Whitepaper: Monitoring a pharmaceutical crystal transformation in situ

Many organic compounds can crystallize in several different forms called polymorphs. Biologically active polymorphs have different solubility and stability and, hence, different bioavailability. The effectiveness of a drug compound depends critically on its particular polymorph.

There are two types of polymorphic systems: monotropic and enantiotropic. In monotropic systems, one polymorph is more stable than the other at all temperatures below the melting point. In enantiotropic systems, one polymorph is more stable than the other above a certain temperature and the other polymorph is more stable below this temperature. In addition, some systems include hydrates and solvates. Hydrates include water in the crystal lattice and solvates include other solvent molecules. Hydrates and solvates are not true polymorphs because they differ by more than just the three-dimensional arrangement of the crystal lattice, but they occur along with polymorphs in real systems and are important to consider in the same context.