Why the First Crystal Form Is Rarely the Right One
When a medicinal chemist synthesises a new active pharmaceutical ingredient for the first time, the crystal form that precipitates out of solution is a function of the solvent system, the temperature, and the rate of crystallisation used. It is rarely the thermodynamically stable form, and it is rarely the form with the best combination of solubility, stability, and processability for a drug product.
Solid state screening is the systematic process of identifying the range of crystal forms a molecule can adopt and selecting the one, or the combination of salt and form, that best supports the target product profile. Getting this right in pre-formulation is one of the highest-value decisions in early drug development. Getting it wrong after GMP manufacturing has started can require a regulatory amendment, a reformulation programme, and a significant delay to the clinical timeline.
What Solid State Screening Covers
Polymorph Screening
Many organic molecules can crystallise in more than one arrangement of molecules in the solid state. These different arrangements, known as polymorphs, have different physical properties including melting point, solubility, dissolution rate, chemical stability, and hygroscopicity. Two polymorphs of the same API can have dramatically different bioavailability profiles. A metastable form that gives excellent dissolution performance in early development may convert to a more stable but less soluble form under the conditions of manufacturing or storage.
Polymorph screening uses a range of crystallisation conditions, solvents, temperatures, and seeding experiments to map the landscape of forms available to a molecule. X-ray powder diffraction (XRPD) is the primary analytical tool for identifying and characterising each form. The ICH Q6A guidance on specifications for new drug substances requires the solid form to be defined and controlled, making this screening work a regulatory necessity as well as a scientific one.
Salt Screening
For ionisable molecules, forming a pharmaceutical salt is often the most effective way to improve aqueous solubility and chemical stability. Salt screening evaluates a range of pharmaceutically acceptable counterions to identify which salt forms are crystalline, physically stable, and give meaningful solubility improvements over the free form. The selection of a salt is also an intellectual property decision: a novel salt form with demonstrated advantages over existing forms may be patentable, extending the effective commercial life of the molecule.
Solvate and Hydrate Assessment
Some molecules preferentially incorporate solvent or water molecules into their crystal lattice, forming solvates or hydrates. These forms can be more stable than anhydrous forms under certain conditions, or they can be a liability if they dehydrate or desolvate during processing. Understanding which solvated forms exist, and under what conditions they are stable, is part of a complete solid state assessment.
Key Solid State Forms and Their Implications
| Form Type | Definition | Key Implication for Development |
| Polymorph | Different crystal packing of the same molecule | Can affect solubility, stability, and bioavailability. Must be controlled in the GMP process. |
| Salt | Ionic form with a pharmaceutically acceptable counterion | Often improves solubility and stability. May be patentable. |
| Cocrystal | Non-ionic multicomponent crystal with a coformer | Useful for non-ionisable molecules. Can improve physical properties. |
| Solvate / Hydrate | Crystal lattice containing solvent or water | Can be stable or unstable depending on humidity. Must be assessed during stability studies. |
| Amorphous | Non-crystalline solid form | Higher energy state, typically higher solubility but lower stability. Requires stabilisation strategy. |
The IP Dimension of Solid State Work
Beyond the formulation and regulatory arguments for thorough solid state screening, there is a commercial argument that is often underappreciated at the early development stage. A patent covering a novel salt form, polymorph, or cocrystal of a development-stage API can significantly extend the period of market exclusivity beyond the composition of matter patent. This is not a secondary consideration. For molecules in highly competitive therapeutic areas, the solid form patent strategy can be as commercially important as the clinical development programme.
Ardena’s solid state research team works closely with clients to ensure that screening work is conducted in a way that generates patentable data where the science supports it, and that the relationship between solid form, IP strategy, and regulatory filing is considered from the outset.
Ardena’s Solid State Research Capabilities in Ghent
Ardena’s solid state research team in Ghent conducts polymorph screening, salt selection, cocrystal screening, and hydrate/solvate assessment using a combination of high-throughput crystallisation and advanced characterisation techniques. XRPD, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and dynamic vapour sorption (DVS) are used to characterise each form and assess its stability under relevant conditions.
The solid state data generated in Ghent is directly connected to the drug product formulation work, meaning that form selection decisions are made with the full development context in mind, not in isolation. This integration is particularly valuable when the optimal form for stability conflicts with the optimal form for dissolution, and a scientifically informed compromise is required.
