salt thereof; a pharmaceutically acceptable carrier; and not more than 0.5% w/w relative to N-ethyl-N-phenyl-1,2,-dihydro-4-hydroxy-5-chloro-1-methyl-2-oxoquinoline-3-carboxamide of 2-Chloro-6-(1-ethyl-N-methyl-2-oxoindoline-3-carboxamido)benzoic acid, 1H,3H-spiro[5-chloro-1-methylquinoline-2,4-dione-3,3′-[1]ethylindolin-[2]-one], or 5-Chloro-N-ethyl-3-hydroxy-1-methyl-2,4-dioxo-N-phenyl-1,2,3,4-tetrahydro-quinoline-3-carboxamide.