Transitions in Crystalline States

Polymorphs are different crystalline forms of the same drug. Because these forms have different free energy or chemical potentials, depending on temperature conditions, transitions between polymorphs occur. Polymorphic transitions during storage may alter critical properties of drugs because the solubility and dissolution rate of drug substances generally

130°C (reduced pressure)

monohydrate M [ » anhydrous II

130°C (reduced pressure)

monohydrate M [ » anhydrous II

Scheme 79. Schematic representation of the polymorphic transitions of cianidanol. (Reproduced from Ref. 581 with permission.)

vary with changes in their crystalline form. From a storage perspective, temperature and humidity affect polymorphic transitions.

Polymorphic transitions were observed between two crystalline forms of benoxapro-fen,578 three forms of bromovalerylurea,579 and two forms of pyridoxal hydrochloride.580 These are just a few examples of the many drug substances exhibiting multiple polymorphic forms. Cianidanol exhibits polymorphic transitions between seven different crystalline forms, depending on temperature and humidity as shown in Scheme 79. However, no difference in dissolution rate was observed among these crystalline forms.581

Transitions between anhydrous and hydrated forms have been reported for many drug substances such as raclopride,582 theophylline,583 584 nitrofurantoin,585 sulfaguanidine,586 and phenobarbital.587 Again, significant differences in solubility can exist between the anhydrous and hydrated forms of the same drug.

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