Discovery and Characterization of a Late-Appearing Polymorph of Famciclovir

High Resolution Image Download MS PowerPoint Slide Famciclovir (FCV), an antiviral drug widely used to treat herpes zoster (HZ), has previously been reported to exhibit four polymorphs and one hydrate, with crystal structures solved only for Form I and the hydrated form. In this work, a detailed investigation of Form I and a new polymorphic form (Form V) of famciclovir was carried out. Crystallization experiments were performed using different solvents, and the reproducible conditions for obtaining polymorph V are presented. Its structure was determined by single-crystal X-ray diffraction (SXRD) for the first time. The infrared spectra of both polymorphs were compared and interpreted with the assistance of a simulated spectrum. The thermal behavior of both polymorphs was investigated by differential scanning calorimetry (DSC), and the solubilities of each form in ethanol were obtained both by the gravimetric method and the polythermal method. The results reveal an enantiotropic relationship between Forms I and V, with Form V being more stable at room temperature. This study highlights the complexity of polymorphism and its implications for pharmaceutical development.