Increasing solubility of enoxacin and norfloxacin by means of salt formation.

A simple method for determining the comparative solubilities of various salt forms of insoluble or slightly soluble drugs is described. FDA-accepted acids, bases, and amino acids are suspended in water with equimolar amounts of a compound having an acidic or basic function. After sonication and filtration, the supernatant is assayed for active drug concentration by ultraviolet spectrophotometry. The method is applied to two slightly soluble antiinfective drugs, enoxacin and norfloxacin, increasing their solubility from less than 0.1 mg/ml to greater than 200 mg/ml in pH 7.4 phosphate buffer.