The Solubility of Drugs

Abstract

In this chapter we will consider the factors controlling the solubility of drugs in solution, in particular the nature of the drug substance, its hydrophobicity, its shape, its surface area, its state of ionisation, the influence of the pH of the medium and the importance of the pKa of the drug. The equation linking solubility to solution pH and drug pKa (equation 5.11) is possibly one of the most important in this book. Experimental methods of measurement of solubility are essential in drug development, as is the ability to predict the solubility of a drug from a knowledge of its chemical structure, recognising hydrophilic and hydrophobic groups and their influence on solubility. How additives, salts, cosolvents, surfactants and other agents affect the solubility of a drug should to an extent be predictable from the theory, bearing in mind the complexity of the body.