The pure polymorphic forms I, II, and III of finasteride were prepared and their purity was confirmed by FTIR, differential scanning calorimetry, and X-ray powder diffraction measurements. The preparation experiments demonstrated that the desolvation process of some finasteride solvates does result not only in the formation of polymorphic forms I and II, but also in obtaining the pure form III. The (13)C cross-polarization magic angle spinning (CP-MAS) and the (15)N CP-MAS spectra can distinguish all three polymorphic forms of finasteride. Additionally, the data point to the presence of only one molecule in crystallographic asymmetric unit of polymorphic forms I and III and two molecules in the form II. The application of electronic circular dichroism (ECD) and vibrational circular dichroism (VCD) spectroscopy for finasteride polymorphic forms shows that the three polymorphs could be distinguished by the characteristic shapes of their VCD spectra in the spectral range 1520-1440 cm(-1). The ECD spectral patterns of all these forms, however, are almost indistinguishable because of their close similarity. Comparison of the (13)C CP-MAS spectra of forms I, II, and III with those reported in the literature indicates that the so-called finasteride "form X" is identical to the previously known finasteride form III. On this basis, the existence of form X was excluded.
Keywords: Calorimetry (DSC); FTIR (Fourier transform infrared spectroscopy); X-ray powder diffractometry; chirality; circular dichroism; finasteride; polymorphism; solid-state NMR.
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