The majority of drugs are administered as solids. This means that apart from the molecular structure of the active ingredient, solid-state properties significantly influence the performance of the final product. New substances with pharmaceutical activity are often weak acids or bases or may have unsuitable solid-state properties. In such cases, it may be preferable to develop the drug as a salt or co-crystal form.
When only amorphous forms of the substance are known, specially designed crystallization screenings can help to find crystalline candidates, often improving the chemical purity and stability. Selection of the best available crystalline candidate should be based on the following physical and chemical parameters:
In addition to its impact on chemical and physical properties, salt and co-crystal formation is also relevant for intellectual property considerations because solids with superior properties can be patent protected.