Disclosed is a salt prepared from the STAT3 inhibitor known as 2-hydroxy-4-[[2-[[(4-methylphenyl)sulfonyl]oxy]acetyl]amino] benzoic acid and 2-amino-2-(hydroxymethyl)-1,3-propanediol. The salt is soluble in water and stable for extended periods of time. Further provided are pharmaceutical compositions comprising the salt and methods of administering the salt to prevent and treat cancer, such as breast cancer.