Dear Colleagues,

Polymorphism, the property of a compound to crystallize in more than one distinct crystal form, plays an indispensable role in researching and developing pharmaceuticals, agrochemicals, materials, and food. Polymorphs exhibit different properties, such as crystal habit, solubility, dissolution rate, melting point, stability, mechanical properties, even bioavailability, which may influence the product quality. Therefore, the study of the behavior of polymorphs can provide a theoretical basis for selecting the optimal solid forms and serve in the polymorphic control and optimization of the products as a primary method. Recently, due to the progress in the crystal engineering for pharmaceutics, the multicomponent molecular crystals for the design of new-generation bioavailable drugs have been intensively applied. The development of this area gave a stimulus for the investigation of different aspects of the polymorphism of multicomponent systems. Revealing the similarity and differences between the properties of the polymorphic forms of the individual compounds and multicomponent systems will make a significant contribution to the understanding of the fundamental principles of the design of pharmaceutical materials with the desired characteristics.

I am happy to invite researchers involved in the field of polymorphism of pharmaceutical systems to present their high-quality results in this forum, which is freely available for open access and discussion. This Special Issue will cover multiple aspects connected with several pertinent topics, including but not limited to:

• Screening the polymorphic forms of drug compounds and multicomponent molecular crystals (drug–drug, drug–coformer) by different methods and approaches (crystallization from solvent(s), sublimation, lyophilization, mechanochemistry, slurry, crystallization from melt and so on).
• Analysis of the crystal structures (by single crystal X-ray) of polymorphic pharmaceutical systems (active pharmaceutical ingredients and their multicomponent crystals). Searching the regularities between the crystal structure characteristics and physicochemical properties/processes, which are important for the pharmaceutical industry: solubility, release, membrane permeability, phase transitions (including fusion processes of various modifications), thermodynamic stability, tabletting and so on. 
• Theoretical calculations/evaluations (by various approaches) of the crystal lattice energies of different polymorphic forms and their relationship with the solubility characteristics and thermodynamic stability.
• In vivo and in vitro studies of polymorphic modifications.

Prof. Dr. German L. Perlovich
Guest Editor

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• polymorphic forms
• drug
• multi-components molecular crystals
• screening
• solubility
• thermodynamic stability
• phase transitions
• release
• membrane permeability