Polymorphism and solid-state characterization of APIs: complete guide to crystal forms
Polymorphism: definition and pharmaceutical importance
Polymorphism describes the phenomenon whereby a chemical substance exists in multiple crystalline solid-state forms that share the same molecular composition but differ in their three-dimensional arrangement within the crystal lattice. For pharmaceutical APIs, polymorphism is of critical importance because different crystal forms can differ substantially in solubility, by factors of 2 to more than 100 times, in melting point by up to 30 degrees Celsius, and in physical stability and processability. The best-known example is ritonavir, whose discovery of polymorphic form II in 1998 led to the product being recalled because the new form had 4 times lower solubility and the capsule formulation was no longer therapeutically effective. Pseudo-polymorphs, or solvates and hydrates, are crystal forms in which solvent or water molecules are incorporated into the crystal lattice and likewise exhibit different physicochemical properties. Regulatory authorities require a complete polymorph characterisation report in registration dossiers and control of the polymorphic form in every production batch.
Crystal form screening
Polymorph screening identifies all relevant solid-state forms of an API via systematic crystallisation experiments across a broad range of solvents, temperatures and solvent concentrations. Automated high-throughput screening platforms such as Crystal16 and Crystalline from Technobis crystallise an API under 16 to 96 conditions simultaneously via temperature profiles and antisolvent additions. Screening methods include cooling crystallisation from solvent solutions, antisolvent precipitation, slurry methods in competing solvents, vapour diffusion and mechanochemical grinding of the solid material. Coordinated characterisation of all forms found via XRPD, DSC and TGA at an early stage makes it possible to fully map the polymorphic landscape before registration studies begin. Per project, typically 3 to 15 new crystal forms are found, of which 30 to 50% are pseudo-polymorphic solvates or hydrates in nature and require specific storage conditions.
XRPD and DSC characterisation
X-Ray Powder Diffraction (XRPD) and Differential Scanning Calorimetry (DSC) are the two primary techniques for solid-state characterisation of API crystal forms in pharmaceutical development. XRPD provides a unique diffractogram fingerprint of each crystal form via Bragg reflections at specific 2-theta angles, with the international Cambridge Structural Database (CSD) containing more than 1 million reference structures. Variable-temperature XRPD from 25 to 200 degrees Celsius monitors polymorphic transitions in situ and is essential for understanding the thermodynamic phase diagram of an API. DSC thermograms reveal the enthalpic signatures of phase transitions, melting points and dehydration processes: every endothermic or exothermic peak between 50 and 300 degrees Celsius represents a crystallographically significant event. Thermogravimetric analysis (TGA) combined with DSC distinguishes solvate desolvation from polymorphic transformation by simultaneously measuring weight loss upon heating, with a resolution of 0.1 microgram on modern TGA instruments.
Solubility and stability per form
The intrinsic solubility of different polymorphic forms differs due to the free energy of the crystal lattice and is related via the Ostwald rule of stages: less stable metastable forms have higher solubility. Solubility measurement via the shake flask method at pH 7.4 phosphate buffer distinguishes the thermodynamic solubility of all forms at 37 degrees Celsius in simulated intestinal medium. Metastable forms with higher solubility can create supersaturation in vivo and thereby increase bioavailability, but the risk of spontaneous conversion to the more stable form during processing or storage must be evaluated. Solid-state stability of polymorphs under accelerated storage at 40 degrees Celsius and 75% RH for 3 months shows whether the selected product form remains stable under normal storage conditions. DSC-based thermodynamic stability analysis via the enthalpy-entropy compensation principle can predict the thermodynamic solubility ratio of two polymorphs without experimental solubility measurements.
Polymorph control in manufacturing
Control of the polymorphic form during commercial API manufacturing is a critical quality requirement that must be captured in the control strategy with in-process monitoring and finished-product analysis. In-process control via Raman spectroscopy or NIR monitoring of the crystallisation step detects polymorphic conversion in real time with a response time of 1 to 5 seconds, enabling rapid process intervention. Seeding with the desired polymorphic form at a relative supersaturation of 5 to 15% above the saturation curve initiates crystallisation of exclusively the desired polymorph, achieving a selectivity above 99%. Storage conditions for the API crystallisate and the finished product must be established and specified based on stability studies to prevent polymorphic conversion during long-term storage. XRPD finished-product analysis on every production batch, with a reference diffractogram and acceptance criteria for peak positions within 0.2 degrees 2-theta, is the legal minimum requirement for polymorph control per the Ph.Eur. monograph on API characterisation.
Conclusion
Polymorphism and solid-state characterisation are fundamental aspects of API development that directly affect therapeutic effectiveness, product stability and regulatory acceptance. Hecht Technology supports polymorph screening studies, characterisation and the implementation of polymorph control in API manufacturing processes in accordance with ICH Q6A and Ph.Eur. Contact us for a polymorphism assessment of your API development project.