Structure, solubility, screening, and synthesis of molecular salts

The preparation of molecular salts as potential delivery vehicles for pharmaceutically active compounds is more common than current appreciation of the phenomena governing the solubility and isolation of salts suggests. In addition, it would appear that there are no reported measurements on a large enough data set for a serious structure-property relationship analysis to have been performed for this class of material. This means that at present, the ability to predict which salt forms will have desirable physical properties is essentially nonexistent. The work reported here sets out to explore these issues using new data on 17 salts obtained from a screen performed on the basic pharmaceutical ephedrine. The importance of solvent choice in salt formation, of salt selection in the control of bioavailability and of ternary phase equilibria in salt isolation and the relationship between a number of measured and calculated crystal properties are illustrated and discussed. The consequences of these relations for the general design, implementation, interpretation, and scale-up of salts screens are also explored.