Crystalline solids

Many drugs exist in the crystalline solid state due to reasons of stability and ease of handling during the various stages of drug development. Crysta lline solids can exist in the form of polymorphs, solvates or hydrates, pha se transitions such as polymorph interconversion, desolvation of solvate, f ormation of hydrate and conversion of crystalline to amorphous form may occ ur during various pharmaceutical processes, which may alter the dissolution rate and transport characteristics of the drug. Hence it is desirable to c hoose the most suitable and stable form of the drug in the initial stages o f drug development. The current focus of research in the solid-state area i s to understand the origins of polymorphism at the molecular level, and to predict and prepare the most stable polymorph of a drug. The recent advance s in computational tools allow the prediction of possible polymorphs of the drug from its molecular structure. Sensitive analytical methods are being developed to understand the nature of polymorphism and to characterize the various crystalline forms of a drug in its dosage form. The aim of this rev iew is to emphasize the recent advances made in the area of prediction and characterization of polymorphs and solvates, to address the current challen ges faced by pharmaceutical scientists and to anticipate future development s. (C) 2001 Elsevier Science B.V. All rights reserved.