Structural studies of two solid steroid compounds

Abstract

Polymorphism (including solvated solid forms) is relatively common among steroids.Understanding the physical properties of each polymorph/ solvate is important for evaluating their relative stability and dissolution rates (related to bioavailability). In pharmaceutical industry powder X-ray Diffraction (XRPD), complemented by thermal methods, is the most widely used technique for characterising solid forms. However, these methods do not directly provide molecular-level information. Solid-state NMR here plays an important leading role. A number of polymorphs and solvates of two steroids (androsterone and finasteride) have been studied. Solid-state NMR and XRPD were used to characterise the hemihydrate form and the anhydrous form 1 of androsterone. XRPD was also used to study the transformation between the hemihydrate and the anhydrous form at room temperature as a function of time. Furthermore, the 13C CPMAS spectra of the two forms were assigned by applying specialised NMR pulse sequences. A number of solid forms of finasteride have been described in the scientific and patent literature. Combined characterisation by solid-state NMR and X-ray diffraction has identified deficiencies in the patent literature, reflecting the limitations of characterisation based on XRPD alone. For instance, one of the patented finasteride "polymorphs" was proved to be a dioxane solvate hydrate. Solid-state NMR and single-crystal XRD have been used to define the structure in a number of newly prepared finasteride solvates. The solvates were found to be all isomorphous with each other as well as with another solvate reported in the literature. They appear to be bis (finasteride) monosolvate monohydrates. Preliminary proton spin-lattice relaxation measurements were done on the solvates and on form 11 (anhydrous). It is suggested that the relaxation is driven by reorientation of the methyl groups rather than by solvent dynamics. A new form X was characterised by solid-state NMR though the sample is of unknown origin. TGA and DSC experiments were also carried out on the various forms.