Salts and Co-Crystals: Best Solid Form

In a well-designed salt/co-crystal screening process, the optimal salt/co-crystal is chosen quickly, based on parameters such as solubility, hygroscopicity, crystallinity, chemical stability and suitability for production.

The benefits of using salts and co-crystals are numerous, some of those are as follows:

• Strengthening of intellectual property
• Purification via (salt or co-crystal) crystallization
• Racemic resolution via diastereomer separation
• Structure elucidation (single crystal XRD)

Dedicated or combined salt and co-crystal programs are targeted to identify the best solid forms for further development based on the following criteria:

• Stability (chemical and physical)
• Solubility/dissolution
• Hygroscopicity
• Melting point
• Polymorph landscape complexity and polymorphic transformations
• Number of polymorphs
• Processability
• Ease of preparation

The basis for selection of possible counter ions or co-crystal formers are toxicological aspects, as defined in the FDA GRAS (generally accepted as safe) list but also the intended application. As for example daily dosage forms have different requirements on a counter ion/co-crystal former than drugs that are dosed once weekly.