Salt and Co-crystal Screening

While salt formation is a clear choice from an intellectual property (IP) standpoint, the crystalline nature of salts is not as obvious. Therefore, identifying crystalline salts can add significant IP value to your program.

Co-crystals present an opportunity to modify the properties of a drug substance. Although more challenging to generate, the effort is often worthwhile. The design path for co-crystals is similar to that of salt screens.

Salt formation is a well proven technology to modify the solid-state properties of a drug substance. Some of the properties which can be modified are listed, but this is not a comprehensive list:

Designing A Screen

There is more to designing a salt screen than a list of counterions. A deep understanding of pKa, route of administration and expected dose are critical to allow a phase appropriate study be conducted. Further analytical tools are required on salt screens beyond the basic solid-state characterisation including but not limited to

• XRPD, TGA, DSC, DVS and PLM as per the batch characterisation
• Multinuclear NMR to assess salt formation and salt stoichiometry
• FT-IR to assess salt formation
• Solubility assessment in relevant media (SGF, FaSSIF, FeSSIF, SLF, SNF SCF etc)
• Dissolution assessment in Pion μDISS with μFLUX if required
• Stability assessment under ICH like conditions in 2 Memmert ICH110 chambers at 25°C/60%RH and 40°C/75%RH