Are metastable polymorphs stable solid solutions?

Polymorphism is the ability of a compound to crystallise in more than one crystal structure. Since most drug compounds are crystalline, discovery and control of polymorphism is a key aspect of drug development and manufacturing.

It is well known in this context that metastable polymorphs are usually discovered first and that stable polymorphs may make an appearance eventually (with time and money). Often, this appearance is linked to an increase in the purity of the active pharmaceutical ingredient (API). Whilst this is general knowledge shared by crystallisation scientists, we currently have a very limited understanding of the fundamental reasons behind it.

In the current project we seek to utilise the above observation to our advantage. We have some experimental and computational evidence that impurities are in fact changing the thermodynamic stability of solid forms through insertion in their crystal lattices (formation of solid solutions). We seek to first test and confirm our hypothesis to then exploit this concept in order to access elusive polymorphs experimentally, be able to produce them reliably and exploit their structure and properties.

Developing a deeper understanding of the impact of impurities in the formation of solid solutions and thus in the realisation of solid forms will have a significant impact in the development of pharmaceuticals, a multi-billion pounds business of great importance to the UK economy.