Abstract

By definition, a salt is formed from the reaction of an acid with a base, and this simple chemical reaction involves either a proton transfer or a neutralizing reaction. Thus a drug, which is either an acid or a base, may form a wide range of salts with appropriate bases or acids, respectively. Many drugs are either weak acids or weak bases and, consequently, can form a range of salts. Salt formation may be used to alter the physicochemical, biopharmaceutical, and processing properties of a drug substance without modifying its fundamental chemical structure. In general, the salts of a drug rarely change its pharmacology; however, the intensity of response may be altered. From a pharmaceutical technology perspective, salt formation is a simple means of endowing a drug having ionizable functional groups with unique properties to overcome some undesirable feature of the parent drug. The drug characteristics required for one dosage form are often quite different from those required for another. In practice, the hydrochloride (HCl) salts of basic drugs and the sodium (Na) salts of acidic drugs are the most common. Salts will differ greatly from the parent drug and also from each other in physicochemical properties, such as melting point, solubility, dissolution rate, hygroscopicity, both physical and chemical stability, crystal form, solution pH, and processability. In addition to alterations in physicochemical properties, salt formation can alter organoleptic properties such as taste and occasionally pharmacological response and toxicity. The early literature on the use of different salt forms to modify the dissolution, solubility, organoleptic properties, stability, absorption, pharmacokinetics, pharmacological response, and toxicity of drugs was reviewed by Berge et al. (1) and Bighley et al. (2). The Handbook of Pharmaceutical Salts has been published by Stahl and Wermuth (3) and presents a detailed and comprehensive review addressing preparation, selection, properties, and use of pharmaceutical salts. The related topic of pharmaceutical cocrystals, in which the interaction of drug with the coformer does not result in ionic interaction/bonds, is the subject of a separate Encyclopedia of Pharmaceutical Technology chapter: “Cocrystals: Design, Properties and Formation Mechanisms.”