The influence of aqueous content in small scale salt screening—Improving hit rate for weakly basic, low solubility drugs

Salt screening and selection is a well established approach for improving the properties of drug candidates, including dissolution rate and bioavailability. Typically during early development only small amounts of compound are available for solid state profiling, including salt screening. In order to probe large areas of experimental space, high-throughput screening is utilized and is often designed in a way to search for suitable crystallization parameters within hundreds or even thousands of conditions. However, the hit rate in these types of screens can be very low.In order to allow for selection of a salt form early within the drug development process whilst using smaller amounts of compounds, a screening procedure taking into account the compounds properties and the driving forces for salt formation is described. Experiments were carried out on the model compounds clotrimazole, cinnarizine itraconazole and atropine. We found an increase in crystalline hit rate for water-insoluble drugs crystallized from solutions that included at least 10% aqueous content.Conversely it was observed that compounds with greater water solubility did not benefit from aqueous content in salt screening, instead organic solvents lead to more crystalline screening hits. Results from four model compounds show that the inclusion of an aqueous component to the salt reaction can enhance the chance of salt formation and significantly improve the crystalline hit rate for low water soluble drugs.