Add time:08/27/2019         Source:sciencedirect.com

Most of the drugs developed in the recent years originally remain as a weak base or acid, but drugs in these forms usually have undesirable pharmaceutical characteristics. Pharmaceutical salts, which are frequently referred to as ionizable drugs, have been complexed with a counterion to produce a neutral complex compound. It became a popular approach because of their capability to produce an improved version of the drugs. The salt-forming agents are appointed by trials, testing, and selections based on their raw material’s cost, acidity/basicity, safety profiles of the ionized form, simplicity of crystallization process, the intended therapeutic use of the drugs, etc. Practically, it is unrealistic to screen all possible counterions in the preparation of salts, but some broad guidelines have been traditionally established to limit the selection process. Firstly, a negatively charged counterion should be used for a drug that is a weak base and positively charged counterion for a weak acid. Secondly, the drug must be completely ionized and in a single state of ionization to allow salt formation. Thirdly, the pKa of the base and the acid should differ minimally by a factor of two, although there are exceptions, where the difference of the pKa is less. Often nonaqueous or mixed solvent systems are used in the crystallization, which can dramatically affect the observed pKa. It has been noted that around 50% of the drug molecules approved by the regulatory authorities and marketed as medicinal products are in the salt form. Thus, proper salt selection has become a common standard operation during drug development process. This chapter focuses on various aspects of the salt selection process of drug candidates from the bench top of the chemist to the successful marketing of the drug product.

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