Changing a drug from its free base or acid to a salt form is commonly done to improve its kinetics, absorption or physicochemical properties (eg, stability, hygroscopicity and flowability). Changing the salt form of a drug is a recognised means of modifying its chemical and biological properties without modifying its structure. Different salts of the same active drug are distinct products with their own chemical and biological profiles that underlie differences in their clinical efficacy and safety.
There is, as yet, no reliable way of predicting exactly what effect changing the salt form of an active drug will have on its biological activity, and the supposition that the same salt form of two related parent compounds will behave in exactly the same way may not be correct. The literature contains many examples of salt forms that differ in the rate of absorption, toxicity and stability of the active drug....
CONCLUSION
Different salt forms of a drug differ in ways that can impact on their clinical efficacy and safety. Changing the salt form varies the solubility and rate of dissolution of a drug, which in turn affects its bioavailability, pharmacokinetic profile, toxicity, and chemical stability. Early selection of an appropriate salt form in the development of a new drug will influence the timely completion of drug development and production, an important factor in accelerating the process of drug discovery.
Substitution of one salt form for another can accelerate the onset and duration of biological activity of a drug and is a recognised means of reducing its toxic potential or improving its chemical stability. It is important to remember, however, that since changing the salt can dramatically change the properties of a drug, every salt form of a drug should be considered as a new medicinal product and tested appropriately before it is released for use in clinical practice.